JSON parsers can disagree when documents use duplicate object names, unusual Unicode escapes, very large numbers, or inputs near implementation limits. A claim that five parsers behaved identically across 24 documents is meaningful only with the documents, parser versions, settings, and a precise definition of “the same.” The standards explain why those details matter—and how to build a comparison that readers can reproduce.
Why do JSON parsers behave differently?
JSON defines a syntax, but it does not settle every question about how a parser turns text into an in-memory value or how a program later serializes that value. Implementations can use different numeric representations, expose object members differently, and impose different limits on input size, nesting, strings, or numeric range.
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RFC 8259, published in December 2017, says a parser “MUST accept all texts that conform to the JSON grammar.” It also permits implementations to accept extensions and to set limits on resources and number handling. As a result, “valid JSON” and “accepted by this parser with these settings” are related but not identical claims. RFC 8259
A comparison should separate at least four outcomes: whether the input is accepted, what value the parser produces, whether serialization preserves the intended meaning, and how errors or partial results are reported. Two parsers that both accept a document may still disagree about its value or produce different output when it is serialized again.
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What happens when a JSON object has duplicate keys?
RFC 8259 says object member names SHOULD be unique, but it does not prescribe one universal resolution for duplicates. It describes receiver behavior as unpredictable: some implementations report only the last name/value pair, others reject or fail, and some expose all pairs. That means a document such as {"role":"user","role":"admin"} can have different consequences depending on the parser and what the application does with its result.
I-JSON, the stricter interoperability profile defined by RFC 7493 in March 2015, prohibits duplicate member names after escape processing. For example, a literal name and an escaped spelling that resolves to the same name are duplicates under that rule. I-JSON also specifies that object-member ordering does not change a message’s meaning. RFC 7493
For systems that pass JSON between services, rejecting duplicate names is often safer than allowing different components to choose different values. The important test result is not merely “accepted”: record whether the parser rejects the object, keeps one value, or preserves multiple pairs.
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Yes, especially when input contains an escaped unpaired UTF-16 surrogate such as "uDEAD". RFC 8259 notes that its grammar can permit sequences that do not encode Unicode characters and warns that receiver behavior for them is unpredictable. RFC 7493 takes a stricter approach for I-JSON: surrogate and noncharacter code points are prohibited in member names and string values, including lone surrogate escapes.
Escaping also makes equality less obvious. A parser may need to compare names after escape processing, not just compare their source spelling. For ordinary interoperable strings, use valid Unicode and UTF-8. I-JSON requires UTF-8 and disallows surrogate code points and noncharacters. RFC 8259 RFC 7493
A 2024 study, “Cross-Language Differential Testing of JSON Parsers,” reported issues among the implementations it tested involving control-character serialization, UTF-16 surrogate-pair handling, U+0000 rejection or truncation, and object-name handling. These are findings about the tested implementations, not a claim that every current parser has those defects. They illustrate why a test should check both the parsed value and whether re-serialized output remains valid JSON. Cross-Language Differential Testing of JSON Parsers
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Why can a large JSON number change when parsed?
JSON’s number grammar allows values that a particular implementation may not represent exactly. RFC 8259 permits limits on numeric range and precision when text is translated into another representation. An implementation using binary floating point, for example, may round a large integer or a decimal with more precision than that representation can retain.
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In a parser comparison, inspect the actual resulting number and its serialized form, not just whether the original text was accepted. Record whether the parser uses integers, decimal arithmetic, binary floating point, or another configured representation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do I test whether two JSON parsers agree?
Define agreement before running the comparison. A single pass/fail label hides important differences: parsers may accept the same input but produce different values, or produce equivalent values but serialize them differently. Keep syntax acceptance, semantic interpretation, round-tripping, and error behavior as separate results.
- Choose the standard or profile. Classify each input as conforming to RFC 8259, conforming to I-JSON, deliberately invalid, or relying on an extension. Do not treat a stricter profile violation as automatically invalid under baseline JSON.
- Record the exact implementations. Name each parser and its library or runtime version, and date the test. A parser-family name alone is not enough to reproduce results.
- Record configuration and limits. Include strictness switches, decoding options, number representation, and limits on size, nesting, strings, and numeric range or precision. Mark a resource-limit rejection separately from a difference in how the input is interpreted.
- Save the exact inputs and expected outcomes. For each document, preserve the bytes or exact text and say what behavior the test is meant to examine: acceptance, resulting value, round-trip output, or error behavior.
- Compare outputs by dimension. Record accepted or rejected status; strings, object members, duplicate-name policy, and number values; whether serialization is valid and preserves meaning; and error category or partial results.
- Publish the comparison table and harness details. Include enough information for another person to run the same documents against the same versions and settings. Avoid describing parsers as identical beyond the tested corpus and dimensions.
A 2024 cross-language differential-testing study demonstrates that this kind of comparison can uncover consequential differences, including transformations that can undermine data integrity when later components trust parser output. A finite test corpus is evidence about the cases tested, not proof of behavior for every possible document. Cross-Language Differential Testing of JSON Parsers
For a comparison of Go implementations, one project’s documentation lists duplicate-name differences among the examined implementations and reports different results on RFC 8259 and I-JSON cases. Treat that as documentation for its listed implementations and versions, not as a permanent ranking of Go parsers. Go JSON interoperability discussion
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How can I make JSON safer to exchange?
- Avoid duplicate object names and use a defined stricter profile, such as I-JSON, when the application needs a narrower interoperability contract.
- Use UTF-8 and valid Unicode strings; avoid lone surrogate escapes and noncharacters where I-JSON compatibility is required.
- Keep numbers within ranges and precision that recipients can represent. Use strings for exact interchange of larger or more precise numeric values.
- At security-sensitive trust boundaries, compare the representation consumed by downstream components and validate that exact representation.
- Use a JSON parser rather than an
eval()-like language facility. RFC 8259 warns that evaluating input as code is unsafe because input can contain executable code. RFC 8259
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