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Spot ("Single Point Of Truth") is a concise, developer-friendly way to describe your API contract.

Leveraging the TypeScript syntax, it lets you describe your API and generate other API contract formats you need (OpenAPI, Swagger, JSON Schema).

You don't need to use TypeScript in your codebase to benefit from using Spot.

Example of an API definition file api.ts which defines a single POST endpoint to create a user:

import { api, endpoint, request, response, body } from "@airtasker/spot";

  name: "My API"
class Api {}

  method: "POST",
  path: "/users"
class CreateUser {
  request(@body body: CreateUserRequest) {}

  @response({ status: 201 })
  response(@body body: CreateUserResponse) {}

interface CreateUserRequest {
  firstName: string;
  lastName: string;

interface CreateUserResponse {
  firstName: string;
  lastName: string;
  role: string;

Getting Started

Get started with writing Spot contracts - Spot Guide

For all available syntax, see Spot Syntax


With yarn installed and initialized add @airtasker/spot to your project:

yarn add @airtasker/spot

You can pass the definition above to a generator by simply running:

npx @airtasker/spot generate --contract api.ts

Why we built Spot

At first glance, you may wonder why we bothered building Spot. Why not use OpenAPI (formely known as Swagger) to describe your API?

At the core, we built Spot because we wanted a better developer experience.

Writing contracts

OpenAPI documents are stored as YAML files, following a very specific schema. You won’t know that you used the wrong field name or forgot to wrap a type definition into a schema object unless you run a good OpenAPI linter. Most developers who aren’t intimately familiar with the OpenAPI specification end up using a visual editor such as Swagger Editor or Stoplight.

Since Spot leverages the TypeScript syntax, all you need is to write valid TypeScript code. Your editor will immediately tell you when your code is invalid. It will tell you what’s missing, and you even get autocomplete for free. We could have picked any other typed language—TypeScript just happened to be one of the most concise and ubiquitous for us.

Reviewing contracts

We believe that API contracts should be checked into Git, or whichever code versioning system you use. In addition, API contracts should be systematically peer reviewed. It’s far too easy for a backend engineer to incorrectly assume what client engineers expect from an endpoint.

Because of their complex nested structure and the richness of the OpenAPI specification, OpenAPI documents can be difficult to review in a pull request. They’re great for machines, but not always for humans.

Spot aims to be as human-readable as possible. We’ve seen developers become a lot more engaged in discussions on pull requests for Spot contracts, compared to our previous OpenAPI documents.

Interoperability with various formats

Depending on what you're trying to achieve (testing, documentation, client code generation…), you'll find tools that only work with OpenAPI 2 (Swagger), and newer tools that only support OpenAPI 3. You may also find tools for a different API ecosystem such as JSON Schema or API Blueprint.

We built Spot with this in mind. Instead of having to juggle various API format converters, Spot can generate every major API document format. This is why we called it "Single Point Of Truth".

oclif Version CircleCI Downloads/week License


To get started and set up an API declaration in the current directory, run:

npx @airtasker/spot init

You can then run a generator with:

npx @airtasker/spot generate --contract api.ts

In Memory Usage

import { Spot } from "@airtasker/spot";

const contract = Spot.parseContract("./api.ts")
const openApi = Spot.OpenApi3.generateOpenAPI3(contract);


  openapi: '3.0.2',
  info: { title: 'my-api', description: undefined, version: '0.0.0' },
  paths: { '/users': { post: [Object] } },
  components: {
    schemas: { CreateUserRequest: [Object], CreateUserResponse: [Object] },
    securitySchemes: undefined
  security: undefined


spot checksum SPOT_CONTRACT

Generate a checksum for a Spot contract

  $ spot checksum SPOT_CONTRACT

  SPOT_CONTRACT  path to Spot contract

  -h, --help  show CLI help

  $ spot checksum api.ts

See code: build/cli/src/commands/checksum.js


Preview Spot contract as OpenAPI3 documentation. The documentation server will start on http://localhost:8080.

