> ## Documentation Index
> Fetch the complete documentation index at: https://redop.useagents.site/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# listen() Options

> Reference for the Redop listen() overloads and transport options for HTTP and stdio.

Use `listen()` to start either the HTTP or stdio transport.

For transport selection guidance and common usage patterns, use [Listening and Transports](/docs/documentation/listening-and-transports).

## Overloads

```ts theme={null}
app.listen();
app.listen(3000);
app.listen(3000, "0.0.0.0");
app.listen({
  port: 3000,
  hostname: "0.0.0.0",
  debug: true,
});
```

## Behavior by call shape

| Call shape                       | Result                                  |
| -------------------------------- | --------------------------------------- |
| `listen()`                       | Starts `stdio`                          |
| `listen(3000)`                   | Starts `http` on port `3000`            |
| `listen(3000, "0.0.0.0")`        | Starts `http` with an explicit hostname |
| `listen({ port: 3000 })`         | Starts `http`                           |
| `listen({ transport: "stdio" })` | Forces `stdio`                          |
| `listen({ transport: "http" })`  | Forces `http`                           |

## Typical HTTP options

```ts theme={null}
app.listen({
  transport: "http",
  port: 3000,
  hostname: "0.0.0.0",
  path: "/mcp",
  health: { path: "/health" },
  debug: true,
  sessionTimeout: 60_000,
  onListen: ({ url }) => {
    console.log(url);
  },
});
```

## Options

### Shared options

| Option      | Notes                                                                                |
| ----------- | ------------------------------------------------------------------------------------ |
| `transport` | `"http"` or `"stdio"`. Defaults to `"http"` when `port` is set, otherwise `"stdio"`. |

### HTTP options

| Option           | Notes                                                                                         |
| ---------------- | --------------------------------------------------------------------------------------------- |
| `port`           | HTTP port. Also available through `listen(3000)`. Defaults to `3000` for HTTP.                |
| `hostname`       | HTTP bind address. Defaults to `127.0.0.1`.                                                   |
| `path`           | MCP endpoint path for HTTP. Defaults to `/mcp`.                                               |
| `health`         | Enable `/health` or provide `{ path }` for a custom health route. Disabled by default.        |
| `debug`          | Writes HTTP transport logs to stderr.                                                         |
| `sessionTimeout` | Legacy HTTP session expiry in milliseconds. Defaults to `60000`. Unused for MCP `2026-07-28`. |
| `listCache`      | Override list/discover cache hints (`ttlMs`, `cacheScope`) for MCP `2026-07-28`.              |
| `onListen`       | Callback fired after the HTTP server starts. Receives `{ hostname, port, url }`.              |
| `cors`           | Enable permissive CORS or provide the typed `CorsOptions` shape.                              |
| `maxBodySize`    | Maximum accepted request body size in bytes in the public type surface.                       |
| `tls`            | TLS options in the public type surface.                                                       |

### Stdio options

| Option      | Notes                                       |
| ----------- | ------------------------------------------- |
| `transport` | Set to `"stdio"` to force stdio explicitly. |

## Current implementation caveats

These reflect the current HTTP implementation, not just the public type surface:

| Option        | Current behavior                                                                                                                                        |
| ------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `cors`        | Present in the type surface. The current HTTP transport always sends a basic permissive CORS response rather than using the full `CorsOptions` shape.   |
| `maxBodySize` | Present in the type surface, but not currently enforced by the HTTP transport.                                                                          |
| `tls`         | Present in the type surface and used when formatting the reported URL, but the current HTTP transport does not start Bun with TLS from this option yet. |

## Defaults

* default transport: `stdio` without `port`, otherwise `http`
* default HTTP hostname: `127.0.0.1`
* default HTTP port: `3000`
* default MCP path: `/mcp`
* default health route: disabled
* default session timeout: `60000`