hono-zod-openapi

Alternative Hono middleware for creating OpenAPI documentation from Zod schemas

Installation

npm install hono-zod-openapi hono zod

Or, if you prefer JSR:

jsr add @paolostyle/hono-zod-openapi

Why?

Hono provides a 3rd-party middleware in their middleware monorepo, which probably works alright, however my issue with this package is that it forces you to write your code in a different manner than you normally would in Hono. Refactoring the app becomes a significant hassle.

This library provides an openApi middleware instead, which you can easily add to your existing codebase, and a createOpenApiDocument function, which will generate an OpenAPI-compliant JSON document and serve it under /doc route of your app (it's configurable, don't worry).

Features

• Super simple usage - just add a middleware and that's it!
• Ability to generate OpenAPI docs both using simple, almost exclusively Zod schema-based notation, or with regular OpenAPI spec
• Request validation - same functionality as @hono/zod-validator (we're using it as a dependency)

Stability

⚠ Warning: This library is still at the early stages although I would consider the documented API rather stable since version 0.2.0. In any case, please be aware that until I release v1.0.0 there might still be some breaking changes between minor versions.

Usage

This library is based on zod-openapi library (not the same one as the official package).

Extending Zod

zod-openapi provides an extension to Zod, which adds a new .openapi() method. hono-zod-openapi reexports the extendZodWithOpenApi method. Call it ideally somewhere in your entry point (e.g. in a file with your main Hono router).

import { z } from 'zod';
import { extendZodWithOpenApi } from 'hono-zod-openapi';

extendZodWithOpenApi(z);

z.string().openapi({ description: 'hello world!', example: 'hello world' });

This is not strictly necessary, but it allows you to add additional information that cannot be represented by just using Zod, or registering OpenAPI components.

Middleware

hono-zod-openapi provides a middleware which you can attach to any endpoint. It accepts a single argument, an object that is mostly the same as the OpenAPI Operation Object. There are 2 main differences:

• a request field, which functions essentially like a condensed version of @hono/zod-validator. For example, { json: z.object() } is equivalent to zValidator('json', z.object()). Passing multiple attributes to the object is equivalent to calling multiple zValidators. At the same time, it will translate the validation schemas to OpenAPI notation. There is no need to use parameters and requestBody fields at all (but it is still possible).

• enhanced responses field which has essentially 4 variants:

  • Passing a Zod schema directly. For simple APIs this is more than enough. description field will be equal to the full HTTP status code (e.g. 200 OK or 500 Internal Server Error) and media type will be inferred based on the passed schema, though it is pretty simple now - for z.string() it will be text/plain, otherwise it's application/json.

    Example:

    openApi({
      responses: {
        200: z
          .object({ wow: z.string() })
          .openapi({ example: { wow: 'this is cool!' } }),
      },
    });

    This will be equivalent to this OpenAPI spec:

    {
      "responses": {
        "200": {
          "description": "200 OK",
          "content": {
            "application/json": {
              "schema": {
                "example": {
                  "wow": "this is cool!"
                },
                "properties": {
                  "wow": {
                    "type": "string"
                  }
                },
                "required": ["wow"],
                "type": "object"
              }
            }
          }
        }
      }
    }

  • "Library notation" - a simplified, flattened format, similar to the official OpenAPI spec, but reduces annoying nesting. Convenient form if you want a custom description or need to pass extra data.

    Example:

    openApi({
      responses: {
        200: {
          // the only required field! Use .openapi() method on the schema to add metadata
          schema: z.string().openapi({
            description: 'HTML code',
            example: '<html><body>hi!</body></html>',
          }),
          // description is optional, as opposed to OpenAPI spec
          description: 'My description',
          // mediaType is optional, it's `text/plain` if schema is z.string()
          // otherwise it's `application/json`, in other scenarios it should be specified
          mediaType: 'text/html',
          // headers field is also optional, but you can also use Zod schema here
          headers: z.object({ 'x-custom': z.string() }),
          // ...you can also pass all the other fields you normally would here in OpenAPI spec
        },
      },
    });

    This will be equivalent to this OpenAPI spec:

    {
      "responses": {
        "200": {
          "content": {
            "text/html": {
              "schema": {
                "description": "HTML code",
                "example": "<html><body>hi!</body></html>",
                "type": "string"
              }
            }
          },
          "description": "My description",
          "headers": {
            "x-custom": {
              "required": true,
              "schema": {
                "type": "string"
              }
            }
          }
        }
      }
    }

  • zod-openapi notation. Mostly useful when you need to have content in multiple formats, or you just want to be as close as possible to the official spec.

