start-core/middleware

Middleware customizes the behavior of server functions and server routes. It is composable — middleware can depend on other middleware to form a chain.

CRITICAL: TypeScript enforces method order: middleware() → inputValidator() → client() → server(). Wrong order causes type errors. CRITICAL: Client context sent via sendContext is NOT validated by default. If you send dynamic user-generated data, validate it in server-side middleware before use.

Two Types of Middleware

Feature	Request Middleware	Server Function Middleware
Scope	All server requests (SSR, routes, functions)	Server functions only
Methods	.server()	.client(), .server()
Input validation	No	Yes (.inputValidator())
Client-side logic	No	Yes
Created with	createMiddleware()	createMiddleware({ type: 'function' })

Request middleware cannot depend on server function middleware. Server function middleware can depend on both types.

Request Middleware

Runs on ALL server requests (SSR, server routes, server functions):

// Use @tanstack/<framework>-start for your framework (react, solid, vue)
import { createMiddleware } from "@tanstack/react-start"

const loggingMiddleware = createMiddleware().server(
  async ({ next, context, request }) => {
    console.log("Request:", request.url)
    const result = await next()
    return result
  },
)

Server Function Middleware

Has both client and server phases:

// Use @tanstack/<framework>-start for your framework (react, solid, vue)
import { createMiddleware } from "@tanstack/react-start"

const authMiddleware = createMiddleware({ type: "function" })
  .client(async ({ next }) => {
    // Runs on client BEFORE the RPC call
    const result = await next()
    // Runs on client AFTER the RPC response
    return result
  })
  .server(async ({ next, context }) => {
    // Runs on server BEFORE the handler
    const result = await next()
    // Runs on server AFTER the handler
    return result
  })

Attaching Middleware to Server Functions

// Use @tanstack/<framework>-start for your framework (react, solid, vue)
import { createServerFn } from "@tanstack/react-start"

const fn = createServerFn()
  .middleware([authMiddleware])
  .handler(async ({ context }) => {
    // context contains data from middleware
    return { user: context.user }
  })

Context Passing via next()

Pass context down the middleware chain:

const authMiddleware = createMiddleware().server(async ({ next, request }) => {
  const session = await getSession(request.headers)
  if (!session) throw new Error("Unauthorized")

  return next({
    context: { session },
  })
})

const roleMiddleware = createMiddleware()
  .middleware([authMiddleware])
  .server(async ({ next, context }) => {
    console.log("Session:", context.session) // typed!
    return next()
  })

Sending Context Between Client and Server

Client → Server (sendContext)

const workspaceMiddleware = createMiddleware({ type: "function" })
  .client(async ({ next, context }) => {
    return next({
      sendContext: {
        workspaceId: context.workspaceId,
      },
    })
  })
  .server(async ({ next, context }) => {
    // workspaceId available here, but VALIDATE IT
    console.log("Workspace:", context.workspaceId)
    return next()
  })

Server → Client (sendContext in server)

const serverTimer = createMiddleware({ type: "function" }).server(
  async ({ next }) => {
    return next({
      sendContext: {
        timeFromServer: new Date(),
      },
    })
  },
)

const clientLogger = createMiddleware({ type: "function" })
  .middleware([serverTimer])
  .client(async ({ next }) => {
    const result = await next()
    console.log("Server time:", result.context.timeFromServer)
    return result
  })

Input Validation in Middleware

import { z } from "zod"
import { zodValidator } from "@tanstack/zod-adapter"

const workspaceMiddleware = createMiddleware({ type: "function" })
  .inputValidator(zodValidator(z.object({ workspaceId: z.string() })))
  .server(async ({ next, data }) => {
    console.log("Workspace:", data.workspaceId)
    return next()
  })

Global Middleware

Create src/start.ts to configure global middleware:

// src/start.ts
// Use @tanstack/<framework>-start for your framework (react, solid, vue)
import { createStart, createMiddleware } from "@tanstack/react-start"

const requestLogger = createMiddleware().server(async ({ next, request }) => {
  console.log(`${request.method} ${request.url}`)
  return next()
})

const functionAuth = createMiddleware({ type: "function" }).server(
  async ({ next }) => {
    // runs for every server function
    return next()
  },
)

export const startInstance = createStart(() => ({
  requestMiddleware: [requestLogger],
  functionMiddleware: [functionAuth],
}))

