diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0856366ee5..12e397512d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -33,6 +33,13 @@
     2. `FIELD_SAME_UTF8_VALIDATION` catches changes to the `utf8_validation`
        feature, which controls validation of string values.
     3. `ENUM_SAME_TYPE` catches changes to an enum's type, open vs. closed.
+- Adds support for extensions to `buf breaking`. All existing rules for
+  fields are now applied to extensions, except for `FIELD_NO_DELETE` (and its
+  variants). There are also new `EXTENSION_NO_DELETE` and
+  `PACKAGE_EXTENSION_NO_DELETE` rules for catching deletions of an extension.
+  The new rules are not active by default in existing v1 and v1beta1
+  configurations, for backwards-compatibility reasons. Migrate your config to
+  v2 to use them.
 - Adds support for top-level extensions to `buf lint`. It previously only
   checked extensions that were defined inside of messages.
 - Adds a new `FIELD_NOT_REQUIRED` lint rule that prevents use of required
diff --git a/private/buf/cmd/buf/buf_test.go b/private/buf/cmd/buf/buf_test.go
index 5f4c6ec93f..f45b9c435a 100644
--- a/private/buf/cmd/buf/buf_test.go
+++ b/private/buf/cmd/buf/buf_test.go
@@ -438,7 +438,7 @@ func TestFailCheckBreaking2(t *testing.T) {
 		t,
 		nil,
 		bufctl.ExitCodeFileAnnotation,
-		filepath.FromSlash(`testdata/protofileref/breaking/a/foo.proto:7:3:Field "2" on message "Foo" changed type from "int32" to "string".`),
+		filepath.FromSlash(`testdata/protofileref/breaking/a/foo.proto:7:3:Field "2" with name "world" on message "Foo" changed type from "int32" to "string".`),
 		"breaking",
 		filepath.Join("testdata", "protofileref", "breaking", "a", "foo.proto"),
 		"--against",
@@ -454,7 +454,7 @@ func TestFailCheckBreaking3(t *testing.T) {
 		bufctl.ExitCodeFileAnnotation,
 		filepath.FromSlash(`
 		<input>:1:1:Previously present file "bar.proto" was deleted.
-		testdata/protofileref/breaking/a/foo.proto:7:3:Field "2" on message "Foo" changed type from "int32" to "string".
+		testdata/protofileref/breaking/a/foo.proto:7:3:Field "2" with name "world" on message "Foo" changed type from "int32" to "string".
 		`),
 		"breaking",
 		filepath.Join("testdata", "protofileref", "breaking", "a", "foo.proto"),
@@ -471,7 +471,7 @@ func TestFailCheckBreaking4(t *testing.T) {
 		bufctl.ExitCodeFileAnnotation,
 		filepath.FromSlash(`
 		testdata/protofileref/breaking/a/bar.proto:5:1:Previously present field "2" with name "value" on message "Bar" was deleted.
-		testdata/protofileref/breaking/a/foo.proto:7:3:Field "2" on message "Foo" changed type from "int32" to "string".
+		testdata/protofileref/breaking/a/foo.proto:7:3:Field "2" with name "world" on message "Foo" changed type from "int32" to "string".
 		`),
 		"breaking",
 		fmt.Sprintf("%s#include_package_files=true", filepath.Join("testdata", "protofileref", "breaking", "a", "foo.proto")),
@@ -488,7 +488,7 @@ func TestFailCheckBreaking5(t *testing.T) {
 		bufctl.ExitCodeFileAnnotation,
 		filepath.FromSlash(`
     <input>:1:1:Previously present file "bar.proto" was deleted.
-		testdata/protofileref/breaking/a/foo.proto:7:3:Field "2" on message "Foo" changed type from "int32" to "string".
+		testdata/protofileref/breaking/a/foo.proto:7:3:Field "2" with name "world" on message "Foo" changed type from "int32" to "string".
 		`),
 		"breaking",
 		filepath.Join("testdata", "protofileref", "breaking", "a", "foo.proto"),
@@ -854,6 +854,7 @@ func TestCheckLsBreakingRulesV2(t *testing.T) {
 	expectedStdout := `
 ID                                              CATEGORIES                      PURPOSE
 ENUM_NO_DELETE                                  FILE                            Checks that enums are not deleted from a given file.
+EXTENSION_NO_DELETE                             FILE                            Checks that extensions are not deleted from a given file.
 FILE_NO_DELETE                                  FILE                            Checks that files are not deleted.
 MESSAGE_NO_DELETE                               FILE                            Checks that messages are not deleted from a given file.
 SERVICE_NO_DELETE                               FILE                            Checks that services are not deleted from a given file.
@@ -905,6 +906,7 @@ RPC_SAME_REQUEST_TYPE                           FILE, PACKAGE, WIRE_JSON, WIRE
 RPC_SAME_RESPONSE_TYPE                          FILE, PACKAGE, WIRE_JSON, WIRE  Checks that rpcs are have the same response type.
 RPC_SAME_SERVER_STREAMING                       FILE, PACKAGE, WIRE_JSON, WIRE  Checks that rpcs have the same server streaming value.
 PACKAGE_ENUM_NO_DELETE                          PACKAGE                         Checks that enums are not deleted from a given package.
+PACKAGE_EXTENSION_NO_DELETE                     PACKAGE                         Checks that extensions are not deleted from a given package.
 PACKAGE_MESSAGE_NO_DELETE                       PACKAGE                         Checks that messages are not deleted from a given package.
 PACKAGE_NO_DELETE                               PACKAGE                         Checks that packages are not deleted.
 PACKAGE_SERVICE_NO_DELETE                       PACKAGE                         Checks that services are not deleted from a given package.
@@ -1905,7 +1907,7 @@ func TestBreakingWithPaths(t *testing.T) {
 		t,
 		nil,
 		bufctl.ExitCodeFileAnnotation,
-		`a/v3/a.proto:6:3:Field "1" on message "Foo" changed type from "string" to "int32".
+		`a/v3/a.proto:6:3:Field "1" with name "key" on message "Foo" changed type from "string" to "int32".
 a/v3/a.proto:7:3:Field "2" with name "Value" on message "Foo" changed option "json_name" from "value" to "Value".
 a/v3/a.proto:7:10:Field "2" on message "Foo" changed name from "value" to "Value".`,
 		"",
diff --git a/private/bufpkg/bufanalysis/bufanalysistesting/bufanalysistesting.go b/private/bufpkg/bufanalysis/bufanalysistesting/bufanalysistesting.go
index 7360cc00f7..8304a55160 100644
--- a/private/bufpkg/bufanalysis/bufanalysistesting/bufanalysistesting.go
+++ b/private/bufpkg/bufanalysis/bufanalysistesting/bufanalysistesting.go
@@ -110,19 +110,33 @@ func AssertFileAnnotationsEqual(
 	actual = normalizeFileAnnotations(t, actual)
 	if !assert.Equal(
 		t,
-		slicesext.Map(expected, func(annotation bufanalysis.FileAnnotation) string { return annotation.String() }),
-		slicesext.Map(actual, func(annotation bufanalysis.FileAnnotation) string { return annotation.String() }),
+		slicesext.Map(expected, bufanalysis.FileAnnotation.String),
+		slicesext.Map(actual, bufanalysis.FileAnnotation.String),
 	) {
-		t.Log("Expecting:")
+		t.Log("If actuals are correct, change expectations to the following:")
 		for _, annotation := range actual {
-			t.Logf("    bufanalysistesting.NewFileAnnotation(t, %q, %d, %d, %d, %d, %q),",
-				annotation.FileInfo().Path(),
-				annotation.StartLine(),
-				annotation.StartColumn(),
-				annotation.EndLine(),
-				annotation.EndColumn(),
-				annotation.Type(),
-			)
+			if annotation.StartLine() == 0 && annotation.StartColumn() == 0 &&
+				annotation.EndLine() == 0 && annotation.EndColumn() == 0 {
+				if annotation.FileInfo().Path() == "" {
+					t.Logf("    bufanalysistesting.NewFileAnnotationNoLocationOrPath(t, %q),",
+						annotation.Type(),
+					)
+				} else {
+					t.Logf("    bufanalysistesting.NewFileAnnotationNoLocation(t, %q, %q),",
+						annotation.FileInfo().Path(),
+						annotation.Type(),
+					)
+				}
+			} else {
+				t.Logf("    bufanalysistesting.NewFileAnnotation(t, %q, %d, %d, %d, %d, %q),",
+					annotation.FileInfo().Path(),
+					annotation.StartLine(),
+					annotation.StartColumn(),
+					annotation.EndLine(),
+					annotation.EndColumn(),
+					annotation.Type(),
+				)
+			}
 		}
 	}
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/bufbreaking_test.go b/private/bufpkg/bufcheck/bufbreaking/bufbreaking_test.go
index 779312957f..c5bd64ff20 100644
--- a/private/bufpkg/bufcheck/bufbreaking/bufbreaking_test.go
+++ b/private/bufpkg/bufcheck/bufbreaking/bufbreaking_test.go
@@ -154,6 +154,19 @@ func TestRunBreakingExtensionMessageNoDelete(t *testing.T) {
 	)
 }
 
+func TestRunBreakingExtensionNoDelete(t *testing.T) {
+	t.Parallel()
+	testBreaking(
+		t,
+		"breaking_extension_no_delete",
+		bufanalysistesting.NewFileAnnotationNoLocation(t, "2.proto", "EXTENSION_NO_DELETE"),
+		bufanalysistesting.NewFileAnnotationNoLocation(t, "2.proto", "EXTENSION_NO_DELETE"),
+		bufanalysistesting.NewFileAnnotationNoLocation(t, "2.proto", "EXTENSION_NO_DELETE"),
+		bufanalysistesting.NewFileAnnotationNoLocation(t, "3.proto", "EXTENSION_NO_DELETE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 8, 3, 14, 4, "EXTENSION_NO_DELETE"),
+	)
+}
+
 func TestRunBreakingFieldNoDelete(t *testing.T) {
 	t.Parallel()
 	testBreaking(
@@ -225,6 +238,8 @@ func TestRunBreakingFieldSameCardinality(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 14, 3, 14, 24, "FIELD_SAME_CARDINALITY"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 70, 3, 70, 26, "FIELD_SAME_CARDINALITY"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 71, 3, 71, 26, "FIELD_SAME_CARDINALITY"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 17, 7, 17, 32, "FIELD_SAME_CARDINALITY"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 25, 3, 25, 20, "FIELD_SAME_CARDINALITY"),
 	)
 }
 
@@ -268,12 +283,19 @@ func TestRunBreakingFieldSameCppStringType(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 31, 34, 31, 52, "FIELD_SAME_CPP_STRING_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 33, 25, 33, 59, "FIELD_SAME_CPP_STRING_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 34, 24, 34, 58, "FIELD_SAME_CPP_STRING_TYPE"),
-		bufanalysistesting.NewFileAnnotation(t, "3.proto", 10, 24, 10, 36, "FIELD_SAME_CPP_STRING_TYPE"),
-		bufanalysistesting.NewFileAnnotation(t, "3.proto", 11, 23, 11, 33, "FIELD_SAME_CPP_STRING_TYPE"),
-		bufanalysistesting.NewFileAnnotation(t, "3.proto", 12, 33, 12, 51, "FIELD_SAME_CPP_STRING_TYPE"),
-		bufanalysistesting.NewFileAnnotation(t, "3.proto", 13, 32, 13, 68, "FIELD_SAME_CPP_STRING_TYPE"),
-		bufanalysistesting.NewFileAnnotation(t, "3.proto", 15, 23, 15, 57, "FIELD_SAME_CPP_STRING_TYPE"),
-		bufanalysistesting.NewFileAnnotation(t, "3.proto", 16, 3, 16, 32, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 43, 7, 43, 37, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 44, 29, 44, 63, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 11, 24, 11, 36, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 12, 23, 12, 33, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 13, 33, 13, 51, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 14, 32, 14, 68, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 16, 23, 16, 57, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 17, 3, 17, 32, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 27, 39, 27, 73, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 28, 7, 28, 29, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 30, 29, 30, 41, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 36, 3, 36, 33, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 40, 3, 40, 34, "FIELD_SAME_CPP_STRING_TYPE"),
 	)
 }
 
@@ -289,6 +311,7 @@ func TestRunBreakingFieldSameCType(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "1.proto", 23, 21, 23, 33, "FIELD_SAME_CPP_STRING_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 49, 28, 49, 48, "FIELD_SAME_CPP_STRING_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 50, 28, 50, 42, "FIELD_SAME_CPP_STRING_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 58, 32, 58, 36, "FIELD_SAME_CPP_STRING_TYPE"),
 	)
 }
 
