structured_output: skip accept_tokens/validate_tokens on terminated grammar by notandruu · Pull Request #42853 · vllm-project/vllm

notandruu · 2026-05-16T21:29:36Z

Summary

When the grammar FSM accepts its final token and transitions to a
terminal state, subsequent calls to accept_tokens() return False.
The scheduler treated that return value as an unexpected grammar
violation, logging an error and marking the request FINISHED_ERROR.

This affects every structured-output request that completes normally:
once the stop token is accepted the grammar is terminated, but
should_advance() has no awareness of is_terminated(), so the next
scheduler step triggers accept_tokens() again and spuriously aborts
the request.

Root cause

should_advance() in StructuredOutputManager returns True
unconditionally for non-reasoning requests, with no check for whether
the grammar has already reached a terminal state. The FSM backend
(XgrammarGrammar.accept_tokens) correctly returns False when called
on a terminated matcher, but the scheduler treated False as a decode
error in all cases.

Fix

Guard all three call sites against is_terminated():

update_from_output (line 1416): if grammar.is_terminated(),
skip accept_tokens(). The grammar constraint is already satisfied;
there is nothing to advance.
update_draft_token_ids (line 1712): if grammar.is_terminated(),
skip validate_tokens(). The constraint is fully satisfied, so all
speculative tokens are structurally valid.
update_draft_token_ids_in_output (line 1745): same guard.

The fix is intentionally minimal — should_advance() is left unchanged
to avoid surprising callers that may rely on its current semantics.

Tests

tests/v1/core/test_structured_output_terminated_grammar.py (new file):

Test	What it verifies
`test_accept_tokens_not_called_when_grammar_terminated`	Termination guard fires; `accept_tokens` never reached; request not `FINISHED_ERROR`
`test_accept_tokens_called_when_grammar_not_terminated`	Active grammars still process tokens normally (regression)
`test_accept_tokens_failure_still_errors_when_not_terminated`	Genuine grammar violations (not terminated, `accept_tokens` → False) still abort
`test_validate_tokens_not_called_when_grammar_terminated_update_draft`	Spec-decode path skips `validate_tokens` on terminated grammar
`test_validate_tokens_called_when_grammar_not_terminated_update_draft`	Spec-decode path calls `validate_tokens` when grammar is active

tests/v1/core/test_scheduler.py:

Added grammar.is_terminated.return_value = False to the existing
test_abort_request_when_structured_output_fsm_cannot_advance so it
exercises the genuine-violation path (not the already-terminated path).

Before / After

Before: any structured-output request that ran to completion would be
marked FINISHED_ERROR on the step following the accepted stop token:

ERROR ... Unexpected: grammar rejected tokens [2] for request req-xyz. Terminating request.

After: the termination guard short-circuits before accept_tokens();
no error is logged; the request finishes with the correct status.

github-actions · 2026-05-16T21:29:43Z

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Code Review

This pull request addresses issue #42619, where the scheduler incorrectly marked requests with a FINISHED_ERROR status by calling accept_tokens() or validate_tokens() on a grammar that had already reached a terminal state. The changes introduce checks for grammar.is_terminated() in the scheduler's update_from_output, update_draft_token_ids, and update_draft_token_ids_in_output methods to prevent these spurious errors. Additionally, a new test suite has been added to verify the fix and ensure regressions are caught. Feedback suggests adding an assertion for metadata.grammar in update_draft_token_ids to maintain consistency with other methods and improve type safety by allowing the removal of type-ignore comments.

gemini-code-assist · 2026-05-16T21:31:29Z

+                # Skip validation when grammar is already terminated; the
+                # constraint has been fully satisfied so all spec tokens are
+                # structurally valid (issue #42619).
+                if not metadata.grammar.is_terminated():  # type: ignore[union-attr]
+                    spec_token_ids = metadata.grammar.validate_tokens(spec_token_ids)  # type: ignore[union-attr]


For consistency and safety, please add an assertion to ensure that metadata.grammar is not None before accessing it. This matches the pattern used in update_from_output (line 1419) and update_draft_token_ids_in_output (line 1748). Adding this assertion also allows for the removal of the type: ignore comments, as the type will be narrowed for the static analyzer.

