name	hono-dev
version	1.0.0
description	Type-safe API development with Hono framework. Covers route chaining for type inference, factory patterns, Zod OpenAPI integration, error handling, RPC client, and testing patterns. Use when building APIs with Hono, using hc client, or when Hono, RPC, OpenAPI, or --hono flag mentioned.

Hono API Development

Route chaining → type-safe RPC → end-to-end types.

Building REST APIs with Hono
Type-safe RPC with hono/client
OpenAPI documentation with Zod
Testing APIs with testClient
When user mentions "Hono", "RPC", "OpenAPI", or uses --hono flag

NOT for: Bun runtime APIs (use bun-dev), other frameworks (Express, Fastify)

Hono v4+ with @hono/zod-openapi v1.0+ Check hono.dev for latest patterns.

Route Chaining — Critical Pattern

Type inference flows through method chain. Break chain = lose types.

// ✅ Chained routes preserve types
const app = new Hono()
  .get('/users', (c) => c.json({ users: [] }))
  .get('/users/:id', (c) => {
    const id = c.req.param('id'); // Typed!
    return c.json({ id });
  })
  .post('/users', async (c) => {
    const body = await c.req.json();
    return c.json({ created: true }, 201);
  });

export type AppType = typeof app; // Full route types!

❌ NEVER break the chain:

const app = new Hono();
app.get('/users', handler1);  // Types LOST!
app.post('/users', handler2);

Path parameters — typed automatically:

.get('/posts/:id/comments/:commentId', (c) => {
  const { id, commentId } = c.req.param(); // Both string
  return c.json({ postId: id, commentId });
})

Query parameters — use Zod for validation:

import { zValidator } from '@hono/zod-validator';
import { z } from 'zod';

const QuerySchema = z.object({
  page: z.coerce.number().int().positive().default(1),
  limit: z.coerce.number().int().positive().max(100).default(20),
});

const app = new Hono()
  .get('/search', zValidator('query', QuerySchema), (c) => {
    const { page, limit } = c.req.valid('query'); // Fully typed!
    return c.json({ page, limit });
  });

Middleware in chain:

const app = new Hono()
  .use('*', logger())
  .use('/api/*', cors())
  .get('/api/public', (c) => c.json({ public: true }))
  .use('/api/admin/*', authMiddleware)
  .get('/api/admin/users', (c) => c.json({ users: [] }));

Factory Pattern — Context Typing

Use createFactory<Env>() to type context variables across middleware and routes.

import { createFactory } from 'hono/factory';
import type { Database } from 'bun:sqlite';

type Env = {
  Variables: {
    user: { id: string; role: 'admin' | 'user' };
    requestId: string;
    db: Database;
  };
};

const factory = createFactory<Env>();

// Typed middleware
const authMiddleware = factory.createMiddleware(async (c, next) => {
  const token = c.req.header('authorization')?.replace('Bearer ', '');
  if (!token) throw new HTTPException(401, { message: 'Unauthorized' });

  const user = await verifyToken(token);
  c.set('user', user); // Type-checked!
  await next();
});

// Typed handlers
const getProfile = factory.createHandlers((c) => {
  const user = c.get('user'); // Typed: { id: string; role: 'admin' | 'user' }
  return c.json({ user });
});

// Assemble app
const app = factory.createApp()
  .use('*', dbMiddleware)
  .use('/api/*', authMiddleware)
  .get('/api/profile', ...getProfile);

export type AppType = typeof app;

Multi-module structure:

// routes/users.ts
export const usersRoute = factory.createApp()
  .get('/', (c) => c.json({ users: [] }))
  .post('/', zValidator('json', CreateUserSchema), async (c) => {
    const data = c.req.valid('json');
    return c.json({ created: true }, 201);
  });

// index.ts
const app = factory.createApp()
  .use('*', dbMiddleware)
  .route('/users', usersRoute)
  .route('/posts', postsRoute);

See factory-pattern.md for advanced patterns.

Error Handling

import { HTTPException } from 'hono/http-exception';

// Throw typed errors
app.get('/users/:id', async (c) => {
  const user = await findUser(c.req.param('id'));
  if (!user) {
    throw new HTTPException(404, { message: 'User not found' });
  }
  return c.json({ user });
});

// Custom error classes
class NotFoundError extends HTTPException {
  constructor(resource: string) {
    super(404, { message: `${resource} not found` });
  }
}

class UnauthorizedError extends HTTPException {
  constructor(message = 'Unauthorized') {
    super(401, { message });
  }
}

// Centralized handler
app.onError((err, c) => {
  if (err instanceof HTTPException) {
    return c.json({ error: err.message }, err.status);
  }
  if (err instanceof ZodError) {
    return c.json({
      error: 'Validation failed',
      issues: err.issues.map(i => ({ path: i.path.join('.'), message: i.message }))
    }, 400);
  }
  const isDev = Bun.env.NODE_ENV !== 'production';
  return c.json({ error: isDev ? err.message : 'Internal server error' }, 500);
});

app.notFound((c) => c.json({ error: 'Not found', path: c.req.path }, 404));

See error-handling.md for patterns.