@@ -297,26 +320,26 @@ func TestRunBreakingFieldSameJavaUTF8Validation(t *testing.T) {
 	testBreaking(
 		t,
 		"breaking_field_same_java_utf8_validation",
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 17, 3, 17, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 18, 3, 18, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 19, 3, 19, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 19, 3, 19, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 28, 3, 28, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 29, 3, 29, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 30, 3, 30, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 30, 3, 30, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 39, 3, 39, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 40, 3, 40, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 41, 3, 41, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 41, 3, 41, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 50, 3, 50, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 51, 3, 51, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 52, 3, 52, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 52, 3, 52, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 61, 3, 61, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 62, 3, 62, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 63, 3, 63, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "1.proto", 63, 3, 63, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 19, 3, 19, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 20, 3, 20, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 21, 3, 21, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 21, 3, 21, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 30, 3, 30, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 31, 3, 31, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 32, 3, 32, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 32, 3, 32, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 42, 3, 42, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 43, 3, 43, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 44, 3, 44, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 44, 3, 44, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 54, 3, 54, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 55, 3, 55, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 56, 3, 56, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 56, 3, 56, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 66, 3, 66, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 67, 3, 67, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 68, 3, 68, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 68, 3, 68, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 5, 1, 5, 38, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 5, 1, 5, 38, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 5, 1, 5, 38, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
@@ -343,10 +366,16 @@ func TestRunBreakingFieldSameJavaUTF8Validation(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "6.proto", 12, 3, 12, 26, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
 		bufanalysistesting.NewFileAnnotation(t, "6.proto", 13, 3, 13, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
 		bufanalysistesting.NewFileAnnotation(t, "6.proto", 13, 3, 13, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "6.proto", 79, 3, 79, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "6.proto", 79, 3, 79, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "6.proto", 90, 3, 90, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
-		bufanalysistesting.NewFileAnnotation(t, "6.proto", 90, 3, 90, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "6.proto", 23, 5, 23, 24, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "6.proto", 24, 5, 24, 33, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "6.proto", 106, 5, 106, 24, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "6.proto", 114, 3, 114, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "6.proto", 114, 3, 114, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "6.proto", 125, 3, 125, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "6.proto", 125, 3, 125, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "6.proto", 134, 3, 134, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "6.proto", 135, 3, 135, 31, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
+		bufanalysistesting.NewFileAnnotation(t, "6.proto", 154, 3, 154, 22, "FIELD_SAME_JAVA_UTF8_VALIDATION"),
 	)
 }
 
@@ -387,6 +416,11 @@ func TestRunBreakingFieldSameDefault(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "1.proto", 104, 27, 104, 42, "FIELD_SAME_DEFAULT"),
 		bufanalysistesting.NewFileAnnotation(t, "1.proto", 109, 5, 109, 26, "FIELD_SAME_DEFAULT"),
 		bufanalysistesting.NewFileAnnotation(t, "1.proto", 111, 27, 111, 40, "FIELD_SAME_DEFAULT"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 140, 30, 140, 45, "FIELD_SAME_DEFAULT"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 141, 5, 141, 27, "FIELD_SAME_DEFAULT"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 148, 29, 148, 42, "FIELD_SAME_DEFAULT"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 150, 28, 150, 43, "FIELD_SAME_DEFAULT"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 153, 29, 153, 42, "FIELD_SAME_DEFAULT"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 6, 18, 6, 33, "FIELD_SAME_DEFAULT"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 8, 17, 8, 30, "FIELD_SAME_DEFAULT"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 10, 17, 10, 32, "FIELD_SAME_DEFAULT"),
@@ -468,8 +502,14 @@ func TestRunBreakingFieldSameJSType(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "1.proto", 13, 22, 13, 40, "FIELD_SAME_JSTYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "1.proto", 14, 7, 14, 21, "FIELD_SAME_JSTYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "1.proto", 22, 20, 22, 38, "FIELD_SAME_JSTYPE"),
-		bufanalysistesting.NewFileAnnotation(t, "2.proto", 49, 27, 49, 45, "FIELD_SAME_JSTYPE"),
-		bufanalysistesting.NewFileAnnotation(t, "2.proto", 50, 3, 50, 26, "FIELD_SAME_JSTYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 51, 27, 51, 45, "FIELD_SAME_JSTYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 52, 3, 52, 26, "FIELD_SAME_JSTYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 57, 32, 57, 50, "FIELD_SAME_JSTYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 58, 34, 58, 52, "FIELD_SAME_JSTYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 59, 5, 59, 34, "FIELD_SAME_JSTYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 68, 29, 68, 47, "FIELD_SAME_JSTYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 69, 29, 69, 47, "FIELD_SAME_JSTYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 70, 3, 70, 28, "FIELD_SAME_JSTYPE"),
 	)
 }
 
@@ -559,7 +599,12 @@ func TestRunBreakingFieldSameName(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "1.proto", 7, 9, 7, 13, "FIELD_SAME_NAME"),
 		bufanalysistesting.NewFileAnnotation(t, "1.proto", 15, 13, 15, 17, "FIELD_SAME_NAME"),
 		bufanalysistesting.NewFileAnnotation(t, "1.proto", 26, 11, 26, 15, "FIELD_SAME_NAME"),
-		bufanalysistesting.NewFileAnnotation(t, "2.proto", 60, 11, 60, 15, "FIELD_SAME_NAME"),
+		bufanalysistesting.NewFileAnnotation(t, "1.proto", 35, 14, 35, 25, "FIELD_SAME_NAME"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 48, 23, 48, 33, "FIELD_SAME_NAME"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 64, 19, 64, 27, "FIELD_SAME_NAME"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 72, 19, 72, 29, "FIELD_SAME_NAME"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 78, 11, 78, 15, "FIELD_SAME_NAME"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 82, 19, 82, 27, "FIELD_SAME_NAME"),
 	)
 }
 
@@ -605,8 +650,13 @@ func TestRunBreakingFieldSameType(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "1.proto", 41, 5, 41, 20, "FIELD_SAME_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 57, 5, 57, 10, "FIELD_SAME_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 58, 5, 58, 9, "FIELD_SAME_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 68, 12, 68, 16, "FIELD_SAME_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "3.proto", 8, 3, 8, 7, "FIELD_SAME_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "3.proto", 9, 3, 9, 7, "FIELD_SAME_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 31, 5, 31, 9, "FIELD_SAME_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 33, 5, 33, 10, "FIELD_SAME_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 38, 3, 38, 8, "FIELD_SAME_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 39, 3, 39, 7, "FIELD_SAME_TYPE"),
 	)
 }
 
@@ -699,6 +749,8 @@ func TestRunBreakingFieldWireCompatibleCardinality(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 14, 3, 14, 24, "FIELD_WIRE_COMPATIBLE_CARDINALITY"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 70, 3, 70, 26, "FIELD_WIRE_COMPATIBLE_CARDINALITY"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 71, 3, 71, 26, "FIELD_WIRE_COMPATIBLE_CARDINALITY"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 17, 7, 17, 32, "FIELD_WIRE_COMPATIBLE_CARDINALITY"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 25, 3, 25, 20, "FIELD_WIRE_COMPATIBLE_CARDINALITY"),
 	)
 }
 
@@ -726,8 +778,11 @@ func TestRunBreakingFieldWireCompatibleType(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 79, 3, 79, 6, "FIELD_WIRE_COMPATIBLE_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 80, 3, 80, 6, "FIELD_WIRE_COMPATIBLE_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 85, 3, 85, 9, "FIELD_WIRE_COMPATIBLE_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 89, 12, 89, 15, "FIELD_WIRE_COMPATIBLE_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "3.proto", 6, 3, 6, 7, "FIELD_WIRE_COMPATIBLE_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "3.proto", 7, 3, 7, 7, "FIELD_WIRE_COMPATIBLE_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 15, 5, 15, 12, "FIELD_WIRE_COMPATIBLE_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 23, 3, 23, 9, "FIELD_WIRE_COMPATIBLE_TYPE"),
 	)
 }
 
@@ -759,6 +814,8 @@ func TestRunBreakingFieldWireJSONCompatibleCardinality(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 14, 3, 14, 24, "FIELD_WIRE_JSON_COMPATIBLE_CARDINALITY"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 70, 3, 70, 26, "FIELD_WIRE_JSON_COMPATIBLE_CARDINALITY"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 71, 3, 71, 26, "FIELD_WIRE_JSON_COMPATIBLE_CARDINALITY"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 17, 7, 17, 32, "FIELD_WIRE_JSON_COMPATIBLE_CARDINALITY"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 25, 3, 25, 20, "FIELD_WIRE_JSON_COMPATIBLE_CARDINALITY"),
 	)
 }
 
@@ -787,8 +844,14 @@ func TestRunBreakingFieldWireJSONCompatibleType(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 83, 3, 83, 6, "FIELD_WIRE_JSON_COMPATIBLE_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 87, 3, 87, 8, "FIELD_WIRE_JSON_COMPATIBLE_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "2.proto", 88, 3, 88, 9, "FIELD_WIRE_JSON_COMPATIBLE_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 92, 12, 92, 15, "FIELD_WIRE_JSON_COMPATIBLE_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "2.proto", 93, 3, 93, 8, "FIELD_WIRE_JSON_COMPATIBLE_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "3.proto", 6, 3, 6, 7, "FIELD_WIRE_JSON_COMPATIBLE_TYPE"),
 		bufanalysistesting.NewFileAnnotation(t, "3.proto", 7, 3, 7, 7, "FIELD_WIRE_JSON_COMPATIBLE_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 14, 5, 14, 10, "FIELD_WIRE_JSON_COMPATIBLE_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 15, 5, 15, 12, "FIELD_WIRE_JSON_COMPATIBLE_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 16, 5, 16, 11, "FIELD_WIRE_JSON_COMPATIBLE_TYPE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 23, 3, 23, 9, "FIELD_WIRE_JSON_COMPATIBLE_TYPE"),
 	)
 }
 
@@ -963,6 +1026,17 @@ func TestRunBreakingOneofNoDelete(t *testing.T) {
 	)
 }
 
+func TestRunBreakingPackageExtensionNoDelete(t *testing.T) {
+	t.Parallel()
+	testBreaking(
+		t,
+		"breaking_package_extension_no_delete",
+		bufanalysistesting.NewFileAnnotationNoLocation(t, "2.proto", "PACKAGE_EXTENSION_NO_DELETE"),
+		bufanalysistesting.NewFileAnnotationNoLocation(t, "3.proto", "PACKAGE_EXTENSION_NO_DELETE"),
+		bufanalysistesting.NewFileAnnotation(t, "3.proto", 8, 3, 14, 4, "PACKAGE_EXTENSION_NO_DELETE"),
+	)
+}
+
 func TestRunBreakingPackageNoDelete(t *testing.T) {
 	t.Parallel()
 	testBreaking(
@@ -983,6 +1057,15 @@ func TestRunBreakingPackageNoDelete(t *testing.T) {
 	)
 }
 
+func TestRunBreakingPackageServiceNoDelete(t *testing.T) {
+	t.Parallel()
+	testBreaking(
+		t,
+		"breaking_package_service_no_delete",
+		bufanalysistesting.NewFileAnnotationNoLocation(t, "1.proto", "PACKAGE_SERVICE_NO_DELETE"),
+	)
+}
+
 func TestRunBreakingReservedEnumNoDelete(t *testing.T) {
 	t.Parallel()
 	testBreaking(
@@ -1089,15 +1172,6 @@ func TestRunBreakingServiceNoDelete(t *testing.T) {
 	)
 }
 
-func TestRunBreakingPackageServiceNoDelete(t *testing.T) {
-	t.Parallel()
-	testBreaking(
-		t,
-		"breaking_package_service_no_delete",
-		bufanalysistesting.NewFileAnnotationNoLocation(t, "1.proto", "PACKAGE_SERVICE_NO_DELETE"),
-	)
-}
-
 func TestRunBreakingIgnoreUnstablePackagesTrue(t *testing.T) {
 	t.Parallel()
 	testBreaking(
diff --git a/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingbuild/bufbreakingbuild.go b/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingbuild/bufbreakingbuild.go
index cebffd9c3a..9fbae52e70 100644
--- a/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingbuild/bufbreakingbuild.go
+++ b/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingbuild/bufbreakingbuild.go
@@ -74,6 +74,12 @@ var (
 		"extension ranges are not deleted from a given message",
 		bufbreakingcheck.CheckExtensionMessageNoDelete,
 	)
+	// ExtensionNoDeleteRuleBuilder is a rule builder.
+	ExtensionNoDeleteRuleBuilder = internal.NewNopRuleBuilder(
+		"EXTENSION_NO_DELETE",
+		"extensions are not deleted from a given file",
+		bufbreakingcheck.CheckExtensionNoDelete,
+	)
 	// FieldNoDeleteRuleBuilder is a rule builder.
 	FieldNoDeleteRuleBuilder = internal.NewNopRuleBuilder(
 		"FIELD_NO_DELETE",
@@ -366,6 +372,12 @@ var (
 		"enums are not deleted from a given package",
 		bufbreakingcheck.CheckPackageEnumNoDelete,
 	)
+	// PackageExtensionNoDeleteRuleBuilder is a rule builder.
+	PackageExtensionNoDeleteRuleBuilder = internal.NewNopRuleBuilder(
+		"PACKAGE_EXTENSION_NO_DELETE",
+		"extensions are not deleted from a given package",
+		bufbreakingcheck.CheckPackageExtensionNoDelete,
+	)
 	// PackageMessageNoDeleteRuleBuilder is a rule builder.
 	PackageMessageNoDeleteRuleBuilder = internal.NewNopRuleBuilder(
 		"PACKAGE_MESSAGE_NO_DELETE",
diff --git a/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingcheck/bufbreakingcheck.go b/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingcheck/bufbreakingcheck.go
index 830b0dbc58..ddfdf33a97 100644
--- a/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingcheck/bufbreakingcheck.go
+++ b/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingcheck/bufbreakingcheck.go
@@ -56,7 +56,7 @@ func checkEnumNoDelete(add addFunc, corpus *corpus, previousFile bufprotosource.
 	for previousNestedName := range previousNestedNameToEnum {
 		if _, ok := nestedNameToEnum[previousNestedName]; !ok {
 			// TODO: search for enum in other files and return that the enum was moved?
-			descriptor, location, err := getDescriptorAndLocationForDeletedEnum(file, previousNestedName)
+			descriptor, location, err := getDescriptorAndLocationForDeletedElement(file, previousNestedName)
 			if err != nil {
 				return err
 			}
@@ -242,6 +242,30 @@ func checkExtensionMessageNoDelete(add addFunc, corpus *corpus, previousMessage
 	return checkTagRanges(add, "extension", message, previousMessage.ExtensionRanges(), message.ExtensionRanges())
 }
 
