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47 lines
1.6 KiB
Text
47 lines
1.6 KiB
Text
---
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title: apps_dev
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sidebarTitle: apps_dev
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---
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# `fastmcp.cli.apps_dev`
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Dev server for previewing FastMCPApp UIs locally.
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Starts the user's MCP server on a configurable port, then starts a lightweight
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Starlette dev server that:
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- Serves a Prefab-based tool picker at GET /
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- Proxies /mcp to the user's server (avoids browser CORS restrictions)
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- Serves the AppBridge host page at GET /launch
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The host page uses @modelcontextprotocol/ext-apps to connect to the MCP server
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and render the selected UI tool inside an iframe.
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Startup sequence
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----------------
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1. Download ext-apps app-bridge.js from npm and patch its bare
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``@modelcontextprotocol/sdk/…`` imports to use concrete esm.sh URLs.
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2. Detect the exact Zod v4 module URL that esm.sh serves for that SDK version
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and build an import-map entry that redirects the broken ``v4.mjs`` (which
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only re-exports ``{z, default}``) to ``v4/classic/index.mjs`` (which
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correctly exports every named Zod v4 function). Import maps apply to the
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full module graph in the document, including cross-origin esm.sh modules.
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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` <sup><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></sup>
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```python
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run_dev_apps(server_spec: str) -> None
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```
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Start the full dev environment for a FastMCPApp server.
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Starts the user's MCP server on *mcp_port*, starts the Prefab dev UI
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on *dev_port* (with an /mcp proxy to the user's server), then opens
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the browser.
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