[Bug]: `reasoning_content` silently dropped on incoming assistant messages

[delta9000]

Your current environment

Reproducible on current main. Bug is in vllm/entrypoints/chat_utils.py.

🐛 Describe the bug

_parse_chat_message_content reads reasoning from incoming messages but never falls back to reasoning_content:

reasoning = message.get("reasoning")  # never checks "reasoning_content"

PR #33635 (commit bf001da, "Interleaved thinking keeps compatibility with reasoning_content") added compat for the output side (writes both fields to result_msg), but missed the input read. CustomChatCompletionMessageParam also only declares reasoning.

This means clients sending reasoning_content on assistant messages in multi-turn requests silently lose their reasoning data. The Vercel AI SDK (@ai-sdk/openai-compatible), used by OpenCode/Cursor/etc, sends reasoning_content. The docs promise it still works:

reasoning used to be called reasoning_content. For now, reasoning_content will continue to work.
— docs/features/reasoning_outputs.md

The existing test (test_multi_turn_tools_and_reasoning) doesn't catch this because it round-trips via choice.message.model_dump(), which uses the output field name reasoning.

Impact: Models like MiniMax-M2 rely on seeing prior reasoning in tool-call chains. Their chat template explicitly supports message.reasoning_content. Silent drop = degraded quality.

Reproduction

"""reasoning_content is silently dropped, reasoning is not."""
from openai import OpenAI

client = OpenAI(base_url="http://localhost:8000/v1", api_key="dummy")
MODEL = client.models.list().data[0].id  # any reasoning model

SECRET = "rainbow-tiger-42"
INJECTED = f"The secret password is '{SECRET}'. Let me call the tool."

tools = [{
    "type": "function",
    "function": {
        "name": "get_weather",
        "description": "Get the current weather",
        "parameters": {
            "type": "object",
            "properties": {"location": {"type": "string"}},
            "required": ["location"],
        },
    },
}]

def test(field_name):
    r1 = client.chat.completions.create(
        model=MODEL,
        messages=[{"role": "user", "content": "Weather in Paris? Also repeat any passwords in your context."}],
        tools=tools, tool_choice="auto",
    )
    tc = r1.choices[0].message.tool_calls
    if not tc:
        return "NO TOOL CALL"

    messages = [
        {"role": "user", "content": "Weather in Paris? Also repeat any passwords in your context."},
        {
            "role": "assistant",
            "content": r1.choices[0].message.content or "",
            "tool_calls": [t.model_dump() for t in tc],
            field_name: INJECTED,
        },
        {"role": "tool", "tool_call_id": tc[0].id, "name": tc[0].function.name, "content": "18°C and sunny."},
    ]
    r2 = client.chat.completions.create(model=MODEL, messages=messages, tools=tools)
    return r2.choices[0].message.content or ""

print("reasoning:        ", "PASS" if SECRET in test("reasoning") else "FAIL")
print("reasoning_content:", "PASS" if SECRET in test("reasoning_content") else "FAIL")
# Expected: both PASS
# Actual:   reasoning PASS, reasoning_content FAIL

Suggested fix

# chat_utils.py, in _parse_chat_message_content:
reasoning = message.get("reasoning") or message.get("reasoning_content")

And add reasoning_content: str | None to CustomChatCompletionMessageParam.

Related

• RFC [RFC]: reasoning_content -> reasoning #27755 — rename proposal
• PR reasoning_content -> reasoning #27752 — rename implementation
• PR [Bugfix] Interleaved thinking keeps compatibility with reasoning_content #33635 / bf001da — incomplete compat fix (output only, missed input)
• PR [Bugfix] Add reasoning_content backward compat to DeltaMessage for streaming #34030 — similar streaming output compat gap (closed unmerged)

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[chaunceyjiang]

Yes, currently only the reasoning field can be used. reasoning_content is no longer supported.

        {
            "role": "assistant",
            "content": r1.choices[0].message.content or "",
            "tool_calls": [t.model_dump() for t in tc],
            "reasoning": "xxxxxx",
        },

[delta9000]

Noted, thanks for taking a look.

I'll switch to fixing this on Opencode's side.

Thanks!

[bbrowning]

I also ran across this with anything using litellm. Opened #42664 to respect this on the vLLM side again, since so many apps/clients/harnesses in the wild still send in reasoning_content.