Why does my JSON look valid but the validator rejects it?

The most likely causes are: (1) invisible Unicode characters, especially zero-width spaces (U+200B) or non-breaking spaces (U+00A0) pasted from word processors; (2) BOM (Byte Order Mark, U+FEFF) at the start of the file — validators should handle it but some older parsers reject it; (3) line endings — lone carriage returns ( , from old Mac systems) are invalid, use or ; (4) control characters — characters U+0000 through U+001F must be escaped.

Can the validator handle streaming JSON or NDJSON?

Yes. When you paste NDJSON (each line is an independent JSON value), the tool detects the format and validates each line independently, reporting errors per line. This is useful for validating log files, streaming API responses, and bulk data exports that use the JSON Lines format.

What is JSON Schema and do I need it?

JSON Schema is a vocabulary that defines the structure and constraints of JSON documents. If you are building or consuming an API, a schema acts as a contract: it specifies which fields are required, their data types, value ranges, regex patterns, and more. Schema validation catches bugs where your API returns or receives data with the wrong shape — before those bugs reach production. Major schema versions include draft-04, draft-06, draft-07, and 2020-12.

Does the validator send my JSON to a server?

No. All validation logic executes entirely in your browser using JavaScript. Your JSON data — whether it contains API keys, credentials, user data, or proprietary configuration — never leaves your machine. This design also means the validator works offline after the page loads.

How do I validate very deeply nested JSON?

Our validator handles nesting up to 1000 levels deep, which exceeds the limit of most JSON parsers (Node.js defaults to 512, browsers typically allow 512-1024). If your JSON exceeds this depth, consider refactoring — deeply nested structures are harder to read, validate, and query. JSONPath or jq can help extract deeply nested values without manual traversal.

Developer Tools2026-05-10

JSON Validator Online: Validate, Debug, and Lint JSON Instantly

A single trailing comma or unquoted key can bring down an entire API response. JSON may look simple — key-value pairs wrapped in curly braces — but its specification (RFC 8259) is surprisingly strict. Our JSON validator catches syntax errors, pinpoints the exact line and column where they occur, and explains what went wrong in plain English. Beyond basic syntax checking, the tool supports JSON Schema validation — give it a schema, and it will verify that your data's structure, types, required fields, and value constraints all match the specification. This is the same validation logic used by API gateways, CI/CD pipelines, and backend frameworks, running right in your browser. For developers working with configuration files (package.json, tsconfig.json, docker-compose.json), API payloads, or data interchange, a fast local validator eliminates the frustration of trial-and-error debugging. No uploads, no server round-trips, no registration — paste your JSON and get results in under 10 milliseconds for files under 100 KB.

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JSON Validator

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Step-by-Step Guide

Paste, Type, or Upload Your JSON

Paste JSON directly into the editor, type it manually, or drag and drop a .json file from your file system. The validator accepts any JSON structure — object, array, string, number, boolean, or null. A line-numbered editor shows your content with monospace formatting for easy visual scanning of deeply nested structures.

Validate and Inspect Errors

Click "Validate" to run the parser. If errors exist, each one is listed with its line number, column position, and a human-readable explanation. For example: "Line 14, Column 3: Unexpected trailing comma after the last property of an object." Click any error to jump the cursor straight to that location in the editor. If validation passes, a green success indicator appears along with statistics — number of keys, nesting depth, and total size in bytes.

Optionally Validate Against a JSON Schema

Paste or upload a JSON Schema (draft-04, draft-06, draft-07, or 2020-12) into the schema panel. The tool then validates your JSON against the schema's constraints: required properties, data types, minimum/maximum values, string patterns (regex), array length bounds, and conditional logic (if/then/else). Schema errors are reported with the same line-level precision as syntax errors.

Tips & Best Practices

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The three most common JSON syntax errors are: (1) trailing commas after the last element in an object or array — JSON forbids this, unlike JavaScript; (2) unquoted object keys — all keys must be double-quoted strings, no exceptions; (3) single-quoted strings — only double quotes are valid JSON, single quotes are JavaScript syntax, not JSON.

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JSON does not support comments. If your configuration file uses // or /* */ comments, it is actually JSONC (JSON with Comments) or JSON5, not standard JSON. Strip comments before validation or use a JSONC-aware tool. VS Code's JSON files support comments only when the file mode is set to "JSON with Comments."

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Large JSON files (>1 MB) should be validated incrementally during development, not after edits accumulate. Set up a watch script with a JSON validator like ajv-cli or jsonlint in your project: `ajv validate -s schema.json -d data.json`. Our online tool handles files up to 10 MB efficiently in the browser.

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JSON Schema draft 2020-12 is the latest version (released June 2022) and adds dynamic references, unevaluatedProperties, and improved annotation collection. Most major API frameworks (FastAPI, Express.js, .NET) support at least draft-07.

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NDJSON (Newline Delimited JSON), also called JSON Lines, stores one JSON object per line and uses the .ndjson or .jsonl extension. It is ideal for streaming and log processing but requires a line-by-line validator — standard JSON parsers will reject a multi-line NDJSON file. Our validator can detect NDJSON format and validate each line independently.

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Always validate API responses in your CI/CD pipeline. A simple `curl -s https://api.example.com/endpoint | python -m json.tool` catches malformed JSON before it reaches production consumers. Our tool's file upload feature works the same way — paste a raw API response and validate it.

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JSON5 (JSON for Humans) extends JSON with trailing commas, unquoted keys, single-quoted strings, comments, and hexadecimal numbers. It is used by Babel, Webpack, and TypeScript compilers for configuration files. If you are writing configuration by hand, JSON5 is far more ergonomic than strict JSON — just remember that consumers expecting standard JSON will reject it.

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For maximum performance, minify JSON before sending it over the network. Our formatter/validator pair lets you validate, then minify in one workflow — the minified output strips all whitespace (saving up to 30-40% of the file size for heavily indented JSON) while preserving structural integrity.

Frequently Asked Questions

JSON is defined by RFC 8259 (published December 2017), which obsoleted RFC 7159 and RFC 4627. RFC 8259 mandates that JSON text be encoded in UTF-8 and clarifies that JSON must be a valid JavaScript value — but it is a subset of JavaScript, not identical to it. Notably, RFC 8259 allows top-level JSON values other than objects and arrays (e.g., a bare string or number), while earlier RFCs did not.

A fast, accurate JSON validator saves hours of debugging by catching syntax errors the moment they happen. Whether you are writing config files, building APIs, or setting up CI checks, our free browser-based validator gives you RFC 8259 compliance, JSON Schema support, and NDJSON handling — all without sending your data anywhere. Paste your JSON and validate it now.

Try this tool for free →open_in_new