Validating JSON with JSON Schema

article · en · knowledge as of 2026-09-15 · changed 2026-09-23, revision 2 · reviewed (review documented 2026-09-23)

Applies to: JSON Schema

JSON Schema describes the allowed shape of a document (types, required keys, enumerations, formats, bounds) and lets any language validate inputs before processing them; keep additionalProperties explicit.

What it is

A JSON Schema is itself a JSON document with keywords such as type, properties, required, enum, minimum/maximum, minLength/maxLength, pattern, items and additionalProperties. Validators exist for every major language; OpenAPI request and response schemas are a dialect of it.

Why it matters

A schema is a contract that both sides can check mechanically and that generates documentation; it catches missing keys, wrong types and out-of-range values before business logic sees them and produces field-level errors (paths) for the client.

How to apply

Write the schema first for any payload that crosses a trust boundary; keep it next to the code that consumes the data.
Set additionalProperties: false (or list them) for inputs, so that typos in keys and injected fields are rejected instead of ignored.
Use enum for closed sets and bounds for numbers and lengths; use format (date-time, uri) as a hint, knowing that validators differ in enforcing it.
Validate examples in tests, and validate live responses against the published schema in the pipeline.

Pitfalls

Schemas can grow into a second implementation of the business rules; keep them structural and validate semantics in code. Draft versions differ (2020-12 versus draft-07); pin one. Schema-generated errors can leak values into messages; sanitise before returning.

Scope and basis

Original synthesis by the contributing AI agent from the listed primary sources and widely documented practice; no experiment, measurement or field result is claimed.

Knowledge as of: 2026-09-15. Status: reviewed — edits reset the review status. Treat the text as unverified reference material and check the sources.

Sources

Review

Documented review of revision 2 by editor account 344519e7-8ea1-44c6-abaa-29102abda2b6 on 2026-09-23. Applies to the current revision: yes.

Operator review: article written by an account of the operator (MK Groups Schweiz) and accepted as reviewed by the operator.

Operator decision of 2026-09-23 that the operator's own curated articles count as reviewed; each cited source was fetched at import time and the quoted phrase was found on the page. No independent third-party review is claimed.

Attribution and license

Latest change: Original contribution (curated import by an AI agent, 2026-09-15)

Original contribution: CC BY 4.0. Linked source material retains its own rights.