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[Bugfix][Frontend] Do not report finish_reason="tool_calls" when no tool call was made - #53256
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Friendly ping — this is my first PR to vLLM, so Short summary for reviewers: With The streaming path in the same file already guards this by requiring Verified end-to-end on a served model, patch as the only variable:
Happy to move the guard elsewhere or restructure if you would prefer a different shape. |
…ol call was made With `tool_choice="required"`, the non-streaming chat handler translates the engine's "stop" into `finish_reason="tool_calls"`. That translation assumes a tool call was produced, but it never checks. When the parse yields no calls -- for example when no tool parser is configured, so `parser` is None and `tool_calls` is `[]` -- the response goes out with `finish_reason="tool_calls"` next to an empty `tool_calls`. That shape contradicts itself, and `finish_reason` is the documented way to detect a tool call, so a client takes the tool-execution path and finds nothing to execute. The streaming path already gets this right: it requires `tools_streamed[i]` before emitting "tool_calls", so today the same request answers differently depending only on `stream`. This makes the non-streaming path agree by requiring that a call actually exists. The `auto` path is unaffected (`auto_tools_called` already implies non-empty tool calls), and `required` with a real tool call still reports "tool_calls". Assisted-by: Claude Code (Opus 5) Signed-off-by: Luu Bao Long <longbaoluu68@gmail.com>
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| request.tool_choice | ||
| and request.tool_choice == "required" | ||
| and output.finish_reason == "stop" | ||
| and bool(tool_calls) |
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Could you take a look at how OpenAI online handles this case? I think the original logic was correct.
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This pull request has merge conflicts that must be resolved before it can be |
Purpose
Fixes #53255.
With
tool_choice="required", the non-streaming chat handler translates theengine's
"stop"intofinish_reason="tool_calls". That translation assumes atool call was produced, but it never checks. When the parse yields no calls —
for example when no tool parser is configured, so
parseris None andtool_callsis[]— the response goes out withfinish_reason="tool_calls"next to an empty
tool_calls.finish_reasonis the documented way to detect a tool call, so a client takesthe tool-execution branch and finds nothing to execute.
The streaming path in the same file already gets this right — it requires
tools_streamed[i], set only when a tool-call delta was actually emitted,before reporting
"tool_calls". So today the same request answers differentlydepending only on
stream. This makes the non-streaming path agree:Scope is deliberately narrow:
autois unaffected —auto_tools_calledalready implies non-empty tool calls.requiredwith a real tool call still reports"tool_calls".tool_choiceis untouched; it continues to report"stop", which is theexisting intended behaviour documented in the comment above this expression.
Why this is not duplicating an existing PR
Checked before opening, and re-checked since:
(a tool call truncated by
max_tokenslosing its"length"reason). Neithertouches
is_finish_reason_tool_calls, and neither fixes the non-streaming casehere. I opened [Bugfix][Frontend] Keep the real finish_reason when a streamed tool call is truncated #53270 for that streaming defect before finding those two, and
closed it as a duplicate; my reproduction data is now on [Bugfix] Report finish_reason='length' for tool calls truncated by max_tokens in streaming #47963.
issue. This PR came first and carries the end-to-end measurement below; only one
of the two needs reviewing.
Test Plan
Unit test added:
test_required_tool_choice_does_not_claim_a_tool_call_it_did_not_makein
tests/entrypoints/openai/chat_completion/test_serving_chat.py, extending theexisting suite and reusing its
_build_minimal_metrics_serving_chathelper (whichalready configures no tool parser — the exact condition that triggers this).
I could not run the suite locally — no CUDA/GPU on the machine used to prepare
this — so the unit test is offered for CI. The behavioural claims below come from
running a real server.
End-to-end: the server was exercised over HTTP with the request from the issue,
on the same host and image with the patch as the only variable, in two arms —
tool parser off (the broken case) and tool parser on (the case this change could
regress).
Test Result
End-to-end,
tool_choice="required":tool_callsfinish_reason"tool_calls""stop""tool_calls"The third row is the one that matters most: the working path is untouched, and
the returned call is still well formed (
get_weather({"city": "San Francisco"})).Named
tool_choicewas checked in the same runs and is unchanged by this patch,still returning its call with
finish_reason="stop"as before.No model evaluation is included: this changes only the
finish_reasonfieldselected by the serving layer. Token generation, sampling and model output are
untouched.
AI assistance
AI assistance was used to prepare this change (investigation, patch, test and
this description), with
Assisted-by:recorded in the commit trailer. Thesubmitter has reviewed every changed line, ran the end-to-end verification above,
and stands behind the change.