Free JSON Schema, OpenAPI, and YAML Validators

How this tool computes its result

Three independent validator tabs in one component. The JSON Schema tab recursively walks a pasted schema checking only structural shape -- type must be a string or array, required must be an array, properties must be an object with each child recursively checked, items recursively checked, and allOf/anyOf/oneOf must be arrays of valid sub-schemas -- with no support for keywords like pattern, enum, or numeric bounds. If an instance JSON is also pasted, it is separately checked against the schema's type/required/properties/items rules only. The OpenAPI tab first tries JSON.parse; on success it checks for an openapi/swagger version field, an info object with title and version, and a paths object. If JSON parsing fails, it falls back to line-based regex checks against raw text (banning tabs, requiring 2-space-multiple indentation, requiring a colon on non-list lines) plus separate regex presence checks for the openapi/swagger, info, paths, title, and version keys. The YAML tab reuses that same line-based regex checker standalone.

Worked example

Pasting the schema {"type":"object","properties":{"name":{"type":"string"}},"required":["name"]} with instance {"name": 123} produces one error: "$.name should be string, got number" -- the required-field check passes since "name" exists, but the type check on that property fails.

When not to use this tool

None of the three validators implement the full spec they claim to check: the JSON Schema check ignores most real JSON Schema keywords, and the OpenAPI YAML fallback is a handful of regex line checks, not an actual YAML parser -- syntactically valid but differently-formatted YAML (flow-style mappings, multi-line strings) can fail checks it should pass.

Common mistakes

• - Pasting OpenAPI as JSON and expecting the same depth as the YAML fallback path -- the JSON branch only checks 4 top-level things with zero per-endpoint validation, so a JSON OpenAPI doc can pass here while being structurally broken deeper in its paths object.
• - Using a tab character anywhere in YAML input -- the checker flags any tab in a line as an error, even inside a quoted string or trailing whitespace, with no exceptions.
• - Expecting instance validation on the JSON Schema tab to run without pasting a schema first, or expecting an error if the schema itself is invalid JSON -- instance checking silently no-ops in both cases rather than surfacing a message.

Related tools