[Bugfix][ResponsesAPI] Fix crash when tool_choice=required exceeds max_output_tokens

[chaunceyjiang]

Purpose

follow up #36841

FIX https://buildkite.com/vllm/ci/builds/56537?group_by=test#019cf9dc-06da-4341-aa86-6e0d6cb06ec8

[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]     return await self.responses_full_generator(
--
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]   File "/usr/local/lib/python3.12/dist-packages/vllm/entrypoints/openai/responses/serving.py", line 711, in responses_full_generator
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]     output = self._make_response_output_items(request, final_output, tokenizer)
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]   File "/usr/local/lib/python3.12/dist-packages/vllm/entrypoints/openai/responses/serving.py", line 904, in _make_response_output_items
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]     return parser.extract_response_outputs(
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]   File "/usr/local/lib/python3.12/dist-packages/vllm/parser/abstract_parser.py", line 325, in extract_response_outputs
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]     tool_calls, content = self._parse_tool_calls(
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]                           ^^^^^^^^^^^^^^^^^^^^^^^
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]   File "/usr/local/lib/python3.12/dist-packages/vllm/parser/abstract_parser.py", line 426, in _parse_tool_calls
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]     tool_calls = TypeAdapter(list[FunctionDefinition]).validate_json(content)
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]   File "/usr/local/lib/python3.12/dist-packages/pydantic/type_adapter.py", line 492, in validate_json
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]     return self.validator.validate_json(
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374]            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[2026-03-17T04:29:59Z] (APIServer pid=2510) ERROR 03-17 04:29:59 [server_utils.py:374] pydantic_core._pydantic_core.ValidationError: 1 validation error for list[function-wrap[__log_extra_fields__()]]

Test Plan

see e2e

Test gpt-5 with openai

response = client.responses.create(
    model="gpt-5",
    input=prompt,
    tools=tools,
    tool_choice="required",
    max_output_tokens=1,
)

Test Result

gpt-5

{
    "id": "resp_0c59c8e31591e43d0069b8f1e2a17c8190bffa061a344becb7",
    "created_at": 1773728226.0,
    "error": null,
    "incomplete_details": {
        "reason": "max_output_tokens"
    },
    "instructions": null,
    "metadata": {},
    "model": "gpt-5",
    "object": "response",
    "output": [
        {
            "id": "rs_0c59c8e31591e43d0069b8f1e3de108190b7204a20852a1dca",
            "summary": [],
            "type": "reasoning",
            "content": null,
            "encrypted_content": null,
            "status": null
        }
    ],
    "parallel_tool_calls": true,
    "temperature": 1.0,
    "tool_choice": "required",
....
}

vllm

{
    "id": "resp_89d52120b02c63ff",
    "created_at": 1773728620.0,
    "error": null,
    "incomplete_details": {
        "reason": "max_output_tokens"
    },
    "instructions": null,
    "metadata": null,
    "model": "my-model",
    "object": "response",
    "output": [
        {
            "id": "rs_a1ff3b9137e892f3",
            "summary": [],
            "type": "reasoning",
            "content": [
                {
                    "text": "The",
                    "type": "reasoning_text"
                }
            ],
            "encrypted_content": null,
            "status": null
        }
    ],
    "parallel_tool_calls": true,
    "temperature": 1.0,
    "tool_choice": "required",

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request addresses a crash in the Responses API when tool_choice="required" and the generated output for the tool call exceeds max_output_tokens. The fix correctly handles potential ValidationError during JSON parsing of the model's output by suppressing the exception. This prevents the crash and ensures that if the tool call JSON is invalid or truncated, no tool call is returned, which is the desired behavior. A new test case is added to validate this fix, confirming that the system remains stable under these conditions.

[chaunceyjiang]

/cc @DarkLight1337 PTAL.