OpenAPI

Installation

@ts-rest/open-api is a library that allows you to generate OpenAPI documents from your ts-rest contracts, it's a great way to expose your API to non-ts-rest clients - for example a public API or a client library.

pnpm add @ts-rest/open-api

bun add @ts-rest/open-api

npm install @ts-rest/open-api

Generating an OpenAPI Document

import { contract } from './contract';
import { generateOpenApi, SchemaTransformerAsync } from '@ts-rest/open-api';

const openApiDocument = generateOpenApi(
  contract,
  {
    info: {
      title: 'Posts API',
      version: '1.0.0',
    },
  },
  {
    schemaTransformer: MY_TRANSFORMER, 
    // Bring your own transformer here
  },
);

That's it! Once you've picked your transformer, you can serve this document however you want, use it for CodeGen for your non-TS clients, or use it to generate a Swagger UI!

Schema Transformers

ts-rest supports multiple schema validation libraries through schema transformers. These transformers convert your schema definitions into OpenAPI-compatible JSON schemas.

There are two flavors of transformers:

Sync transformers (SchemaTransformerSync) - for libraries that can transform schemas synchronously
Async transformers (SchemaTransformerAsync) - for libraries that require asynchronous transformation

For backwards compatibility, ts-rest currently defaults to a built-in Zod 3 transformer (ZOD_3_SCHEMA_TRANSFORMER). However, this built-in support will be removed in v4, and you should start providing your own transformer.

Zod 3

Transformer Implementation

import { SchemaTransformer } from './types';
import { generateSchema } from '@anatine/zod-openapi';
import { z } from 'zod';

export const ZOD_3_SYNC: SchemaTransformerSync = ({ schema }) => {
  if (schema instanceof z.ZodAny) {
    return generateSchema(schema);
  }
  return null;
};

Adding Metadata to Contracts

For Zod 3, you can extend your schemas with additional OpenAPI information using @anatine/zod-openapi. This allows you to add title, description, and example fields to improve the quality and clarity of the generated documentation.

First, extend Zod with extendZodWithOpenApi(z) when defining the schemas for your contract:

import { initContract } from '@ts-rest/core';
import { z } from 'zod';
import { extendZodWithOpenApi } from '@anatine/zod-openapi';

extendZodWithOpenApi(z);

const c = initContract();

export const contract = c.router({
  getUser: {
    method: 'GET',
    path: '/users/:id',
    pathParams: z.object({
      id: z.string().openapi({
        description: "The user's ID",
      }),
    }),
    responses: {
      200: z.object({
        id: z.string().uuid().openapi({
          title: 'Unique ID',
          description: 'A UUID generated by the server',
        }),
        name: z.string(),
        phoneNumber: z.string().min(10).openapi({
          description: 'US phone numbers only',
          example: '555-555-5555',
        }),
      }),
    },
  },
});

See the official @anatine/zod-openapi docs for more information.

All Zod schemas defined in your contract can benefit from the additional OpenAPI schema, including pathParams, queryParams, and responses. These keys are all used when generating the OpenAPI JSON file. This could improve the quality of your generated documentation.

Adding Examples to the Media Type

In order to add examples to the media type rather than to the schema itself (this is useful if you want to show multiple examples), we have added a mediaExamples property to the .openapi() method options. This will only work for the schemas of the body, responses and individual query parameters if you are using jsonQuery option.

You can see an example of its usage in the code snippet above.

Zod 4

Transformer Implementation

import { z } from 'zod/v4';
import { SchemaTransformerAsync } from './types';
import convert from '@openapi-contrib/json-schema-to-openapi-schema';

export const ZOD_4_ASYNC: SchemaTransformerAsync = async ({ schema }) => {
  if (schema instanceof z.core.$ZodAny) {
    const jsonSchema = z.toJSONSchema(schema);

    return await convert(jsonSchema);
  }

  return null;
};

Valibot

Transformer Implementation

import { isStandardSchema } from '@ts-rest/core';
import type { SchemaTransformerAsync } from '@ts-rest/open-api';
import { toJsonSchema } from '@valibot/to-json-schema';
import { convert } from '@openapi-contrib/json-schema-to-openapi-schema';

export const VALIBOT_ASYNC: SchemaTransformerAsync = async ({ schema }) => {
  if (isStandardSchema(schema) && schema['~standard'].vendor === 'valibot') {
    const jsonSchema = toJsonSchema(schema as any, { errorMode: 'ignore' });
    return await convert(jsonSchema);
  }

  return null;
};

