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# TanStack Start adapter

The `@mastra/tanstack-start` package mounts Mastra in a TanStack Start application. See [Server Adapters](https://mastra.ai/docs/server/server-adapters) for general adapter concepts.

## Installation

Install the TanStack Start adapter and its Hono peer dependency:

**npm**:

```bash
npm install @mastra/tanstack-start@latest hono
```

**pnpm**:

```bash
pnpm add @mastra/tanstack-start@latest hono
```

**Yarn**:

```bash
yarn add @mastra/tanstack-start@latest hono
```

**Bun**:

```bash
bun add @mastra/tanstack-start@latest hono
```

## Usage example

Mount the adapter in a catch-all splat route:

```typescript
import { createStartRouteHandler } from '@mastra/tanstack-start'
import { createFileRoute } from '@tanstack/react-router'
import { mastra } from '../../mastra'

export const Route = createFileRoute('/api/$')({
  server: {
    handlers: createStartRouteHandler({ mastra }),
  },
})
```

The `prefix` option defaults to `/api` and must match the splat route's mount path. For example, when mounting at `src/routes/api/mastra/$.ts`, use `createStartRouteHandler({ mastra, prefix: '/api/mastra' })`.

## Signature

```typescript
function createStartRouteHandler(options: StartRouteHandlerOptions): StartRouteHandlers
```

## Parameters

**mastra** (`Mastra`): Mastra instance whose server configuration and registered resources are exposed

**tools** (`ToolsInput`): Additional tools to register with the server (Default: `{}`)

**prefix** (`string`): API route prefix, which must match the splat route mount path (Default: `'/api'`)

## Return value

Returns a `StartRouteHandlers` object with `GET`, `POST`, `PUT`, `DELETE`, `PATCH`, `OPTIONS`, and `HEAD` properties. Each property is a handler with this signature:

```typescript
(context: StartHandlerContext) => Response | Promise<Response>
```

The context type contains the incoming request and the splat route parameters:

```typescript
type StartHandlerContext = {
  request: Request
  params: Record<string, string>
}
```

## Serverless deployments & lazy initialization

The adapter exports its route handlers synchronously, then creates the underlying Hono app and `MastraServer` when the first request arrives. That initialized app is reused for subsequent requests in the same process, which supports serverless runtimes without top-level asynchronous initialization.

It uses the same Hono-based `MastraServer` core as the [Next.js adapter](https://mastra.ai/reference/server/next-adapter). A2A endpoints use an in-memory task store, so task state lasts only for the lifetime of the process and isn't shared between serverless instances.

## Body size limit

Request bodies have a default limit of 4.5 MB. Requests that exceed the limit receive `{ error: 'Request body too large' }`.

Set `server.bodySizeLimit` on the `Mastra` instance to configure the limit.

## Middleware

Middleware configured with `server.middleware` or `setServerMiddleware()` runs through the adapter's Hono-based server, except on framework-public routes such as those marked `requiresAuth: false`. See [Server middleware](https://mastra.ai/docs/server/middleware).

## Related

- [Server Adapters](https://mastra.ai/docs/server/server-adapters)
- [MastraServer](https://mastra.ai/reference/server/mastra-server)