+// CheckExtensionNoDelete is a check function.
+var CheckExtensionNoDelete = newFilePairCheckFunc(checkExtensionNoDelete)
+
+func checkExtensionNoDelete(add addFunc, corpus *corpus, previousFile bufprotosource.File, file bufprotosource.File) error {
+	previousNestedNameToExtension, err := bufprotosource.NestedNameToExtension(previousFile)
+	if err != nil {
+		return err
+	}
+	nestedNameToExtension, err := bufprotosource.NestedNameToExtension(file)
+	if err != nil {
+		return err
+	}
+	for previousNestedName := range previousNestedNameToExtension {
+		if _, ok := nestedNameToExtension[previousNestedName]; !ok {
+			descriptor, location, err := getDescriptorAndLocationForDeletedElement(file, previousNestedName)
+			if err != nil {
+				return err
+			}
+			add(descriptor, nil, location, `Previously present extension %q was deleted from file.`, previousNestedName)
+		}
+	}
+	return nil
+}
+
 // CheckFieldNoDelete is a check function.
 var CheckFieldNoDelete = newMessagePairCheckFunc(checkFieldNoDelete)
 
@@ -275,8 +299,6 @@ func checkFieldNoDeleteWithRules(add addFunc, previousMessage bufprotosource.Mes
 	for previousNumber, previousField := range previousNumberToField {
 		if _, ok := numberToField[previousNumber]; !ok {
 			if !isDeletedFieldAllowedWithRules(previousField, message, allowIfNumberReserved, allowIfNameReserved) {
-				// otherwise prints as hex
-				previousNumberString := strconv.FormatInt(int64(previousNumber), 10)
 				suffix := ""
 				if allowIfNumberReserved && allowIfNameReserved {
 					return errors.New("both allowIfNumberReserved and allowIfNameReserved set")
@@ -287,7 +309,17 @@ func checkFieldNoDeleteWithRules(add addFunc, previousMessage bufprotosource.Mes
 				if allowIfNameReserved {
 					suffix = fmt.Sprintf(` without reserving the name %q`, previousField.Name())
 				}
-				add(message, nil, message.Location(), `Previously present field %q with name %q on message %q was deleted%s.`, previousNumberString, previousField.Name(), message.Name(), suffix)
+				description := fieldDescription(previousField)
+				// Description will start with capital letter; lower-case it
+				// to better fit in this message.
+				description = strings.ToLower(description[:1]) + description[1:]
+				add(
+					message,
+					nil,
+					message.Location(),
+					`Previously present %s was deleted%s.`,
+					description,
+					suffix)
 			}
 		}
 	}
@@ -323,11 +355,9 @@ func checkFieldSameCardinality(
 	previousCardinality := getCardinality(previousDescriptor)
 	currentCardinality := getCardinality(descriptor)
 	if previousCardinality != currentCardinality {
-		// otherwise prints as hex
-		numberString := strconv.FormatInt(int64(field.Number()), 10)
 		add(field, nil, field.Location(),
-			`Field %q on message %q changed cardinality from %q to %q.`,
-			numberString, field.ParentMessage().Name(),
+			`%s changed cardinality from %q to %q.`,
+			fieldDescription(field),
 			previousCardinality,
 			currentCardinality,
 		)
@@ -367,8 +397,6 @@ func checkFieldSameCppStringType(
 	if (previousStringType != stringType || previousIsStringPiece != isStringPiece) &&
 		// it is NOT breaking to move from string_piece -> string
 		!(previousIsStringPiece && stringType == protobuf.CppFeatures_STRING) {
-		// otherwise prints as hex
-		numberString := strconv.FormatInt(int64(field.Number()), 10)
 		var previousType, currentType fmt.Stringer
 		if previousIsStringPiece {
 			previousType = descriptorpb.FieldOptions_STRING_PIECE
@@ -384,10 +412,8 @@ func checkFieldSameCppStringType(
 			field,
 			nil,
 			withBackupLocation(field.CTypeLocation(), fieldCppStringTypeLocation(field), field.Location()),
-			`Field %q with name %q on message %q changed C++ string type from %q to %q.`,
-			numberString,
-			field.Name(),
-			field.ParentMessage().Name(),
+			`%s changed C++ string type from %q to %q.`,
+			fieldDescription(field),
 			previousType,
 			currentType,
 		)
@@ -419,16 +445,12 @@ func checkFieldSameJavaUTF8Validation(
 		return err
 	}
 	if previousValidation != validation {
-		// otherwise prints as hex
-		numberString := strconv.FormatInt(int64(field.Number()), 10)
 		add(
 			field,
 			nil,
 			withBackupLocation(field.File().JavaStringCheckUtf8Location(), fieldJavaUTF8ValidationLocation(field), field.Location()),
-			`Field %q with name %q on message %q changed Java string UTF8 validation from %q to %q.`,
-			numberString,
-			field.Name(),
-			field.ParentMessage().Name(),
+			`%s changed Java string UTF8 validation from %q to %q.`,
+			fieldDescription(field),
 			previousValidation,
 			validation,
 		)
@@ -457,16 +479,12 @@ func checkFieldSameDefault(
 		return nil
 	}
 	if !defaultsEqual(previousDefault, currentDefault) {
-		// otherwise prints as hex
-		numberString := strconv.FormatInt(int64(field.Number()), 10)
 		add(
 			field,
 			nil,
 			withBackupLocation(field.DefaultLocation(), field.Location()),
-			`Field %q with name %q on message %q changed default value from %v to %v.`,
-			numberString,
-			field.Name(),
-			field.ParentMessage().Name(),
+			`% changed default value from %v to %v.`,
+			fieldDescription(field),
 			previousDefault.printable,
 			currentDefault.printable,
 		)
@@ -478,10 +496,15 @@ func checkFieldSameDefault(
 var CheckFieldSameJSONName = newFieldPairCheckFunc(checkFieldSameJSONName)
 
 func checkFieldSameJSONName(add addFunc, corpus *corpus, previousField bufprotosource.Field, field bufprotosource.Field) error {
+	if previousField.Extendee() != "" {
+		// JSON name can't be set explicitly for extensions
+		return nil
+	}
 	if previousField.JSONName() != field.JSONName() {
-		// otherwise prints as hex
-		numberString := strconv.FormatInt(int64(field.Number()), 10)
-		add(field, nil, withBackupLocation(field.JSONNameLocation(), field.Location()), `Field %q with name %q on message %q changed option "json_name" from %q to %q.`, numberString, field.Name(), field.ParentMessage().Name(), previousField.JSONName(), field.JSONName())
+		add(field, nil, withBackupLocation(field.JSONNameLocation(), field.Location()),
+			`%s changed option "json_name" from %q to %q.`,
+			fieldDescription(field),
+			previousField.JSONName(), field.JSONName())
 	}
 	return nil
 }
@@ -490,10 +513,15 @@ func checkFieldSameJSONName(add addFunc, corpus *corpus, previousField bufprotos
 var CheckFieldSameJSType = newFieldPairCheckFunc(checkFieldSameJSType)
 
 func checkFieldSameJSType(add addFunc, corpus *corpus, previousField bufprotosource.Field, field bufprotosource.Field) error {
+	if !is64bitInteger(previousField.Type()) || !is64bitInteger(field.Type()) {
+		// this check only applies to 64-bit integer fields
+		return nil
+	}
 	if previousField.JSType() != field.JSType() {
-		// otherwise prints as hex
-		numberString := strconv.FormatInt(int64(field.Number()), 10)
-		add(field, nil, withBackupLocation(field.JSTypeLocation(), field.Location()), `Field %q with name %q on message %q changed option "jstype" from %q to %q.`, numberString, field.Name(), field.ParentMessage().Name(), previousField.JSType().String(), field.JSType().String())
+		add(field, nil, withBackupLocation(field.JSTypeLocation(), field.Location()),
+			`%s changed option "jstype" from %q to %q.`,
+			fieldDescription(field),
+			previousField.JSType().String(), field.JSType().String())
 	}
 	return nil
 }
@@ -502,10 +530,19 @@ func checkFieldSameJSType(add addFunc, corpus *corpus, previousField bufprotosou
 var CheckFieldSameName = newFieldPairCheckFunc(checkFieldSameName)
 
 func checkFieldSameName(add addFunc, corpus *corpus, previousField bufprotosource.Field, field bufprotosource.Field) error {
-	if previousField.Name() != field.Name() {
-		// otherwise prints as hex
-		numberString := strconv.FormatInt(int64(field.Number()), 10)
-		add(field, nil, field.NameLocation(), `Field %q on message %q changed name from %q to %q.`, numberString, field.ParentMessage().Name(), previousField.Name(), field.Name())
+	var previousName, name string
+	if previousField.Extendee() != "" {
+		previousName = previousField.FullName()
+		name = field.FullName()
+	} else {
+		previousName = previousField.Name()
+		name = field.Name()
+	}
+	if previousName != name {
+		add(field, nil, field.NameLocation(),
+			`%s changed name from %q to %q.`,
+			fieldDescriptionWithName(field, ""), // don't include name in description
+			previousName, name)
 	}
 	return nil
 }
@@ -514,6 +551,10 @@ func checkFieldSameName(add addFunc, corpus *corpus, previousField bufprotosourc
 var CheckFieldSameOneof = newFieldPairCheckFunc(checkFieldSameOneof)
 
 func checkFieldSameOneof(add addFunc, corpus *corpus, previousField bufprotosource.Field, field bufprotosource.Field) error {
+	if previousField.Extendee() != "" {
+		// extensions can't be defined inside oneofs
+		return nil
+	}
 	previousOneof := previousField.Oneof()
 	if previousOneof != nil {
 		previousOneofDescriptor, err := previousOneof.AsDescriptor()
@@ -547,9 +588,10 @@ func checkFieldSameOneof(add addFunc, corpus *corpus, previousField bufprotosour
 	}
 	if previousInsideOneof && insideOneof {
 		if previousOneof.Name() != oneof.Name() {
-			// otherwise prints as hex
-			numberString := strconv.FormatInt(int64(field.Number()), 10)
-			add(field, nil, field.Location(), `Field %q on message %q moved from oneof %q to oneof %q.`, numberString, field.ParentMessage().Name(), previousOneof.Name(), oneof.Name())
+			add(field, nil, field.Location(),
+				`%sq moved from oneof %q to oneof %q.`,
+				fieldDescription(field),
+				previousOneof.Name(), oneof.Name())
 		}
 		return nil
 	}
@@ -560,9 +602,10 @@ func checkFieldSameOneof(add addFunc, corpus *corpus, previousField bufprotosour
 		previous = "outside"
 		current = "inside"
 	}
-	// otherwise prints as hex
-	numberString := strconv.FormatInt(int64(field.Number()), 10)
-	add(field, nil, field.Location(), `Field %q on message %q moved from %s to %s a oneof.`, numberString, field.ParentMessage().Name(), previous, current)
+	add(field, nil, field.Location(),
+		`%s moved from %s to %s a oneof.`,
+		fieldDescription(field),
+		previous, current)
 	return nil
 }
 
@@ -626,16 +669,12 @@ func checkFieldSameUTF8Validation(
 	}
 	utf8Validation := descriptorpb.FeatureSet_Utf8Validation(val.Enum())
 	if previousUTF8Validation != utf8Validation {
-		// otherwise prints as hex
-		numberString := strconv.FormatInt(int64(field.Number()), 10)
 		add(
 			field,
 			nil,
 			withBackupLocation(field.Features().UTF8ValidationLocation(), field.Location()),
-			`Field %q with name %q on message %q changed UTF8 validation from %v to %v.`,
-			numberString,
-			field.Name(),
-			field.ParentMessage().Name(),
+			`%s changed UTF8 validation from %v to %v.`,
+			fieldDescription(field),
 			previousUTF8Validation,
 			utf8Validation,
 		)
@@ -667,11 +706,9 @@ func checkFieldWireCompatibleCardinality(
 	previousCardinality := getCardinality(previousDescriptor)
 	currentCardinality := getCardinality(descriptor)
 	if cardinalityToWireCompatiblityGroup[previousCardinality] != cardinalityToWireCompatiblityGroup[currentCardinality] {
-		// otherwise prints as hex
-		numberString := strconv.FormatInt(int64(field.Number()), 10)
 		add(field, nil, field.Location(),
-			`Field %q on message %q changed cardinality from %q to %q.`,
-			numberString, field.ParentMessage().Name(),
+			`%s changed cardinality from %q to %q.`,
+			fieldDescription(field),
 			previousCardinality,
 			currentCardinality,
 		)
@@ -758,11 +795,9 @@ func checkFieldWireJSONCompatibleCardinality(
 	previousCardinality := getCardinality(previousDescriptor)
 	currentCardinality := getCardinality(descriptor)
 	if cardinalityToWireJSONCompatiblityGroup[previousCardinality] != cardinalityToWireJSONCompatiblityGroup[currentCardinality] {
-		// otherwise prints as hex
-		numberString := strconv.FormatInt(int64(field.Number()), 10)
 		add(field, nil, field.Location(),
-			`Field %q on message %q changed cardinality from %q to %q.`,
-			numberString, field.ParentMessage().Name(),
+			`%s changed cardinality from %q to %q.`,
+			fieldDescription(field),
 			previousCardinality,
 			currentCardinality,
 		)
@@ -875,15 +910,12 @@ func addFieldChangedType(
 	default:
 		fieldLocation = field.TypeLocation()
 	}
-	// otherwise prints as hex
-	previousNumberString := strconv.FormatInt(int64(previousField.Number()), 10)
 	add(
 		field,
 		nil,
 		fieldLocation,
-		`Field %q on message %q changed type from %q to %q.%s`,
-		previousNumberString,
-		field.ParentMessage().Name(),
+		`%s changed type from %q to %q.%s`,
+		fieldDescription(field),
 		fieldDescriptorTypePrettyString(previousDescriptor),
 		fieldDescriptorTypePrettyString(descriptor),
 		combinedExtraMessage,
@@ -891,15 +923,12 @@ func addFieldChangedType(
 }
 
 func addEnumGroupMessageFieldChangedTypeName(add addFunc, previousField bufprotosource.Field, field bufprotosource.Field) {
-	// otherwise prints as hex
-	numberString := strconv.FormatInt(int64(previousField.Number()), 10)
 	add(
 		field,
 		nil,
 		field.TypeNameLocation(),
-		`Field %q on message %q changed type from %q to %q.`,
-		numberString,
-		field.ParentMessage().Name(),
+		`%s changed type from %q to %q.`,
+		fieldDescription(field),
 		strings.TrimPrefix(previousField.TypeName(), "."),
 		strings.TrimPrefix(field.TypeName(), "."),
 	)
@@ -1252,7 +1281,7 @@ func checkPackageEnumNoDelete(add addFunc, corpus *corpus) error {
 					file, ok := filePathToFile[previousEnum.File().Path()]
 					if ok {
 						// File exists, try to get a location to attach the error to.
-						descriptor, location, err := getDescriptorAndLocationForDeletedEnum(file, previousNestedName)
+						descriptor, location, err := getDescriptorAndLocationForDeletedElement(file, previousNestedName)
 						if err != nil {
 							return err
 						}
@@ -1271,6 +1300,54 @@ func checkPackageEnumNoDelete(add addFunc, corpus *corpus) error {
 	return nil
 }
 
