I turned my Hono API into an MCP server by changing one import

Developer Serik Shaikamalov released hono-mcp-bridge, an open-source library that turns an existing Hono API into a Model Context Protocol server by swapping the zValidator import and calling mountMcp(app). The bridge reads each route's Zod validation schemas, converts them to JSON Schema with Zod 4's z.toJSONSchema, and exposes every endpoint as an MCP tool that routes through the app's normal validation, auth, logging and rate limiting. It supports include/exclude/method filtering and per-route hiding, and currently requires Zod v4 with no SSE support.

I have a travel-agency backend on Hono https://hono.dev : tours, bookings, payments. I wanted AI agents to use it too, so that a manager could tell Claude "find March tours under $2000 and book the first one for Ali Aliyev" and Claude would call the right endpoints itself. The standard way to do that is MCP https://modelcontextprotocol.io : a server tells the agent which tools exist, what they do and which parameters they take, as JSON Schema. The usual approach is to write that server next to your API and describe every endpoint a second time: server.registerTool 'search tours', { description: 'Search tours', inputSchema: { month: z.number , maxPrice: z.number .optional }, }, async args = { / call the same logic as GET /tours again / } But my routes already have those schemas: app.get '/tours', zValidator 'query', z.object { month: z.coerce.number .min 1 .max 12 .describe 'Departure month' , maxPrice: z.coerce.number .optional , } , c = c.json searchTours c.req.valid 'query' , Method, path, parameters, types, even descriptions: everything an agent needs is already there. So I built hono-mcp-bridge https://github.com/serikshaikamalov/hono-mcp-bridge : js - import { zValidator } from '@hono/zod-validator' + import { zValidator, mountMcp } from 'hono-mcp-bridge' + mountMcp app // POST /mcp: every route is now a tool zValidator . @hono/zod-validator and attaches the schema to the returned middleware with a symbol. Same signature, same types, so c.req.valid 'query' is still fully typed. app.routes . { method, path, handler } . Group by route, read the schemas back, convert them with Zod 4's z.toJSONSchema . GET /tours/:id becomes get tours by id with input { params, query, body } . Request passed to app.fetch . Validation, auth middleware the Authorization and Cookie headers are forwarded , logging and rate limits all apply. If the agent sends bad input, it gets your zod 400 back as isError: true , reads it and retries. @modelcontextprotocol/sdk client to check compatibility. claude mcp add --transport http tours http://localhost:3000/mcp Find March tours under $2000 and book the first one for Ali Aliyev, +77001234567 → get tours {"query": {"month": 3, "maxPrice": 2000}} ← {"tours": {"id":1,"title":"Umrah, 10 days","price":1890} } → post bookings {"body": {"tourId": 1, "fullName": "Ali Aliyev", "phone": "+77001234567"}} ← {"bookingId":"20aa50cd-...", ...} Agents act with the credentials of whoever connected them. Use include , exclude or methods: 'GET' to limit what is exposed, hide single routes with describeTool { hidden: true } , and set allowedOrigins when serving on localhost. Your API still decides who can do what. hono-mcp-server : wrap each route in registerTool with its own schema. hono-gql-mcp : one GraphQL tool derived from @hono/zod-openapi . @hono/mcp : transport only. fastapi-mcp does the same job for Python and FastAPI. No SSE or server-initiated messages yet, zod v4 only. Next up: @hono/zod-openapi , Valibot and ArkType via Standard Schema, and outputSchema . npm i hono-mcp-bridge GitHub: https://github.com/serikshaikamalov/hono-mcp-bridge https://github.com/serikshaikamalov/hono-mcp-bridge . Issues and PRs welcome. I'd love to hear how you handle permissions when exposing your API to agents.