docs/apps/architecture.mdx
import { VersionBadge } from '/snippets/version-badge.mdx'
<VersionBadge version="3.2.0" />You don't need this page to build apps. It's for when something isn't rendering the way you expect, when UI tool calls aren't reaching your server, or when you're writing custom HTML apps and need to understand the protocol directly.
An MCP app moves through five stages from Python to pixels:
Python components → JSON tree → structuredContent → Renderer iframe → Host UI
You write Prefab components. FastMCP serializes them to a JSON component tree and delivers it as structuredContent on the tool result. The host loads the Prefab renderer in a sandboxed iframe, pushes the JSON in, and the renderer paints the UI. If the UI calls server tools, it talks back through the same postMessage channel.
The sections below walk each stage.
When you mark a tool with app=True or @app.ui(), FastMCP wires up the metadata and renderer resource that the protocol requires.
app=True flagapp on @mcp.tool accepts True, an AppConfig, or a dict. When you pass True, FastMCP checks whether the tool's return type is a Prefab type (PrefabApp, Component, or unions containing them). If it qualifies, FastMCP expands True into a full AppConfig — setting the renderer URI, CSP headers, and visibility — and stores it in the tool's meta["ui"] dict.
This expansion also registers the shared Prefab renderer resource (below). The tool and the renderer are linked through a resourceUri field in the metadata: the tool says "render me with ui://prefab/renderer.html" and the host fetches that resource when it displays the result.
Type inference works the same way. If the return type is a Prefab type and you haven't set app explicitly, FastMCP auto-wires the metadata as if you'd written app=True.
FastMCPApp uses the same mechanism but adds two things. First, it tags every tool — both @app.ui() entry points and @app.tool() backends — with meta["fastmcp"]["app"] set to the app's name. That tag lets the server identify which app a tool belongs to when routing UI calls.
Second, it sets meta["ui"]["visibility"] to control who can see each tool. Entry points default to ["model"] (LLM-visible). Backend tools default to ["app"] (UI-only). Hosts use this to filter the tool list.
When a Prefab tool runs, its return value — a PrefabApp or a bare Component — becomes a JSON blob the renderer can interpret.
PrefabApp.to_json()The entry point is PrefabApp.to_json(). It walks the component tree and produces a JSON object with three top-level keys: view (the component tree), state (initial state values), and _meta (routing metadata).
FastMCP passes a tool_resolver callback to to_json(). Whenever the tree contains a CallTool action that references a function (not a string), the resolver converts it to a ResolvedTool with the function's registered name. This is how CallTool(save_contact) becomes CallTool("save_contact") on the wire. The resolver also handles unwrap_result — a flag telling the renderer to unwrap single-value results from the {"result": value} envelope FastMCP uses for schema compliance.
_meta.fastmcp.app tagAfter to_json() produces the tree, FastMCP injects _meta.fastmcp.app with the app's name (if the tool belongs to a FastMCPApp). This tag rides along inside structuredContent all the way to the renderer.
When the renderer calls a backend tool, it includes _meta.fastmcp.app in the CallTool request. The server sees this tag and routes the call through a special path that bypasses transforms (below).
The final tool result has two parts: content (a list of TextContent blocks for the LLM) and structuredContent (the JSON tree for the renderer). By default, Prefab tools send "[Rendered Prefab UI]" as the text content — just enough for the LLM to know something was rendered. If you return a ToolResult explicitly, you control both halves.
Normal tool calls go through the provider chain, which applies transforms (namespace prefixes, visibility filters) before resolving by name. App UI calls need a different path.
get_app_tool bypassBackend tools are typically hidden from the model (visibility=["app"]). Visibility transforms would filter them out of normal resolution. And namespace transforms might rename them — save_contact becomes contacts_save_contact — while the renderer still uses the original name.
get_app_tool solves both problems. When the server sees _meta.fastmcp.app on an incoming CallTool request, it calls get_app_tool(app_name, tool_name) instead of the normal get_tool(name). This walks the provider tree directly, skipping transforms. It finds the tool by its original registered name and verifies that its meta["fastmcp"]["app"] matches the expected app.
That's why CallTool("save_contact") keeps working when the server is mounted under a namespace. The renderer sends the original name plus the app identity; the server uses get_app_tool to find it without transforms in the way.
Authorization still applies. get_app_tool bypasses transforms but runs auth checks against the tool's auth config before executing.
get_app_tool is defined on the Provider base class and overridden by aggregate and wrapped providers. Aggregate providers fan out the lookup across child providers in parallel. Wrapped providers (like FastMCPProvider, which wraps a nested FastMCP server) delegate to the inner server's get_app_tool. Backend tools are reachable through any depth of composition.
The Prefab renderer is a self-contained JavaScript application that interprets the JSON component tree and renders it as a React UI.
FastMCP registers the renderer as a ui://prefab/renderer.html resource with MIME type text/html;profile=mcp-app. The HTML is bundled inside the prefab-ui Python package; get_renderer_html() returns it as a string. All Prefab tools on a server share this single resource.
The resource also carries CSP metadata (via get_renderer_csp()) declaring the CDN domains the renderer needs. Hosts use this to configure the iframe's Content Security Policy.
postMessage communicationThe renderer lives in a sandboxed iframe and communicates with the host using postMessage. The protocol follows the MCP Apps extension spec:
The host pushes the tool result (with structuredContent) into the iframe. The renderer parses the component tree, initializes state, and renders the UI. When the user interacts — submitting a form, clicking a button — and the interaction triggers a CallTool action, the renderer sends a callServerTool message back to the host via postMessage. The host forwards it as a regular MCP tools/call request to the server, including _meta.fastmcp.app for routing.
The response flows back the same way: server → host → iframe via postMessage, and the renderer updates state with the result.
The @modelcontextprotocol/ext-apps JavaScript SDK provides the App class (sometimes called AppBridge) that manages the postMessage handshake. It handles connection negotiation, tool result delivery, server tool calls, and host context (safe area insets, theme preferences). The Prefab renderer uses it internally; you only touch it directly when building custom HTML apps.
fastmcp dev apps simulates the host-side behavior locally without a real MCP client.
Two HTTP servers. Your MCP server runs on port 8000 with the Streamable HTTP transport. The dev UI runs on port 8080 and serves a picker page that lists your app tools.
A reverse proxy at /mcp on the dev server forwards requests to your MCP server. This matters because the renderer iframe runs on localhost:8080 and your MCP server runs on localhost:8000 — without the proxy, the renderer's callServerTool requests would be cross-origin and the browser would block them. The proxy keeps everything same-origin from the iframe's perspective.
When you select a tool and click launch, the dev UI calls the tool through the proxy, receives the structuredContent response, and opens a new tab. That tab loads the tool's renderer resource (via the proxy), creates an AppBridge, and pushes the tool result into the renderer. From here on it matches what a real host provides: the renderer displays the UI, and any CallTool actions route back through the proxy to your server.
Auto-reload is on by default, so changes to your server code restart the MCP server automatically. The dev UI keeps running — relaunch the tool to see changes.