OpenAI's new flagship models (gpt-5.x) return 404 'This is not a chat
model' on /v1/chat/completions and are only reachable via /v1/responses.
Add a dedicated _stream_openai_responses path in ExternalProviderClient
that:
- Translates outbound messages into the Responses shape: system messages
are folded into the top-level 'instructions' field, user/assistant
messages become {role, content} items with input_text / input_image
content parts (data URLs and https URLs both pass through).
- Drops presence_penalty / top_k / frequency_penalty, none of which the
Responses contract accepts.
- Translates inbound SSE events back into OpenAI Chat Completions
chunks so the frontend keeps a single SSE shape:
response.output_text.delta -> delta chunk with content
response.completed -> chunk with finish_reason='stop'
response.incomplete -> chunk with finish_reason='length'
response.failed / error -> propagated error SSE line
Stream terminates with data: [DONE] (Responses emits this verbatim).
stream_chat_completion dispatches all provider_type='openai' calls to
this path; other OpenAI-compatible providers (mistral, gemini, etc.)
continue to use /v1/chat/completions.
Frontend provider-capabilities map updated to hide presence_penalty for
OpenAI in the chat settings panel, matching the new request contract.
Includes unit coverage in tests/test_openai_responses_translation.py
exercising the request body translation, image-part rewriting, and
SSE-to-chat-completions translation via httpx.MockTransport.
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