Validate __proto__ properties against raw input before Zod parsing

[schani]

Problem

A __proto__ entry in properties never validated its value, and required: ["__proto__"] was only enforced for schemas without an explicit type:

const schema = convertJsonSchemaToZod({
    type: "object",
    properties: { ["__proto__"]: { type: "number" } },
});
schema.safeParse(JSON.parse('{"__proto__": "bad"}')).success; // was true — now false

const schema2 = convertJsonSchemaToZod({ type: "object", required: ["__proto__"] });
schema2.safeParse({}).success; // was true — now false

Root cause: Zod strips own __proto__ keys when building parse output (prototype-pollution defense), and all of this library's refinement-based validation runs on that output — by the time any check executes, the key is gone.

Fix

All of the logic lives in a single new ProtoPropertyHandler (the old ProtoRequiredHandler special case is deleted); no other handler needed changes beyond comment updates. For schemas that constrain an own __proto__ key (via required or a properties entry), the handler replaces the converted schema wholesale:

1. It builds a reduced copy of the JSON schema: __proto__ is dropped from required, its properties entry is relaxed to the boolean true schema (so additionalProperties still treats it as a declared property), and keywords that must observe raw object keys (enum, const, minProperties, maxProperties, propertyNames, dependentRequired, dependentSchemas, and sibling references) are removed. The copy is built with rest/spread and Object.fromEntries so no __proto__ setter is ever invoked.
2. The reduced schema is converted normally via the existing re-entrant convertJsonSchemaToZod (it no longer constrains __proto__, so the recursion terminates immediately).
3. Everything removed from the reduced schema is checked against the raw input in front of it:

z.any().superRefine(rawChecks).pipe(convertJsonSchemaToZod(reducedSchema))

z.any() passes the input through untouched, so the checks see the own __proto__ key; .pipe() then runs the reduced schema, so every other constraint still applies and the parse output remains pollution-safe (own __proto__ keys are still stripped from output — verified by a test). The raw checks cover: required presence and the __proto__ value schema (only for non-null, non-array objects; boolean schemas like false work), full enum/const deep-equality semantics, minProperties/maxProperties, propertyNames, dependentRequired, dependentSchemas, and sibling references. Keys are always read via Object.getOwnPropertyDescriptor / Object.prototype.hasOwnProperty.call, never through the __proto__ accessor; the schema's own properties.__proto__ entry is likewise read via a property descriptor so schemas arriving from JSON.parse work.

Details:

• Both issue cases now work: properties["__proto__"] value schemas and required: ["__proto__"], on typed and untyped object schemas.
• ProtoPropertyHandler runs after all other refinements (before MetadataHandler, so .describe() still lands on the outermost schema — covered by a check that description survives).
• The previously-skipped official-suite case properties | properties whose names are Javascript object property names | __proto__ not valid now passes and was removed from failing-tests-skip-list.json (104 → 103 skips). The sibling required.json __proto__ group stays green.
• README "Special Property Support" and "Known Limitations" updated: the remaining gaps are that jsonSchemaObjectToZodRawShape still skips __proto__ entries, patternProperties/additionalProperties schemas still do not apply to the key in the deferred refinement path, and sibling composition keywords still evaluate after Zod strips the key.

Testing

Tests were written first and confirmed to fail on the unfixed code (7 failures reproducing both issue cases plus variants), then the fix was implemented:

• Rewrote the two tests in src/object-properties.test.ts that previously asserted the limitation.
• New src/proto-properties.test.ts covering: untyped/typed required and properties cases, combined required + value schema, __proto__ required alongside ordinary keys, schemas arriving via JSON.parse, boolean/undefined property schemas, non-object inputs, pollution-safety of parse output, enum/const/minProperties/maxProperties interaction (including the empty-enum special case), additionalProperties: false with a declared __proto__ property, interaction with the required-with-default pipe from Enforce required for properties that have a default #51, sibling references, propertyNames, dependentRequired, and dependentSchemas.
• Full suite after merging current main: 1567 passed | 103 skipped, with 100% line and branch coverage.
• The simplified implementation was differentially tested against the previous multi-file implementation of this PR: 546 schema/input pairs across 26 __proto__-related schemas produced identical results.
• npm run build succeeds; issue examples verified against the built CJS and ESM bundles.

Fixes #41

🤖 Generated with Claude Code

[schani]

This doesn't fix all the cases, and doing that seems like a huge headache. So for the time being we're punting on this. Will document in the README if it's not already.