+// CheckPackageExtensionNoDelete is a check function.
+var CheckPackageExtensionNoDelete = newFilesCheckFunc(checkPackageExtensionNoDelete)
+
+func checkPackageExtensionNoDelete(add addFunc, corpus *corpus) error {
+	previousPackageToNestedNameToExtension, err := bufprotosource.PackageToNestedNameToExtension(corpus.previousFiles...)
+	if err != nil {
+		return err
+	}
+	packageToNestedNameToExtension, err := bufprotosource.PackageToNestedNameToExtension(corpus.files...)
+	if err != nil {
+		return err
+	}
+	// caching across loops
+	var filePathToFile map[string]bufprotosource.File
+	for previousPackage, previousNestedNameToExtension := range previousPackageToNestedNameToExtension {
+		if nestedNameToExtension, ok := packageToNestedNameToExtension[previousPackage]; ok {
+			for previousNestedName, previousExtension := range previousNestedNameToExtension {
+				if _, ok := nestedNameToExtension[previousNestedName]; !ok {
+					// if cache not populated, populate it
+					if filePathToFile == nil {
+						filePathToFile, err = bufprotosource.FilePathToFile(corpus.files...)
+						if err != nil {
+							return err
+						}
+					}
+					// Check if the file still exists.
+					file, ok := filePathToFile[previousExtension.File().Path()]
+					if ok {
+						// File exists, try to get a location to attach the error to.
+						descriptor, location, err := getDescriptorAndLocationForDeletedElement(file, previousNestedName)
+						if err != nil {
+							return err
+						}
+						add(descriptor, nil, location, `Previously present extension %q was deleted from package %q.`, previousNestedName, previousPackage)
+					} else {
+						// File does not exist, we don't know where the enum was deleted from.
+						// Add the previous enum to check for ignores. This means that if
+						// ignore_unstable_packages is set, this will be triggered if the
+						// previous enum was in an unstable package.
+						add(nil, []bufprotosource.Descriptor{previousExtension}, nil, `Previously present extension %q was deleted from package %q.`, previousNestedName, previousPackage)
+					}
+				}
+			}
+		}
+	}
+	return nil
+}
+
 // CheckPackageMessageNoDelete is a check function.
 var CheckPackageMessageNoDelete = newFilesCheckFunc(checkPackageMessageNoDelete)
 
diff --git a/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingcheck/util.go b/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingcheck/util.go
index 2396a6a67b..c92cd81b93 100644
--- a/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingcheck/util.go
+++ b/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingcheck/util.go
@@ -17,6 +17,7 @@ package bufbreakingcheck
 import (
 	"fmt"
 	"sort"
+	"strconv"
 	"strings"
 
 	"github.com/bufbuild/buf/private/bufpkg/bufanalysis"
@@ -236,25 +237,56 @@ func newMessagePairCheckFunc(
 func newFieldPairCheckFunc(
 	f func(addFunc, *corpus, bufprotosource.Field, bufprotosource.Field) error,
 ) func(string, internal.IgnoreFunc, []bufprotosource.File, []bufprotosource.File) ([]bufanalysis.FileAnnotation, error) {
-	return newMessagePairCheckFunc(
-		func(add addFunc, corpus *corpus, previousMessage bufprotosource.Message, message bufprotosource.Message) error {
-			previousNumberToField, err := bufprotosource.NumberToMessageField(previousMessage)
-			if err != nil {
-				return err
-			}
-			numberToField, err := bufprotosource.NumberToMessageField(message)
-			if err != nil {
-				return err
-			}
-			for previousNumber, previousField := range previousNumberToField {
-				if field, ok := numberToField[previousNumber]; ok {
-					if err := f(add, corpus, previousField, field); err != nil {
+	return combine(
+		// Regular fields
+		newMessagePairCheckFunc(
+			func(add addFunc, corpus *corpus, previousMessage bufprotosource.Message, message bufprotosource.Message) error {
+				previousNumberToField, err := bufprotosource.NumberToMessageField(previousMessage)
+				if err != nil {
+					return err
+				}
+				numberToField, err := bufprotosource.NumberToMessageField(message)
+				if err != nil {
+					return err
+				}
+				for previousNumber, previousField := range previousNumberToField {
+					if field, ok := numberToField[previousNumber]; ok {
+						if err := f(add, corpus, previousField, field); err != nil {
+							return err
+						}
+					}
+				}
+				return nil
+			},
+		),
+		// And extension fields
+		newFilesCheckFunc(
+			func(add addFunc, corpus *corpus) error {
+				previousTypeToNumberToField := make(map[string]map[int]bufprotosource.Field)
+				for _, previousFile := range corpus.previousFiles {
+					if err := addToTypeToNumberToExtension(previousFile, previousTypeToNumberToField); err != nil {
 						return err
 					}
 				}
-			}
-			return nil
-		},
+				typeToNumberToField := make(map[string]map[int]bufprotosource.Field)
+				for _, file := range corpus.files {
+					if err := addToTypeToNumberToExtension(file, typeToNumberToField); err != nil {
+						return err
+					}
+				}
+				for previousType, previousNumberToField := range previousTypeToNumberToField {
+					numberToField := typeToNumberToField[previousType]
+					for previousNumber, previousField := range previousNumberToField {
+						if field, ok := numberToField[previousNumber]; ok {
+							if err := f(add, corpus, previousField, field); err != nil {
+								return err
+							}
+						}
+					}
+				}
+				return nil
+			},
+		),
 	)
 }
 
@@ -326,7 +358,23 @@ func newMethodPairCheckFunc(
 	)
 }
 
-func getDescriptorAndLocationForDeletedEnum(file bufprotosource.File, previousNestedName string) (bufprotosource.Descriptor, bufprotosource.Location, error) {
+func combine(
+	checks ...func(string, internal.IgnoreFunc, []bufprotosource.File, []bufprotosource.File) ([]bufanalysis.FileAnnotation, error),
+) func(string, internal.IgnoreFunc, []bufprotosource.File, []bufprotosource.File) ([]bufanalysis.FileAnnotation, error) {
+	return func(id string, ignoreFunc internal.IgnoreFunc, previousFiles, files []bufprotosource.File) ([]bufanalysis.FileAnnotation, error) {
+		var annotations []bufanalysis.FileAnnotation
+		for _, check := range checks {
+			checkAnnotations, err := check(id, ignoreFunc, previousFiles, files)
+			if err != nil {
+				return nil, err
+			}
+			annotations = append(annotations, checkAnnotations...)
+		}
+		return annotations, nil
+	}
+}
+
+func getDescriptorAndLocationForDeletedElement(file bufprotosource.File, previousNestedName string) (bufprotosource.Descriptor, bufprotosource.Location, error) {
 	if strings.Contains(previousNestedName, ".") {
 		nestedNameToMessage, err := bufprotosource.NestedNameToMessage(file)
 		if err != nil {
@@ -488,3 +536,67 @@ func getCustomFeatureLocation(field bufprotosource.Field, extension protoreflect
 	}
 	return field.OptionLocation(featureField, int32(extension.Number()), int32(feature.Number()))
 }
+
+func fieldDescription(field bufprotosource.Field) string {
+	var name string
+	if field.Extendee() != "" {
+		// extensions are known by fully-qualified name
+		name = field.FullName()
+	} else {
+		name = field.Name()
+	}
+	return fieldDescriptionWithName(field, name)
+}
+
+func fieldDescriptionWithName(field bufprotosource.Field, name string) string {
+	if name != "" {
+		name = fmt.Sprintf(" with name %q", name)
+	}
+	// otherwise prints as hex
+	numberString := strconv.FormatInt(int64(field.Number()), 10)
+	var kind, message string
+	if field.Extendee() != "" {
+		kind = "Extension"
+		message = field.Extendee()
+	} else {
+		kind = "Field"
+		message = field.ParentMessage().Name()
+	}
+	return fmt.Sprintf("%s %q%s on message %q", kind, numberString, name, message)
+}
+
+func addToTypeToNumberToExtension(container bufprotosource.ContainerDescriptor, typeToNumberToExt map[string]map[int]bufprotosource.Field) error {
+	for _, extension := range container.Extensions() {
+		numberToExt := typeToNumberToExt[extension.Extendee()]
+		if numberToExt == nil {
+			numberToExt = make(map[int]bufprotosource.Field)
+			typeToNumberToExt[extension.Extendee()] = numberToExt
+		}
+		if existing, ok := numberToExt[extension.Number()]; ok {
+			return fmt.Errorf("duplicate extension %d of %s: %s in %q and %s in %q",
+				extension.Number(), extension.Extendee(),
+				existing.FullName(), existing.File().Path(),
+				extension.FullName(), extension.File().Path())
+		}
+		numberToExt[extension.Number()] = extension
+	}
+	for _, message := range container.Messages() {
+		if err := addToTypeToNumberToExtension(message, typeToNumberToExt); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func is64bitInteger(fieldType descriptorpb.FieldDescriptorProto_Type) bool {
+	switch fieldType {
+	case descriptorpb.FieldDescriptorProto_TYPE_INT64,
+		descriptorpb.FieldDescriptorProto_TYPE_SINT64,
+		descriptorpb.FieldDescriptorProto_TYPE_UINT64,
+		descriptorpb.FieldDescriptorProto_TYPE_FIXED64,
+		descriptorpb.FieldDescriptorProto_TYPE_SFIXED64:
+		return true
+	default:
+		return false
+	}
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingv2/bufbreakingv2.go b/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingv2/bufbreakingv2.go
index 4aa617cecf..d73093b571 100644
--- a/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingv2/bufbreakingv2.go
+++ b/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingv2/bufbreakingv2.go
@@ -31,6 +31,11 @@ import (
 // values are a feature of proto2 and editions, but not in
 // proto3 syntax.)
 //
+// Adds new EXTENSION_NO_DELETE and PACKAGE_EXTENSION_NO_DELETE
+// checks, to make sure that extensions are not deleted from a
+// file or package, respectively. (In previous versions, an
+// extension being deleted was simply undetected.)
+//
 // Removes the following deprecated checks (but retains their
 // replacements):
 //   - FIELD_SAME_CTYPE
diff --git a/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingv2/vars.go b/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingv2/vars.go
index 5fbfde0c5c..7d2c8e9785 100644
--- a/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingv2/vars.go
+++ b/private/bufpkg/bufcheck/bufbreaking/internal/bufbreakingv2/vars.go
@@ -30,6 +30,7 @@ var (
 		bufbreakingbuild.EnumValueNoDeleteUnlessNumberReservedRuleBuilder,
 		bufbreakingbuild.EnumValueSameNameRuleBuilder,
 		bufbreakingbuild.ExtensionMessageNoDeleteRuleBuilder,
+		bufbreakingbuild.ExtensionNoDeleteRuleBuilder,
 		bufbreakingbuild.FieldNoDeleteRuleBuilder,
 		bufbreakingbuild.FieldNoDeleteUnlessNameReservedRuleBuilder,
 		bufbreakingbuild.FieldNoDeleteUnlessNumberReservedRuleBuilder,
@@ -73,6 +74,7 @@ var (
 		bufbreakingbuild.MessageSameRequiredFieldsRuleBuilder,
 		bufbreakingbuild.OneofNoDeleteRuleBuilder,
 		bufbreakingbuild.PackageEnumNoDeleteRuleBuilder,
+		bufbreakingbuild.PackageExtensionNoDeleteRuleBuilder,
 		bufbreakingbuild.PackageMessageNoDeleteRuleBuilder,
 		bufbreakingbuild.PackageNoDeleteRuleBuilder,
 		bufbreakingbuild.PackageServiceNoDeleteRuleBuilder,
@@ -124,6 +126,9 @@ var (
 			"FILE",
 			"PACKAGE",
 		},
+		"EXTENSION_NO_DELETE": {
+			"FILE",
+		},
 		"FIELD_NO_DELETE": {
 			"FILE",
 			"PACKAGE",
@@ -300,6 +305,9 @@ var (
 		"PACKAGE_ENUM_NO_DELETE": {
 			"PACKAGE",
 		},
+		"PACKAGE_EXTENSION_NO_DELETE": {
+			"PACKAGE",
+		},
 		"PACKAGE_MESSAGE_NO_DELETE": {
 			"PACKAGE",
 		},
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/1.proto
new file mode 100644
index 0000000000..c4b0a38d51
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/1.proto
@@ -0,0 +1,37 @@
+syntax = "proto3";
+
+package a;
+
+message Two {
+  int32 one = 1;
+  int32 two = 2;
+  int32 three = 3;
+}
+
+message Three {
+  message Four {
+    message Five {
+      int32 one = 1;
+      int32 two = 2;
+      int32 three = 3;
+    }
+    message Six {
+      int32 one = 1;
+      int32 two = 2;
+      int32 three = 3;
+    }
+  }
+  message Seven {
+    int32 one = 1;
+    int32 two = 2;
+    int32 three = 3;
+  }
+  message Eight {
+    int32 one = 1;
+    int32 two = 2;
+    int32 three = 3;
+  }
+  int32 one = 1;
+  int32 two = 2;
+  int32 three = 3;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/2.proto
new file mode 100644
index 0000000000..5fa7742702
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/2.proto
@@ -0,0 +1,25 @@
+syntax = "proto2";
+
+package a;
+
+import "3.proto";
+import "4.proto";
+
+message One {
+  optional int32 one = 1;
+  optional int32 two = 2;
+}
+
+message Nine {
+  optional int32 one = 1;
+  optional int32 three = 3;
+}
+
+extend Foo {
+  optional bytes meta = 20;
+  optional Foo ch = 22;
+}
+
+extend b.Fizz {
+  optional b.Fizz child = 22;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/3.proto
new file mode 100644
index 0000000000..5fe583c710
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/3.proto
@@ -0,0 +1,20 @@
+syntax = "proto2";
+
+package b;
+
+message Fizz {
+  extensions 10 to 100;
+
+  message Buzz {
+    optional int32 len = 1;
+
+    extend Fizz {
+      optional string str = 10;
+    }
+  }
+}
+
+extend Fizz {
+  optional bytes meta = 20;
+  repeated uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/4.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/4.proto
new file mode 100644
index 0000000000..721ba92620
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/4.proto
@@ -0,0 +1,16 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    optional int32 len = 1;
+
+    extend Foo {
+      optional string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/buf.yaml b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/buf.yaml
new file mode 100644
index 0000000000..4c07ede4e4
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_extension_no_delete/buf.yaml
@@ -0,0 +1,4 @@
+version: v2
+breaking:
+  use:
+    - EXTENSION_NO_DELETE
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_no_delete/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_no_delete/3.proto
new file mode 100644
index 0000000000..a4ecd5c3b4
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_no_delete/3.proto
@@ -0,0 +1,17 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+  message Bar {
+    optional int32 len = 1;
+    extend Foo {
+      optional string str = 10;
+    }
+  }
+}
+
+extend Foo {
+  repeated uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_no_delete_unless_name_reserved/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_no_delete_unless_name_reserved/3.proto
new file mode 100644
index 0000000000..a4ecd5c3b4
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_no_delete_unless_name_reserved/3.proto
@@ -0,0 +1,17 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+  message Bar {
+    optional int32 len = 1;
+    extend Foo {
+      optional string str = 10;
+    }
+  }
+}
+
+extend Foo {
+  repeated uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_no_delete_unless_number_reserved/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_no_delete_unless_number_reserved/3.proto
new file mode 100644
index 0000000000..a4ecd5c3b4
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_no_delete_unless_number_reserved/3.proto
@@ -0,0 +1,17 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+  message Bar {
+    optional int32 len = 1;
+    extend Foo {
+      optional string str = 10;
+    }
+  }
+}
+
+extend Foo {
+  repeated uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_cardinality/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_cardinality/3.proto
index 8115aee21b..e5c7186484 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_cardinality/3.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_cardinality/3.proto
@@ -6,3 +6,21 @@ message Ten2 {
   repeated int64 one = 1;
   int64 two = 2 [features.field_presence = LEGACY_REQUIRED];
 }
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    int32 len = 1;
+
+    extend Foo {
+      repeated string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Foo {
+  bytes meta = 20;
+  uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_cpp_string_type/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_cpp_string_type/2.proto
index be7db0c60c..2ab9f8cb82 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_cpp_string_type/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_cpp_string_type/2.proto
@@ -33,4 +33,19 @@ message Bar {
   string default5 = 35 [features.(pb.cpp).string_type=VIEW];
   bytes default6 = 36 [features.(pb.cpp).string_type=CORD];
   repeated string default7 = 37;
+
+  extensions 100 to 300;
+
+  message Frobnitz {
+    extend Bar {
+      repeated string ext_str1 = 100;
+      string ext_str2 = 101;
+      repeated bytes ext_byt1 = 102;
+      bytes ext_byt2 = 103 [features.(pb.cpp).string_type=VIEW];
+    }
+  }
+}
+
+extend Bar {
+  repeated string ext_str1 = 200;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_cpp_string_type/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_cpp_string_type/3.proto
index 43f4a47252..dd39615f0e 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_cpp_string_type/3.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_cpp_string_type/3.proto
@@ -2,6 +2,7 @@ edition = "2023";
 