  $ spot docs SPOT_CONTRACT

  SPOT_CONTRACT  path to Spot contract

  -h, --help       show CLI help
  -p, --port=port  [default: 8080] Documentation server port

  $ spot docs api.ts

See code: build/cli/src/commands/docs.js

spot generate

Runs a generator on an API. Used to produce client libraries, server boilerplates and well-known API contract formats such as OpenAPI.

  $ spot generate

  -c, --contract=contract    (required) Path to a TypeScript Contract definition
  -g, --generator=generator  Generator to run
  -h, --help                 show CLI help
  -l, --language=language    Language to generate
  -o, --out=out              Directory in which to output generated files

  $ spot generate --contract api.ts --language yaml --generator openapi3 --out output/

See code: build/cli/src/commands/generate.js

spot help [COMMAND]

display help for spot

  $ spot help [COMMAND]

  COMMAND  command to show help for

  --all  see all commands in CLI

See code: @oclif/plugin-help

spot init

Generates the boilerplate for an API.

  $ spot init

  -h, --help  show CLI help

  $ spot init
  Generated the following files:
  - api.ts
  - tsconfig.json
  - package.json

See code: build/cli/src/commands/init.js


Lint a Spot contract

  $ spot lint SPOT_CONTRACT

  SPOT_CONTRACT  path to Spot contract

  -h, --help                                                     show CLI help
  --has-discriminator=(error|warn|off)                           Setting for has-discriminator
  --has-request-payload=(error|warn|off)                         Setting for has-request-payload
  --has-response=(error|warn|off)                                Setting for has-response
  --has-response-payload=(error|warn|off)                        Setting for has-response-payload
  --no-inline-objects-within-unions=(error|warn|off)             Setting for no-inline-objects-within-unions
  --no-nullable-arrays=(error|warn|off)                          Setting for no-nullable-arrays
  --no-nullable-fields-within-request-bodies=(error|warn|off)    Setting for no-nullable-fields-within-request-bodies
  --no-omittable-fields-within-response-bodies=(error|warn|off)  Setting for no-omittable-fields-within-response-bodies
  --no-trailing-forward-slash=(error|warn|off)                   Setting for no-trailing-forward-slash

  $ spot lint api.ts
  $ spot lint --has-descriminator=error
  $ spot lint --no-nullable-arrays=off

See code: build/cli/src/commands/lint.js


Run a mock server based on a Spot contract

  $ spot mock SPOT_CONTRACT

  SPOT_CONTRACT  path to Spot contract

  -h, --help                                   show CLI help
  -p, --port=port                              (required) [default: 3010] Port on which to run the mock server
  --pathPrefix=pathPrefix                      Prefix to prepend to each endpoint path

  --proxyBaseUrl=proxyBaseUrl                  If set, the server will act as a proxy and fetch data from the given
                                               remote server instead of mocking it

  --proxyFallbackBaseUrl=proxyFallbackBaseUrl  Like proxyBaseUrl, except used when the requested API does not match
                                               defined SPOT contract. If unset, 404 will always be returned.

  --proxyMockBaseUrl=proxyMockBaseUrl          Like proxyBaseUrl, except used to proxy draft endpoints instead of
                                               returning mocked responses.

  $ spot mock api.ts

See code: build/cli/src/commands/mock.js

spot validate SPOT_CONTRACT

Validate a Spot contract

  $ spot validate SPOT_CONTRACT

  SPOT_CONTRACT  path to Spot contract

  -h, --help  show CLI help

  $ spot validate api.ts

See code: build/cli/src/commands/validate.js

spot validation-server SPOT_CONTRACT

Start the spot contract validation server

  $ spot validation-server SPOT_CONTRACT

  SPOT_CONTRACT  path to Spot contract

  -h, --help       show CLI help
  -p, --port=port  [default: 5907] The port where application will be available

  $ spot validation-server api.ts

See code: build/cli/src/commands/validation-server.js