Using Middleware with Server Routes

All handlers in a route

export const Route = createFileRoute("/api/users")({
  server: {
    middleware: [authMiddleware],
    handlers: {
      GET: async ({ context }) => Response.json(context.user),
      POST: async ({ request }) => {
        /* ... */
      },
    },
  },
})

Specific handlers only

export const Route = createFileRoute("/api/users")({
  server: {
    handlers: ({ createHandlers }) =>
      createHandlers({
        GET: async () => Response.json({ public: true }),
        POST: {
          middleware: [authMiddleware],
          handler: async ({ context }) => {
            return Response.json({ user: context.session.user })
          },
        },
      }),
  },
})

Middleware Factories

Create parameterized middleware for reusable patterns like authorization:

const authMiddleware = createMiddleware().server(async ({ next, request }) => {
  const session = await auth.getSession({ headers: request.headers })
  if (!session) throw new Error("Unauthorized")
  return next({ context: { session } })
})

type Permissions = Record<string, string[]>

function authorizationMiddleware(permissions: Permissions) {
  return createMiddleware({ type: "function" })
    .middleware([authMiddleware])
    .server(async ({ next, context }) => {
      const granted = await auth.hasPermission(context.session, permissions)
      if (!granted) throw new Error("Forbidden")
      return next()
    })
}

// Usage
const getClients = createServerFn()
  .middleware([authorizationMiddleware({ client: ["read"] })])
  .handler(async () => {
    return { message: "The user can read clients." }
  })

Custom Headers and Fetch

Setting headers from client middleware

const authMiddleware = createMiddleware({ type: "function" }).client(
  async ({ next }) => {
    return next({
      headers: { Authorization: `Bearer ${getToken()}` },
    })
  },
)

Headers merge across middleware. Later middleware overrides earlier. Call-site headers override all middleware headers.

Custom fetch

// Use @tanstack/<framework>-start for your framework (react, solid, vue)
import type { CustomFetch } from "@tanstack/react-start"

const loggingMiddleware = createMiddleware({ type: "function" }).client(
  async ({ next }) => {
    const customFetch: CustomFetch = async (url, init) => {
      console.log("Request:", url)
      return fetch(url, init)
    }
    return next({ fetch: customFetch })
  },
)

Fetch precedence (highest to lowest): call site → later middleware → earlier middleware → createStart global → default fetch.

Common Mistakes

1. HIGH: Trusting client sendContext without validation

// WRONG — client can send arbitrary data
.server(async ({ next, context }) => {
  await db.query(`SELECT * FROM workspace_${context.workspaceId}`)
  return next()
})

// CORRECT — validate before use
.server(async ({ next, context }) => {
  const workspaceId = z.string().uuid().parse(context.workspaceId)
  await db.query('SELECT * FROM workspaces WHERE id = $1', [workspaceId])
  return next()
})

2. MEDIUM: Confusing request vs server function middleware

Request middleware runs on ALL requests (SSR, routes, functions). Server function middleware runs only for createServerFn calls and has .client() method.

3. HIGH: Browser APIs in .client() crash during SSR

During SSR, .client() callbacks run on the server. Browser-only APIs like localStorage or window will throw ReferenceError:

// WRONG — localStorage doesn't exist on the server during SSR
const middleware = createMiddleware({ type: "function" }).client(
  async ({ next }) => {
    const token = localStorage.getItem("token")
    return next({ sendContext: { token } })
  },
)

// CORRECT — use cookies/headers or guard with typeof window check
const middleware = createMiddleware({ type: "function" }).client(
  async ({ next }) => {
    const token =
      typeof window !== "undefined" ? localStorage.getItem("token") : null
    return next({ sendContext: { token } })
  },
)

4. MEDIUM: Wrong method order

// WRONG — type error
createMiddleware({ type: 'function' })
  .server(() => { ... })
  .client(() => { ... })

// CORRECT — middleware → inputValidator → client → server
createMiddleware({ type: 'function' })
  .middleware([dep])
  .inputValidator(schema)
  .client(({ next }) => next())
  .server(({ next }) => next())

Cross-References

• start-core/server-functions — what middleware wraps
• start-core/server-routes — middleware on API endpoints