 package a;
 
+import "2.proto";
 import "google/protobuf/cpp_features.proto";
 
 option features.(pb.cpp).string_type=CORD;
@@ -18,4 +19,23 @@ message Baz {
   map<string, Baz> map1 = 11;
   map<uint32, string> map2 = 12;
   map<int64, bytes> map3 = 13;
+
+  extensions 100 to 200;
+
+  message Fizzbuzz {
+    extend Baz {
+      repeated string ext_str1 = 100 [features.(pb.cpp).string_type=CORD];
+      string ext_str2 = 101;
+      repeated bytes ext_byt1 = 102 [features.(pb.cpp).string_type=VIEW];
+      bytes ext_byt2 = 103 [ctype=STRING];
+    }
+  }
+}
+
+extend Baz {
+  repeated string ext_str = 200;
+}
+
+extend Bar {
+  repeated string ext_str2 = 201;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_ctype/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_ctype/2.proto
index 6b96d0903b..97d3135566 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_ctype/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_ctype/2.proto
@@ -49,3 +49,11 @@ message Nine {
   optional string one = 1 [ctype = STRING_PIECE];
   optional string two = 2 [ctype = STRING];
 }
+
+message Ten2 {
+  extensions 1 to 100;
+}
+
+extend Ten2 {
+  optional string ten_one = 1 [ctype = CORD];
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_default/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_default/1.proto
index 72df2636b0..0986ca76c0 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_default/1.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_default/1.proto
@@ -133,4 +133,23 @@ message Baz {
   repeated uint64 uints = 3;
   optional Bar bar = 4;
   repeated Bar bars = 5;
-}
\ No newline at end of file
+
+  extensions 100 to 200;
+
+  extend Baz {
+    optional string s = 200 [default = "abc"];
+    optional bool b = 199;
+    optional float f32 = 198 [default = 123.0];
+    optional double f = 196 [default = 0.1020304];
+  }
+}
+
+extend Baz {
+  optional uint32 u1 = 100 [default = 123];
+  optional uint32 u2 = 101 [default = 9876];
+  optional bytes b1 = 102 [default = "abc"];
+  optional bytes b2 = 103 [default = "a1b2c3"];
+  repeated Baz bb1 = 104;
+  optional string s1 = 105 [default = "0"];
+  optional string s2 = 106 [default = "xyz"];
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_default/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_default/2.proto
index 0bdc0851d8..44212a0aee 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_default/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_default/2.proto
@@ -1,6 +1,6 @@
 edition = "2023";
-
 package a;
+import "google/protobuf/descriptor.proto";
 
 message Fizz {
   string s1 = 1 [default = "abc"];
@@ -97,3 +97,9 @@ message Buzz {
   Buzz buzz = 4;
   repeated Buzz buzzes = 5;
 }
+
+extend google.protobuf.MessageOptions {
+  uint32 num = 10000 [default = 0];
+  repeated string strings = 10101;
+  Buzz buzz = 20202;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_java_utf8_validation/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_java_utf8_validation/1.proto
index 3bc0df1942..9488ab7d9e 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_java_utf8_validation/1.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_java_utf8_validation/1.proto
@@ -3,6 +3,7 @@ syntax = "proto2";
 package a;
 
 message A1 {
+  extensions 100 to 1000;
   optional string s1 = 1;
   repeated string s2 = 2;
   map<string, string> s3 = 3;
@@ -14,6 +15,7 @@ message A1 {
 }
 
 message B1 {
+  extensions 100 to 1000;
   optional string s1 = 1;
   repeated string s2 = 2;
   map<string, string> s3 = 3;
@@ -36,6 +38,7 @@ message C1 {
 }
 
 message D1 {
+  extensions 100 to 1000;
   optional string s1 = 1;
   repeated string s2 = 2;
   map<string, string> s3 = 3;
@@ -47,6 +50,7 @@ message D1 {
 }
 
 message E1 {
+  extensions 100 to 1000;
   optional string s1 = 1;
   repeated string s2 = 2;
   map<string, string> s3 = 3;
@@ -58,6 +62,7 @@ message E1 {
 }
 
 message F1 {
+  extensions 100 to 1000;
   optional string s1 = 1;
   repeated string s2 = 2;
   map<string, string> s3 = 3;
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_java_utf8_validation/6.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_java_utf8_validation/6.proto
index 104516b73b..832989c5d3 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_java_utf8_validation/6.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_java_utf8_validation/6.proto
@@ -1,7 +1,7 @@
 edition = "2023";
 
 package a;
-
+import "1.proto";
 import "google/protobuf/java_features.proto";
 
 option features.utf8_validation = NONE;
@@ -16,6 +16,13 @@ message A6 {
   repeated double other2 = 12;
   map<uint64, bytes> other3 = 13;
   A6 other4 = 14;
+
+  extensions 100 to 1000;
+
+  extend A1 {
+    string a1_s1 = 100;
+    repeated string a1_s2 = 101;
+  }
 }
 
 message B6 {
@@ -27,6 +34,13 @@ message B6 {
   repeated double other2 = 12;
   map<uint64, bytes> other3 = 13;
   B6 other4 = 14;
+
+  extensions 100 to 1000;
+
+  extend B1 {
+    string b1_s1 = 100;
+    repeated string b1_s2 = 101;
+  }
 }
 
 message C6 {
@@ -49,6 +63,13 @@ message D6 {
   repeated double other2 = 12;
   map<uint64, bytes> other3 = 13;
   D6 other4 = 14;
+
+  extensions 100 to 1000;
+
+  extend D1 {
+    string d1_s1 = 100;
+    repeated string d1_s2 = 101;
+  }
 }
 
 message E6 {
@@ -60,6 +81,13 @@ message E6 {
   repeated double other2 = 12;
   map<uint64, bytes> other3 = 13;
   E6 other4 = 14;
+
+  extensions 100 to 1000;
+
+  extend E1 {
+    string e1_s1 = 100;
+    repeated string e1_s2 = 101;
+  }
 }
 
 message F6 {
@@ -71,6 +99,13 @@ message F6 {
   repeated double other2 = 12;
   map<uint64, bytes> other3 = 13;
   F6 other4 = 14;
+
+  extensions 100 to 1000;
+
+  extend F1 {
+    string f1_s1 = 100;
+    repeated string f1_s2 = 101;
+  }
 }
 
 message D8 {
@@ -94,3 +129,28 @@ message E8 {
   map<uint64, bytes> other3 = 13;
   E8 other4 = 14;
 }
+
+extend A6 {
+  string a6_s1 = 100;
+  repeated string a6_s2 = 101;
+}
+
+extend B6 {
+  string b6_s1 = 100;
+  repeated string b6_s2 = 101;
+}
+
+extend D6 {
+  string d6_s1 = 100;
+  repeated string d6_s2 = 101;
+}
+
+extend E6 {
+  string e6_s1 = 100;
+  repeated string e6_s2 = 101;
+}
+
+extend F6 {
+  string f6_s1 = 100;
+  repeated string f6_s2 = 101;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_jstype/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_jstype/2.proto
index 6aa2334f2c..40e4ba5da3 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_jstype/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_jstype/2.proto
@@ -1,6 +1,6 @@
 syntax = "proto2";
-
 package a;
+import "google/protobuf/descriptor.proto";
 
 message One {
   optional int64 one = 1;
@@ -43,9 +43,30 @@ message Three2 {
 message Nine2 {
   optional int64 one = 1 [jstype = JS_NUMBER];
   required int64 two = 2 [jstype = JS_STRING];
+
+  extensions 100 to 1000;
 }
 
 message Nine {
   optional int64 one = 1 [jstype = JS_NUMBER];
   optional int64 two = 2;
+
+  extend google.protobuf.FileOptions {
+    optional uint64 f1 = 10001 [jstype = JS_NORMAL];
+    optional sint64 f2 = 10002 [jstype = JS_STRING];
+    optional int64 f3 = 10003 [jstype = JS_STRING];
+    optional fixed64 f4 = 10004 [jstype = JS_NUMBER];
+    optional sfixed64 f5 = 10005;
+  }
+}
+
+extend Nine2 {
+  optional sint64 s1 = 101 [jstype = JS_NORMAL];
+  optional sint64 s2 = 102 [jstype = JS_NUMBER];
+  optional sint64 s3 = 103 [jstype = JS_STRING];
+  optional sint64 s4 = 104;
+  optional sint64 s5 = 105 [jstype = JS_STRING];
+  optional sint64 s6 = 106 [jstype = JS_NUMBER];
+  optional sint64 s7 = 107;
+  optional sint64 s8 = 108;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_name/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_name/1.proto
index 4975740a07..a84f16cebc 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_name/1.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_name/1.proto
@@ -1,6 +1,6 @@
 syntax = "proto3";
-
 package a;
+import "google/protobuf/descriptor.proto";
 