Zod OpenAPI

import { createRoute, OpenAPIHono, z } from '@hono/zod-openapi';
import { swaggerUI } from '@hono/swagger-ui';

const UserSchema = z.object({
  id: z.string().uuid(),
  email: z.string().email(),
  name: z.string().min(1).max(100),
}).openapi('User');

const route = createRoute({
  method: 'get',
  path: '/users/{id}',
  request: {
    params: z.object({ id: z.string().uuid() }),
  },
  responses: {
    200: {
      content: { 'application/json': { schema: UserSchema } },
      description: 'User found',
    },
    404: {
      content: { 'application/json': { schema: z.object({ error: z.string() }) } },
      description: 'User not found',
    },
  },
  tags: ['Users'],
  summary: 'Get user by ID',
});

const app = new OpenAPIHono();

app.openapi(route, (c) => {
  const { id } = c.req.valid('param'); // Typed!
  const user = db.query('SELECT * FROM users WHERE id = ?').get(id);
  if (!user) return c.json({ error: 'User not found' }, 404);
  return c.json(user, 200);
});

// Swagger UI
app.get('/docs', swaggerUI({ url: '/openapi.json' }));
app.doc('/openapi.json', {
  openapi: '3.1.0',
  info: { title: 'API', version: '1.0.0' },
});

See zod-openapi.md for complete patterns.

RPC Client — End-to-End Types

// Server
const app = new Hono()
  .get('/posts', (c) => c.json({ posts: [] }))
  .get('/posts/:id', (c) => c.json({ id: c.req.param('id') }))
  .post('/posts', zValidator('json', CreatePostSchema), async (c) => {
    const data = c.req.valid('json');
    return c.json({ id: '123', ...data }, 201);
  });

export type AppType = typeof app;

// Client
import { hc } from 'hono/client';
import type { AppType } from './server';

const client = hc<AppType>('http://localhost:3000');

// GET request
const res = await client.posts.$get();
const data = await res.json(); // Typed: { posts: any[] }

// GET with params
const res2 = await client.posts[':id'].$get({ param: { id: '123' } });

// POST request
const res3 = await client.posts.$post({
  json: { title: 'Hello', content: 'World' }
});

// With headers
const res4 = await client.posts.$get({}, {
  headers: { Authorization: 'Bearer token' }
});

Testing with testClient

import { describe, expect, test, beforeEach, afterEach } from 'bun:test';
import { testClient } from 'hono/testing';
import { Database } from 'bun:sqlite';
import app from './server';

describe('API Tests', () => {
  let db: Database;

  beforeEach(() => {
    db = new Database(':memory:');
    db.run('CREATE TABLE posts (id TEXT PRIMARY KEY, title TEXT, content TEXT)');
  });

  afterEach(() => {
    db.close();
  });

  const client = testClient(app);

  test('GET /posts returns posts', async () => {
    const res = await client.posts.$get();
    expect(res.status).toBe(200);
    const data = await res.json();
    expect(data).toHaveProperty('posts');
  });

  test('POST /posts creates post', async () => {
    const res = await client.posts.$post({
      json: { title: 'Test', content: 'Content' }
    });
    expect(res.status).toBe(201);
    const data = await res.json();
    expect(data).toMatchObject({ title: 'Test' });
  });

  test('Protected route requires auth', async () => {
    const res = await client.api.profile.$get();
    expect(res.status).toBe(401);
  });

  test('Protected route accepts valid token', async () => {
    const res = await client.api.profile.$get({}, {
      headers: { Authorization: 'Bearer valid-token' }
    });
    expect(res.status).toBe(200);
  });
});

See testing-patterns.md for complete patterns.

Middleware Patterns

// Logging
import { logger } from 'hono/logger';
app.use('*', logger());

// CORS
import { cors } from 'hono/cors';
app.use('/api/*', cors({
  origin: ['http://localhost:3000'],
  credentials: true,
}));

// Rate limiting
const rateLimiter = factory.createMiddleware(async (c, next) => {
  const ip = c.req.header('x-forwarded-for') || 'unknown';
  const key = `rate:${ip}`;
  const count = await cache.incr(key);

  if (count === 1) await cache.expire(key, 60);
  if (count > 100) {
    throw new HTTPException(429, { message: 'Rate limit exceeded' });
  }

  await next();
});

// Request ID
const requestId = factory.createMiddleware(async (c, next) => {
  c.set('requestId', crypto.randomUUID());
  await next();
  c.res.headers.set('x-request-id', c.get('requestId'));
});

Rules

ALWAYS:

Chain routes for type inference → .get().post().put()
Export type AppType = typeof app for RPC client
Use createFactory<Env>() for typed context variables
Validate with Zod schemas via zValidator
Handle errors with HTTPException and centralized onError
Test with testClient for type safety

NEVER:

Break method chain with variable assignment between routes
Use any types — let Hono infer or define explicitly
Use JSON.parse(await c.req.text()) — use c.req.json() or Zod validator
Skip request validation on user input
Expose stack traces in production

When Type Errors Occur:

Check route chaining not broken
Verify export type AppType matches actual app
Ensure middleware uses createFactory for context types
Check client using correct param, query, or json keys

References

Examples:

typed-routes.md — Complete route chaining examples
testing-patterns.md — Testing with testClient

References:

factory-pattern.md — Context typing with createFactory
zod-openapi.md — OpenAPI integration patterns
error-handling.md — HTTPException and error patterns
middleware.md — Auth, logging, CORS patterns

External:

Hono: https://hono.dev
@hono/zod-openapi: https://github.com/honojs/middleware/tree/main/packages/zod-openapi

hono-dev

Install Skill

SKILL.md