fix: import OpenAPI 3.1/3.2 contentEncoding base64 as bytes with @encode decorator

.chronus/changes/fix-base64-content-encoding-import-2025-12-5-18-32-55.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/openapi3"
+---
+
+Import OpenAPI 3.1/3.2 schemas with contentEncoding: base64 as bytes type with `@encode("base64", string)` decorator

packages/openapi3/src/cli/actions/convert/generators/generate-types.ts

@@ -572,6 +572,12 @@ function getNumberType(schema: SupportedOpenAPISchema): string {
 
 function getStringType(schema: SupportedOpenAPISchema): string {
   const format = schema.format ?? "";
+
+  // Handle contentEncoding: base64 for OpenAPI 3.1+ (indicates binary data encoded as base64 string)
+  if ("contentEncoding" in schema && schema.contentEncoding === "base64") {
+    return "bytes";
+  }
+
   let type = "string";
   switch (format) {
     case "binary":

packages/openapi3/src/cli/actions/convert/utils/decorators.ts

@@ -305,6 +305,14 @@ function getStringSchemaDecorators(schema: OpenAPI3Schema | OpenAPISchema3_1) {
     decorators.push({ name: "format", args: [schema.format] });
   }
 
+  // Handle contentEncoding: base64 for OpenAPI 3.1+ (indicates binary data encoded as base64 string)
+  if ("contentEncoding" in schema && schema.contentEncoding === "base64") {
+    decorators.push({
+      name: "encode",
+      args: [createTSValue(`"base64"`), createTSValue("string")],
+    });
+  }
+
   return decorators;
 }
 

packages/openapi3/test/tsp-openapi3/data-types.test.ts

@@ -75,6 +75,24 @@ describe("converts top-level schemas", () => {
     ]);
   });
 
+  it("handles contentEncoding base64 as bytes with @encode decorator", async () => {
+    const serviceNamespace = await tspForOpenAPI3({
+      schemas: {
+        Base64Encoded: {
+          type: "string",
+          contentEncoding: "base64",
+        } as any,
+      },
+    });
+
+    const scalars = serviceNamespace.scalars;
+    /* @encode("base64", string) scalar Base64Encoded extends bytes; */
+    expect(scalars.get("Base64Encoded")?.baseScalar?.name).toBe("bytes");
+    expectDecorators(scalars.get("Base64Encoded")!.decorators, [
+      { name: "encode", args: ["base64", { kind: "Scalar", name: "string" }] },
+    ]);
+  });
+
   it("handles arrays", async () => {
     const serviceNamespace = await tspForOpenAPI3({
       schemas: {

packages/openapi3/test/tsp-openapi3/generate-type.test.ts

@@ -3,10 +3,10 @@ import { formatTypeSpec } from "@typespec/compiler";
 import { strictEqual } from "node:assert";
 import { beforeAll, describe, it } from "vitest";
 import { Context, createContext } from "../../src/cli/actions/convert/utils/context.js";
-import { OpenAPI3Document, OpenAPI3Schema, Refable } from "../../src/types.js";
+import { OpenAPI3Document, OpenAPI3Schema, OpenAPISchema3_1, Refable } from "../../src/types.js";
 
 interface TestScenario {
-  schema: Refable<OpenAPI3Schema>;
+  schema: Refable<OpenAPI3Schema | OpenAPISchema3_1>;
   expected: string;
 }
 
@@ -62,6 +62,8 @@ const testScenarios: TestScenario[] = [
   },
   { schema: { type: "string", format: "binary" }, expected: "bytes" },
   { schema: { type: "string", format: "byte" }, expected: "bytes" },
+  // OpenAPI 3.1+ contentEncoding: base64 should be bytes
+  { schema: { type: "string", contentEncoding: "base64" }, expected: "bytes" },
   { schema: { type: "string", format: "date" }, expected: "plainDate" },
   { schema: { type: "string", format: "date-time" }, expected: "utcDateTime" },
   { schema: { type: "string", format: "duration" }, expected: "duration" },