oav

OpenAPI 3.0 / 3.1 / 3.2 HTTP request and response validator for JavaScript and TypeScript services. Two primary drivers:

• Tenant overrides over a base spec. When tenants extend a shared API (adding a required header on one route, refining a schema, requiring auth where the base spec didn't), they need to document those changes in the spec they ship, not as application-side patches. applyOverlays rewrites the document at load time. Custom keywords, formats, and dialects plug into the compiler the same way, so per-tenant validation rules don't require forking. See docs/overlays.md.
• Validators that fit in microservice runners. oav compile-spec openapi.yaml emits a single zero-dependency ES module exposing the full validator surface. Targets Cloudflare Workers, Vercel Edge, Lambda@Edge, Deno Deploy: runtimes where new Function() is unavailable, or where dependency footprint matters. --only "POST /pets" (repeatable) scopes the output to specific operations without touching the source spec.

Errors come back as a flat list of typed leaves (code, path, message, params) by default, or a nested tree on request. One validator call covers the full HTTP frame: method, path, parameters, body, content type, status, and headers.

If you only need generic JSON Schema validation across many drafts, start with Ajv. If you want a one-line Express middleware with file upload and auth handler conveniences built in, start with express-openapi-validator. See docs/comparison.md for the feature map.

Install

oav is the package shorthand used throughout the docs. Install uses the scoped npm package names shown below.

| You need | Package choice | | ------------------------------------------------ | ------------------------------- | | YAML specs, HTTP/YAML readers, and the oav CLI | oav | | JSON or pre-parsed specs with zero runtime deps | oav-core | | Express 4 request middleware | oav + oav-express4 | | Express 5 request middleware | oav + oav-express5 | | Fastify preValidation hook | oav + oav-fastify | | Edge/serverless validator emitted at build time | oav as a dev/build dependency |

oav is a superset of oav-core: same programmatic surface plus YAML readers and the oav CLI. The CLI's commander and esbuild deps load on demand from the binary entry, never from the library entrypoints, so application bundles tree-shake them out.

npm install @aahoughton/oav            # YAML + CLI
npm install @aahoughton/oav-core       # JSON only, zero runtime deps
npm install @aahoughton/oav-express4   # Express 4 adapter (transitively pulls oav-core)
npm install @aahoughton/oav-express5   # Express 5 adapter
npm install @aahoughton/oav-fastify    # Fastify adapter

oav re-exports oav-core at five subpath entrypoints (/schema, /spec, /overlay-spec, /formats, /core); on the lean package, substitute oav-core in imports that don't touch the YAML readers (createYamlFileReader, createSmartHttpReader) or the CLI. See docs/modules.md for what each subpath exports.

Quick start

Express

import express from "express";
import { createValidator, createYamlFileReader } from "@aahoughton/oav";
import { loadSpec } from "@aahoughton/oav/spec";
import { validateRequests } from "@aahoughton/oav-express5";

const { document } = await loadSpec({
  reader: createYamlFileReader(),
  entry: "openapi.yaml",
});
const validator = createValidator(document);

const app = express();
app.use(express.json());
app.use(validateRequests(validator));

app.post("/pets", (req, res) => res.json({ ok: true }));

Invalid requests receive an application/problem+json response. Valid requests continue to your route handlers. Express 4 uses the same shape with oav-express4; Fastify uses oav-fastify as a preValidation hook. See docs/integration.md.

Framework-agnostic

import { createValidator, createYamlFileReader, formatText } from "@aahoughton/oav";
import { loadSpec } from "@aahoughton/oav/spec";

const { document } = await loadSpec({
  reader: createYamlFileReader(),
  entry: "openapi.yaml",
});
const validator = createValidator(document);

const result = validator.validateRequest({
  method: "POST",
  path: "/pets",
  contentType: "application/json",
  headers: { "x-tenant": "acme" },
  body: { name: "Fido" },
});

if (!result.valid) console.error(formatText(result.errors));

For a multi-file spec or a spec hosted over HTTP, compose readers: composeReaders([createYamlFileReader(), createSmartHttpReader(), createFileReader()]) handles local YAML, remote JSON / YAML, and local JSON transparently.

validateRequest / validateResponse return { valid: true } on success, or { valid: false, errors, truncated } on failure. The zero-config default is a flat errors list that stops at the first problem (maxErrors: 1); raise maxErrors to collect more, or pass output: "tree" for a nested error tree under error (or output: "predicate" for a bare boolean). Every error carries a stable code (e.g. "type", "required", "content-type", "oneOf"), a path rooted at the HTTP frame (e.g. ["body", "pets", 3, "name"]), a human-readable message, and a machine-readable params object whose shape per code is documented in BuiltInErrorParams.

Runnable end-to-end demos in examples/: custom formats, custom keywords, cross-field constraints, error budgets, version differences, overlays, and spec-derived middleware config.

