feat: add OpenAPI 3.1 support

[reuvenharrison]

Summary

Comprehensive OpenAPI 3.1 / JSON Schema 2020-12 support. This consolidates and supersedes PRs #1102, #1114, #1118–#1124.

Core features

• Type arrays with null support (e.g., ["string", "null"])
• JSON Schema 2020-12 keywords: const, examples, prefixItems, contains, minContains, maxContains, patternProperties, dependentSchemas, propertyNames, unevaluatedItems, unevaluatedProperties
• Conditional keywords: if/then/else, dependentRequired
• Identity/referencing keywords: $id, $anchor, $dynamicRef, $dynamicAnchor
• Content vocabulary: contentMediaType, contentEncoding, contentSchema
• Core keywords: $schema, $comment, $defs
• ExclusiveBound union type: supports both boolean (3.0) and numeric (3.1) exclusive bounds
• Webhooks with $ref resolution
• Version detection: IsOpenAPI3_0(), IsOpenAPI3_1()
• Info.Summary and License.Identifier fields

Validation

• JSON Schema 2020-12 validator via EnableJSONSchema2020() (per-field) and EnableJSONSchema2020Validation() (document-level)
• Auto-detection: doc.Validate() automatically enables JSON Schema 2020-12 mode for OpenAPI 3.1 documents
• Built-in validator: const keyword enforcement, IsEmpty() awareness of all new fields
• $ref resolution in loader for all new schema fields (including $defs)
• Schema.validate() recursion into all new sub-schema fields
• transformOpenAPIToJSONSchema recursion into all nested schemas (including $defs)
• Allow $ref alongside other keywords (legal in 3.1)
• Handle "null" type in schema validation

Backward compatibility

• 100% backward compatible with OpenAPI 3.0
• ExclusiveBound transparently handles both boolean and numeric forms
• openapi2conv updated to convert between ExclusiveBound and boolean, preserving numeric bound values

Commit history

1. feat: add OpenAPI 3.1 support — base implementation (Types, Schema fields, Webhooks, JSON Schema validator, version helpers)
2. fix: resolve $ref in OpenAPI 3.1 schema fields — loader handles refs in prefixItems, contains, patternProperties, etc.
3. fix: recurse into OpenAPI 3.1 fields in transformOpenAPIToJSONSchema — transform handles all nested schemas
4. fix: validate sub-schemas in OpenAPI 3.1 schema fields — Schema.validate() recurses into new fields
5. feat: add const keyword validation to built-in validator — const enforcement without EnableJSONSchema2020()
6. fix: improve OpenAPI 3.1 spec compliance — paths optional in 3.1, mutualTLS, license url/identifier exclusivity, auto-enable JSON Schema 2020-12 in openapi3filter
7. feat: add remaining JSON Schema 2020-12 keywords — $id, $anchor, $dynamicRef, $dynamicAnchor, contentMediaType, contentEncoding, contentSchema, discriminator for anyOf
8. style: fix go fmt formatting
9. fix: resolve 8 correctness issues — IsEmpty(), JSONLookup(), validate() array items, transform bugs, MarshalYAML, const comparison, webhooks ordering
10. fix: resolve 8 additional issues — $comment keyword, PrefixItems type consistency, exclusiveBoundToBool data loss, auto-enable 3.1 validation, test coverage gaps, README breaking changes
11. feat: add $schema, $defs keywords and fix remaining issues — per-schema dialect, local schema definitions, jsonSchemaDialect URI validation, discriminator refactor

Open issues for complete 3.1 support

The following items are known gaps that can be addressed in follow-up PRs:

Missing features

• $schema keyword — Added per-schema dialect declaration.
• $defs keyword — Added with full support (struct, marshal, unmarshal, IsEmpty, JSONLookup, validate, loader, transform).
• $comment keyword — Added as first-class field.
• $dynamicRef / $dynamicAnchor not resolved — Parsed and serialized but the loader does not resolve them. Schemas using $dynamicRef for recursive references will not work.
• Built-in validator ignores most 3.1 keywords — Only validates const. Other 3.1 keywords are silently ignored without EnableJSONSchema2020().
• contentMediaType / contentEncoding not validated at runtime — Spec-compliant (annotation-only in 2020-12), but some users may expect validation.
• Missing pathItems in Components struct — OAS 3.1 added components/pathItems. Completely absent — silently dropped during parsing.

