Hono Fieldguide

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", or "OpenAPI"

NOT for: Bun runtime APIs (use bun-fieldguide), 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