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fix(frontend): let vLLM tool-call structural tags start inside reasoning - #16061
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WalkthroughThe vLLM chat processor now reads a runtime setting that excludes reasoning from tool-call structural tags. During preprocessing, it checks whether the rendered prompt leaves reasoning open and selects the corresponding structural-tag guidance. ChangesTool-call reasoning guidance
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Severity of issue fixed: Medium Merge Risk: 🟡 Moderate · up to Marker-free prompts may receive incompatible tool-call guidance, putting thinking-mode tool calls at risk. Resolve the prompt-state handling before merging unless this behavior is explicitly accepted. Pre-merge checks |
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| Docstring Coverage |  |
Docstring coverage is 27.59% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 29 functions across 5 files. | Write docstrings for the functions missing them to satisfy the coverage threshold. |
✅ Passed checks (4 passed)
| Check name | Status | Explanation |
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| Linked Issues check |  |
Issue #16060 requires the Python vLLM path to allow an open reasoning block to close before a tool-call structural tag applies. preprocess_chat_request now evaluates the rendered prompt, the reasoni… |
| Out of Scope Changes check | |
The changed implementation files define or propagate the worker reasoning policy. The added policy and processor tests verify the behavior required by issue #16060. The reported changes contain no dem… |
| Title check | |
The title clearly and concisely describes the main fix: allowing vLLM tool-call structural tags to begin inside reasoning. |
| Description check | |
The description includes all required sections, explains the problem and implementation, identifies review starting points, links issue #16060, and provides validation results. |
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Review comments at @components/src/dynamo/frontend/prepost.py:
- Line 553: Update _prompt_leaves_reasoning_open() to distinguish prompts with
neither reasoning delimiter from prompts that leave reasoning open, so
marker-free prompts do not return True. Ensure structural-tag guidance for
deepseek_v4_1 still permits the model to emit the opening marker.
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📒 Files selected for processing (5)
components/src/dynamo/frontend/prepost.pycomponents/src/dynamo/frontend/structural_tag_policy.pycomponents/src/dynamo/frontend/tests/test_structural_tag_policy.pycomponents/src/dynamo/frontend/tests/test_vllm_processor_unit.pycomponents/src/dynamo/frontend/vllm_processor.py
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/ok to test f00609b |
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Previously reported defects still present:
- Original discussion: Still present: _prompt_leaves_reasoning_open() treats any prompt where the reasoning parser reports no end marker as starting inside reasoning. For marker-free prompts, vLLM-style is_reasoning_end() implementations return false even though the prompt did not open a reasoning block, so this can incorrectly request reasoning-prefixed structural tags instead of guidance that permits the model to start reasoning normally.
- Original discussion: The prior marker-free prompt concern is still present at the reviewed head:
_prompt_leaves_reasoning_open()still returnsnot parser.is_reasoning_end(prompt_token_ids)without distinguishing a prompt with no reasoning delimiter from one that actually leaves reasoning open. - Original discussion: The prior prompt-state concern is still present at the reviewed head:
_prompt_leaves_reasoning_open()returnsnot parser.is_reasoning_end(prompt_token_ids), so a reasoning parser that returnsFalsefor a prompt with neither reasoning delimiter is treated the same as an actually open reasoning block. - Original discussion: The unresolved marker-free prompt concern is still present:
_prompt_leaves_reasoning_open()returnsnot parser.is_reasoning_end(prompt_token_ids), and vLLM's base thinking parser returnsFalsewhen neither the reasoning start nor end delimiter appears in the prompt. For reasoning-enabled templates where generation is expected to emit the opening marker itself, this makes the frontend passstarts_in_reasoning=Trueand build a structural tag that expects a reasoning close before the tool-call grammar, so the model cannot first emit the opener. The fix should distinguish an actually open prompt from a delimiter-free prompt before selecting reasoning-prefixed structural-tag guidance.
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✅ Dynamo PR CI passed — run 38017237271 (attempt 1) on
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I reproduced the issue and validated the fix live using Qwen3.5-9B on H100 NVL, with an ordinary Issue: Python vLLM hardcodes Fix: replace that hardcoded value with a decision based on the request’s rendered prompt and the deployment’s existing reasoning-gate capability. When reasoning is open and the engine does not defer grammar enforcement, the tag permits reasoning to finish before the tool call. Tag construction happens after prompt rendering/tokenization, before generation. Live validationTested the #16061 patch at
Versions: vLLM 0.31.0, SGLang 0.5.21, TRT-LLM 1.3.0rc29.