    Example:

    openApi({
      responses: {
        200: {
          // required
          description: 'Success response',
          content: {
            'application/json': {
              schema: z.object({ welcome: z.string() }),
            },
          },
          // ...you can also pass all the other fields you normally would here in OpenAPI spec
        },
      },
    });

    This will be equivalent to this OpenAPI spec:

    {
      "responses": {
        "200": {
          "content": {
            "application/json": {
              "schema": {
                "properties": {
                  "welcome": {
                    "type": "string"
                  }
                },
                "required": ["welcome"],
                "type": "object"
              }
            }
          },
          "description": "Success response"
        }
      }
    }

  • Classic OpenAPI spec notation: just refer to the official spec. Not recommended but it also just works.

Since the object can get pretty large, you can use defineOpenApiOperation function to get the autocomplete in the IDE.

Simple example:

import { Hono } from 'hono';
import { z } from 'zod';
import { createOpenApiDocument, openApi } from 'hono-zod-openapi';

export const app = new Hono().get(
  '/user',
  openApi({
    tags: ['User'],
    responses: {
      200: z.object({ hi: z.string() }).openapi({ example: { hi: 'user' } }),
    },
    request: {
      query: z.object({ id: z.string() }),
    },
  }),
  (c) => {
    // works identically to @hono/zod-validator
    const { id } = c.req.valid('query');
    return c.json({ hi: id }, 200);
  },
);

// this will add a `GET /doc` route to the `app` router
createOpenApiDocument(app, {
  info: {
    title: 'Example API',
    version: '1.0.0',
  },
});

Calling GET /doc will result in this response:

{
  "info": {
    "title": "Example API",
    "version": "1.0.0"
  },
  "openapi": "3.1.0",
  "paths": {
    "/user": {
      "get": {
        "tags": ["User"],
        "parameters": [
          {
            "in": "query",
            "name": "id",
            "required": true,
            "schema": {
              "type": "string"
            }
          }
        ],
        "responses": {
          "200": {
            "content": {
              "application/json": {
                "schema": {
                  "example": {
                    "hi": "user"
                  },
                  "properties": {
                    "hi": {
                      "type": "string"
                    }
                  },
                  "required": ["hi"],
                  "type": "object"
                }
              }
            },
            "description": "200 OK"
          }
        }
      }
    }
  }
}

Recipes

Authentication

Generally you just need to follow one of the Authentication guides here, depending on the type of authentication you're using.

Bearer Auth example:

const app = new Hono().get(
  '/example',
  openApi({
    responses: {
      200: z.object({}),
    },
    security: [{ bearerAuth: [] }],
  }),
  async (c) => {
    return c.json({}, 200);
  },
);

createOpenApiDocument(app, {
  info: {
    title: 'Some API',
    version: '0.0.1',
  },
  components: {
    securitySchemes: {
      bearerAuth: {
        type: 'http',
        scheme: 'bearer',
      },
    },
  },
  // if you use bearer auth in every endpoint, you can add
  // this here instead of adding `security` to every route:
  // security: [{ bearerAuth: [] }],
});

Reusing common fields

Adding the same fields to various routes over an over can be a bit tedious. You can create your own typesafe wrapper, which will provide the fields shared by multiple endpoints. For example, if a lot of your endpoints require a security field and a tag, you can create a function like this:

const taggedAuthRoute = <T extends HonoOpenApiRequestSchemas>(
  doc: HonoOpenApiOperation<T>,
) => {
  return defineOpenApiOperation({
    ...doc,
    tags: ['MyTag'],
    security: [{ apiKey: [] }],
  });
};

and use it with openApi middleware:

openApi(
  taggedAuthRoute({
    request: {
      json: z.object({ field: z.number() }),
    },
  }),
);

API

createOpenApiDocument

function createOpenApiDocument(
  router: Hono,
  document: Omit<ZodOpenApiObject, 'openapi'>,
  { addRoute = true, routeName = '/doc' }: Settings = {},
): ReturnType<typeof createDocument>;

Call this function after you defined your Hono app to generate the OpenAPI document and host it under /doc route by default. info field in the second argument is required by the OpenAPI specification. You can pass there also any other fields available in the OpenAPI specification, e.g. servers, security or components.

