feat(router): add OpenAI Responses API and Home Assistant model support (#374)

[sheepdestroyer]

Summary

This PR addresses issue #374 by implementing OpenAI Responses API support, streaming SSE tool call argument event synthesis, and Home Assistant model aliases required by Home Assistant's openai_conversation integration.

Changes Included

1. Responses API Endpoints:

   • Added POST /v1/responses and POST /responses handlers in router/main.py proxying requests to LiteLLM's /v1/responses endpoint.
   • Preserves model triage classification when auto-routing models (llm-routing-auto-free, llm-routing-ollama) are requested.
   • Parses input_text and text content part types, extracts the latest user turn in reverse order, and joins multi-part strings with space separators.
   • Validates upstream HTTP status codes before streaming, returning proper HTTP errors instead of masking 200 streams.
   • Synthesizes response.function_call_arguments.delta and response.function_call_arguments.done SSE events during streaming for Home Assistant compatibility.

2. Model Aliases & Configuration:

   • Added local-qwen-3.6-hass, gpt-4o-mini, and gpt-4o model definitions to litellm/config.yaml with enable_thinking: false and supports_reasoning: false.
   • Added models to public_model_groups so GET /v1/models lists them.
   • Included new model aliases in DIRECT_TIERS in router/main.py to bypass classifier overhead when specified directly.

3. Tool Support & E2E Validation:

   • Verified and enabled support for Home Assistant Assist actions (function tool type returning function_call output), code_interpreter, and web_search.
   • Added test_ha_tool_calling to scripts/verification/verify_canonical_endpoints.py testing E2E tool calling across local-qwen-3.6-hass, gpt-4o-mini, and gpt-4o for both /v1/responses and /v1/chat/completions.

4. Testing & Documentation:

   • Expanded automated pytest suite in router/tests/test_responses_api.py covering model routing, Responses API requests, parameterized tool calls, streaming SSE deltas, input extraction, and error handling (372/372 tests passing).
   • Added section 9e to README.md detailing Home Assistant configuration and capability requirements.

Closes#374
Supersedes #382

Summary by Sourcery

Add OpenAI-compatible Responses and audio proxy endpoints and extend model support and testing for Home Assistant integration.

New Features:

• Expose OpenAI Responses API endpoints that proxy to LiteLLM, including support for streaming and Home Assistant-compatible tool/function calls.
• Add audio proxy routes that forward speech-to-text and text-to-speech requests to LiteLLM with basic path validation.

Enhancements:

• Extend direct tier model list and LiteLLM configuration to include Home Assistant-focused and OpenAI-style model aliases as public model groups.
• Adjust whisper-related transcription models to use the correct API base path for audio requests.

Documentation:

• Update README links to point to canonical GitHub locations and add documentation describing Home Assistant integration and Responses API support.

Tests:

• Add a pytest suite for the Responses API covering routing, input extraction, tool usage, streaming SSE behavior, and error handling.
• Introduce an end-to-end verification test for Home Assistant tool calling across Responses and Chat Completions APIs and register it in the canonical verification runner.

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📥 Commits

Reviewing files that changed from the base of the PR and between dcdd018 and 76f3f43.

📒 Files selected for processing (5)

• README.md
• litellm/config.yaml
• router/main.py
• router/tests/test_responses_api.py
• scripts/verification/verify_canonical_endpoints.py

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