 message Two {
   int32 one = 1;
@@ -30,8 +30,17 @@ message Three {
     int32 one = 1;
     int32 two = 2;
     int32 three = 3;
+
+    extend google.protobuf.MessageOptions {
+      string msg_str_opt = 10101;
+    }
   }
   int32 one = 1;
   int32 two = 2;
   int32 three = 3;
 }
+
+
+extend google.protobuf.FileOptions {
+  string file_str_opt = 10101;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_name/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_name/2.proto
index eb4361be69..df173dd0db 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_name/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_name/2.proto
@@ -1,57 +1,75 @@
-syntax = "proto3";
+syntax = "proto2";
 
 package a;
 
 message One {
-  int32 one = 1;
-  int32 two = 2;
-  int32 three = 3;
+  optional int32 one = 1;
+  optional int32 two = 2;
+  optional int32 three = 3;
 }
 
 message One2 {
-  int32 one = 1;
-  int32 two = 2;
-  int32 three = 3;
+  optional int32 one = 1;
+  optional int32 two = 2;
+  optional int32 three = 3;
 }
 
 message Two2 {
-  int32 one = 1;
-  int32 two = 2;
-  int32 three = 3;
+  optional int32 one = 1;
+  optional int32 two = 2;
+  optional int32 three = 3;
 }
 
 message Three2 {
   message Four2 {
     message Five2 {
-      int32 one = 1;
-      int32 two = 2;
-      int32 three = 3;
+      optional int32 one = 1;
+      optional int32 two = 2;
+      optional int32 three = 3;
     }
     message Six2 {
-      int32 one = 1;
-      int32 two = 2;
-      int32 three = 3;
+      optional int32 one = 1;
+      optional int32 two = 2;
+      optional int32 three = 3;
     }
   }
   message Seven2 {
-    int32 one = 1;
-    int32 two = 2;
-    int32 three = 3;
+    optional int32 one = 1;
+    optional int32 two = 2;
+    optional int32 three = 3;
   }
   message Eight2 {
-    int32 one = 1;
-    int32 two = 2;
-    int32 three = 3;
+    optional int32 one = 1;
+    optional int32 two = 2;
+    optional int32 three = 3;
+
+    extend Three2 {
+      repeated uint64 uint_opt = 101;
+      optional string str_option = 102;
+    }
   }
-  int32 one = 1;
-  int32 two = 2;
-  int32 three = 3;
+  optional int32 one = 1;
+  optional int32 two = 2;
+  optional int32 three = 3;
+
+  extensions 100 to 1000;
 }
 
 message Nine2 {
-  int32 one = 1;
-  int32 two = 2;
-  int32 three = 3;
+  optional int32 one = 1;
+  optional int32 two = 2;
+  optional int32 three = 3;
+
+  extend Three2 {
+    optional bool bool_opt = 103;
+  }
+
+  extensions 100 to 1000;
+}
+
+extend Nine2 {
+  repeated uint64 uint_opt = 101;
+  optional string str_option = 102;
 }
 
 message Nine {
@@ -60,4 +78,7 @@ message Nine {
     int32 four = 2;
     int32 three = 3;
   }
+  extend Nine2 {
+    optional bool bool_opt = 103;
+  }
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_type/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_type/2.proto
index 9af1a018ce..48c00aadf1 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_type/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_type/2.proto
@@ -1,6 +1,6 @@
 syntax = "proto2";
-
 package a;
+import "google/protobuf/descriptor.proto";
 
 message One {
   optional int32 one = 1;
@@ -63,3 +63,7 @@ message Ten {
   repeated int64 one = 1;
   required int64 two = 2;
 }
+
+extend google.protobuf.FieldOptions {
+  repeated Nine ten_option = 10101;
+}
\ No newline at end of file
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_type/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_type/3.proto
index 9d309ce149..39e4e16561 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_type/3.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_same_type/3.proto
@@ -9,6 +9,8 @@ message One3 {
   One3 delimited = 2;
   One3 normal_b = 3;
   One3 delimited_b = 4 [features.message_encoding=DELIMITED];
+
+  extensions 100 to 1000;
 }
 
 message Nine2 {
@@ -22,4 +24,18 @@ message Ten2 {
   Foo foo = 1 [features.message_encoding=DELIMITED];
   message Foo {
   }
+
+  extensions 100 to 1000;
+
+  extend Ten2 {
+    bool ten2_str = 100;
+    Ten2 ten2_msg = 101;
+    int64 ten2_uint32 = 102;
+  }
+}
+
+extend One3 {
+  bytes one3_str = 100;
+  One3 one3_msg = 101 [features.message_encoding=DELIMITED];
+  uint32 one3_uint32 = 102;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_compatible_cardinality/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_compatible_cardinality/3.proto
index 8115aee21b..e5c7186484 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_compatible_cardinality/3.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_compatible_cardinality/3.proto
@@ -6,3 +6,21 @@ message Ten2 {
   repeated int64 one = 1;
   int64 two = 2 [features.field_presence = LEGACY_REQUIRED];
 }
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    int32 len = 1;
+
+    extend Foo {
+      repeated string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Foo {
+  bytes meta = 20;
+  uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_compatible_type/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_compatible_type/2.proto
index 4e916f3fa9..c1070f3fb8 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_compatible_type/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_compatible_type/2.proto
@@ -1,6 +1,6 @@
 syntax = "proto3";
-
 package a;
+import "google/protobuf/descriptor.proto";
 
 message CompatiblePrimitives {
   bool bool_field_1 = 1;
@@ -84,3 +84,9 @@ message StringBytes {
   bytes string_field_1 = 1;
   string bytes_field_1 = 2;
 }
+
+extend google.protobuf.FieldOptions {
+  repeated Bat bat_option = 10101;
+  bytes str_option = 10102;
+  bytes byt_option = 10103;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_compatible_type/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_compatible_type/3.proto
index 3a6e0a2034..eae0c3ffe7 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_compatible_type/3.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_compatible_type/3.proto
@@ -7,4 +7,18 @@ message Message {
   Message delimited = 2;
   Message normal_b = 3;
   Message delimited_b = 4 [features.message_encoding=DELIMITED];
+
+  extensions 100 to 1000;
+
+  extend Message {
+    bytes str = 100;
+    Message msg = 101 [features.message_encoding=DELIMITED];
+    uint64 uint64 = 102;
+  }
+}
+
+extend Message {
+  string str = 200;
+  Message msg = 201;
+  sint32 sint32 = 202;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_json_compatible_cardinality/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_json_compatible_cardinality/3.proto
index 8115aee21b..e5c7186484 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_json_compatible_cardinality/3.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_json_compatible_cardinality/3.proto
@@ -6,3 +6,21 @@ message Ten2 {
   repeated int64 one = 1;
   int64 two = 2 [features.field_presence = LEGACY_REQUIRED];
 }
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    int32 len = 1;
+
+    extend Foo {
+      repeated string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Foo {
+  bytes meta = 20;
+  uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_json_compatible_type/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_json_compatible_type/2.proto
index 95ed57b8fb..48399ce3f2 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_json_compatible_type/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_json_compatible_type/2.proto
@@ -1,6 +1,6 @@
 syntax = "proto3";
-
 package a;
+import "google/protobuf/descriptor.proto";
 
 message CompatiblePrimitives {
   uint32 uint32_field_1 = 1;
@@ -87,3 +87,9 @@ message StringBytes {
   bytes string_field_1 = 1;
   string bytes_field_1 = 2;
 }
+
+extend google.protobuf.FieldOptions {
+  repeated Bat bat_option = 10101;
+  bytes str_option = 10102;
+  bytes byt_option = 10103;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_json_compatible_type/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_json_compatible_type/3.proto
index 3a6e0a2034..eae0c3ffe7 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_json_compatible_type/3.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_field_wire_json_compatible_type/3.proto
@@ -7,4 +7,18 @@ message Message {
   Message delimited = 2;
   Message normal_b = 3;
   Message delimited_b = 4 [features.message_encoding=DELIMITED];
+
+  extensions 100 to 1000;
+
+  extend Message {
+    bytes str = 100;
+    Message msg = 101 [features.message_encoding=DELIMITED];
+    uint64 uint64 = 102;
+  }
+}
+
+extend Message {
+  string str = 200;
+  Message msg = 201;
+  sint32 sint32 = 202;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/1.proto
new file mode 100644
index 0000000000..c4b0a38d51
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/1.proto
@@ -0,0 +1,37 @@
+syntax = "proto3";
+
+package a;
+
+message Two {
+  int32 one = 1;
+  int32 two = 2;
+  int32 three = 3;
+}
+
+message Three {
+  message Four {
+    message Five {
+      int32 one = 1;
+      int32 two = 2;
+      int32 three = 3;
+    }
+    message Six {
+      int32 one = 1;
+      int32 two = 2;
+      int32 three = 3;
+    }
+  }
+  message Seven {
+    int32 one = 1;
+    int32 two = 2;
+    int32 three = 3;
+  }
+  message Eight {
+    int32 one = 1;
+    int32 two = 2;
+    int32 three = 3;
+  }
+  int32 one = 1;
+  int32 two = 2;
+  int32 three = 3;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/2.proto
new file mode 100644
index 0000000000..5fa7742702
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/2.proto
@@ -0,0 +1,25 @@
+syntax = "proto2";
+
+package a;
+
+import "3.proto";
+import "4.proto";
+
+message One {
+  optional int32 one = 1;
+  optional int32 two = 2;
+}
+
+message Nine {
+  optional int32 one = 1;
+  optional int32 three = 3;
+}
+
+extend Foo {
+  optional bytes meta = 20;
+  optional Foo ch = 22;
+}
+
+extend b.Fizz {
+  optional b.Fizz child = 22;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/3.proto
new file mode 100644
index 0000000000..5fe583c710
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/3.proto
@@ -0,0 +1,20 @@
+syntax = "proto2";
+
+package b;
+
+message Fizz {
+  extensions 10 to 100;
+
+  message Buzz {
+    optional int32 len = 1;
+
+    extend Fizz {
+      optional string str = 10;
+    }
+  }
+}
+
+extend Fizz {
+  optional bytes meta = 20;
+  repeated uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/4.proto b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/4.proto
new file mode 100644
index 0000000000..721ba92620
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/4.proto
@@ -0,0 +1,16 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    optional int32 len = 1;
+
+    extend Foo {
+      optional string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/buf.yaml b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/buf.yaml
new file mode 100644
index 0000000000..a4b55888dc
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata/breaking_package_extension_no_delete/buf.yaml
@@ -0,0 +1,4 @@
+version: v2
+breaking:
+  use:
+    - PACKAGE_EXTENSION_NO_DELETE
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_extension_no_delete/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_extension_no_delete/1.proto
new file mode 100644
index 0000000000..e44c469ede
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_extension_no_delete/1.proto
@@ -0,0 +1,49 @@
+syntax = "proto3";
+
+package a;
+
+message One {
+  int32 one = 1;
+  int32 two = 2;
+  int32 three = 3;
+}
+
+message Two {
+  int32 one = 1;
+  int32 two = 2;
+  int32 three = 3;
+}
+
+message Three {
+  message Four {
+    message Five {
+      int32 one = 1;
+      int32 two = 2;
+      int32 three = 3;
+    }
+    message Six {
+      int32 one = 1;
+      int32 two = 2;
+      int32 three = 3;
+    }
+  }
+  message Seven {
+    int32 one = 1;
+    int32 two = 2;
+    int32 three = 3;
+  }
+  message Eight {
+    int32 one = 1;
+    int32 two = 2;
+    int32 three = 3;
+  }
+  int32 one = 1;
+  int32 two = 2;
+  int32 three = 3;
+}
+
+message Nine {
+  int32 one = 1;
+  int32 two = 2;
+  int32 three = 3;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_extension_no_delete/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_extension_no_delete/2.proto
new file mode 100644
index 0000000000..d9826fc340
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_extension_no_delete/2.proto
@@ -0,0 +1,22 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    optional int32 len = 1;
+
+    extend Foo {
+      optional string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Foo {
+  optional bytes meta = 20;
+  repeated uint64 tags = 21;
+  optional Foo ch = 22;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_extension_no_delete/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_extension_no_delete/3.proto
new file mode 100644
index 0000000000..af0b4ef3f4
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_extension_no_delete/3.proto
@@ -0,0 +1,22 @@
+syntax = "proto2";
+
+package b;
+
+message Fizz {
+  extensions 10 to 100;
+
+  message Buzz {
+    optional int32 len = 1;
+
+    extend Fizz {
+      optional string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Fizz {
+  optional bytes meta = 20;
+  repeated uint64 tags = 21;
+  optional Fizz child = 22;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_no_delete/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_no_delete/3.proto
new file mode 100644
index 0000000000..aabbf2b6d3
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_no_delete/3.proto
@@ -0,0 +1,21 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    optional int32 len = 1;
+
+    extend Foo {
+      optional string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Foo {
+  optional bytes meta = 20;
+  repeated uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_no_delete_unless_name_reserved/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_no_delete_unless_name_reserved/3.proto
new file mode 100644
index 0000000000..aabbf2b6d3
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_no_delete_unless_name_reserved/3.proto
@@ -0,0 +1,21 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    optional int32 len = 1;
+
+    extend Foo {
+      optional string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Foo {
+  optional bytes meta = 20;
+  repeated uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_no_delete_unless_number_reserved/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_no_delete_unless_number_reserved/3.proto
new file mode 100644
index 0000000000..aabbf2b6d3
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_no_delete_unless_number_reserved/3.proto
@@ -0,0 +1,21 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    optional int32 len = 1;
+
+    extend Foo {
+      optional string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Foo {
+  optional bytes meta = 20;
+  repeated uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_cardinality/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_cardinality/3.proto
new file mode 100644
index 0000000000..aabbf2b6d3
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_cardinality/3.proto
@@ -0,0 +1,21 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    optional int32 len = 1;
+
+    extend Foo {
+      optional string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Foo {
+  optional bytes meta = 20;
+  repeated uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_cpp_string_type/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_cpp_string_type/2.proto
index 761ba4f594..995cb27d66 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_cpp_string_type/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_cpp_string_type/2.proto
@@ -33,4 +33,20 @@ message Bar {
   string default5 = 35;
   bytes default6 = 36;
   repeated string default7 = 37;
+
+  extensions 100 to 300;
+
+  message Frobnitz {
+    extend Bar {
+      repeated string ext_str1 = 100;
+      string ext_str2 = 101 [ctype=STRING_PIECE];
+      repeated bytes ext_byt1 = 102 [features.(pb.cpp).string_type=VIEW];
+      bytes ext_byt2 = 103;
+    }
+  }
 }
+
+extend Bar {
+  repeated string ext_str1 = 200 [features.(pb.cpp).string_type=STRING];
+  repeated string ext_str2 = 201;
+}
\ No newline at end of file
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_cpp_string_type/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_cpp_string_type/3.proto
index 87442acd54..291719237f 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_cpp_string_type/3.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_cpp_string_type/3.proto
@@ -18,4 +18,19 @@ message Baz {
   map<string, Baz> map1 = 11;
   map<uint32, string> map2 = 12;
   map<int64, bytes> map3 = 13;
+
+  extensions 100 to 200;
+
+  message Fizzbuzz {
+    extend Baz {
+      repeated string ext_str1 = 100;
+      string ext_str2 = 101 [ctype=STRING_PIECE];
+      repeated bytes ext_byt1 = 102 [features.(pb.cpp).string_type=VIEW];
+      bytes ext_byt2 = 103;
+    }
+  }
+}
+
+extend Baz {
+  repeated string ext_str = 200;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_ctype/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_ctype/2.proto
index 072de89958..492288a991 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_ctype/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_ctype/2.proto
@@ -39,3 +39,11 @@ message Nine2 {
   optional string one = 1 [ctype = STRING_PIECE];
   required string two = 2 [ctype = CORD];
 }
+
+message Ten2 {
+  extensions 1 to 100;
+}
+
+extend Ten2 {
+  optional string ten_one = 1 [ctype = STRING_PIECE];
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_default/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_default/1.proto
index 8876de0551..49e7becc79 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_default/1.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_default/1.proto
@@ -133,4 +133,24 @@ message Baz {
   repeated uint64 uints = 3;
   optional Bar bar = 4;
   repeated Bar bars = 5;
+
+  extensions 100 to 200;
+
+  extend Baz {
+    optional string s = 200;
+    optional bool b = 199 [default = true];
+    optional fixed32 f32 = 198 [default = 123];
+    optional sint64 s64 = 197 [default = -456];
+    optional float f = 196 [default = 0.1020304];
+  }
+}
+
+extend Baz {
+  optional uint32 u1 = 100 [default = 3456];
+  optional uint32 u2 = 101 [default = 9876];
+  optional bytes b1 = 102 [default = "abcdef"];
+  optional bytes b2 = 103 [default = "a1b2c3"];
+  repeated Baz bb1 = 104;
+  optional string s1 = 105 [default = "abc"];
+  optional string s2 = 106 [default = "xyz"];
 }
\ No newline at end of file
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_default/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_default/2.proto
index 41d7dd44a0..b0daf4439e 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_default/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_default/2.proto
@@ -2,6 +2,8 @@ syntax = "proto3";
 
