[Structured Output] Compose auto tool-call grammar with response_format schema - #48157
[Structured Output] Compose auto tool-call grammar with response_format schema#48157pablopupo wants to merge 2 commits into
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…at schema Add compose_tool_call_or_schema to the structural tag registry so a tool_choice="auto" request that also sets response_format can either emit a tool call or a schema-constrained final answer, rather than dropping the schema. It builds an OrFormat whose tool branch is forced to require at least one tool call, so the schema branch stays reachable. Prototype for issue vllm-project#39929, proven with a model-free xgrammar compile test covering the hermes and minimax builders. Signed-off-by: pablopupo <145598901+pablopupo@users.noreply.github.com>
…quests Add get_composed_structural_tag to the registry and a hook in DelegatingParser._apply_structural_tag so a tool_choice="auto" request that also sets response_format (json_schema, json_object, or the Responses API's text.format) gets the schema composed with the tool-call grammar for hermes and minimax instead of being dropped. A shared helper reads the schema off both ChatCompletionRequest and ResponsesRequest, and composition is skipped whenever a reasoning parser is active, since a think preamble would not match the schema answer branch. Extends the composition tests with a json_object any-JSON case and a non-strict-tools case proving get_composed_structural_tag bypasses the strict-tools short circuit that get_model_structural_tag applies, and adds request-level tests through the real DelegatingParser/registry path proving the wiring composes for hermes, leaves non-composable models and response_format untouched, and falls back when reasoning is active. Continues the prototype for issue vllm-project#39929. Signed-off-by: pablopupo <145598901+pablopupo@users.noreply.github.com>
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Purpose
A request that sets
tool_choiceto auto and also asks for aresponse_formatschema cannot get both today. As soon as the server builds a grammar for tool calling, the schema is dropped, so an agent that wants to either call a tool or answer with a structured payload loses one side of that. This is the case behind #39929, and it shows up in practice because the OpenAI Agents SDK sends the agent'soutput_typeschema asresponse_formaton every model call, including turns where the model should be free to call a tool.This wires up a composed grammar for that case. It builds an
OrFormatwith the tool-call grammar as one branch and theresponse_formatschema as the other, and the tool branch is forced to require at least one tool call. Without that forcing an auto tool grammar also matches plain free text with no call at all, which would make the schema branch unreachable and the whole constraint vacuous.The composition applies to hermes and minimax for now, the two formats where vLLM builds its own structural tag and can express that "at least one call" requirement. Every other model, and any request with an active reasoning parser, falls back to current behavior, since a model that reasons before answering would emit a think preamble the schema branch could never match. Both of those are left as follow-ups rather than handled here.
This is additive and does not depend on #47439, which is a separate, more general drop-the-constraint fallback for the models this PR does not touch.
Test Plan
pytest tests/tool_parsers/test_composed_tool_schema_grammar.py tests/tool_parsers/test_structural_tag_registry.py tests/tool_use/test_tool_parser_adjust_request.pyThe grammar-level tests build the real registry tags for hermes and minimax, compose them, compile with xgrammar, and assert acceptance of a tool call and of schema-valid JSON, and rejection of schema-invalid JSON and free text, plus the bare
json_objectcase. The request-level tests drive the real parser and registry to confirm composition happens for a composable model and clearsresponse_format, is left untouched for a model without a vLLM-owned builder, and is skipped when a reasoning parser is attached.tests/tool_use/test_chat_completions.py::test_response_format_with_tool_choice_autoexercises the end to end path against a live server.Test Result
66 passedacross the grammar-level and request-level suites. Reverting the two source files while keeping the tests makes the two composable-model request tests fail (the schema branch is not composed andresponse_formatis not cleared) while the fallback tests stay green, confirming the wiring is what drives the behavior.pre-commitruff and mypy hooks pass. The chat completions end to end test needs a model server and was not run locally.