Others

You can create transformers for any schema validation library by implementing either SchemaTransformerSync or SchemaTransformerAsync:

import type {
  SchemaTransformerSync,
  SchemaTransformerAsync,
} from '@ts-rest/open-api';

// Sync transformer example
const MY_SYNC_TRANSFORMER: SchemaTransformerSync = ({ schema }) => {
  // Check if this is your schema type
  if (isMySchemaType(schema)) {
    // Transform to OpenAPI schema
    return transform(schema);
  }

  // Return null if not your schema type
  return null;
};

// Async transformer example
const MY_ASYNC_TRANSFORMER: SchemaTransformerAsync = async ({ schema }) => {
  // Check if this is your schema type
  if (isMySchemaType(schema)) {
    // Transform to OpenAPI schema (async)
    return await transform(schema);
  }

  // Return null if not your schema type
  return null;
};

The transformer receives a schema parameter and should:

Check if the schema belongs to your validation library
If yes, transform it to an OpenAPI-compatible schema object
If no, return null

Extending Operations with Additional OpenAPI Fields

We do not provide first-party support to set all possible OpenAPI fields on the operations such as the security field. In addition, you may have some specific needs to modify the fields already set by ts-rest such as the tags field.

Therefore, we have provided an operationMapper option to allow you to modify the OpenAPI fields of the operations. This is a callback function, that will receive the operation object, the contract endpoint, and the operation ID, and must return a valid OpenAPI operation object. A common way to provide data to this function is to utilize the metadata field of the contract endpoint. However, feel free to come up with a different solution to doing this if you would not like to include this data in your contracts.

const hasCustomTags = (
  metadata: unknown,
): metadata is { openApiTags: string[] } => {
  return (
    !!metadata && typeof metadata === 'object' && 'openApiTags' in metadata
  );
};

const hasSecurity = (
  metadata: unknown,
): metadata is { openApiSecurity: SecurityRequirementObject[] } => {
  return (
    !!metadata && typeof metadata === 'object' && 'openApiSecurity' in metadata
  );
};

const apiDoc = generateOpenApi(
  router,
  {
    info: { title: 'Blog API', version: '0.1' },
    components: {
      securitySchemes: {
        BasicAuth: {
          type: 'http',
          scheme: 'basic',
        },
      },
    },
  },
  {
    operationMapper: (operation, appRoute, id) => ({
      ...operation,
      ...(hasCustomTags(appRoute.metadata)
        ? {
            tags: appRoute.metadata.openApiTags,
          }
        : {}),
      ...(hasSecurity(appRoute.metadata)
        ? {
            security: appRoute.metadata.openApiSecurity,
          }
        : {}),
      // You can also use the operation ID for custom logic
      description: `${operation.description || ''} (Operation: ${id})`,
    }),
  },
);

Enabling `operationId`s (Recommended!)

You can set setOperationId to either true or concatenated-path to set operationIds on your endpoints.

In the case of setting it to true, it will use only the endpoint name from your contract. You have to ensure that the endpoint names are unique across the entire contract.

In the case of setting it to concatenated-path, it will use the endpoint name concatenated with the path through the nested contract. This is useful when you have multiple endpoints with the same name but different paths. This will result in longer but more descriptive operationIds.

const openApiSchema = generateOpenApi(
  postsApi,
  {
    info: {
      title: 'Posts API',
      version: '1.0.0',
    },
  },
  {
    setOperationId: true, 
    // setOperationId: 'concatenated-path',
  },
);

Below is an example of what the OpenAPI document would look like with operationId's enabled:

{
  "openapi": "3.0.2",
  "paths": {
    "/posts": {
      "get": {
        "description": "Get all posts",
        "tags": [],
        "parameters": [
          {
            "name": "userId",
            "in": "query",
            "schema": {
              "type": "number"
            }
          }
        ],
        "operationId": "getPosts", 
        // or if using concatenated-path
        "operationId": "posts.getPosts", 
        "responses": {

JSON Query Params

If you've enabled JSON Query params for your server and client, you can enable jsonQuery to mark the query params as application/json in the OpenAPI document:

const openApiSchema = generateOpenApi(
  postsApi,
  {
    info: {
      title: 'Posts API',
      version: '1.0.0',
    },
  },
  {
    jsonQuery: true, 
  },
);

You'll want to do this to let your non ts-rest clients know that they should send the query params as JSON.

OpenAPI

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