[asanomitakanori]

Summary

• Add a "gemma" JSON-schema style that emits Gemma 4's tool-call argument spelling: unquoted object keys and strings delimited by the single <|"|> token with no escape sequences, at every nesting level.
• Re-enable the gemma_4 builtin structural tag (auto / required / forced tool choice, <|channel>thought reasoning prefix, parallel_tool_calls), which has been unregistered with a "we will support it later" TODO since refactor: unify reasoning parameter and rename model keys #609.
• Validate against the released Gemma-4 chat templates in the alignment test, plus token-level matcher tests.

Closes #587. Continues #684 by @iwanhae, which was closed unmerged after the converter was rewritten to build grammar trees directly (#745) and the branch fell into conflict; the original implementation and authorship are preserved through Co-authored-by.

Why

Engines currently fall back to unconstrained decoding for Gemma 4 tool calls because the existing converters cannot express <|"|>-delimited strings. This blocks tool_choice="required" and JSON-schema-constrained arguments for Gemma 4 in SGLang and vLLM (vllm-project/vllm#53363, sgl-project/sglang users on #684).

What changed since #684

Ported to the current int32_t-based converter API and reworked around JSONSchemaFormat.excludes (#912):

• The delimiter is registered as an exclusion. GemmaToolCallingConverter appends <|"|> to excludes_, so:
  • pattern / format / minLength-maxLength string bodies are intersected with "does not contain <|"|>" through ExcludingString. A permissive pattern such as .* can no longer close the string early — this was the documented limitation of feat: support Gemma tool calling format with <|"|> string delimiters  #684. (Built-in format regexes the FSM engine cannot build keep the CFG expansion, as in the JSON converter.)
  • const / enum literals that would have to spell the delimiter become unsatisfiable instead of emitting a string that parses differently.
  • Declared property names are excluded from additionalProperties keys (the original PR fell back to the plain identifier pattern).
  • Caller-provided JSONSchemaFormat.excludes work for style="gemma" too.
• Two small virtual hooks on JSONSchemaConverter: CreatePatternKeyRule (replaces the local lambda in GenerateObject) and CreatePropertyNamesKeyRule. Formats with unquoted keys override them to emit bare key bodies for patternProperties / propertyNames; the JSON and XML converters keep their behaviour unchanged.
• Length-bounded strings are built as (.|\n){m,n} intersected with the exclusion, so they count codepoints and allow lone < / delimiter prefixes without hand-written lookahead rules.
• The alignment test renders the vendored Gemma-4 templates with jinja2 under the GEMMA: model-id prefix (no HF token or Gemma-4-capable transformers needed), reusing the tokenizer prompt-diff path; a token-gated drift guard checks the vendored templates against google/gemma-4-E2B-it / google/gemma-4-31B-it.
• Docs (structural_tag.md, tool_calling_and_reasoning.md), the TypeScript JSONSchemaFormat.style union, and the style error message are updated.

Test plan

• tests/python/test_function_calling_converter.py -k gemma (59 cases: strings, scalars, nesting, any, enum/const incl. large-int literal preservation, additionalProperties, patternProperties, propertyNames, length bounds incl. multi-byte and newline, .* pattern cannot emit the delimiter, format: date, excludes)
• tests/python/test_builtin_structural_tag.py (612 passed incl. the shared get_*_structural_tag matrix, a single-token <|"|> / <|tool_call> tokenizer walk, and the auto-dispatch regression)
• tests/python/test_builtin_structural_tag_alignment.py -k gemma (34 passed: e2b + 31b templates × thinking on/off × tool scenarios, plus the drift guard against the official templates with a HF token)
• Full test_function_calling_converter.py, test_structural_tag_converter.py, test_json_schema_excludes.py, test_grammar_matcher_json_schema.py, test_json_schema_converter.py: 2933 passed (-m "not hf_token_required")
• pre-commit run (black, isort, clang-format 19.1.7, taplo) and ruff check pass

Known limitations

• A patternProperties / propertyNames key pattern whose language contains : cannot round-trip, because Gemma keys are unquoted (same as feat: support Gemma tool calling format with <|"|> string delimiters  #684).
• Numbers nested inside const objects/arrays are re-serialized through picojson; top-level scalar literals keep the schema's own text.

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