Ajv (Another JSON Schema Validator)
Fastest JSON Schema validator for JavaScript. Compiles JSON Schema drafts 04, 06, 07, 2019-09, and 2020-12 into optimized validation functions. Used by many tools (webpack, eslint, ajv-formats) for configuration and data validation. Supports custom keywords, formats, and error messages. The standard JSON Schema validation library for the Node.js ecosystem.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
Local computation. MIT licensed. Regular security audits. ReDoS-safe format validators when using ajv-formats. No remote calls.
⚡ Reliability
Best When
You need high-performance JSON Schema (draft-07, draft-2020-12) validation, OpenAPI schema compliance, or have existing JSON Schema files you want to enforce.
Avoid When
You're starting a new TypeScript project without existing JSON Schema — Zod provides better TypeScript integration with schema-derived types without a separate JSON Schema definition.
Use Cases
- • Validate API request/response bodies against JSON Schema for agent API clients and servers
- • Validate configuration files and environment variables before agent startup using compiled schemas
- • Enforce data contracts between agent components by validating message schemas at runtime
- • Validate OpenAPI response bodies against schemas for agent integration testing
- • Generate type-safe validation in TypeScript agent pipelines with compile-time schema type inference
Not For
- • Simple input validation — Zod or Yup offer more ergonomic APIs with TypeScript inference for application-level validation
- • Runtime type checking in TypeScript — use Zod for validator+type inference in one; Ajv requires separate type definitions
- • Human-readable error messages out of the box — Ajv errors are verbose JSON; use ajv-errors or transform errors for user-facing messages
Interface
Authentication
Local library — no authentication required. MIT licensed.
Pricing
MIT licensed. Zero cost.
Agent Metadata
Known Gotchas
- ⚠ Ajv v8 dropped support for JSON Schema draft-04 by default — use new Ajv04() from 'ajv/dist/draft-04' for draft-04 schemas; breaking change from v6
- ⚠ Compile schemas once at startup — ajv.compile(schema) is expensive; store compiled validator in a variable and reuse rather than compiling on each request
- ⚠ format validation (email, date, uri) is NOT included by default in v8 — install 'ajv-formats' package and apply: const ajv = new Ajv(); addFormats(ajv)
- ⚠ additionalProperties: false in schemas rejects extra fields — common source of validation failures when API responses have undocumented fields; use allowUnionTypes option
- ⚠ Ajv errors are on the validate.errors property AFTER calling validate() — validate() returns boolean, errors are stored as side effect: const valid = validate(data); if (!valid) console.log(validate.errors)
- ⚠ TypeScript users: ajv-validator/ajv provides TypeScript types but schema-to-type inference is not built-in — use TypeBox or Zod for automatic TypeScript type derivation from schemas
Alternatives
Full Evaluation Report
Detailed scoring breakdown, competitive positioning, security analysis, and improvement recommendations for Ajv (Another JSON Schema Validator).
Scores are editorial opinions as of 2026-03-06.