New in reflex-enterprise v0.9.4.

Extending the MCP Server

Beyond the built-in tools and reflex:// resources, you can register your own tools, resources, and prompts on the underlying FastMCP server. There are two ways in, and both hand you the same instance.

At build time — configure=

Pass MCPPlugin(configure=...) a callable (or a "module.function" import path). It is invoked as configure(server) once the built-in tools and resources are registered and before the server is mounted, so whatever you add is serving from the first request:

import reflex_enterprise as rxe


def customize(server):
    @server.tool()
    def ping() -> str:
        """Health check."""
        return "pong"

    @server.resource("config://version")
    def version() -> str:
        """The deployed app version."""
        return "1.0.0"


config = rxe.Config(
    app_name="my_app",
    plugins=[rxe.MCPPlugin(configure=customize)],
)

A tool or resource that declares a Context-typed parameter has it injected by FastMCP, giving you the active request and session — the same mechanism the built-in tools use to resolve the caller's session from their bearer token.

At runtime — get_mcp_server()

To register from application code instead, grab the active server with rxe.get_mcp_server(). The server is built when the app is compiled, so this has to run after that — a lifespan task is the natural home, since it starts once the backend comes up:

from contextlib import asynccontextmanager

import reflex_enterprise as rxe


@asynccontextmanager
async def register_mcp_tools(app):
    """Add app-defined tools to the MCP server at startup."""
    server = rxe.get_mcp_server()

    @server.tool()
    def ping() -> str:
        """Health check."""
        return "pong"

    yield


app = rxe.App()
app.register_lifespan_task(register_mcp_tools)

# `get_mcp_server()` raises a `RuntimeError` until the app has compiled.
That rules out calling it at module scope, including as a bare `@rxe.get_mcp_server().tool()` decorator — at import time there is no server yet. Register from a lifespan task as above, or use `configure=` for setup-time customization. The same applies to the equivalent `plugin.get_mcp_server()` instance method.

If you register tools after the transport is already serving, connected clients pick them up on their next tools/list.

Related

• Auto MCP: the built-in tools and resources your additions sit alongside.
• Custom MCP resources: the simpler path when all you want is a read-only view of session state.