Overlay quickstart

applyOverlays rewrites the document in memory before the validator is constructed. Typical shapes:

import { applyOverlays } from "@aahoughton/oav/spec";
import type { SpecOverlay } from "@aahoughton/oav/spec";

// Add a deployment-specific server. `addServers` appends; `servers`
// replaces wholesale.
const eu: SpecOverlay = {
  addServers: [{ url: "https://eu.api.example.com" }],
};

// Require an API key on POST /pets without forking the base spec.
const requireKey: SpecOverlay = {
  overrides: {
    "/pets": {
      operations: { post: { addSecurity: [{ apiKey: [] }] } },
    },
  },
};

// Apply one rule to every operation matching a tag (walks paths and
// webhooks).
const lockInternals: SpecOverlay = {
  modifyOperations: [
    {
      where: { tags: ["internal"] },
      apply: { addSecurity: [{ internalKey: [] }] },
    },
  ],
};

// Tighten an upstream schema; the original definition still applies.
const requirePetId: SpecOverlay = {
  extendSchemas: { Pet: { required: ["id"] } },
};

const patched = applyOverlays(document, [eu, requireKey, lockInternals, requirePetId]);
const validator = createValidator(patched);

The full verb surface (component-bucket fan-out, predicate iterators, operation-level metadata) is documented in docs/overlays.md; cross-cutting integration shapes live in docs/integration.md.

Where to go next

| Task | Read | | ----------------------------------------- | ---------------------------------------------------------------------------------------------------------------- | | Wire into Express, Fastify, Next.js, Hono | docs/integration.md | | Patch a spec you do not own | docs/overlays.md | | Emit standalone validators | packages/cli/README.md | | Compare against Ajv and other tools | docs/comparison.md | | Migrate from express-openapi-validator | docs/migration-from-eov.md | | Use custom formats, keywords, or limits | docs/configuration.md |

How it compares

The JavaScript ecosystem already has solid OpenAPI validation tools: Ajv for JSON Schema, express-openapi-validator for Express, openapi-backend for operationId routing plus validation, and smaller request/response validators for custom stacks. oav is aimed at HTTP-aware validation with structured errors, overlays, and standalone OpenAPI validator output. See docs/comparison.md for the feature map, and docs/migration-from-eov.md if you are migrating from express-openapi-validator.

On raw speed, oav wins some and loses some against Ajv, and for most services the difference is immaterial. oav compiles schemas far faster (tens of microseconds against milliseconds), validates competitively on typical request bodies, and carries a slightly smaller steady-state memory footprint than express-openapi-validator. Ajv is faster on fast-fail validation of some ordinary object shapes. At normal request volumes (a validation or two per request, thousands of requests per second) these gaps are nanoseconds per call and vanish into everything else a handler does. They only start to matter if you validate at extreme volume or against pathological shapes (very large uniqueItems arrays, very long length-bounded strings).

For the host-stamped per-shape numbers, the memory comparison, and the methodology, see docs/comparison.md. Raw benchmark data lives in performance/.

Conformance

The conformance/ sub-package drives the compiler and CLI against the upstream JSON Schema 2020-12 Test Suite, a set of OpenAPI 3.0 / 3.1 / 3.2 petstore scenarios, and a handful of real-world specs (Stripe, GitHub, DigitalOcean, Twilio, Asana, Box, Adyen) that have to load and compile without error. See conformance/REPORT.md for pass / fail counts by category.

Categories oav does not aim to cover:

• $dynamicRef with runtime dynamic-scope rebinding (oav resolves statically against the anchor map).
• The optional/format/* subtree (format is annotation-only by default per JSON Schema 2020-12 §6.3).
• A small tail of isolated optional cases (float-overflow handling, external-ref loading tied to dynamic scope).

OpenAPI specs hand-authored or generated for typical APIs rarely touch any of these. If they matter for your use case, the report lays out which tests fail and why.

CLI

oav resolve openapi.yaml
oav validate openapi.yaml --request req.http
oav validate openapi.yaml --path "POST /pets" --body payload.json
oav validate openapi.yaml --path "GET /pets" --response --status 200 --body resp.json
oav compile-schema schema.json -o validator.mjs             # JSON Schema -> standalone validator
oav compile-spec openapi.yaml  -o validator.mjs             # OpenAPI   -> standalone HTTP validator (edge / Lambda)

Flags: --format text|json|summary, --depth n, --overlay file (repeatable), -o file, --quiet, --dialect (compile-schema / compile-spec), --requests-only (compile-spec), --only METHOD PATH (compile-spec, repeatable). See packages/cli/README.md for the full surface, the .http file format, and both compile commands' output contracts.

