[Bugfix] Structural-tag grammar omits reasoning prefix, crashing forced tool_choice when thinking is enabled

[flobernd]

Summary

When a request combines a constrained tool_choice (a named function, or "required") with reasoning/thinking enabled, generation crashes at the reasoning→tool boundary:

• non-streaming → HTTP 500 InternalServerError
• streaming → 200, reasoning is streamed, then an SSE error chunk is injected exactly where the tool-call arguments should begin; the request is aborted with no tool call.

Server logs show the structured-output FSM rejecting the boundary tokens:

ERROR backend_xgrammar.py:162] Failed to advance FSM for request ... for tokens 13. Please file an issue.
ERROR scheduler.py:1611] Unexpected: grammar rejected tokens [13, 154842, 154843, 2911, 10711, 154847] for request ... Terminating request.

Decoding those token ids on GLM-5.2 gives:

13      '.'           (last token of the reasoning text)
154842  '</think>'    (end of reasoning)
154843  '<tool_call>' (start of the GLM-native tool call)
2911    'web'
10711   '_search'
154847  '<arg_key>'

i.e. a single (speculative-decode) batch that spans the reasoning → </think> → tool_call transition is handed to a grammar that cannot accept it.

Only the combination fails; each axis alone is fine:

tool_choice	thinking	result
named function	ON	FAIL (500 / mid-stream error)
"required"	ON	FAIL (identical)
"auto"	ON	pass
named function	OFF	pass

Root cause

For models whose tool parser declares a structural_tag_model (e.g. GLM: structural_tag_model = "glm_4_7"), a constrained tool_choice is enforced with an xgrammar structural tag (gated by VLLM_ENFORCE_STRICT_TOOL_CALLING, default True). The tag is built in DelegatingParser._apply_structural_tag (vllm/parser/abstract_parser.py), which hardcodes reasoning=False:

structure_tag = self._tool_parser.get_structural_tag(request, reasoning=False)

xgrammar's get_glm_4_7_structural_tag (and the other model templates) use that flag to decide whether the grammar includes the reasoning section:

# xgrammar/builtin_structural_tag.py
if not reasoning:
    return StructuralTag(format=suffix_tag)            # tool-call only, NO </think>
prefix_tag = TagFormat(begin="", content=AnyTextFormat(), end=THINK_TAG_END)
return StructuralTag(format=SequenceFormat(elements=[prefix_tag, suffix_tag]))

So with reasoning=False the grammar models only the tool-call format and has no </think>-terminated reasoning prefix. When thinking is enabled the model legitimately emits ...reasoning... </think> <tool_call>..., which the grammar cannot accept.

This is made fatal by the engine's structured-output reasoning gate. The STRUCTURAL_TAG path has a deliberate same-step advance under speculative decoding (vllm/v1/structured_output/__init__.py):

# Structural tags ... model phased output (e.g. thinking tag -> answer tag),
# and speculative decoding must run grammar.validate_tokens on draft tokens
# produced immediately after that transition.
if (self.vllm_config.speculative_config is not None
        and structured_req.structured_output_key[0] == StructuredOutputOptions.STRUCTURAL_TAG):
    return True

That code assumes the structural tag models the reasoning phase. Because _apply_structural_tag built it without one, the boundary batch ['.', '</think>', '<tool_call>', ...] is fed to a grammar whose start state only accepts <tool_call> → accept_tokens fails → the request is terminated.

Note: the --structured-outputs-config.enable_in_reasoning=True flag is not a fix here — it forces the (reasoning-less) grammar from token 0 and suppresses thinking entirely.

Fix

Two coupled changes, both in DelegatingParser._apply_structural_tag:

1. Build the structural tag with reasoning=<thinking enabled> instead of hardcoding False, so the grammar models the ...</think> reasoning prefix when the model will think. Thinking-disabled requests keep reasoning=False (unchanged behaviour). The reasoning state comes from a new _reasoning_enabled() helper: the parser (and its reasoning parser) is built per request with that request's chat_template_kwargs, so it already knows whether thinking is on.

2. When the tag includes the reasoning phase, mark the grammar as reasoning-handling (request._grammar_from_tool_parser = True) so the engine enforces the grammar from the first token (reasoning_ended=True) rather than deferring past a reasoning section. This is required because change #1 alone is not sufficient: the engine's reasoning gate leaves the tokens sampled right after </think> unmasked, so under a forced/required tool_choice the model can sample prose instead of <tool_call> (e.g. emit </think>I'll search...), and the subsequent grammar.accept_tokens rejects it → another 500. With the grammar enforced from token 0 and an unconstrained AnyText reasoning prefix, thinking is not restricted, but the tool call is forced the instant </think> is emitted. This reuses the exact signal the Mistral grammar path already uses to mean "the grammar handles reasoning"; it is a ChatCompletionRequest private attr (guarded with hasattr).

Plus a one-line support change:

3. vllm/parser/engine/adapters.py — expose thinking_enabled on ParserEngineReasoningAdapter, mirroring the wrapped parser engine (which derives it from chat_template_kwargs such as enable_thinking), so _reasoning_enabled() can read it. Defaults True when unavailable (xgrammar's own default).

