[adimalkar]

Fixes #967

What

A string schema whose format isn't one of the built-in formats compiled without any message, and the format was then ignored. {"type": "string", "format": "non_existing_format"} accepts "hello", so a constraint the schema asked for disappears and nobody notices.

This PR logs a warning when that happens:

json_schema_converter.cc:1155: Warning: format "non_existing_format" is not supported; ignoring format

Why a warning and not an error

The issue asks for a compile-time error. I went with a warning, as explained in #967 (comment):

• In JSON Schema 2020-12, format is an annotation by default, and validators may ignore formats they don't know. The existing tests from Warn when pattern is combined with minLength/maxLength in string schemas (silently dropped today) #896 already treat an unrecognized format as "a plain annotation".
• Schema generators emit formats xgrammar doesn't implement. Pydantic, for example, emits password for SecretStr, binary for bytes and path for Path. An error would make those schemas, which compile today, fail.

Glad to switch to an error, or an error only under strict_mode, if maintainers would rather.

How

The warning lives in SchemaParser::ParseString, next to the minLength/maxLength warning from #896, for the same reason that one does: every converter goes through ParseString, including the XML tool-calling converter, which overrides GenerateString. It reuses JSONSchemaConverter::IsBuiltinFormat. It's an unconditional XGRAMMAR_LOG(WARNING) rather than WarnUnsupportedKeywords, because nothing calls that function with verbose=true, so its warnings never print.

The grammar doesn't change.

Tests

Added to tests/python/test_json_schema_converter.py, next to the #896 tests and using the same _compile_and_capture helper:

• an unknown format warns: alone, as password, combined with pattern, and combined with maxLength
• the built-in formats (email, date, date-time, uuid, uri) don't warn
• the XML tool-calling path (qwen_xml) warns too
• an unknown-format string accepts exactly what a plain string accepts, which pins the "grammar unchanged" behaviour

Locally, tests/python/test_json_schema_converter.py gives 627 passed. The whole tests/python directory gives 4524 passed. The only 26 failures are in test_builtin_structural_tag_alignment.py and come from the optional cohere_melody package, which isn't installed here. pre-commit and ruff check pass.

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