[dinwwwh]

Requested by dinwwwh · project thread

After #87, an anyOf of null and a branch that can't take nullable: true stays an anyOf pair in 3.0. That covers a $ref (Zod Ref.nullable(), pydantic Optional[Model], oRPC's self-referencing Category.parent) and a typed enum (z.enum([...]).nullable()). NSwag 14.7 reads that pair as an empty class, so dotnet clients fail at runtime. It reads a oneOf as the nullable type and round-trips.

The oneOf means the same thing here. Null matches only the null branch. A branch with a type and no nullable: true never matches null. So at most one branch matches any value.

• rejectsNull(schema): a record with no nullable: true whose type is a string other than "null", or a non-empty array of strings without "null".
• convertSchemaRef records each $ref it keeps as written when its target rejects null. It checks the 3.1 target, because in a cycle the converted target may not be finished yet.
• mergeNullable: when the merge doesn't apply, oneOf is unset, and the other branch is a recorded $ref or rejects null, the anyOf becomes a oneOf.

Other generators the issue hunt checked (openapi-typescript, hey-api, oapi-codegen, datamodel-codegen, openapi-python-client, Kiota, openapi-generator typescript-fetch) produce the same code either way.

Tests: the typed enum/allOf/not branches and the keyword-clash case now expect oneOf. A dangling $ref, a branch with nullable, and an untyped branch stay anyOf. So does a pair beside an existing oneOf. A new test covers a $ref to a component typed object (becomes oneOf), and to components typed ["object", "null"] or carrying a 3.0-style nullable (stay anyOf). The oRPC snapshot changes in two places: Category.parent and the Planet | null response. README row updated. The 3.1 to 3.0 bench shows no change beyond noise.

---

Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[codspeed]

Merging this PR will not alter performance

✅ 21 untouched benchmarks

---

_{Comparing claude/project-thread-o6uyg9 (4fb6fc0) with main (30a428b)}

[pullfrog]

✅ No new issues found.

Reviewed changes

• anyOf + null → oneOf: mergeNullable now rewrites the pair to a oneOf when the non-null branch rejects null, so NSwag 14.7 reads it as nullable instead of an empty class.
• rejectsNull helper: identifies a branch that rejects null by checking for a non-null type and no nullable: true; type is conjunctive in JSON Schema, so this holds even when a sibling enum/const contains null.
• NULL_REJECTING_REFS tracking: convertSchemaRef records each kept {$ref} whose 3.1 target rejects null, checked against the source target since a cycle can meet its conversion unfinished.
• Tests + README: typed enum/allOf/not branches and the keyword-clash case now expect oneOf; a new $ref test covers a type: object target (becomes oneOf) versus ["object","null"] / 3.0-style nullable (stay anyOf). Two oRPC snapshot positions updated.

I verified the oneOf rewrite is acceptance-preserving: onlyNull() matches exactly null, and the recorded/predicted branch never matches null, so the two branches are disjoint and oneOf ≡ anyOf. A specialist pass enumerated 225k+ direct schemas and 230k+ ref cases against a 3.0 evaluator with no counterexample. Full suite (256 tests) and uncheck (oxlint + oxfmt + tsc) pass.

  ^{｜ View workflow run ｜ Using deepseek-v4.1-flash (free via Pullfrog for OSS) ｜ 𝕏}

[pullfrog]

✅ No new issues found.

Reviewed changes

• README row clarified (delta since prior review): packages/downgrader/README.md row 183 now reads "{ type: "null" } and one other branch, as Zod writes a nullable object, array, integer, enum, or $ref", and the 3.0 side now qualifies the nullable: true merge with "when it has a single type". No code change; the wording matches mergeNullable/rejectsNull behavior and the $ref-kept-as-written case.

No new behavioral surface in this commit — the production change in 05bf13f was already approved in the prior review (review #5637779763).

  ^{｜ View workflow run ｜ Using deepseek-v4.1-flash (free via Pullfrog for OSS) ｜ 𝕏}