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Apps are FastMCP’s third pillar: server-side UI built on the official MCP Apps extension (io.modelcontextprotocol/ui). A FastMCPApp registers tools whose results render as interactive components inside the host conversation — with full graceful degradation in clients that don’t support UI. 
import { FastMCPApp, Column, Text } from '@prefecthq/fastmcp-ts/server'

const app = new FastMCPApp({ name: 'search-app', version: '1.0.0' })

app.entrypoint(
  { name: 'search', description: 'Search the product catalog' },
  () => Column({}, [Text('Product Search')])
)
This page is under construction. The outline below sketches what it will cover.
  • Entrypoints vs. backend tools — entrypoint() registers a tool visible to the LLM, auto-linked to a ui:// resource; backendTool() registers a tool callable only from within the rendered UI (visibility: ['app']).
  • Capability negotiation — the UI extension is advertised only when UI components exist; clients that haven’t negotiated it get a plain-text fallback instead of structured UI content.
  • Visibility model — ToolConfig.ui.visibility (['model'], ['app'], or both) controls what the LLM sees in tools/list.
  • Mounting apps — server.addProvider(app) / parent.mount(app.server); mount-prefix-aware action references via actionRef.