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4.6.1 • Public • Published

Fastify OpenApi Glue

CI status Coverage Status NPM version npm

A plugin for fastify to autogenerate a configuration based on a OpenApi(v2/v3) specification.

It aims at facilitating "design first" API development i.e. you write or obtain an API specification and use that to generate code. Given an OpenApi specification Fastify-openapi-glue handles the fastify configuration of routes and validation schemas etc. You can also generate your own project from a OpenApi specification.


If you are upgrading from a previous major version of fastify-openapi-glue then please check out


npm i fastify-openapi-glue --save



Add the plugin to your project with register and pass it some basic options and you are done !

import openapiGlue from "fastify-openapi-glue";
import { Service } from "./service.js";
import { Security } from "./security.js";

const options = {
  specification: `${currentDir}/petstore-openapi.v3.json`,
  serviceHandlers: new Service(),
  securityHandlers: new Security(),
  prefix: "v1",

fastify.register(openapiGlue, options);

All schema and routes will be taken from the OpenApi specification listed in the options. No need to specify them in your code.


  • specification: this can be a JSON object, or the name of a JSON or YAML file containing a valid OpenApi(v2/v3) file
  • serviceHandlers: this can be a javascript object or class instance. See the serviceHandlers documentation for more details.
  • securityHandlers: this can be a javascript object or class instance. See the securityHandlers documentation for more details.
  • prefix: this is a string that can be used to prefix the routes, it is passed verbatim to fastify. E.g. if the path to your operation is specified as "/operation" then a prefix of "v1" will make it available at "/v1/operation". This setting overrules any "basePath" setting in a v2 specification. See the servers documentation for more details on using prefix with a v3 specification.
  • operationResolver: a custom operation resolver function, (operationId, method, openapiPath) => handler | routeOptions where method is the uppercase HTTP method (e.g. "GET") and openapiPath is the path taken from the specification without prefix (e.g. "/operation"). Mutually exclusive with serviceHandlers. See the operationResolver documentation for more details.
  • addEmptySchema: a boolean that allows empty bodies schemas to be passed through. This might be useful for status codes like 204 or 304. Default is false.

specification and either serviceHandlers or operationResolver are mandatory, securityHandlers and prefix are optional. See the examples section for a demo.

Please be aware that this will refer to your serviceHandlers object or your securityHandler object and not to Fastify as explained in the bindings documentation

OpenAPI extensions

The OpenAPI specification supports extending an API spec to describe extra functionality that isn't covered by the official specification. Extensions start with x- (e.g., x-myapp-logo) and can contain a primitive, an array, an object, or null.

The following extensions are provided by the plugin:

  • x-fastify-config (object): any properties will be added to the routeOptions.config property of the Fastify route.

    For example, if you wanted to use the fastify-raw-body plugin to compute a checksum of the request body, you could add the following extension to your OpenAPI spec to signal the plugin to specially handle this route:

          operationId: processWebhook
            rawBody: true
              description: Webhook processed successfully
  • x-no-fastify-config (true): this will ignore this specific route as if it was not present in your OpenAPI specification:

        operationId: processWebhook
        x-no-fastify-config: true
            description: Webhook processed successfully

You can also set custom OpenAPI extensions (e.g., x-myapp-foo) for use within your app's implementation. These properties are passed through unmodified to the Fastify route on {req,reply}.routeOptions.config. Extensions specified on a schema are also accessible (e.g., routeOptions.schema.body or routeOptions.schema.responses[<statusCode>]).


To make life even more easy there is the openapi-glue cli. The openapi-glue cli takes a valid OpenApi (v2/v3) file (JSON or YAML) and generates a project including a fastify plugin that you can use on any fastify server, a stub of the serviceHandlers class and a skeleton of a test harness to test the plugin.


  openapi-glue [options] <OpenApi specification>

or if you don't have openapi-glue installed:

  npx github:seriousme/fastify-openapi-glue <OpenApi specification>

This will generate a project based on the provided OpenApi specification. Any existing files in the project folder will be overwritten! See the generator examples section for a demo.


  -p <name>                   The name of the project to generate
  --projectName=<name>        [default: generated-javascript-project]

  -b <dir> --baseDir=<dir>    Directory to generate the project in.
                              This directory must already exist.
                              [default: "."]

The following options are only usefull for testing the openapi-glue plugin:
  -c --checksumOnly           Don't generate the project on disk but
                              return checksums only.
  -l --localPlugin            Use a local path to the plugin.

See the generator example section for a demo.


Clone this repository and run npm i


Executing npm start will start fastify on localhost port 3000 with the routes extracted from the petstore example and the accompanying serviceHandlers definition

  "statusCode": 400,
  "error": "Bad Request",
  "message": "params.petId should be integer"
  "statusCode": 500,
  "error": "Internal Server Error",
  "message": "Operation findPetsByStatus not implemented"
  "statusCode": 500,
  "error": "Internal Server Error",
  "message":"\"name\" is required!"

as the pet returned by service.js does not match the response schema.


The folder examples/generated-javascript-project contains the result of running openapi-glue -l --baseDir=examples examples/petstore/petstore-swagger.v2.yaml. The generated code can be started using npm start in examples/generated-javascript-project (you will need to run npm i in the generated folder first)


  • the plugin ignores information in a v3 specification under server/url as there could be multiple values here, use the prefix option if you need to prefix your routes. See the servers documentation for more details.
  • fastify only supports application/json and text/plain out of the box. The default charset is utf-8. If you need to support different content types, you can use the fastify addContentTypeParser API.
  • fastify will by default coerce types, e.g when you expect a number a string like "1" will also pass validation, this can be reconfigured, see Validation and Serialization.
  • fastify only supports one schema per route. So while the v3 standard allows for multiple content types per route, each with their own schema this is currently not going to work with fastify. Potential workarounds include a custom content type parser and merging schemas upfront using JSON schema oneOf.
  • the plugin aims to follow fastify and does not compensate for features that are possible according to the OpenApi specification but not possible in standard fastify (without plugins). This will keep the plugin lightweigth and maintainable. E.g. Fastify does not support cookie validation, while OpenApi v3 does.
  • if you have special needs on querystring handling (e.g. arrays, objects etc) then fastify supports a custom querystring parser. You might need to pass the AJV option coerceTypes: 'array' as an option to Fastify.
  • the plugin is an ECMAscript Module (aka ESM). If you are using Typescript then make sure that you have read: to avoid any confusion.
  • If you want to use a specification that consists of multiple files then please check out the page on subschemas
  • Fastify uses AJV strict mode in validating schemas. If you get an error like ....due to error strict mode: unknown keyword: "..." then please check out the page on AJV strict mode


  • contributions are always welcome.
  • if you plan on submitting new features then please create an issue first to discuss and avoid disappointments.
  • main development is done on the master branch therefore PRs to that branch are preferred.
  • please make sure you have run npm test before you submit a PR.


Fastify-openapi-glue is the successor to the now deprecated fastify-swaggergen project. Main difference is that it:

  • aims to support OpenApi and not just Swagger V2 (hence the name change)
  • does not include fastify-swagger support anymore. If you need to show the swagger UI you can include it yourself. Removing the swagger UI clears up a number of dependencies.


Licensed under MIT

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