# Express Adapter

The `@mastra/express` package provides a server adapter for running Mastra with [Express](https://expressjs.com).

> **Info:** For general adapter concepts (constructor options, initialization flow, etc.), see [Server Adapters](https://mastra.ai/docs/server/server-adapters).

## Installation

Install the Express adapter and Express framework:

**npm**:

```bash
npm install @mastra/express@latest express
```

**pnpm**:

```bash
pnpm add @mastra/express@latest express
```

**Yarn**:

```bash
yarn add @mastra/express@latest express
```

**Bun**:

```bash
bun add @mastra/express@latest express
```

## Usage example

```typescript
import express from 'express';
import { MastraServer } from '@mastra/express';
import { mastra } from './mastra';

const app = express();
app.use(express.json()); // Required for body parsing

const server = new MastraServer({ app, mastra });
await server.init();

app.listen(4111, () => {
  console.log('Server running on port 4111');
});
```

> **Note:** Express requires `express.json()` middleware for JSON body parsing. Add it before creating the `MastraServer`.

## Constructor parameters

**app:** (`Application`): Express app instance

**mastra:** (`Mastra`): Mastra instance

**prefix?:** (`string`): Route path prefix (e.g., \`/api/v2\`) (Default: `''`)

**openapiPath?:** (`string`): Path to serve OpenAPI spec (e.g., \`/openapi.json\`) (Default: `''`)

**bodyLimitOptions?:** (`{ maxSize: number, onError: (err) => unknown }`): Request body size limits

**streamOptions?:** (`{ redact?: boolean }`): Stream redaction config. When true, redacts sensitive data from streams. (Default: `{ redact: true }`)

**customRouteAuthConfig?:** (`Map<string, boolean>`): Per-route auth overrides. Keys are \`METHOD:PATH\` (e.g., \`GET:/api/health\`). Value \`false\` makes route public, \`true\` requires auth.

**tools?:** (`Record<string, Tool>`): Available tools for the server

**taskStore?:** (`InMemoryTaskStore`): Task store for A2A (Agent-to-Agent) operations

**mcpOptions?:** (`MCPOptions`): MCP transport options. Set \`serverless: true\` for stateless environments like Cloudflare Workers or Vercel Edge.

## Differences from Hono

| Aspect               | Express                        | Hono                  |
| -------------------- | ------------------------------ | --------------------- |
| Body parsing         | Requires `express.json()`      | Handled by framework  |
| Context storage      | `res.locals`                   | `c.get()` / `c.set()` |
| Middleware signature | `(req, res, next)`             | `(c, next)`           |
| Streaming            | `res.write()` / `res.end()`    | `stream()` helper     |
| AbortSignal          | Created from `req.on('close')` | `c.req.raw.signal`    |

## Adding custom routes

Add routes directly to the Express app:

```typescript
const app = express();
app.use(express.json());

const server = new MastraServer({ app, mastra });

// Before init - runs before Mastra middleware
app.get('/early-health', (req, res) => res.json({ status: 'ok' }));

await server.init();

// After init - has access to Mastra context
app.get('/custom', (req, res) => {
  const mastraInstance = res.locals.mastra;
  res.json({ agents: Object.keys(mastraInstance.listAgents()) });
});

app.listen(4111);
```

> **Tip:** Routes added before `init()` run without Mastra context. Add routes after `init()` to access the Mastra instance and request context.

## Accessing context

In Express middleware and routes, access Mastra context via `res.locals`:

```typescript
app.get('/custom', (req, res) => {
  const mastra = res.locals.mastra;
  const requestContext = res.locals.requestContext;
  const abortSignal = res.locals.abortSignal;

  const agent = mastra.getAgent('myAgent');
  res.json({ agent: agent.name });
});
```

Available properties on `res.locals`:

| Key                     | Description                             |
| ----------------------- | --------------------------------------- |
| `mastra`                | Mastra instance                         |
| `requestContext`        | Request context map                     |
| `abortSignal`           | Request cancellation signal             |
| `tools`                 | Available tools                         |
| `taskStore`             | Task store for A2A operations           |
| `customRouteAuthConfig` | Per-route auth overrides                |
| `user`                  | Authenticated user (if auth configured) |

## Adding middleware

Add Express middleware before or after `init()`:

```typescript
const app = express();
app.use(express.json());

// Middleware before init
app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();
});

const server = new MastraServer({ app, mastra });
await server.init();

// Middleware after init has access to Mastra context
app.use((req, res, next) => {
  const mastra = res.locals.mastra;
  next();
});
```

## Manual initialization

For custom middleware ordering, call each method separately instead of `init()`. See [manual initialization](https://mastra.ai/docs/server/server-adapters) for details.

## Examples

- [Express Adapter](https://github.com/mastra-ai/mastra/tree/main/examples/server-express-adapter) - Basic Express server setup

## Related

- [Server Adapters](https://mastra.ai/docs/server/server-adapters) - Shared adapter concepts
- [MastraServer Reference](https://mastra.ai/reference/server/mastra-server) - Full API reference
- [createRoute() Reference](https://mastra.ai/reference/server/create-route) - Creating type-safe custom routes