Bugs / correctness

• Schema.JSONLookup() missing all new 3.1 fields — All fields added.
• IsEmpty() missing checks for most new 3.1 fields — All fields added.
• validate() requires items for type array even in 3.1 — Relaxed.
• transformOpenAPIToJSONSchema does not clean up exclusiveMinimum: false — Fixed.
• transformOpenAPIToJSONSchema drops nullable: true without type — Fixed.
• MarshalYAML unconditionally emits "paths": null for 3.1 docs — Fixed.
• visitConstOperation uses == for json.Number — Fixed.
• Webhooks validation iterates non-deterministically — Fixed.
• exclusiveBoundToBool loses constraint data — Fixed.
• doc.Validate() does not auto-enable 3.1 mode — Fixed.
• jsonSchemaDialect URI validation is a no-op — Now requires scheme.
• Built-in validator applies items to ALL array elements, ignoring prefixItems — In 3.1, items applies only beyond the prefixItems tuple.
• patternProperties omission changes additionalProperties semantics — additionalProperties: false incorrectly rejects properties matching pattern properties.
• Silent fallback in visitJSONWithJSONSchema — Silently falls back to built-in validator when JSON Schema compilation fails.
• Const: nil cannot express "value must be null" — Go nil is the zero value for any.

Breaking API changes

• ExclusiveMin/ExclusiveMax type changed — Documented in README.
• PrefixItems type changed — Documented in README.

Code quality

• PrefixItems type inconsistency — Changed to SchemaRefs.
• Discriminator logic duplicated for anyOf — Refactored into resolveDiscriminatorRef helper.

Test coverage

• No ref resolution test for ContentSchema — Added.
• No ref resolution test for If/Then/Else — Added.
• No transformOpenAPIToJSONSchema test for ContentSchema — Added.
• No Schema.validate() test for ContentSchema — Added.

Test plan

• Type arrays: serialization, deserialization, helper methods, backward compatibility
• Webhooks: marshal/unmarshal, validation, JSONLookup
• Version detection: IsOpenAPI3_0, IsOpenAPI3_1, migration scenarios
• JSON Schema 2020-12 validator: type, number, object, array, oneOf/anyOf/allOf/not, exclusive bounds, const, nullable
• if/then/else: conditional validation, marshal round-trip, unresolved ref detection
• dependentRequired: conditional property requirements
• Loader: $ref resolution in all 3.1 schema fields + if/then/else + contentSchema
• Schema.validate(): unresolved ref detection in all new sub-schema fields + contentSchema
• Const: string, number, boolean, null, object, type+const, multiError
• Full test suite passes (go test ./...)

Supersedes

Closes #1102, closes #1114, closes #1118, closes #1119, closes #1120, closes #1121, closes #1122, closes #1123, closes #1124

🤖 Generated with Claude Code

Co-Authored-By: Chance Kirsch <>
Co-Authored-By: RobbertDM <>

[reuvenharrison]

Open Issues Status Update

22 of 30 issues resolved across commits 9–11.

Resolved issues (22)

Bugs / correctness (11 fixed)

• ✅ Schema.JSONLookup() — all 23 missing JSON Schema 2020-12 fields added
• ✅ IsEmpty() — missing checks for PrefixItems, Contains, MinContains, MaxContains, PatternProperties, DependentSchemas, PropertyNames, UnevaluatedItems, UnevaluatedProperties, Examples
• ✅ validate() array items — relaxed when jsonSchema2020ValidationEnabled or prefixItems present
• ✅ transformOpenAPIToJSONSchema exclusiveMinimum:false — boolean false now cleaned up
• ✅ transformOpenAPIToJSONSchema nullable without type — now sets type: ["null"]
• ✅ MarshalYAML paths:null — only emits paths when non-nil
• ✅ visitConstOperation json.Number — uses reflect.DeepEqual
• ✅ Webhooks validation ordering — uses componentNames() for deterministic iteration
• ✅ exclusiveBoundToBool data loss — numeric bound value preserved in OAS 2.0 conversion
• ✅ doc.Validate() auto-enable 3.1 — auto-enables EnableJSONSchema2020Validation() for 3.1 docs
• ✅ jsonSchemaDialect URI validation — now requires a scheme

