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JSON Syntax Checker

A JSON syntax checker confirms that your JSON follows the rules of the JSON specification: double-quoted keys, no trailing commas, correctly nested objects and arrays, matched brackets, and valid backslash escapes inside strings.

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Finds all syntax errors in one pass

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Shows line and column of each error

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Clear plain-language error descriptions

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No coding required

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The Six Most Common JSON Syntax Errors and What Causes Each One

Trailing commas are the single most common JSON syntax error in real codebases, by a wide margin. They appear when a developer copies an object literal from JavaScript or TypeScript source code (where trailing commas have been legal since ES5 and are actively encouraged by style guides like Prettier) or when a template engine generates JSON and forgets to suppress the comma after the final item in a loop. The parser response is typically "Unexpected token }" or "Unexpected token ]" because the parser sees a closing bracket immediately after a comma and expects another value to follow. The fix is always the same: delete the comma sitting directly before the closing bracket or brace. An example of the error: the object {"name": "Alice", "age": 30,} contains a trailing comma after 30 that makes the entire structure invalid.

Single quotes and unquoted keys are the second and third most frequent errors, and both come from the same root confusion between JSON and JavaScript. In JSON, every string, including every property name, must be wrapped in double quotes. Writing {name: "Alice"} with an unquoted key or {"name": 'Alice'} with a single-quoted value will both fail JSON parsing immediately. This happens routinely when data is copied from Python dictionary representations, Ruby hash literals, JavaScript object literals, or YAML documents where the rules are looser. The fix is mechanical: wrap every property name in double quotes and replace every single-quoted string value with a double-quoted one. A regular expression substitution can sometimes handle the bulk replacement, though you have to watch for apostrophes embedded inside string content.

The remaining common errors form a short list. Missing commas between elements such as {"a": 1 "b": 2} cause the parser to expect a comma after the first value and find an unexpected string instead. Mismatched brackets produce either "Unexpected end of input" (an opening { without a closing }) or "Unexpected token" when an extra closing bracket appears. Unterminated strings, where a value opens with a double quote but the closing quote is missing or escaped incorrectly, trigger a parse failure that may consume the rest of the file before erroring. Invalid escape sequences inside strings, such as \x followed by a hex code, fail because the JSON specification permits only a fixed set of escapes: \", \\, \/, \b, \f, \n, \r, \t, and \uXXXX for any other Unicode code point. A syntax checker catches every category in one pass.

Integrating a syntax check into a continuous integration pipeline pays for itself within the first failed build it prevents from reaching the main branch. A typical setup adds a step to GitHub Actions, GitLab CI, or CircleCI that runs python3 -m json.tool or jq . over every changed JSON file in the diff and fails the job on the first non-zero exit code. For repositories with many JSON files, a find command piped into xargs runs the same check across the entire tree in seconds. Some teams go further and add a pre-commit hook using the pre-commit framework with the check-json hook from the standard library, which catches the failure on the developer's machine before the commit lands in version control at all. Both approaches share the same underlying syntax validation logic that FixTools applies on the browser side. The earlier in the pipeline you catch the failure, the cheaper the fix.

RFC 8259 Section 2: JSON Grammar JSON.org syntax diagrams MDN: Working with JSON

How to use this tool

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Paste your JSON and use the Validate button to run a full syntax check.

How It Works

Step-by-step guide to json syntax checker:

1
Paste your JSON
Copy the JSON string you want to check from wherever it currently lives, whether a code editor, a network response, a generated artifact, or a database column, and paste the full text into the FixTools input area. Pasting the raw text rather than a formatted preview ensures the checker sees the exact bytes your downstream parser will see.
2
Run the syntax check
Click Validate and FixTools immediately parses your JSON against the RFC 8259 grammar using the browser native engine. Every syntax violation surfaces in the output panel with a specific line, column, and explanation. Because parsing happens locally, the response is essentially instant for any payload that fits in browser memory comfortably.
3
Fix and recheck
Address each error at the reported position by removing the trailing comma, adding the missing quote, balancing the bracket, or whatever the diagnosis indicates. Click Validate again after each fix. JSON parsers stop at the first fatal error, so a clean pass after one fix may reveal additional issues further down that were previously hidden behind the first failure.