 package a;
 
+import "google/protobuf/descriptor.proto";
+
 message Fizz {
   string s1 = 1;
   string s2 = 2;
@@ -97,3 +99,9 @@ message Buzz {
   Buzz buzz = 4;
   repeated Buzz buzzes = 5;
 }
+
+extend google.protobuf.MessageOptions {
+  uint32 num = 10000;
+  repeated string strings = 10101;
+  Buzz buzz = 20202;
+}
\ No newline at end of file
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/1.proto
index 1914278342..fb266a0d65 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/1.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/1.proto
@@ -1,8 +1,8 @@
 syntax = "proto2";
-
 package a;
 
 message A1 {
+  extensions 100 to 1000;
   optional string s1 = 1;
   repeated string s2 = 2;
   map<string, string> s3 = 3;
@@ -66,4 +66,16 @@ message A6 {
   repeated double other2 = 12;
   map<uint64, bytes> other3 = 13;
   optional A6 other4 = 14;
+
+  extensions 100 to 1000;
+
+  extend A1 {
+    optional string a1_s1 = 100;
+    repeated string a1_s2 = 101;
+  }
+}
+
+extend A6 {
+  optional string a6_s1 = 100;
+  repeated string a6_s2 = 101;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/2.proto
index a81733ef26..4614e98d7e 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/2.proto
@@ -1,10 +1,10 @@
 syntax = "proto2";
-
 package a;
 
 option java_string_check_utf8 = true;
 
 message B1 {
+  extensions 100 to 1000;
   optional string s1 = 1;
   repeated string s2 = 2;
   map<string, string> s3 = 3;
@@ -68,4 +68,16 @@ message B6 {
   repeated double other2 = 12;
   map<uint64, bytes> other3 = 13;
   optional B6 other4 = 14;
+
+  extensions 100 to 1000;
+
+  extend B1 {
+    optional string b1_s1 = 100;
+    repeated string b1_s2 = 101;
+  }
+}
+
+extend B6 {
+  optional string b6_s1 = 100;
+  repeated string b6_s2 = 101;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/4.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/4.proto
index ac264e3b7f..5b77348220 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/4.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/4.proto
@@ -1,10 +1,10 @@
 edition = "2023";
-
 package a;
 
 import "google/protobuf/java_features.proto";
 
 message D1 {
+  extensions 100 to 1000;
   string s1 = 1;
   repeated string s2 = 2;
   map<string, string> s3 = 3;
@@ -68,6 +68,18 @@ message D6 {
   repeated double other2 = 12;
   map<uint64, bytes> other3 = 13;
   D6 other4 = 14;
+
+  extensions 100 to 1000;
+
+  extend D1 {
+    string d1_s1 = 100 [features.(pb.java).utf8_validation=VERIFY];
+    repeated string d1_s2 = 101;
+  }
+}
+
+extend D6 {
+  string d6_s1 = 100 [features.(pb.java).utf8_validation=VERIFY];
+  repeated string d6_s2 = 101;
 }
 
 message D7 {
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/5.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/5.proto
index c1edc6f4a8..a7564be7eb 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/5.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/5.proto
@@ -1,5 +1,4 @@
 edition = "2023";
-
 package a;
 
 import "google/protobuf/java_features.proto";
@@ -7,6 +6,7 @@ import "google/protobuf/java_features.proto";
 option features.(pb.java).utf8_validation = VERIFY;
 
 message E1 {
+  extensions 100 to 1000;
   string s1 = 1;
   repeated string s2 = 2;
   map<string, string> s3 = 3;
@@ -70,6 +70,18 @@ message E6 {
   repeated double other2 = 12;
   map<uint64, bytes> other3 = 13;
   E6 other4 = 14;
+
+  extensions 100 to 1000;
+
+  extend E1 {
+    string e1_s1 = 100 [features.(pb.java).utf8_validation=DEFAULT];
+    repeated string e1_s2 = 101;
+  }
+}
+
+extend E6 {
+  string e6_s1 = 100 [features.(pb.java).utf8_validation=DEFAULT];
+  repeated string e6_s2 = 101;
 }
 
 message E7 {
@@ -92,4 +104,6 @@ message E8 {
   repeated double other2 = 12;
   map<uint64, bytes> other3 = 13;
   E8 other4 = 14;
+
+  extensions 100 to 1000;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/6.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/6.proto
index 9666b11da4..3bc4318e0f 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/6.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_java_utf8_validation/6.proto
@@ -1,5 +1,4 @@
 edition = "2023";
-
 package a;
 
 import "google/protobuf/java_features.proto";
@@ -8,6 +7,7 @@ option features.utf8_validation = NONE;
 option features.(pb.java).utf8_validation = VERIFY;
 
 message F1 {
+  extensions 100 to 1000;
   string s1 = 1;
   repeated string s2 = 2;
   map<string, string> s3 = 3;
@@ -71,6 +71,18 @@ message F6 {
   repeated double other2 = 12;
   map<uint64, bytes> other3 = 13;
   F6 other4 = 14;
+
+  extensions 100 to 1000;
+
+  extend F1 {
+    string f1_s1 = 100 [features.(pb.java).utf8_validation=DEFAULT];
+    repeated string f1_s2 = 101;
+  }
+}
+
+extend F6 {
+  string f6_s1 = 100 [features.(pb.java).utf8_validation=DEFAULT];
+  repeated string f6_s2 = 101;
 }
 
 message F7 {
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_jstype/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_jstype/1.proto
index 519ec3a5d3..0f4cb0c7d1 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_jstype/1.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_jstype/1.proto
@@ -1,6 +1,6 @@
 syntax = "proto3";
-
 package a;
+import "google/protobuf/descriptor.proto";
 
 message One {
   int64 one = 1 [jstype = JS_NORMAL];
@@ -38,4 +38,12 @@ message Three {
 message Nine {
   int64 one = 1 [jstype = JS_NORMAL];
   int64 two = 2 [jstype = JS_STRING];
+
+  extend google.protobuf.FileOptions {
+    uint64 f1 = 10001;
+    sint64 f2 = 10002 [jstype = JS_STRING];
+    int64 f3 = 10003 [jstype = JS_NUMBER];
+    fixed64 f4 = 10004 [jstype = JS_NORMAL];
+    sfixed64 f5 = 10005 [jstype = JS_STRING];
+  }
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_jstype/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_jstype/2.proto
index 8629da4224..307cc03058 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_jstype/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_jstype/2.proto
@@ -38,4 +38,17 @@ message Three2 {
 message Nine2 {
   optional int64 one = 1 [jstype = JS_NUMBER];
   required int64 two = 2 [jstype = JS_STRING];
+
+  extensions 100 to 1000;
+}
+
+extend Nine2 {
+  optional sint64 s1 = 101 [jstype = JS_NORMAL];
+  optional sint64 s2 = 102 [jstype = JS_NUMBER];
+  optional sint64 s3 = 103 [jstype = JS_STRING];
+  optional sint64 s4 = 104;
+  optional sint64 s5 = 105;
+  optional sint64 s6 = 106 [jstype = JS_STRING];
+  optional sint64 s7 = 107 [jstype = JS_NUMBER];
+  optional sint64 s8 = 108 [jstype = JS_NORMAL];
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_name/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_name/1.proto
index e44c469ede..ce7c0f65f8 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_name/1.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_name/1.proto
@@ -1,6 +1,6 @@
 syntax = "proto3";
-
 package a;
+import "google/protobuf/descriptor.proto";
 
 message One {
   int32 one = 1;
@@ -46,4 +46,12 @@ message Nine {
   int32 one = 1;
   int32 two = 2;
   int32 three = 3;
+
+  extend google.protobuf.MessageOptions {
+    string msg_str_opt = 10101;
+  }
+}
+
+extend google.protobuf.FileOptions {
+  string file_str_opt = 10101;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_name/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_name/2.proto
index ea9ae23a30..8b7a70bf7f 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_name/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_name/2.proto
@@ -1,49 +1,64 @@
-syntax = "proto3";
+syntax = "proto2";
 
 package a;
 
 message One2 {
-  int32 one = 1;
-  int32 two = 2;
-  int32 three = 3;
+  optional int32 one = 1;
+  optional int32 two = 2;
+  optional int32 three = 3;
 }
 
 message Two2 {
-  int32 one = 1;
-  int32 two = 2;
-  int32 three = 3;
+  optional int32 one = 1;
+  optional int32 two = 2;
+  optional int32 three = 3;
 }
 
 message Three2 {
   message Four2 {
     message Five2 {
-      int32 one = 1;
-      int32 two = 2;
-      int32 three = 3;
+      optional int32 one = 1;
+      optional int32 two = 2;
+      optional int32 three = 3;
     }
     message Six2 {
-      int32 one = 1;
-      int32 two = 2;
-      int32 three = 3;
+      optional int32 one = 1;
+      optional int32 two = 2;
+      optional int32 three = 3;
     }
   }
   message Seven2 {
-    int32 one = 1;
-    int32 two = 2;
-    int32 three = 3;
+    optional int32 one = 1;
+    optional int32 two = 2;
+    optional int32 three = 3;
   }
   message Eight2 {
-    int32 one = 1;
-    int32 two = 2;
-    int32 three = 3;
+    optional int32 one = 1;
+    optional int32 two = 2;
+    optional int32 three = 3;
+    extend Three2 {
+      repeated uint64 uint_opt = 101;
+      optional string str_opt = 102;
+      optional bool bool_opt = 103;
+    }
   }
-  int32 one = 1;
-  int32 two = 2;
-  int32 three = 3;
+  optional int32 one = 1;
+  optional int32 two = 2;
+  optional int32 three = 3;
+
+  extensions 100 to 1000;
 }
 
