hono-fieldguide

name	hono-fieldguide
description	This skill should be used when building APIs with Hono, using hc client, implementing OpenAPI, or when Hono, RPC, or type-safe API are mentioned.
metadata	{"version":"1.0.1"}

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", or "OpenAPI"

NOT for: Bun runtime APIs (use bun-fieldguide), 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.( {
   user = c.(); 
   c.({ user });
});


 app = factory
  .()
  .(, dbMiddleware)
  .(, authMiddleware)
  .(, ...getProfile);

   =  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  ) {
     c.({ : err. }, err.);
  }
   (err  ) {
     c.(
      {
        : ,
        : err..( ({
          : i..(),
          : i.,
        })),
      },
      
    );
  }
   isDev = .. !== ;
   c.({ : isDev ? err. :  }, );
});

app.( c.({ : , : c.. }, ));

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() }) },
      },
      : ,
    },
  },
  : [],
  : ,
});

 app =  ();

app.(route,  {
   { id } = c..(); 
   user = db.().(id);
   (!user)  c.({ :  }, );
   c.(user, );
});


app.(, ({ :  }));
app.(, {
  : ,
  : { : , :  },
});

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..$get();
 data =  res.(); 


 res2 =  client.[].$get({ : { :  } });


 res3 =  client..$post({
  : { : , :  },
});


 res4 =  client..$get(
  {},
  {
    : { :  },
  }
);

</rpc_client>

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",  () => {
     res =  client..$post({
      : { : , :  },
    });
    (res.).();
     data =  res.();
    (data).({ :  });
  });

  (,  () => {
     res =  client...$get();
    (res.).();
  });

  (,  () => {
     res =  client...$get(
      {},
      {
        : { :  },
      }
    );
    (res.).();
  });
});

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( (c, next) => {
  c.(, crypto.());
   ();
  c...(, c.());
});

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