Real-world examples

Common situations where this approach makes a real difference:

API developer

A developer is writing integration tests for a payment API and one specific test payload keeps coming back as a 400 Bad Request from the server with no useful body. They paste the request JSON into FixTools and the syntax checker immediately reports a missing comma between two properties: "currency": "USD" and "amount": 100 sitting on consecutive lines with no separator. The eye glides over the gap, but the parser does not. The checker points to the exact column, the fix takes ten seconds, and the test moves into the passing column without any further investigation needed.

Configuration manager

A systems administrator edits a JSON-based application config file in vi during an incident and saves it quickly. The app then refuses to start, dumping a parse error stack trace that gives only a byte offset rather than a line number. Pasting the file into the FixTools syntax checker reveals an unquoted key on line 34: environment: "production" should have been "environment": "production". The fix is a two-character edit and the service comes back up. The lesson sticks, and future edits route through the validator before commit.

Data migration engineer

A data engineer is migrating records out of a legacy system that exported data as a Python dict literal rather than proper JSON. The export uses single-quoted strings throughout and trailing commas at the end of every object. Running the full file through the FixTools syntax checker confirms 47 single-quote instances and 12 trailing commas as the only structural issues. The engineer writes a targeted find-and-replace script tuned to those exact patterns, runs it once, revalidates the output, and the migration loader accepts the cleaned file on the first attempt without further hand editing.

Technical writer

A technical writer is producing API documentation and including JSON request and response examples throughout the guide. Before publishing each section, they paste every example through the FixTools syntax checker as part of a final review. One example contains an unquoted key that was copied straight from a cURL command line where the shell quoting hid the issue. The checker catches the bad key before the documentation goes live, preventing every developer who reads that page from copying a broken snippet into their own code and filing a support ticket about it later.

Pro tips

Get better results with these expert suggestions:

Fix the first error first

JSON parsers stop at the first fatal syntax error they encounter, which means fixing that error and revalidating will sometimes surface additional issues that were sitting silently further down the file. Always address the lowest line-number error first, then run the check again. Trying to fix multiple errors at once without revalidating in between is a reliable way to introduce new mistakes while clearing old ones, so resist the urge to batch the edits.

Check escape sequences in strings

Invalid escape sequences inside JSON strings are easy to miss visually because they look fine until a parser objects. The valid escape sequences in JSON are exactly: \", \\, \/, \b, \f, \n, \r, \t, and \uXXXX for any other character. A backslash followed by anything else, such as \x41 for a hex character or \p for a Unicode property, is a syntax error even though the surrounding string content looks completely normal. When the parser reports an error inside a quoted string, check the escapes first.

Confirm bracket nesting with formatting

If you suspect a missing bracket somewhere in a large file, paste your JSON into the FixTools formatter first (assuming the input is close enough to valid that the formatter can do partial work). The indented output makes nesting levels visually obvious and exposes unclosed objects or arrays that are essentially invisible in single-line or lightly-formatted JSON. The structure should descend and ascend symmetrically, and any place where the indentation looks lopsided usually points at the missing closing character.

Copy raw source, not rendered output

When copying JSON from a browser page, a PDF document, or a word processor, the rich text rendering layer can silently replace straight double quotes with curly typographic quotes (U+201C and U+201D). These look identical to the eye but fail JSON parsing immediately because the parser expects U+0022. Always copy from a plain text source: a terminal, a code editor, a raw network response panel, or a curl output. If a paste keeps failing for no obvious reason, suspect the quote characters first.

FAQ

Frequently asked questions

The most common errors in roughly descending order are: trailing commas after the last element in an object or array such as {"a":1,} or [1,2,]; single quotes instead of double quotes around strings or property names; unquoted property keys; missing commas between sibling elements; mismatched or missing opening and closing brackets and braces; unterminated strings where a closing double quote is missing; and invalid backslash escape sequences inside string values. Trailing commas are by far the most frequent because they are valid in JavaScript, Python, and most modern programming languages, so muscle memory keeps producing them in JSON contexts where they are not allowed.