Versions

createValidator reads the spec's openapi string once at construction and picks the matching dialect. No per-request branching.

| Spec | Dialect | Notes | | ----- | --------------------- | ----------------------------------------------------------- | | 3.0.x | OAS 3.0 Schema Object | nullable, boolean exclusiveMin/Max, sibling-$ref drop | | 3.1.x | JSON Schema 2020-12 | Assertive format | | 3.2.x | JSON Schema 2020-12 | Same as 3.1 + the QUERY HTTP method |

3.2 coverage is the Schema Object (unchanged from 3.1) plus QUERY. Other 3.2 document-level additions (additionalOperations, in: querystring, streaming media types) aren't recognized yet.

Override via createValidator(spec, { dialect }) to force or customize one of the built-in dialects (jsonSchemaDialect, openapi31Dialect, oas30Dialect). Unknown / missing openapi strings fall back to the 3.1 dialect by default; configure with onUnknownVersion: "throw" | "warn" | "fallback31".

Swagger 2.0 specs aren't supported directly: createValidator throws on swagger: "2.0" documents. Convert to OpenAPI 3.0 first with swagger2openapi and pass the 3.0 output to createValidator:

npx swagger2openapi swagger.json -o openapi.json

Configuring the validator

createValidator(spec, options) accepts options for dialect override, custom formats and keywords, error budget, strict-mode lint, security shape-checking, ignored paths, and version-mismatch policy. See docs/configuration.md for the option table, custom-keyword recipe, and bounded-error-collection details. The canonical contract is the ValidatorOptions TSDoc.

Framework integration

oav is a validator, not a middleware package: you write a short adapter between your framework and validateRequest / validateResponse, or use one of the companion adapter packages. An inline Express 5 adapter is about this long:

import { allowHeaderFor, httpStatusFor, toProblemDetails } from "@aahoughton/oav";

app.use(async (req, res, next) => {
  const result = validator.validateRequest({
    method: req.method,
    path: req.path,
    query: req.query as Record<string, string | string[]>,
    headers: req.headers as Record<string, string | string[]>,
    contentType: req.get("content-type") ?? undefined,
    body: req.body,
  });
  if (result.valid) return next();
  const allow = allowHeaderFor(result.errors);
  if (allow !== undefined) res.setHeader("Allow", allow);
  res
    .status(httpStatusFor(result.errors))
    .type("application/problem+json")
    .json(toProblemDetails(result.errors, { instance: req.originalUrl }));
});

httpStatusFor, allowHeaderFor, and toProblemDetails accept either the flat errors list or a tree error, so this wiring is the same whichever output the validator uses.

Companion adapter packages cover common request-validation wiring: oav-express4, oav-express5, and oav-fastify. They share export names and option shapes; only the framework type differs.

For Next.js, Hono, Bun, Deno, and custom frameworks, use the framework-agnostic validateRequest / validateResponse calls or the Fetch helpers (validateFetchRequest, validateFetchResponse). See docs/integration.md for body parsing, response validation, uploads, security, ignored paths, and custom error envelopes.

oav is not a drop-in replacement for express-openapi-validator. The adapters cover request validation; response validation, auth dispatch, upload parsing, and custom error envelopes stay explicit in your application. In return, the validator does not mutate req or res, OpenAPI 3.0 behavior is built into the dialect, and failures come back as structured errors (a flat list by default, a nested tree on request) rather than framework-specific error classes.

Known limitations

Runtime behavior corners. For feature-scope tradeoffs against Ajv and OpenAPI middleware packages (draft versions, $data, async validation, response interception, upload helpers), see docs/comparison.md.

• $dynamicRef behaves like $ref with anchor lookup; no runtime dynamic-scope traversal.
• style: deepObject query parameters support only single-level nesting (obj[key]=value); OpenAPI 3.0–3.2 don't define nested semantics.
• pattern keywords and format: "regex" compile to the JavaScript built-in RegExp, which has no execution timeout. If your OpenAPI spec is attacker-controlled (e.g. multi-tenant upload), a catastrophic pattern like (a+)+$ is a ReDoS vector against any string the validator checks. Pass a regexCompiler to createValidator to plug in re2 or a complexity-checking engine; see "Hardening against untrusted regex patterns" .
• Recursive schemas validate by recursing on the JavaScript call stack. Unbounded, a deeply nested payload (a few thousand levels, only a few KB on the wire) can exhaust the stack and throw RangeError: Maximum call stack size exceeded. Set the maxDepth option (CompileOptions / ValidatorOptions) to bound recursion at the validator: a payload past the cap fails as a depth error (HTTP 400) instead of crashing. For untrusted input set maxDepth, and optionally cap nesting at the parse boundary as a backstop; see "Guarding against deeply nested payloads" .

Contributing

See CONTRIBUTING.md for branch / PR / release flow. Development workflow (lint / typecheck / test / build) and the conformance and performance sub-packages are described there and in CLAUDE.md.

License

MIT. See LICENSE.