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Why do JSON parsers behave differently?
JSON’s core grammar is standardized, but a parser’s observed behavior also depends on implementation choices, configuration, and resource limits. RFC 8259, published in December 2017, says a parser “MUST accept all texts that conform to the JSON grammar.” It also permits implementation limits on text size, nesting depth, numeric range or precision, and string length or contents. As a result, a grammar-conforming document can still exceed a particular parser’s configured limits.
It also matters what is being compared. One parser may accept an input another rejects; two may accept it but produce different values; or both may produce the same value while serializing it differently. An error, a partial result, or invalid JSON emitted during a round trip is a separate outcome again. A single pass/fail result can hide these distinctions.
What happens when a JSON object has duplicate keys?
RFC 8259 says object member names SHOULD be unique, but it does not define one universal resolution rule when they are not. The standard describes receiver behavior as unpredictable: implementations may keep only the last name/value pair, reject or fail on the object, or expose all pairs.
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I-JSON, a stricter profile defined in RFC 7493 in March 2015, prohibits duplicate member names after escape processing. That detail matters: names that look different in source text can decode to the same name. If a system depends on a unique value for each key, reject duplicates or enforce a clearly defined profile before different components interpret the same document.
Can JSON parsers interpret Unicode escapes differently?
They can, particularly at the edge of what the standards permit. RFC 8259 warns that an escaped unpaired UTF-16 surrogate, such as "uDEAD", can lead to unpredictable receiver behavior. I-JSON excludes surrogate code points and noncharacters from member names and string values, including lone surrogates represented with escapes.
A 2024 cross-language differential-testing study reported specific problems among the implementations it tested: incorrect handling of a UTF-16 surrogate pair, rejection or truncation of U+0000, and serialization of a valid escaped control character into invalid raw output. These are findings about those tested implementations, not a claim that every current parser has the same defects. The study also reported malformed output or changed object structure for some object names. Such cases matter when a parsed value is later trusted by another component.
Why can a large JSON number change when parsed?
JSON’s number grammar does not guarantee that every implementation preserves every number’s exact mathematical value. RFC 8259 permits limits on numeric range and precision when a parser translates a number into its internal representation. A parser might round a large integer, lose precision, or reject a number beyond its configured range.
RFC 7493 notes that binary64 is widely available and advises against assuming receivers can process numbers with greater magnitude or precision. For I-JSON, its stated upper positive integer limit for which a sender can expect exact treatment is 9007199254740991. If exact interchange of larger integers or more precise decimal values is required, the profile recommends representing them as strings and defining how consumers should interpret them.
How do I test whether two JSON parsers agree?
Define agreement before running the comparison. Separate syntax acceptance from semantic values, serialized output, and errors; then record enough detail for another person to reproduce each result.
- Identify the implementations. Record each parser library or runtime, exact release, and test date. A language or parser-family name alone is not precise enough.
- Record configuration and environment. Include strictness switches, decoding options, number representation, and resource limits, as well as relevant runtime details.
- Classify every input. Mark whether it conforms to RFC 8259, to the stricter I-JSON profile, or is deliberately invalid or an extension. Do not treat rejection of invalid input as equivalent to disagreement on valid input.
- Specify what counts as a match. Compare acceptance, resulting strings and object members, duplicate-name policy, numeric value and representation, and error category. For round trips, check both whether meaning was preserved and whether the serialized output is valid JSON.
- Report limits and outcomes per case. Include exact input documents, results, and whether an error returned a partial value. Distinguish a size, nesting, string-length, or numeric limit from a different interpretation of the document.
A finite collection of test documents can demonstrate agreement on those documents under the recorded conditions; it cannot prove that parsers behave identically on every possible JSON text. A 2024 study, “Cross-Language Differential Testing of JSON Parsers”, shows how differential testing can uncover consequential edge cases, but does not independently establish the particular 24-document, six-parser result described in the headline.
A Go-focused comparison likewise documents differences among the particular implementations it lists, including duplicate-name behavior and results on RFC 8259 and I-JSON cases. Its results should be read as project documentation for those implementations and versions, not as a timeless ranking of Go parsers: Go parser comparison and discussion.
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How can you reduce interoperability and security risk?
- Avoid duplicate object names when data crosses language or organizational boundaries.
- Use UTF-8 and agree on a profile such as I-JSON when stricter shared rules are needed.
- Keep numeric values within ranges and precision that all recipients can represent, or encode exact large values as strings under an agreed convention.
- At trust boundaries, test the exact parser and configuration in use, and validate the representation downstream components actually consume.
- Use a JSON parser rather than an
eval()-like language facility. RFC 8259 warns that JSON input may contain executable code that makes such evaluation unsafe.
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