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 ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/cli/apps_dev.py#L1690" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

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.