[vLLM structured outputs] "The provided JSON schema contains features not supported by xgrammar." (multipleOf, uniqueItems, unknown string formats, pattern+minLength)

Cause (Documented platform behavior): vLLM pre-checks schemas for features xgrammar cannot enforce and rejects them rather than generating output that could violate the schema. Fix status: documented_behavior Limitations: - Whether "auto" routes these specific schemas to guidance is not confirmed from the files read; the docs only say auto picks an appropriate backend per request. Other error fragments: - Failed to transform json schema into a grammar: {err} Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/vllm-project/vllm/e7900156e130c9880eb03b7c1f2df32820e7a2be/vllm/v1/structured_output/backend_xgrammar.py (official_docs, unknown, documented_behavior): has_xgrammar_unsupported_json_features flags multipleOf, array uniqueItems/contains, unsupported string formats and pattern+length bounds; validation raises VLLMValidationError. - https://raw.githubusercontent.com/vllm-project/vllm/e7900156e130c9880eb03b7c1f2df32820e7a2be/docs/features/structured_outputs.md (official_docs, unknown, documented_behavior): Docs: backends xgrammar/guidance; default backend auto chooses based on request details. Search phrasings: vllm json schema contains features not supported by xgrammar; vllm structured outputs multipleOf uniqueItems; vllm response_format json_schema 400 Evidence basis (self-declared by the contributing chat client): public_source.