Missing features (3 added)

• ✅ $schema keyword — per-schema dialect declaration
• ✅ $comment keyword — first-class field for round-trip fidelity
• ✅ $defs keyword — local reusable schema definitions with full ref resolution

Code quality (2 fixed)

• ✅ PrefixItems type — changed from []*SchemaRef to SchemaRefs
• ✅ Discriminator duplication — refactored into resolveDiscriminatorRef helper

Breaking API changes (2 documented)

• ✅ ExclusiveMin/ExclusiveMax type change — documented in README v0.132.0
• ✅ PrefixItems type change — documented in README v0.132.0

Test coverage (4 added)

• ✅ Ref resolution test for ContentSchema
• ✅ Ref resolution test for If/Then/Else
• ✅ transformOpenAPIToJSONSchema test for ContentSchema
• ✅ Schema.validate() test for ContentSchema

Remaining open issues (8)

These are larger-scope items for follow-up PRs:

#	Issue	Category
1	$dynamicRef/$dynamicAnchor not resolved by loader	Missing feature
2	Built-in validator ignores most 3.1 keywords (needs EnableJSONSchema2020())	Missing feature
3	pathItems missing from Components struct	Missing feature
4	Built-in validator applies items to ALL elements, ignoring prefixItems	Bug
5	patternProperties omission changes additionalProperties semantics	Bug
6	Silent fallback in visitJSONWithJSONSchema when compilation fails	Bug
7	Const: nil cannot express "value must be null"	Bug
8	contentMediaType/contentEncoding not validated at runtime (spec-compliant)	Missing feature

[reuvenharrison]

Coverage vs Pierre Fenoll's Task List

Assessment of this PR against the tasks outlined by Pierre for complete OpenAPI 3.1 support:

Task	Status	Notes
De/serialization	Complete	All 3.1 fields with full round-trip (marshal/unmarshal/JSONLookup/IsEmpty)
Document validation	Complete	All fields validated, auto-detect 3.1, $ref resolution in loader
JSON Schema validation	Mostly complete	External lib (santhosh-tekuri/jsonschema/v6) chosen; 3.1 keywords delegated to it. Built-in validator only handles const.
3.0 ↔ 3.1 conversion	Minimal	Only ExclusiveBound conversion in openapi2conv. No general downgrade/upgrade path.
openapi3gen (reflection)	Not started	Zero changes to openapi3gen/
openapi3filter (middleware)	Minimal	Auto-enables JSON Schema 2020-12 mode for 3.1 specs in ValidateParameter, ValidateRequestBody, ValidateResponse
3.0/3.1 coexistence	Complete	IsOpenAPI3_0()/IsOpenAPI3_1(), ExclusiveBound union type, omitempty on all 3.1 fields
Tests	Comprehensive	7 new test files, 2 testdata files

What this PR delivers (~60-70% of the full task list)

• Full data model for all JSON Schema 2020-12 keywords (26+ new Schema fields)
• Document-level features (Webhooks, JSONSchemaDialect, Info.Summary, License.Identifier, mutualTLS)
• External JSON Schema 2020-12 validator with automatic 3.1 detection
• 100% backward compatible with OpenAPI 3.0

Remaining items for follow-up PRs

1. openapi3gen — Reflection-based generation of 3.1 schemas from Go types (largest gap)
2. 3.0 ↔ 3.1 conversion — Downgrade path (prefixItems→items, type arrays→nullable, $defs→definitions) and upgrade path
3. openapi3filter — Deeper middleware integration beyond auto-enabling 3.1 mode
4. Built-in validator — Currently only validates const; other 3.1 keywords require EnableJSONSchema2020()
5. $dynamicRef resolution — Parsed/serialized but loader does not resolve dynamic references
6. components/pathItems — OAS 3.1 added this field; currently silently dropped during parsing