Validate JSON against a JSON Schema, see errors inline.
Updated
Quick start
Paste JSON, see parse status, structure stats and duplicate-key warnings. Optionally check against a JSON Schema.
Drop your JSON into the textarea. Parse errors show line and column; valid JSON shows a green badge.
Counts of each value type plus max depth — useful for spotting accidentally-deep nesting or unexpected null fields.
Enable the schema panel and paste a JSON Schema to validate types, required fields, ranges and enums.
In-depth guide
Paste JSON and immediately see whether it parses, where the error is if not, what's inside (counts of objects, arrays, strings, numbers, max depth), and whether any keys are duplicated. Optionally check it against a JSON Schema document.
RFC 8259 is strict. Common mistakes that fail validation:
[1, 2, 3,] is invalid.0123 is invalid (octal-looking).RFC 8259 says implementations "should" reject duplicate object keys but most don't. JSON.parse keeps the last value, jq the first, Python the last, Go errors out — every parser is different.
If your config has accidentally duplicated keys, the value you see in production may not be the value in your file. This tool re-scans the raw text and flags duplicates so you catch them before they ship.
Toggle the schema panel and paste a JSON Schema document. We support a practical subset suitable for shape-checking API payloads:
type — string, number, integer, boolean, null, array, object (or array of these).required — array of property names that must be present.properties — recursive schemas keyed by property name.items — recursive schema for each array element.enum, minimum, maximum, minLength, maxLength.For richer validation (oneOf, allOf, $ref, format) use a library like ajv inside your codebase.
When to reach for it. Use it to confirm a payload is well-formed JSON and to jump straight to the line and column of a syntax error.
When something else is better. Well-formed isn’t the same as schema-valid — it won’t check that fields match your API contract.
The pitfall to watch. The reported error position points at where parsing failed, which is often just after the real mistake (a missing comma on the line above).
Everything runs on your device. The values you enter are processed locally in this browser tab — EpitomeTool does not send your input to a server, store it, or log it. That means you can use the tool offline once the page has loaded, and refreshing the tab wipes the slate.
No. Parsing and validation run in your browser. Useful for checking sensitive payloads without uploading them to a hosted validator.
The text parses as JSON per the official grammar (RFC 8259) — balanced braces, proper string escaping, no trailing commas, no comments. The tool also surfaces a tree-walk summary: counts of each value type and maximum nesting depth.
JSON technically allows duplicate keys, but JSON.parse silently keeps only the last value — which is almost always a bug. We re-scan the raw text and flag duplicates so you can spot them before they cause data loss.
A practical subset: type, required, properties, items, enum, minimum, maximum, minLength, maxLength. Enough to validate common API payloads. For full Draft-07 / 2020-12 support use a dedicated validator like ajv.
We extract the byte offset from the engine's error message and convert it to line and column numbers. Note: V8 (Chrome) gives a position; Firefox gives a line/column directly — both work.
Not supported — this tool validates strict JSON only. Strip comments and trailing commas before pasting.
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