Nice fix for the OpenAI path — we hit the same bug in production (langchain-openai 1.1.x with_structured_output returned plain text and pydantic json_invalid).

One additional gap worth flagging: the Anthropic-platform path has the symmetric problem. After CC→Responses conversion, ResponsesToAnthropicRequest in backend/internal/pkg/apicompat/responses_to_anthropic_request.go doesn't consume req.Text.Format, and AnthropicOutputConfig in types.go only has Effort (no Format field). So even if this PR lands, a response_format through a group whose platform is Anthropic (handler: GatewayService.ForwardAsChatCompletions → Anthropic) still gets dropped before reaching Claude.

Anthropic shipped structured outputs GA on 2025-11 (output_config.format, supports type: "json_schema" and type: "json" — note json not json_object). Mapping is straightforward:

Responses text.format {type: json_schema, schema, name, strict}
  → Anthropic output_config.format {type: json_schema, schema}  // name/strict dropped
Responses text.format {type: json_object}
  → Anthropic output_config.format {type: json}

Docs: https://platform.claude.com/docs/en/build-with-claude/structured-outputs

Happy to send a follow-up PR for the Anthropic path once this lands, or include it here if you'd prefer one unified PR — let me know which is easier to review.