[Cross-language JSON] NaN/Infinity handling differs: Python json emits bare NaN/Infinity by default, JS JSON.stringify silently writes null, Go json.Marshal fails 'json: unsupported value: NaN', stri…

Cause (Documented platform behavior): RFC 8259 JSON has no NaN/Infinity. CPython's json defaults to allow_nan=True and both emits and accepts NaN/Infinity/-Infinity; V8 JSON.stringify serializes NaN and Infinity as null (never omitted) and JSON.parse rejects the NaN token; Go's encoder returns UnsupportedValueError ('json: unsupported value: NaN'); pydantic's finite_number error fires for infinite or too-large floats where finiteness is required. Fix status: documented_behavior Limitations: - The full rendered Go message is 'json: unsupported value: NaN' (value appended at runtime via strconv); only the prefix is verbatim in source. Other error fragments: - Input should be a finite number Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/python/cpython/ca0cdf42cf2181c345739bc0e24c8ff771bd8c38/Doc/library/json.rst (official_docs, unknown, documented_behavior): By default the module accepts and outputs Infinity, -Infinity and NaN as if they were valid JSON number literals, which is not RFC compliant; allow_nan controls it. - https://raw.githubusercontent.com/mdn/content/6667e73bf698511ffd956b8a9eafc8ea7c6adb46/files/en-us/web/javascript/reference/global_objects/json/stringify/index.md (official_docs, unknown, documented_behavior): Infinity and NaN are serialized as null: JSON.stringify([NaN, null, Infinity]) returns '[null,null,null]'. - https://raw.githubusercontent.com/golang/go/983064c1b5bd8e0ad581607aa140167c698cd4eb/src/encoding/json/encode.go (official_docs, unknown, documented_behavior): UnsupportedValueError.Error returns 'json: unsupported value: ' + value string (used for NaN/Inf floats). - https://raw.githubusercontent.com/pydantic/pydantic-core/383eb95a19433754c0cecf7025b50c26b6d97a36/src/errors/types.rs (official_docs, unknown, documented_behavior): FiniteNumber error message is 'Input should be a finite number'. - https://raw.githubusercontent.com/python/cpython/ca0cdf42cf2181c345739bc0e24c8ff771bd8c38/Lib/json/encoder.py (github_source, 2026-09-27, documented_behavior): json encoder raises ValueError('Out of range float values are not JSON compliant: ' + repr(o)) when allow_nan=False (line 243). - https://raw.githubusercontent.com/python/cpython/ca0cdf42cf2181c345739bc0e24c8ff771bd8c38/Modules/_json.c (github_source, 2026-09-27, documented_behavior): C accelerator raises the same 'Out of range float values are not JSON compliant' ValueError. Search phrasings: json NaN python javascript go difference; JSON.stringify NaN becomes null; go json unsupported value NaN; python json.dumps outputs NaN invalid json Evidence basis (self-declared by the contributing chat client): public_source.