Validation

Grammar-level replay through the real xgrammar matcher:

# reasoning=False (current): grammar rejects the FIRST reasoning token
  [reasoning=False] REJECTED at idx 0 token=40 'I'

# reasoning=True (fix): accepts the full reasoning -> </think> -> tool-call seq
  [reasoning=True]  ACCEPTED full reasoning->tool-call sequence

# reasoning=True grammar, state right AFTER </think> (forced tool_choice):
  reasoning prefix (18 toks incl </think>) fully accepted: True
  after </think>:  accept '<tool_call>' = True   accept 'I' (prose) = False
  #allowed tokens right after </think>: 3 of 154856   (tool call forced)
  allowed mid-reasoning: 154879 of 154856             (thinking unconstrained)

End-to-end against the running server:

• 4-arm matrix (temp 0): the two fatal arms (forced/required + thinking) go from HTTP 500 / mid-stream abort to valid tool calls with reasoning.
• Sampling + streaming stress test (forced tool_choice, temp 0.7 and 1.0, streaming and non-streaming, 120 requests) — 0 failures after the fix (was ~1/12 before). Reasoning is still emitted (93–338 chars/req) and the forced tool call is always valid JSON.

Verified on vLLM 0.11.2.dev (commit base e8788e3) serving GLM-5.2-NVFP4, launched with --tool-call-parser glm47 --reasoning-parser glm45 --enable-auto-tool-choice, TP=8, MTP speculative decoding (5 tokens).

Not addressed (orthogonal, pre-existing)

Under tool_choice:"required" + thinking, GLM may emit an unbounded chain of tool calls (the grammar legally permits >= 1); with a tight max_tokens the trailing call is truncated to invalid JSON. This is model behaviour under required, independent of this fix (a named/forced single function is unaffected).

Related issues

• [Bug]: 400 response when using Qwen3 + reasoning + tool calling + tool_choice "required" #19051 — Qwen3 + reasoning + tool_choice:"required" parse failure (closed, PR [Bugfix]: Fix the incompatibility issue with tool_choice 'required' when Thinking is enabled #19075 fixed only "required", not the grammar/reasoning interaction).
• [Bug]: 500 internal error with mistral reasoning parser and tool choice required #25253 — Mistral reasoning + "required" → HTTP 500 (closed).
• [Feature]: Support structured output and tool call together #16313 — structured output + tool call coexistence.

This patch addresses the structural-tag grammar construction, which is a different code path from the above and applies to any model that pairs a structural_tag_model tool parser with a reasoning parser.

Suggested regression test

Compile the model structural tag with reasoning=True and assert an xgrammar matcher accepts a ...</think><tool_call>... sequence; assert _apply_structural_tag requests reasoning=True when the reasoning parser reports thinking_enabled and reasoning=False otherwise.

🤖 Generated with Claude Code

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[mgoin]

Can you clean up the comments in this PR? They are quite specific and sloppy

[flobernd]

Trimmed in 80e6fe4. Kept the non-obvious rationale (per-request parser construction, the PrivateAttr guard, why the grammar is enforced from the first token) and dropped the restatement and symptom narration. Logic unchanged.

[OriNachum]

We hit the same root defect through a third trigger arm not covered in this PR's description: tool_choice: "auto" with per-tool "strict": true (VLLM_ENFORCE_STRICT_TOOL_CALLING default-on).

_apply_structural_tag (vllm/parser/abstract_parser.py, still on main today) calls get_structural_tag(request, reasoning=False) unconditionally, so on a thinking model (Qwen3.6-27B, --reasoning-parser=qwen3, --tool-call-parser=qwen3_coder, build 0.23.1rc1.dev672+g93d8f834d) the grammar cannot accept the </think> special token. Non-streaming requests die with HTTP 500:

ERROR backend_xgrammar.py:162 Failed to advance FSM for request ... for tokens 248069. Please file an issue.
ERROR scheduler.py:1630 Unexpected: grammar rejected tokens [248069, 271, 248058, 198] for request ... Terminating request.

(that's </think>, \n\n, <tool_call>, \n — the reasoning→tool boundary). #46118 appears to be the same crash observed through the MTP lens; same token id 248069.

Two data points that may help this PR:

• get_model_structural_tag(..., reasoning=True) already produces a correct think-aware tag in this build (any_text → </think> → "\n\n" → triggered <tool_call> tags), and the identical request with chat_template_kwargs={"enable_thinking": false} returns a clean constrained tool call — so the omission of the reasoning prefix at the call site is the whole defect for this arm too.
• Constrained decoding composes fine with MTP speculative decoding once the grammar is think-aware: we measured 100% draft acceptance on the strict legs.

Downstream we've worked around it with a --tool-parser-plugin subclass that overrides get_structural_tag to derive reasoning from the request's effective enable_thinking (defaulting to on, since a reasoning=True grammar requires a closing </think> and must not be forced onto no-think requests) — happy to share details or test this PR against our repro if useful.

(Written with AI assistance — Claude.)

[bbartels]

Downstream we've worked around it with a --tool-parser-plugin subclass that overrides get_structural_tag to derive reasoning from the request's effective enable_thinking (defaulting to on, since a reasoning=True grammar requires a closing and must not be forced onto no-think requests) — happy to share details or test this PR against our repro if useful.

@OriNachum Would you mind making that plugin open somewhere? Would love to deploy this as a fix