Suggested change

# Skip validation when grammar is already terminated; the

# constraint has been fully satisfied so all spec tokens are

# structurally valid (issue #42619).

if not metadata.grammar.is_terminated(): # type: ignore[union-attr]

spec_token_ids = metadata.grammar.validate_tokens(spec_token_ids) # type: ignore[union-attr]

assert metadata is not None and metadata.grammar is not None

# Skip validation when grammar is already terminated; the

# constraint has been fully satisfied so all spec tokens are

# structurally valid (issue #42619).

if not metadata.grammar.is_terminated():

spec_token_ids = metadata.grammar.validate_tokens(spec_token_ids)

Done — added assert metadata is not None and metadata.grammar is not None before the is_terminated() check in update_draft_token_ids, and removed the now-redundant # type: ignore[union-attr] comments. Matches the pattern in update_draft_token_ids_in_output.

…rammar When the grammar FSM transitions to a terminal state (e.g. after accepting the EOS token), subsequent calls to accept_tokens() return False. The scheduler interpreted that as an unexpected grammar violation and set the request to FINISHED_ERROR, producing spurious errors and corrupted outputs for every structured-output request that completes normally. Root cause: should_advance() has no awareness of is_terminated(), so the scheduler kept calling accept_tokens() every step after completion. Fix: - scheduler.update_from_output: guard accept_tokens() behind grammar.is_terminated() — skip, not error, when already done. - scheduler.update_draft_token_ids: guard validate_tokens() the same way; a terminated grammar implies the constraint is fully satisfied, so all spec tokens are structurally valid. - scheduler.update_draft_token_ids_in_output: same guard. Tests: - test_accept_tokens_not_called_when_grammar_terminated: verifies the termination guard fires and accept_tokens is never reached. - test_accept_tokens_called_when_grammar_not_terminated: regression guard — active grammars still process tokens normally. - test_accept_tokens_failure_still_errors_when_not_terminated: genuine grammar violations (not terminated, accept fails) still abort. - test_validate_tokens_not_called_when_grammar_terminated_update_draft: spec-decode path skips validate_tokens on terminated grammar. - test_validate_tokens_called_when_grammar_not_terminated_update_draft: spec-decode path calls validate_tokens when grammar is active. Fixes vllm-project#42619 Signed-off-by: Andrew Liu <andrewjliu22@berkeley.edu> Signed-off-by: Andrew Liu <andrewjliu22@gmail.com>

notandruu · 2026-05-18T04:06:04Z

Could a maintainer add the verified or ready label to unblock the author trust gate? Happy to make any changes needed.

notandruu · 2026-06-27T10:32:52Z

Ping — could a maintainer add the verified or ready label to unblock CI? Happy to make any changes needed.

mergify · 2026-07-04T09:21:28Z

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @notandruu.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

gaby · 2026-08-15T09:55:34Z

@notandruu Rebase your branch, it has conflicts.

sfeng33 · 2026-08-19T18:00:52Z

Superseded by #52805

notandruu requested review from ApostaC, WoosukKwon, alexm-redhat, heheda12345, njhill, orozery, robertgshaw2-redhat and ywang96 as code owners May 16, 2026 21:29

mergify Bot added the v1 label May 16, 2026

gemini-code-assist Bot reviewed May 16, 2026

View reviewed changes

notandruu force-pushed the fix/structured-output-terminated-matcher branch from 266c707 to a75cfc5 Compare May 17, 2026 06:52

notandruu force-pushed the fix/structured-output-terminated-matcher branch from a75cfc5 to df94584 Compare May 18, 2026 03:59

mergify Bot added the needs-rebase label Jul 4, 2026

arquiteto-raiz mentioned this pull request Aug 1, 2026

[Bug]: Structured-output scheduler can keep advancing a terminated xgrammar matcher #42619

Closed

1 task

volkanncicek mentioned this pull request Aug 13, 2026

[Bug]: Engine core livelock (100% CPU, no crash) with MTP speculative decoding + xgrammar structured outputs - regression from v0.24.0 #49210

Open

sfeng33 closed this Aug 19, 2026

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structured_output: skip accept_tokens/validate_tokens on terminated grammar - #42853

structured_output: skip accept_tokens/validate_tokens on terminated grammar#42853
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notandruu:fix/structured-output-terminated-matcher

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notandruu commented May 16, 2026

Summary

Root cause

Fix

Tests

Before / After

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