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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.

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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.

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Functions

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run_dev_apps 

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.