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fix(parser): enforce required tool choice with gemma4 tool parser - #53765
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Flagging an important note before review. On the parent issue #53363, a community contributor (sudhirpol522) reported a fix in flight via XGrammar (#684 upstream) that constrains generation with a grammar so gemma4 tool calls parse correctly. This PR takes a different, narrower approach: post-parse enforcement that raises when So this is not a duplicate of that work, but it is a weaker guarantee than grammar-constraint. If the XGrammar path is preferred, please treat this as a stopgap and I am happy to close or reshape it. Raising it proactively so maintainers can pick the direction. |
…roject#53363) tool_choice='required' was not enforced for gemma4 because Gemma4EngineToolParser sets supports_required_and_named=False and skips structured output JSON so the model can emit native <|tool_call> syntax. Under required, the model could free-generate prose with no constraint at all. Add a post-parse enforcement check that raises when a required tool choice produced no tool call. It runs on the unified engine parser (parse and parse_delta at finish) and on the standalone tool parser extract_tool_calls. The native <|tool_call> path is untouched, so the speculative decoding fix that motivated the original skip is preserved. Regression tests cover required plus gemma4 for non-streaming and streaming paths. Fixes defect 1 of issue vllm-project#53363. Signed-off-by: Laura López Real <113060513+laulopezreal@users.noreply.github.com>
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Hi @aarnphm @chaunceyjiang @sfeng33 @bbrowning — this is ready for review when you have a moment. Summary:
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Summary
Fixes Defect 1 of #53363. With the gemma4 tool parser,
tool_choice="required"was not enforced: the model could free-generate plain prose instead of a tool call.Root cause
Gemma4EngineToolParsersetssupports_required_and_named = Falseand intentionally skips structured-output JSON for required/named tool choice so the model can emit its native<|tool_call>syntax (this skip exists to avoid a crash under speculative decoding). Underrequiredthat meant no constraint at all, and the parser fell through to the auto-tool-call path, so prose-only responses passed through unconstrained.Fix
Add a post-parse enforcement check that raises when a required tool choice produced no tool call:
vllm/parser/gemma4.py: newToolChoiceRequiredNotEnforcedError, anenforce_required_tool_call()helper, andparse/parse_deltaoverrides onGemma4Parserthat raise whentool_choice="required"yields no tool call.vllm/tool_parsers/gemma4_engine_tool_parser.py:extract_tool_callsoverride applying the same enforcement on the standalone tool-parser path.The native
<|tool_call>path is untouched, so the speculative-decoding crash fix that motivated the original structured-output skip is preserved.Scope
Defect 1 only.
gemma4_engine_reasoning_parser.pyand thefinish_reasonlabeling logic are untouched (Defect 2, the emptytool_callswithfinish_reason="tool_calls"labeling, is covered by #53255 / #53256 / #53270).Tests
tests/tool_parsers/test_gemma4_required_enforcement.pywith 8 tests covering required + gemma4 across non-streaming and streaming, on both the unified engine path and the standalone tool-parser path.tests/parser/andtests/tool_parsers/(3 skipped, 34 xfailed). The only errors are pre-existing and unrelated: 15 intest_llama3_json_tool_parser.pyfrom a gated HF repo 401, and one device-detection test that passes withVLLM_TARGET_DEVICE=cpu.Not a duplicate: no open PR addresses this fix (verified via
gh pr listand the issue timeline before opening).