feat(openai): add chat/completions api_format config option #902
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StreamEndEvent.usage can be null when providers don't include usage data in their final stream chunk, causing a TypeError downstream. Add `?? new Usage(0, 0)` fallback to emitStreamEndEvent() in all providers missing it, matching the existing pattern in the OpenAI stream handler.
Add an `api_format` config option to the OpenAI driver that allows switching from the default `/responses` endpoint to `/chat/completions`. This enables using Prism with OpenAI-compatible backends like vLLM, LiteLLM, and LocalAI that only implement the chat/completions API. Set `OPENAI_API_FORMAT=chat_completions` in your env to use it. Only text, structured, and stream methods dispatch conditionally — other modalities (embeddings, images, moderation, TTS, STT) already use standard endpoints that work with compatible backends as-is.
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Summary
api_formatconfig option to the OpenAI driver (default:responses, alternative:chat_completions)/chat/completionstext(),structured(), andstream()dispatch conditionally — other modalities (embeddings, images, moderation, TTS, STT) already use standard endpoints that work as-isUsage
New files
Handlers/ChatCompletions/Text.php,Structured.php,Stream.php— chat/completions protocol handlersMaps/ChatCompletionsMessageMap.php— standard message format (texttype, flat assistant content)Maps/ChatCompletionsFinishReasonMap.php— mapsstop/tool_calls/lengthMaps/ChatCompletionsToolMap.php,ChatCompletionsToolChoiceMap.php— nestedfunctionkey formatMaps/ChatCompletionsImageMapper.php—image_urlformatRefs #900
Test plan
OPENAI_API_FORMAT=chat_completionswith a LiteLLM/vLLM proxy