[Fix] Work around xgrammar 0.2.1 negative integer minimum in Kimi-K3 structural tags

[gilfordting]

Motivation

XGrammar 0.2.1 (the version pinned in python/pyproject.toml) miscompiles a JSON schema integer with a one-sided negative lower bound: {"type": "integer", "minimum": -N} produces a grammar that accepts the incomplete literal "-" and rejects every valid negative integer. Any Kimi-K3 tool whose parameter schema uses such a bound gets structurally-invalid tool calls under constrained decoding.

Modifications

kimik3_structural_tag.py::_value_format: when an integer schema has only a negative lower bound (no maximum/exclusiveMaximum/multipleOf), split the range at zero into anyOf: [{... maximum: -1}, {... minimum: 0}]. Semantically identical schema, but each branch is a form the converter compiles correctly. Schemas with an upper bound or multipleOf are left untouched.

Unit tests cover the rewrite and the untouched cases (test_kimik3_structural_tag.py, runs against the real pinned xgrammar).

This is deliberately scoped as a converter-side workaround; the underlying bug belongs to xgrammar's JSON-schema-to-grammar conversion and the workaround can be dropped once the pin moves past a fixed release.

Accuracy Tests

N/A — affects only grammar compilation for constrained tool-call decoding; the accepted language is the schema's original semantics.

Speed Tests and Profiling

N/A.

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.

🤖 Generated with Claude Code

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CI States

Latest PR Test (Base): ❌ Run #31746756454
Latest PR Test (Extra): ❌ Run #31746756300

[gongy]

/rerun-test test_protocol.py test_json_mode.py test_constrained_decoding.py

[github-actions]

Results for /rerun-test test_protocol.py test_json_mode.py test_constrained_decoding.py:

🚀 ubuntu-latest (1 test): ✅ View workflow run

cd test/ && python3 registered/unit/entrypoints/openai/test_protocol.py

🚀 1-gpu-5090 (1 test): ✅ View workflow run

cd test/ && python3 registered/constrained_decoding/test_constrained_decoding.py

⛔ test_json_mode.py: No test file found matching test_json_mode.py under test/registered/ or python/sglang/multimodal_gen/test/.

[gongy]

/rerun-test test/registered/unit/function_call/test_json_schema_constraint.py

[github-actions]

Results for /rerun-test test/registered/unit/function_call/test_json_schema_constraint.py:

🚀 ubuntu-latest (1 test): ✅ View workflow run

cd test/ && python3 registered/unit/function_call/test_json_schema_constraint.py