Hono API Development

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

<when_to_use>

• 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) </when_to_use>

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

Route Chaining — Critical Pattern

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

<route_chaining>

// ✅ 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: [] }));

</route_chaining>

Factory Pattern — Context Typing

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

<factory_pattern>

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. </factory_pattern>

Error Handling

<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. </error_handling>

Zod OpenAPI

<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. </zod_openapi>

RPC Client — End-to-End Types

<rpc_client>

// 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' }
});

</rpc_client>

Testing with testClient

<testing> ```typescript 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](examples/testing-patterns.md) for complete patterns.
</testing>

## Middleware Patterns

<middleware>
```typescript
// 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'));
});

</middleware> <rules> ## 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 </rules>

<references> ## 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 </references>