---
title: apps_dev
sidebarTitle: apps_dev
---

# `fastmcp.cli.apps_dev`


Dev server for previewing FastMCPApp UIs locally.

Starts the user's MCP server on a configurable port, then starts a lightweight
Starlette dev server that:

  - Serves a Prefab-based tool picker at GET /
  - Proxies /mcp to the user's server (avoids browser CORS restrictions)
  - Serves the AppBridge host page at GET /launch

The host page uses @modelcontextprotocol/ext-apps to connect to the MCP server
and render the selected UI tool inside an iframe.

Startup sequence
----------------
1. Download ext-apps app-bridge.js from npm and patch its bare
   ``@modelcontextprotocol/sdk/…`` imports to use concrete esm.sh URLs.
2. Detect the exact Zod v4 module URL that esm.sh serves for that SDK version
   and build an import-map entry that redirects the broken ``v4.mjs`` (which
   only re-exports ``{z, default}``) to ``v4/classic/index.mjs`` (which
   correctly exports every named Zod v4 function).  Import maps apply to the
   full module graph in the document, including cross-origin esm.sh modules.
3. Serve both the patched JS and the import-map JSON from the dev server.


## Functions

### `run_dev_apps` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/cli/apps_dev.py#L825" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
run_dev_apps(server_spec: str) -> None
```


Start the full dev environment for a FastMCPApp server.

Starts the user's MCP server on *mcp_port*, starts the Prefab dev UI
on *dev_port* (with an /mcp proxy to the user's server), then opens
the browser.

