fix: Always set additionalProperties=false for strict schema compliance

[yashwantbezawada]

Summary

Fixes #2740

This PR resolves an issue where Pydantic models using ConfigDict(extra="allow") fail when used with structured output because additionalProperties is not forced to false.

Problem

The OpenAI API requires "additionalProperties": false in JSON schemas for structured output (response_format). When Pydantic models use extra="allow", the generated schema includes "additionalProperties": true, which causes the API to reject the request with:

BadRequestError: Error code: 400 - {'error': {'message': "Invalid schema for response_format 'MyClass': In context=(), 'additionalProperties' is required to be supplied and to be false.", ...}}

Root Cause

The _ensure_strict_json_schema() function (line 50) only set additionalProperties = false when the key was not already present:

if typ == "object" and "additionalProperties" not in json_schema:
    json_schema["additionalProperties"] = False

This conditional approach failed for Pydantic models with extra="allow" because those models already have "additionalProperties": true in their schema.

Solution

Modified the code to always set additionalProperties = false for object types, regardless of whether the key already exists:

if typ == "object":
    # Always set additionalProperties to False for strict schema compliance.
    # The OpenAI API requires additionalProperties=false for structured output,
    # even if Pydantic models use extra="allow" which sets it to True.
    json_schema["additionalProperties"] = False

Changes

1. Modified src/openai/lib/_pydantic.py: Updated _ensure_strict_json_schema() to unconditionally set additionalProperties = false
2. Added test case: New test_pydantic_extra_allow() in tests/lib/test_pydantic.py to verify the fix

Testing

Manual Testing

Reproduced the issue from #2740 and verified the fix resolves it:

Before fix:

schema = to_strict_json_schema(MyClassWithExtraAllow)
# {"additionalProperties": true, ...}  ❌ Fails API validation

After fix:

schema = to_strict_json_schema(MyClassWithExtraAllow)
# {"additionalProperties": false, ...}  ✅ Passes API validation

Test Coverage

Added comprehensive test that verifies:

• Models with extra="allow" correctly generate additionalProperties: false
• Schema structure remains valid (type, properties, required fields)
• Regression protection for this specific issue

Backward Compatibility

This change is backward compatible:

• No changes to public API
• Only affects internal schema generation
• Models without extra="allow" already had additionalProperties: false
• Models with extra="allow" now work correctly (previously failed)

References

• Issue: Improper handling of pydantic extra="allow" #2740
• Related forum discussion: https://community.openai.com/t/additionalproperties-error-when-unpacking-list-of-one-pydantic-object-in-union/953872/2

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[chatgpt-codex-connector]

Preserve schemas for typed dictionaries when enforcing strictness

Overwriting additionalProperties unconditionally means any object schema that intentionally defines a value schema for dynamic keys (e.g. Pydantic fields of type Dict[str, T]) now loses that schema and becomes additionalProperties: False. These dict fields previously produced {"additionalProperties": {…}} so the model could accept arbitrary keys whose values match the inner schema; after this change the generated schema forbids all keys, so structured outputs containing mappings can never validate even though the underlying model allows them. Consider only forcing False when the value is a boolean and leaving structured additionalProperties schemas intact.

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