 message Nine2 {
-  int32 one = 1;
-  int32 two = 2;
-  int32 three = 3;
+  optional int32 one = 1;
+  optional int32 two = 2;
+  optional int32 three = 3;
+
+  extensions 100 to 1000;
+}
+
+extend Nine2 {
+  repeated uint64 uint_opt = 101;
+  optional string str_opt = 102;
+  optional bool bool_opt = 103;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_type/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_type/1.proto
index ea1190d417..16112834ea 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_type/1.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_type/1.proto
@@ -1,6 +1,6 @@
 syntax = "proto3";
-
 package a;
+import "google/protobuf/descriptor.proto";
 
 message One {
   int32 one = 1;
@@ -68,3 +68,7 @@ message Ten {
   int64 one = 1;
   int64 two = 2;
 }
+
+extend google.protobuf.FieldOptions {
+  repeated Ten ten_option = 10101;
+}
\ No newline at end of file
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_type/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_type/2.proto
index 3153e6a452..236885969e 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_type/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_type/2.proto
@@ -57,4 +57,12 @@ message Nine2 {
 message Ten2 {
   optional group Foo = 1 {
   }
+
+  extensions 100 to 1000;
+
+  extend Ten2 {
+    optional string ten2_str = 100;
+    optional Ten2 ten2_msg = 101;
+    optional uint32 ten2_uint32 = 102;
+  }
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_type/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_type/3.proto
index 45c2618d23..c65eee157a 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_type/3.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_same_type/3.proto
@@ -7,4 +7,12 @@ message One3 {
   One3 delimited = 2 [features.message_encoding=DELIMITED];
   One3 normal_b = 3;
   One3 delimited_b = 4 [features.message_encoding=DELIMITED];
+
+  extensions 100 to 1000;
 }
+
+extend One3 {
+  string one3_str = 100;
+  One3 one3_msg = 101;
+  uint32 one3_uint32 = 102;
+}
\ No newline at end of file
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_compatible_cardinality/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_compatible_cardinality/3.proto
new file mode 100644
index 0000000000..aabbf2b6d3
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_compatible_cardinality/3.proto
@@ -0,0 +1,21 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    optional int32 len = 1;
+
+    extend Foo {
+      optional string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Foo {
+  optional bytes meta = 20;
+  repeated uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_compatible_type/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_compatible_type/1.proto
index b8ac336ef9..a09d820120 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_compatible_type/1.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_compatible_type/1.proto
@@ -1,6 +1,6 @@
 syntax = "proto3";
-
 package a;
+import "google/protobuf/descriptor.proto";
 
 message CompatiblePrimitives {
   int32 int32_field_1 = 1;
@@ -83,3 +83,9 @@ message StringBytes {
   string string_field_1 = 1;
   bytes bytes_field_1 = 2;
 }
+
+extend google.protobuf.FieldOptions {
+  repeated Bar bar_option = 10101;
+  string str_option = 10102;
+  bytes byt_option = 10103;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_compatible_type/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_compatible_type/2.proto
index 0fe8765bd1..35545844a8 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_compatible_type/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_compatible_type/2.proto
@@ -7,4 +7,18 @@ message Message {
   Message delimited = 2 [features.message_encoding=DELIMITED];
   Message normal_b = 3;
   Message delimited_b = 4 [features.message_encoding=DELIMITED];
+
+  extensions 100 to 1000;
+
+  extend Message {
+    string str = 100;
+    Message msg = 101;
+    uint32 uint32 = 102;
+  }
+}
+
+extend Message {
+  string str = 200;
+  Message msg = 201;
+  uint32 uint32 = 202;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_json_compatible_cardinality/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_json_compatible_cardinality/3.proto
new file mode 100644
index 0000000000..aabbf2b6d3
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_json_compatible_cardinality/3.proto
@@ -0,0 +1,21 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    optional int32 len = 1;
+
+    extend Foo {
+      optional string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Foo {
+  optional bytes meta = 20;
+  repeated uint64 tags = 21;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_json_compatible_type/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_json_compatible_type/1.proto
index db70d36483..cc6a5142f2 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_json_compatible_type/1.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_json_compatible_type/1.proto
@@ -1,6 +1,6 @@
 syntax = "proto3";
-
 package a;
+import "google/protobuf/descriptor.proto";
 
 message CompatiblePrimitives {
   int32 int32_field_1 = 1;
@@ -86,3 +86,9 @@ message StringBytes {
   string string_field_1 = 1;
   bytes bytes_field_1 = 2;
 }
+
+extend google.protobuf.FieldOptions {
+  repeated Bar bar_option = 10101;
+  string str_option = 10102;
+  bytes byt_option = 10103;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_json_compatible_type/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_json_compatible_type/2.proto
index 0fe8765bd1..35545844a8 100644
--- a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_json_compatible_type/2.proto
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_field_wire_json_compatible_type/2.proto
@@ -7,4 +7,18 @@ message Message {
   Message delimited = 2 [features.message_encoding=DELIMITED];
   Message normal_b = 3;
   Message delimited_b = 4 [features.message_encoding=DELIMITED];
+
+  extensions 100 to 1000;
+
+  extend Message {
+    string str = 100;
+    Message msg = 101;
+    uint32 uint32 = 102;
+  }
+}
+
+extend Message {
+  string str = 200;
+  Message msg = 201;
+  uint32 uint32 = 202;
 }
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_package_extension_no_delete/1.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_package_extension_no_delete/1.proto
new file mode 100644
index 0000000000..e44c469ede
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_package_extension_no_delete/1.proto
@@ -0,0 +1,49 @@
+syntax = "proto3";
+
+package a;
+
+message One {
+  int32 one = 1;
+  int32 two = 2;
+  int32 three = 3;
+}
+
+message Two {
+  int32 one = 1;
+  int32 two = 2;
+  int32 three = 3;
+}
+
+message Three {
+  message Four {
+    message Five {
+      int32 one = 1;
+      int32 two = 2;
+      int32 three = 3;
+    }
+    message Six {
+      int32 one = 1;
+      int32 two = 2;
+      int32 three = 3;
+    }
+  }
+  message Seven {
+    int32 one = 1;
+    int32 two = 2;
+    int32 three = 3;
+  }
+  message Eight {
+    int32 one = 1;
+    int32 two = 2;
+    int32 three = 3;
+  }
+  int32 one = 1;
+  int32 two = 2;
+  int32 three = 3;
+}
+
+message Nine {
+  int32 one = 1;
+  int32 two = 2;
+  int32 three = 3;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_package_extension_no_delete/2.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_package_extension_no_delete/2.proto
new file mode 100644
index 0000000000..d9826fc340
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_package_extension_no_delete/2.proto
@@ -0,0 +1,22 @@
+syntax = "proto2";
+
+package a;
+
+message Foo {
+  extensions 10 to 100;
+
+  message Bar {
+    optional int32 len = 1;
+
+    extend Foo {
+      optional string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Foo {
+  optional bytes meta = 20;
+  repeated uint64 tags = 21;
+  optional Foo ch = 22;
+}
diff --git a/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_package_extension_no_delete/3.proto b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_package_extension_no_delete/3.proto
new file mode 100644
index 0000000000..af0b4ef3f4
--- /dev/null
+++ b/private/bufpkg/bufcheck/bufbreaking/testdata_previous/breaking_package_extension_no_delete/3.proto
@@ -0,0 +1,22 @@
+syntax = "proto2";
+
+package b;
+
+message Fizz {
+  extensions 10 to 100;
+
+  message Buzz {
+    optional int32 len = 1;
+
+    extend Fizz {
+      optional string str = 10;
+      repeated string labels = 11;
+    }
+  }
+}
+
+extend Fizz {
+  optional bytes meta = 20;
+  repeated uint64 tags = 21;
+  optional Fizz child = 22;
+}
diff --git a/private/bufpkg/bufprotosource/bufprotosource.go b/private/bufpkg/bufprotosource/bufprotosource.go
index ceca0153c6..86f9570f01 100644
--- a/private/bufpkg/bufprotosource/bufprotosource.go
+++ b/private/bufpkg/bufprotosource/bufprotosource.go
@@ -121,6 +121,7 @@ type NamedDescriptor interface {
 type ContainerDescriptor interface {
 	Enums() []Enum
 	Messages() []Message
+	Extensions() []Field
 }
 
 // OptionExtensionDescriptor contains options and option extensions.
@@ -585,7 +586,7 @@ func PackageToFiles(files ...File) (map[string][]File, error) {
 
 // ForEachEnum calls f on each Enum in the given ContainerDescriptor, including nested Enums.
 //
-// Returns error and stops iterating if f returns error
+// Returns error and stops iterating if f returns error.
 // Never returns error unless f returns error.
 func ForEachEnum(f func(Enum) error, containerDescriptor ContainerDescriptor) error {
 	for _, enum := range containerDescriptor.Enums() {
@@ -601,6 +602,25 @@ func ForEachEnum(f func(Enum) error, containerDescriptor ContainerDescriptor) er
 	return nil
 }
 
+// ForEachExtension calls f on each extension Field in the given ContainerDescriptor,
+// including nested extensions.
+//
+// Returns error and stops iterating if f returns error.
+// Never returns error unless f returns error.
+func ForEachExtension(f func(Field) error, containerDescriptor ContainerDescriptor) error {
+	for _, extension := range containerDescriptor.Extensions() {
+		if err := f(extension); err != nil {
+			return err
+		}
+	}
+	for _, message := range containerDescriptor.Messages() {
+		if err := ForEachExtension(f, message); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 // ForEachMessage calls f on each Message in the given ContainerDescriptor, including nested Messages.
 //
 // Returns error and stops iterating if f returns error
@@ -640,6 +660,30 @@ func NestedNameToEnum(containerDescriptor ContainerDescriptor) (map[string]Enum,
 	return nestedNameToEnum, nil
 }
 
+// NestedNameToExtension maps the Enums in the ContainerDescriptor to a map from
+// nested name to extension Field.
+//
+// Returns error if extensions do not have unique nested names within the
+// ContainerDescriptor, which should generally never happen for properly-formed
+// ContainerDescriptors.
+func NestedNameToExtension(containerDescriptor ContainerDescriptor) (map[string]Field, error) {
+	nestedNameToExtension := make(map[string]Field)
+	if err := ForEachExtension(
+		func(extension Field) error {
+			nestedName := extension.NestedName()
+			if _, ok := nestedNameToExtension[nestedName]; ok {
+				return fmt.Errorf("duplicate extension: %q", nestedName)
+			}
+			nestedNameToExtension[nestedName] = extension
+			return nil
+		},
+		containerDescriptor,
+	); err != nil {
+		return nil, err
+	}
+	return nestedNameToExtension, nil
+}
+
 // FullNameToEnum maps the Enums in the Files to a map from full name to enum.
 //
 // Returns error if the Enums do not have unique full names within the Files,
@@ -695,6 +739,37 @@ func PackageToNestedNameToEnum(files ...File) (map[string]map[string]Enum, error
 	return packageToNestedNameToEnum, nil
 }
 
+// PackageToNestedNameToExtension maps the extension Fields in the Files to a map
+// from package to nested name to Field.
+//
+// Returns error if the extension do not have unique nested names within the packages,
+// which should generally never happen for properly-formed Files.
+func PackageToNestedNameToExtension(files ...File) (map[string]map[string]Field, error) {
+	packageToNestedNameToExtension := make(map[string]map[string]Field)
+	for _, file := range files {
+		if err := ForEachExtension(
+			func(enum Field) error {
+				pkg := enum.File().Package()
+				nestedName := enum.NestedName()
+				nestedNameToExtension, ok := packageToNestedNameToExtension[pkg]
+				if !ok {
+					nestedNameToExtension = make(map[string]Field)
+					packageToNestedNameToExtension[pkg] = nestedNameToExtension
+				}
+				if _, ok := nestedNameToExtension[nestedName]; ok {
+					return fmt.Errorf("duplicate extension in package %q: %q", pkg, nestedName)
+				}
+				nestedNameToExtension[nestedName] = enum
+				return nil
+			},
+			file,
+		); err != nil {
+			return nil, err
+		}
+	}
+	return packageToNestedNameToExtension, nil
+}
+
 // NameToEnumValue maps the EnumValues in the Enum to a map from name to EnumValue.
 //
 // Returns error if the EnumValues do not have unique names within the Enum,
@@ -815,8 +890,7 @@ func PackageToNestedNameToMessage(files ...File) (map[string]map[string]Message,
 
 // NumberToMessageField maps the Fields in the Message to a map from number to Field.
 //
-// TODO: is this right?
-// Includes extensions.
+// Does not includes extensions.
 //
 // Returns error if the Fields do not have unique numbers within the Message,
 // which should generally never happen for properly-formed Messages.
@@ -829,27 +903,13 @@ func NumberToMessageField(message Message) (map[int]Field, error) {
 		}
 		numberToMessageField[number] = messageField
 	}
-	for _, messageField := range message.Extensions() {
-		if messageField.Extendee() != message.FullName() {
-			// TODO: ideally we want this field to be returned when
-			// the Extendee message is passed into some function,
-			// need to investigate what index is necessary for that.
-			continue
-		}
-		number := messageField.Number()
-		if _, ok := numberToMessageField[number]; ok {
-			return nil, fmt.Errorf("duplicate message field: %d", number)
-		}
-		numberToMessageField[number] = messageField
-	}
 	return numberToMessageField, nil
 }
 
 // NumberToMessageFieldForLabel maps the Fields with the given label in the message
 // to a map from number to Field.
 //
-// TODO: is this right?
-// Includes extensions.
+// Does not includes extensions.
 //
 // Returns error if the Fields do not have unique numbers within the Message,
 // which should generally never happen for properly-formed Messages.