Every pass returned exactly one |
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Hi @fengyuli-dev thanks a lot for catching this and fixing it! |
With structural tags on (the default since ai-dynamo#13654), the Python vLLM chat processor always built the tool-call tag with reasoning=False, i.e. for the output after the reasoning block. When the chat template ends the generation prompt inside an open reasoning block (DeepSeek V4.1 with thinking on, the default) and the vLLM worker does not hold the grammar back until reasoning ends, the grammar applies from the first generated token and excludes the end-of-reasoning marker. The model can then never close its reasoning, so a request with tools and tool_choice=auto runs to max_tokens with no tool call. The vLLM worker already publishes tool_call_structural_tag_excludes_reasoning and the Rust preprocessor already uses it (starts_in_reasoning). Do the same in the Python vLLM path: render first, and build the tag with reasoning=True when a reasoning parser runs for the request, does not find the end of reasoning in the prompt, and the worker does not exclude reasoning. Signed-off-by: Fengyu Li <fengyul@nvidia.com>
…opener Address review: is_reasoning_end() is also false for a prompt with no reasoning marker (models that emit their own opener), which would have built a reasoning prefix that forbids that opener. Mirror the Rust preprocessor's prompt_injected_reasoning_start instead: the rendered prompt, trailing whitespace ignored, must end with the reasoning parser's reasoning_start_str. Signed-off-by: Fengyu Li <fengyul@nvidia.com>
ai-dynamo#13903 added the canonical structural_tag.reasoning_boundary setting and resolves it in the Rust preprocessor; the Python vLLM processor only warned that non-default values are unsupported. Resolve it the same way: backend leaves reasoning to the engine, auto follows the worker's published flag, and structural_tag models reasoning unless the backend already excludes it. Also drop a test-helper docstring that restated the code. Signed-off-by: Fengyu Li <fengyul@nvidia.com>
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Thanks @indrajit96, conflict is addressed Not sure why this PR is flagged as external contribution though :). In this case, could you please re-trigger the CI and merge on my behalf? |
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/ok to test fedd6ea |
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@indrajit96 Thanks, appreciate the quick turnaround. Could you please help with the merge? |
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Overview:
The Python vLLM chat processor built every tool-call structural tag with
reasoning=False. When the chat template leaves generation inside an open reasoning block (DeepSeek V4.1 with thinking on, its default) and the vLLM worker does not hold the grammar back until reasoning ends, the tag forbids</think>from the first token, so thinking-mode tool calls withtool_choice: "auto"never complete. Structural tags became the default in #13654, which turned this into a default-config failure. This PR makes the Python vLLM path build the tag the same way the Rust preprocessor already does.Details:
structural_tag_policy.runtime_structural_tag_excludes_reasoning()readstool_call_structural_tag_excludes_reasoning, which the vLLM worker already publishes (publish_vllm_structural_tag_reasoning_policy) and the Rust preprocessor already uses.preprocess_chat_requestrenders the prompt before building tool guidance (rendering does not depend on it) and passesstarts_in_reasoning= a reasoning parser runs for the request and the rendered prompt, trailing whitespace ignored, ends with that parser'sreasoning_start_strand the worker does not exclude reasoning. This mirrorsprompt_injected_reasoning_startandstarts_in_reasoningin the Rust preprocessor (lib/llm/src/preprocessor.rs,lib/llm/src/preprocessor/structural_tag.rs). Prompts without a reasoning marker (models that emit their own opener) keep the post-reasoning tag.build_tool_call_guided_decodingpasses it asreasoning=to vLLM'sget_model_structural_tag(xgrammar then prefixes the tag with the open reasoning block ending at</think>), or to the parser'sget_structural_taghook. The parser hook is called exactly as before when no reasoning prefix is needed.VllmProcessorreads the flag from the model card's runtime config next to the other structural-tag options.Behavior is unchanged when structural tags are off, without tools, without a reasoning parser, when the prompt does not end with the reasoning opener (thinking off, or models that emit their own opener), and when the worker gates the grammar itself. As in the Rust preprocessor, a worker that does not publish the flag is treated as not excluding reasoning.
Validation
DeepSeek-V4.1-Flash, DEP4 on 4x B200, vLLM nightly 81198e97, Dynamo 689a20b with these three files applied; same request: one
get_weathertool,tool_choice: auto; validated at head 58c53a9:triggered_tagsexcluding<think>/</think>sequence[tag(begin="", end="</think>"), triggered_tags]--reasoning-parser deepseek_v41(engine gates)triggered_tags(unchanged)test_structural_tag_policy.pyandtest_vllm_processor_unit.py: 237 passed in that image. New tests cover prompt-opened (with and without trailing whitespace), engine-gated, prompt-closed, marker-free prompt, parser without an opener, no reasoning parser, and the registry path.Where should the reviewer start?
components/src/dynamo/frontend/prepost.py:preprocess_chat_request(render moved ahead ofbuild_tool_call_guided_decoding) and_prompt_leaves_reasoning_open.Related Issues