Yes, absolutely. Valid JSON syntax means the file can be parsed without error, but the parsed data may not match what your application actually expects. Your app might expect a field called "userId" but the JSON contains "user_id" because the producer used a different naming convention. It might expect a number but the JSON contains a quoted string that happens to look like a number. These are semantic or schema-level problems that pure syntax checking does not detect. For structural validation against expected shape and types, use a JSON Schema validator with a defined schema document.

For JSON specifically, the two terms are nearly identical in practice. A syntax checker confirms that the JSON parses without error. A linter might additionally flag style issues such as inconsistent indentation, keys that are not in alphabetical order, or trailing whitespace. But since JSON has no executable semantics for a linter to analyse, linting JSON essentially reduces to syntax correctness with optional style enforcement layered on top. Both tools ultimately answer the same core question: does this document conform to the JSON specification published as RFC 8259, and is it safe to hand off to a downstream parser?

That error means the parser reached a closing brace at a point where it was still expecting either a value or a property name. The usual culprit is a trailing comma sitting just before the closing brace: the parser sees the comma, expects another property to follow, and then finds the closing } instead of a key. Remove the comma that appears directly before the closing brace and the error goes away. The error message points at the } character because that is where the parser actually gave up, but the real mistake is one character earlier in the file.

A valid JSON document, per RFC 8259, contains exactly one top-level value. If you have multiple JSON objects that need to be checked together, they must be wrapped inside an array, like [{...},{...}], or you must check each one individually as a separate input. A file containing multiple JSON objects on separate lines without a wrapping array is using the NDJSON or JSON Lines format, which is a popular convention for streaming but is not standard JSON. A standard syntax checker will fail at the start of the second object because the grammar does not permit it.

No, and the omission is deliberate. RFC 8259 explicitly does not permit any form of comments in JSON, neither C-style block comments nor // line comments nor # hash comments. The decision was made early in the format design to keep JSON minimal and unambiguous across every parser implementation. If you need to annotate a JSON config file with explanatory notes, consider JSONC (JSON with Comments) used by VS Code config files, or JSON5 which permits comments and trailing commas. Both are supersets accepted by specific parsers that opt in, but neither is accepted by the standard JSON.parse function.

Yes, and this trips up generated content frequently. RFC 8259 section 7 states that characters in the range U+0000 through U+001F, which includes tab, newline, and carriage return, must be escaped when they appear inside JSON string values. A literal newline embedded directly inside a string is invalid. The correct representations are \n for a newline character, \t for a tab, and \r for a carriage return. Other control characters use the \uXXXX format with the four-digit hex code. Pasting JSON that contains unescaped control characters will fail the syntax check with a specific message about the offending byte.

Paste the JSON into a formatter first, even if you suspect it is broken. If the file is close to valid, the formatter will still indent most of it and the nesting structure becomes visually obvious. A missing closing bracket somewhere in the middle causes all subsequent content to appear one indent level deeper than it should be, which jumps out on inspection. Alternatively, most syntax checkers report "Unexpected end of input" at the very end of the file when a closing bracket is missing, so working backwards from the end while counting opens versus closes lands you on the unclosed structure quickly.

When the engine error message is sparse, layer additional tools on top of the raw output. A hex viewer such as xxd or the built-in hex mode in VS Code reveals invisible characters, BOM bytes, and encoding issues that look identical in a normal editor view. A JSON formatter that indents the document by nesting level exposes structural imbalance because subsequent content sits at the wrong indent depth when a bracket is missing. A diff tool comparing the failing document against a known-good template highlights exactly which bytes diverge. For programmatic debugging, jq with the --debug flag emits trace events as it parses, which helps narrow down where the structure becomes ambiguous. FixTools combines several of these views into one screen so you do not have to switch tools repeatedly while iterating.