[Bugfix][Responses API] Fix streaming tool calls on /v1/responses

[hnt2601]

Two bugs made streaming function calling unusable on the Responses API for any tool-call parser that relies on special-token delimiters (Gemma4), and for any parser when tool_choice="required" is combined with stream=True.

1. Gemma4 tool calls leak as plain text via response.output_text.delta

Gemma4ToolParser.adjust_request guarded the skip_special_tokens = False line with isinstance(request, ChatCompletionRequest), so a ResponsesRequest carrying tools kept the default skip_special_tokens = True. The tokenizer then stripped the Gemma4 delimiters (<|tool_call>, <tool_call|>, <|"|>) from the detokenized text before the parser saw them, and
Gemma4ToolParser.extract_tool_calls_streaming took the self.tool_call_start_token not in current_text branch and emitted the raw call:fn{...} body via response.output_text.delta instead of response.function_call_arguments.delta.

Fix: drop the isinstance guard so both ChatCompletionRequest and ResponsesRequest get skip_special_tokens = False, matching the pattern already used by FunctionGemmaToolParser.adjust_request.

2. tool_choice="required" + stream=True crashes on /v1/responses

ToolParser.adjust_request built ResponseTextConfig in two steps (bare constructor, then request.text.format = ...). Under Pydantic v2 the post-init field assignment is not tracked in __fields_set__, which can drop the nested config from model_dump(...) and surface downstream as ValidationError: schema field required when the initial ResponseCreatedEvent is serialized. The same call site also passed a description="Response format for tool calling" kwarg that is not semantically a tool schema description.

Fix: use a single-shot ResponseTextConfig(format=...) constructor so format is part of __fields_set__, and drop the description kwarg.

Tests

Added tests/tool_use/test_gemma4_responses_adjust_request.py with two unit regressions:

• test_gemma4_adjust_request_sets_skip_special_tokens_on_responses: asserts Gemma4ToolParser.adjust_request flips skip_special_tokens=False for a ResponsesRequest with tools.
• test_tool_parser_adjust_request_builds_valid_response_text_config: asserts the dumped ResponseTextConfig (with by_alias=True) has format.type=="json_schema", contains the nested schema key, and does not leak the old "Response format for tool calling" string.

Both tests fail on main and pass after this change. End-to-end curl verification against a live Gemma4 server (--tool-call-parser gemma4 --enable-auto-tool-choice on a single H100) confirms response.function_call_arguments.delta events are now emitted and no call:get_weather{...} text leaks via response.output_text.delta.

Purpose

Test Plan

Test Result

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[gemini-code-assist]

Code Review

This pull request addresses two bugs in the /v1/responses path that affected streaming tool calls. It updates Gemma4ToolParser to ensure skip_special_tokens is disabled for both ChatCompletionRequest and ResponsesRequest, preventing the removal of necessary tool-call delimiters. Additionally, it refactors ToolParser.adjust_request to use single-shot initialization for ResponseTextConfig, ensuring compatibility with Pydantic v2's field tracking and removing an unsupported description parameter. New regression tests have been added to verify these fixes. I have no feedback to provide.

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[chaunceyjiang]

Thanks.

This PR looks good.

In fact, the tool_choice="required" + stream=True combination on /v1/responses has not been officially implemented yet.

[mergify]

Documentation preview: https://vllm--39892.org.readthedocs.build/en/39892/

[mergify]

Hi @hnt2601, the pre-commit checks have failed. Please run:

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[ehfd]

@sfeng33 @chaunceyjiang

[ehfd]

@bbrowning

[sfeng33]

@chaunceyjiang would you have more feedback on this PR?

[chaunceyjiang]

LGTM