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[Bugfix][Structured Outputs] Support new-format structural tags in the guidance backend (strict/required/named tool calling) - #55153
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Tool parsers emit structural tags in the new xgrammar-style format
({"type": "structural_tag", "format": {...}}, built by
vllm/tool_parsers/structural_tag_registry.py), but the guidance backend
only parsed the legacy {"triggers": [...], "structures": [...]} shape and
crashed with KeyError: 'triggers' (surfaced as "Invalid grammar
specification: 'triggers'") on every tool-calling request that produces a
structural tag: tool_choice="auto" with a strict:true tool,
tool_choice="required", and named tool choice.
Translate the shapes the tool-parser registry emits into llguidance
grammars:
- triggered_tags (auto): free text plus trigger-dispatched tags, via a
generated Lark grammar mirroring llguidance.StructTag.to_grammar;
- tags_with_separator (required/named): one-or-more tags with no free
text, exactly one when stop_after_first is set;
- any_text: unconstrained output.
Tag ends accept the closing marker both spelled in raw bytes and as the
tokenizer's special token (models emit </tool_call> as a special token;
a bytes-only grammar rejects it at runtime, which under speculative
decoding surfaces as "grammar rejected tokens" request termination).
Embedded tag schemas are passed to llguidance as real JSON schemas with
the backend's whitespace defaults, so tool-call arguments are actually
constrained. Unsupported format shapes raise VLLMValidationError with a
clear message. The legacy path is unchanged.
Co-authored-by: Claude <noreply@anthropic.com>
Signed-off-by: Hugo Abonizio <hugo@maritaca.ai>
llguidance.StructTag.grammar expects a JSON schema (or a Lark grammar),
but the legacy structural-tag path passed it the output of
LLMatcher.grammar_from_json_schema — a serialized {"grammars": [...]}
envelope. llguidance silently interprets that envelope as a schema with
no keywords, so the content of every legacy structural tag was
effectively unconstrained (any JSON accepted regardless of the tag's
schema).
Embed the tag's actual schema with the backend's whitespace defaults
instead, the same way the new-format path does. Behavioral change:
legacy structural-tag requests that previously generated unconstrained
content are now actually constrained by their declared schema.
Co-authored-by: Claude <noreply@anthropic.com>
Signed-off-by: Hugo Abonizio <hugo@maritaca.ai>
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What's the plan with this PR? We have been patching all our DeepSeek-Flash deployments for the last two months with these changes in order to reduce the failure rate with tool calling. Are there any alternatives? |
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Update: we've since switched back to xgrammar (with spec decode + async scheduling) and it works fine for us. Given that and the recent changes on main, I don't think this PR is useful anymore. @sfbemerk this PR doesn't cover DeepSeek-V4.1's tag formats anyway. On the Rust frontend, #61892 and #62456 (in v0.33.0) switch to xgrammar by default, which fixes forced tool choice. @kreeuwijk xgrammar fails to compile the lookahead |
FIX #55152
Summary
With
--structured-outputs-config '{"backend": "guidance"}', every tool-calling request that produces a structural tag —tool_choice="auto"with astrict: truetool,tool_choice="required", or a named tool — fails at validation withInvalid grammar specification: 'triggers': the tool parsers emit the new xgrammar-style format ({"type": "structural_tag", "format": {...}}) while the guidance backend only parsed the legacy{"triggers": [...], "structures": [...]}shape.Two commits, deliberately separable:
Commit 1 — support the new format (the bug fix). Translates the shapes the tool-parser registry emits into llguidance grammars:
triggered_tags(auto: free text + trigger-dispatched tags, Lark mirroringllguidance.StructTag.to_grammar),tags_with_separator(required: one-or-more tags, no free text; named: exactly one), andany_text. Tag ends accept the closing marker both spelled in raw bytes and as the tokenizer's special token — models emit</tool_call>as a special token, and a bytes-only grammar rejects it at runtime, which under speculative decoding surfaces asgrammar rejected tokensrequest termination. Unsupported shapes raiseVLLMValidationErrorwith a clear message. Legacy path untouched.Commit 2 — legacy path: pass real schemas to llguidance.
StructTag.grammarexpects a JSON schema, but the legacy path passed the serialized{"grammars": [...]}envelope fromgrammar_from_json_schema, which llguidance silently treats as an empty schema — legacy structural-tag content was never actually constrained. This is a behavioral fix (previously-unconstrained requests become constrained), hence the separate commit; happy to split it into its own PR if preferred.Testing
New
tests/v1/structured_output/test_backend_guidance_structural_tag.py(CPU-only, Qwen3-0.6B tokenizer): special-token and byte-spelled closings accepted, argument schemas enforced, required forbids free text, named stops after one call, unsupported shapes rejected, legacy compiles and (commit 2) enforces its schema. 8/8 passing on the v0.28.0 image with these files overlaid; the legacy-enforcement test fails without commit 2, as expected.Commit 1 is additionally validated in production: it has been serving live traffic on one of our deployments (guidance backend + speculative decoding + async scheduling) with zero grammar errors.
AI assistance disclosure
This PR was developed with AI assistance (Claude); the fix and tests were reviewed, validated and are submitted by a human contributor. Commits carry the corresponding
Co-authored-bytrailer per the contributing guidelines.