[sherlockwu]

With thinking enabled and tool_choice="auto", Kimi K3 can return only reasoning: no answer and no tool call.

Example

{
  "model": "<your-served-kimi-k3-model>",
  "messages": [
    {"role": "user", "content": "What is the weather in San Francisco? Use the weather tool."}
  ],
  "chat_template_kwargs": {"thinking": true},
  "tool_choice": "auto",
  "tools": [{
    "type": "function",
    "function": {
      "name": "get_weather",
      "parameters": {
        "type": "object",
        "properties": {"city": {"type": "string"}},
        "required": ["city"]
      }
    }
  }],
  "stream": true,
  "stream_options": {"include_usage": true}
}

Before: the client gets reasoning_content chunks until the token budget runs out, then finish_reason: "length". It gets no content and no tool_calls.

After: the model finishes thinking, then calls get_weather({"city": "San Francisco"}). tool_choice="auto" still allows a plain answer instead.

Why it happens

For auto, the frontend sends a tool-call grammar (a structural tag) to the worker. Kimi K3's grammar forbids the end-of-thinking marker <|close|>think<|sep|>. The frontend also sent require_reasoning=False, so the grammar applied from the first token, while the model was still thinking. The model could not close its thinking block. It escaped only occasionally, by jumping straight into a tools block.

Fix

_guided_output_requires_reasoning now returns True for an auto request whose guided decoding is a tool-call structural tag. SGLang then holds the grammar back until thinking ends. This matches SGLang's serving_chat.py, which sets require_reasoning = thinking_mode for every tool_choice.

Unchanged:

• required/named tool choices.
• An explicit response_format, including the gpt-oss exception.

Note: the SGLang worker must run with SGLang's own --reasoning-parser. --dyn-reasoning-parser alone does not enable the backend that holds the grammar back. The same already applies to required/named tool choices.

Testing

test_sglang_processor_unit.py:

• gate cases for an auto structural tag, including response_format: text and the gpt-oss structural_tag exception;
• a Kimi K3 pool-worker test using the example above: thinking=true gives require_reasoning=True, thinking=false gives False. The thinking=true case fails without the fix.

291 passed. The two kimi_k2 auto failures also fail on main with this SGLang checkout (get_model_structural_tag rejects parallel_tool_calls). Not yet verified against a live Kimi K3 worker.

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👋 Hi sherlockwu! Thank you for contributing to ai-dynamo/dynamo.

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Walkthrough

Guided-decoding settings now affect reasoning checks in both SGLang preprocessing paths. Tool-call constraint generation also supports passing thinking mode to parser methods that accept it.

Changes

Guided Tool Reasoning

Layer / File(s)	Summary
Guided-decoding reasoning checks components/src/dynamo/frontend/sglang_prepost.py, components/src/dynamo/frontend/sglang_processor.py, components/src/dynamo/frontend/tests/test_sglang_processor_unit.py	The reasoning helper checks guided-decoding settings, and both preprocessing paths pass those settings to it. Tests cover forced, named, and automatic tool choices.
Thinking-mode structural-tag constraints components/src/dynamo/frontend/sglang_prepost.py, components/src/dynamo/frontend/tests/test_sglang_processor_unit.py	The constraint builder accepts a thinking_mode flag and passes it to supported parser methods. Preprocessing sets the flag when no reasoning parser is configured and reasoning is enabled. A test checks the Kimi K3 structural-tag prefix.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🔵 Low · up to 8127c

Some client-specified structural-tag requests may now be handled as requiring reasoning, including gpt-oss requests. This narrow regression merits a fix or explicit acceptance, but its downstream impact is not established.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check	⚠️ Warning	The description explains the problem, fix, behavior before and after, and testing. However, it omits the required Related Issues section and does not include the required Details or Where should the r…	Add the required Related Issues section and select either a linked issue with a valid issue number or confirm that no related issue exists. Add Details and Where should the reviewer start sections, or map the existing content to those headi…

✅ Passed checks (3 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the main change: gating SGLang auto tool-call grammars on reasoning.

Full details: Description check

Explanation

The description explains the problem, fix, behavior before and after, and testing. However, it omits the required Related Issues section and does not include the required Details or Where should the reviewer start sections.

Resolution

Add the required Related Issues section and select either a linked issue with a valid issue number or confirm that no related issue exists. Add Details and Where should the reviewer start sections, or map the existing content to those headings.

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Actionable comments posted: 1

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Inline comments:
In `@components/src/dynamo/frontend/sglang_prepost.py`:
- Around line 365-366: Restrict the structural-tag gate in the
reasoning-detection function to automatically generated tool-call guidance:
exclude requests whose response_format is a client-specified structural_tag
format, while preserving the automatic guidance behavior and existing precedence
rules.

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📥 Commits

Reviewing files that changed from the base of the PR and between ad8778f and 8127caf.

📒 Files selected for processing (3)

• components/src/dynamo/frontend/sglang_prepost.py
• components/src/dynamo/frontend/sglang_processor.py
• components/src/dynamo/frontend/tests/test_sglang_processor_unit.py

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[rmccorm4]

/ok to test 716eac6

[dynamo-review-agent]

Previously reported defects still present:

• Original discussion: The ({}, False, False) parameter in test_auto_tool_structural_tag_requires_effective_reasoning is still present at components/src/dynamo/frontend/tests/test_sglang_processor_unit.py:1160. It remains redundant with test_kimi_k3_auto_tool_sets_reasoning_gate_pool[False], which drives the same auto-tool structural-tag guidance through _preprocess_worker with reasoning disabled and verifies require_reasoning stays false.

[rmccorm4]

/ok to test fafa1f6

[ishandhanani]

/ok-to-test 63ca14f