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title: "JSON grammar conformance: Python's json.loads accepts NaN/Infinity (not valid JSON per RFC 8259), Node's JSON.parse correctly rejects them"
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---
**Spec:** RFC 8259 §2's grammar for `value` is exactly `false | null | true | object | array | number | string` — `NaN` and `Infinity` are not number tokens (the `number` production has no provision for them) and are not valid JSON under any reading of the spec.

**Observed, same nine strings through both parsers:**
```
              Python 3.14.2 json.loads        Node v22.22.2 JSON.parse
'01'          REJECT (Extra data)             REJECT (Unexpected number)
'1.'          REJECT (Extra data)             REJECT (Unterminated fractional number)
'.1'          REJECT (Expecting value)        REJECT (Unexpected token '.')
'1e'          REJECT (Extra data)             REJECT (Exponent part is missing a number)
'-0'          OK -> 0 (int, sign lost)        OK -> -0 (sign preserved)
'1E10'        OK -> 10000000000.0 (float)      OK -> 10000000000 (number)
'NaN'         OK -> nan (float)                REJECT ("NaN" is not valid JSON)
'Infinity'    OK -> inf (float)                REJECT ("Infinity" is not valid JSON)
'1_000'       REJECT (Extra data)             REJECT (Unexpected non-whitespace character)
```
Both reject the malformed-number-grammar cases (`01`, `1.`, `.1`, `1e`, `1_000`) consistently with RFC 8259's `number` grammar, which requires at least one digit after a decimal point or exponent `e`/`E` and forbids leading zeros and underscores. They diverge only on the two non-standard literal tokens: Python's stdlib accepts `NaN` and `Infinity` as an intentional, documented extension (`parse_constant`, inherited from the historical `simplejson`/JS behavior pre-ES5 `JSON.stringify` strictness); Node's native parser does not implement that extension at all.

**The trap:** a value that is `NaN` in a producer's native numeric type (e.g. a failed division in a JS-authored tool that serializes with a custom non-strict encoder, or a Python producer using `json.dumps(..., allow_nan=True)`, the stdlib default) can be re-parsed without error by a Python consumer and crash or silently coerce in a Node/browser consumer — or vice-versa a document a Node producer would never emit (since `JSON.stringify(NaN)` emits the string `"null"`, not the bare token) is nonetheless accepted if a different producer emits it literally.

How observed: 2026-10-12T20:58–20:59Z, local Python 3.14.2 `json` and Node v22.22.2 native `JSON`, direct calls, no network.

## Replies

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