[ochprince]

Summary

Closes #45273

This backport fixes multi-turn OpenAI /v1/responses failures on v0.19.1 when used with Cursor Codex and GLM-5.1-FP8. Single-turn requests work; multi-turn sessions with assistant history and tool calls fail with 400/500 errors.

1. Root cause

On v0.19.1, the Responses API request pipeline does not handle Codex-compatible multi-turn input items correctly:

Layer	Problem
responses/protocol.py	function_call_parsing only handles function_call; historical assistant / reasoning messages fail Pydantic Union validation (Field required: status) → 400 Bad Request
responses/utils.py	_construct_single_message_from_response_item assumes non-empty content[0] → 500 IndexError when Codex sends empty assistant content
responses/utils.py	Stateful multi-turn leaks prior system instructions; tools=[] not handled consistently
responses/api_router.py	Response serialization missing by_alias=True
tool_parsers/glm4_moe_tool_parser.py	Expects Chat Completions tool schema (tool.function.name) but Responses API passes FunctionTool (tool.name) → streaming tool-call crash
entrypoints/chat_utils.py	json.loads on malformed historical function_call.arguments → JSONDecodeError: Extra data

v0.21.0 partially fixed assistant parsing via input_item_parsing, but GLM-5.1 deployments are pinned to v0.19.1 due to model compatibility.

2. Fix approach

Minimal backport from v0.21.0 patterns plus Codex-specific hardening:

• protocol.py: Replace function_call_parsing with input_item_parsing; add check_tool_usage; normalize function_call.arguments
• utils.py: Defensive empty-content handling; filter leaked system messages; fix construct_tool_dicts for empty tools
• api_router.py: Add by_alias=True to JSON/SSE responses; fix serialize_message return type
• glm4_moe_tool_parser.py: Support both FunctionTool and Chat Completions tool formats via _extract_tool_info
• chat_utils.py: Add _parse_tool_call_arguments with raw_decode fallback for trailing extra data

3. Test verification

Production integration (GLM-5.1-FP8, 2×8×H800, v0.19.1 Docker):

• Multi-turn Codex conversations with assistant history — no 400/500
• Long-running agent sessions with tool calls — stable
• Before/after screenshots in [Bug]: v0.19.1 /v1/responses fails for multi-turn Codex requests (GLM-5.1) #45273

Unit tests added:

pytest tests/entrypoints/openai/responses/test_function_call_parsing.py \
       tests/entrypoints/openai/responses/test_responses_utils.py \
       tests/tool_use/test_responses_request_validations.py \
       tests/tool_parsers/test_glm4_moe_tool_parser.py -v

Minimal curl repro (Symptom 1):

curl -X POST http://<host>:8001/v1/responses \
  -H "Content-Type: application/json" \
  -d '{
    "model": "GLM-5.1-FP8",
    "input": [
      {"type": "message", "role": "user",
       "content": [{"type": "input_text", "text": "Hello"}]},
      {"type": "message", "role": "assistant",
       "content": [{"type": "output_text", "text": "Hi there!"}]},
      {"type": "message", "role": "user",
       "content": [{"type": "input_text", "text": "Continue"}]}
    ]
  }'

Before patch: 400 Bad Request. After patch: 200 OK.

4. Compatibility

• Changes are scoped to Responses API request parsing, response serialization, GLM tool parser compatibility, and tool-call argument normalization
• Does not modify model kernels, scheduler, or inference logic
• Chat Completions API behavior is unchanged
• Other models using Responses API with similar multi-turn input formats should also benefit

5. AI-assisted disclosure

This patch was developed with AI assistance (Cursor). All changes were reviewed and validated end-to-end on a production GLM-5.1 deployment. Commits include Co-authored-by: Cursor <cursoragent@cursor.com> trailers per project policy.

Changed files

 vllm/entrypoints/openai/responses/protocol.py
 vllm/entrypoints/openai/responses/utils.py
 vllm/entrypoints/openai/responses/api_router.py
 vllm/tool_parsers/glm4_moe_tool_parser.py
 vllm/entrypoints/chat_utils.py
 + 4 test files

Test plan

• Multi-turn Codex session with GLM-5.1-FP8 on v0.19.1 (production H800 cluster)
• Multi-turn tool calling via Codex
• curl repro for assistant history 400 → 200
• CI unit tests (added; pending upstream CI run)

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[bbartels]

Dont think vllm tends to do backporting once a new minor version got released

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