Examples:

• typical usage:

  createOpenApiDocument(app, {
    info: {
      title: 'Example API',
      version: '1.0.0',
    },
  });

• add the route under /openApi route:

  createOpenApiDocument(
    app,
    {
      info: {
        title: 'Example API',
        version: '1.0.0',
      },
    },
    { routeName: '/openApi' },
  );

• don't add the route, just get the OpenAPI document as an object

  const openApiDoc = createOpenApiDocument(
    app,
    {
      info: {
        title: 'Example API',
        version: '1.0.0',
      },
    },
    { addRoute: false },
  );

openApi

function openApi<Req extends RequestSchemas, E extends Env, P extends string>(
  operation: Operation<Req>,
): MiddlewareHandler<E, P, Values<Req>>;

A Hono middleware used to document a given endpoint. Refer to the Middleware section above to see the usage examples.

defineOpenApiOperation

A no-op function, used to ensure proper validator's type inference and provide autocomplete in cases where you don't want to define the spec inline.

Example:

const operation = defineOpenApiOperation({
  responses: {
    200: z.object({ name: z.string() }),
  },
  request: {
    json: z.object({ email: z.string() }),
  },
});

const app = new Hono().post('/user', openApi(operation), async (c) => {
  const { name } = c.req.valid('json');

  return c.json({ name }, 200);
});

This will result in this JSON document:

{
  "info": {
    "title": "Example API",
    "version": "1.0.0"
  },
  "openapi": "3.1.0",
  "paths": {
    "/user": {
      "get": {
        "parameters": [
          {
            "in": "cookie",
            "name": "session",
            "required": true,
            "schema": {
              "type": "string"
            }
          }
        ],
        "requestBody": {
          "content": {
            "application/json": {
              "schema": {
                "properties": {
                  "email": {
                    "type": "string"
                  }
                },
                "required": ["email"],
                "type": "object"
              }
            }
          }
        },
        "responses": {
          "200": {
            "content": {
              "application/json": {
                "schema": {
                  "properties": {
                    "name": {
                      "type": "string"
                    }
                  },
                  "required": ["name"],
                  "type": "object"
                }
              }
            },
            "description": "200 OK"
          },
          "400": {
            "content": {
              "application/xml": {
                "schema": {
                  "properties": {
                    "message": {
                      "type": "string"
                    }
                  },
                  "required": ["message"],
                  "type": "object"
                }
              }
            },
            "description": "Custom description"
          },
          "401": {
            "content": {
              "application/json": {
                "schema": {
                  "properties": {
                    "message": {
                      "type": "string"
                    }
                  },
                  "required": ["message"],
                  "type": "object"
                }
              }
            },
            "description": "Required description"
          },
          "404": {
            "content": {
              "application/json": {
                "schema": {
                  "properties": {
                    "message": {
                      "type": "string"
                    }
                  },
                  "required": ["message"],
                  "type": "object"
                }
              }
            },
            "description": "Not found"
          }
        },
        "tags": ["User"]
      }
    }
  }
}

Runtime compatibility

While this package should work in Bun, Deno, Cloudflare Workers and browsers (as I'm not using any platform specific APIs and I do not plan to), the codebase is currently tested against Node.js 18.x, 20.x and 22.x. I haven't found any tools that would help with cross-platform testing that wouldn't incur significant maintenance burden.

For now I managed to successfully run the tests with Bun test runner with some grepping and used the lib in Cloudflare Workers and everything seemed to work fine. If you are using the library in non-Node runtime and encountered some bugs, please consider creating an issue.