Tool schemas cause 400 errors on strict validators (Codex, Vertex AI)

[sametakofficial]

Built-in tool schemas (like question) break on providers with strict JSON Schema validation — Codex, Vertex AI OpenAI-compatible, SGLang, etc.

The question tool has two problems:

• additionalProperties: false is set but multiple isn't in required, which strict validators reject
• Zod .meta() injects $schema and ref keywords that aren't valid in OpenAI function parameter schemas

Error from Codex:

Invalid schema for function 'question': In context=('properties', 'questions',
'items'), 'required' is required to be supplied and to be an array including
every key in properties. Missing 'multiple'.

Related: #11413, #8184

[github-actions]

This issue appears to be closely related to several existing issues addressing the same root cause - missing 'required' field in tool schemas and invalid Zod meta fields:

• Claude API rejects tool schemas: missing 'required' field causes 'JSON schema is invalid' error #13618: Claude API rejects tool schemas with missing 'required' field
• "question" tool schema causes "400 Bad Request" with Gemini/Vertex AI` #11413: The specific 'question' tool schema issue on Vertex AI/Gemini
• Strict JSON Schema validation fail on backends like SGLang (Missing required field in empty tool parameters) #8184: SGLang strict validation failures with empty object parameters

All of these stem from the same problem in the schema transformation layer where object-based schemas don't include an explicit 'required' field and Zod's `` metadata isn't being stripped. Consider consolidating these issues or using one as the canonical issue for the fix across all providers.