Description

Summary
from fastmcp import FastMCP in v3.0.2 takes 2.5–5.5 seconds even for simple stdio tool servers that use no auth, no Redis, no task scheduling, and no runtime type checking. The overhead comes from eager imports of heavy transitive dependencies that are never needed at runtime.

This is different from #2205 (which focused on mcp.types). The MCP SDK has since fixed their lazy-loading (#1508). The remaining bottleneck is FastMCP's own dependency chain.

Environment

• FastMCP 3.0.2 / MCP SDK 1.26.0
• Python 3.13.9, Windows 11
• MetaMcp: HTTP multiplexer proxying 19 MCP backends via create_proxy()

Impact
We proxy 19 backends via create_proxy(). Each stdio backend spawns a Python process that imports FastMCP. Total cold-start cost across all backends: 100+ seconds. This delays tool availability for clients after a restart.

Profiling Data

Profiled with python -X importtime on 3 real-world servers:

Breakdown: What FastMCP imports eagerly but never uses

Total avoidable overhead: ~4.3 seconds for a simple FastMCP("my-server") + @mcp.tool() server.

Self-time leaders

Suggested fix

Lazy-import the following behind their actual use sites:

1. Auth stack (fastmcp.server.auth.oauth_proxy, authlib, cryptography, httpx) → only load when OAuth is configured. Saves ~1.1s
2. docket + key_value + redis → only load when storage backend is configured or dependency injection is used. Saves ~550ms
3. beartype → only load when runtime type checking is actually enabled. Saves ~520ms
4. mcp.client.session → only load in proxy/client contexts, not for pure server usage. Saves ~1.4s

This would bring import fastmcp from ~2.5s down to ~0.3–0.5s for the common case of a simple stdio tool server.

Example Code

# This alone costs 2.5s+ even though no auth, Redis, or beartype is needed:
from fastmcp import FastMCP

mcp = FastMCP("my-simple-tool-server")

@mcp.tool()
def hello(name: str) -> str:
    return f"Hello {name}"

Version Information

FastMCP version: 3.0.2
MCP version: 1.26.0
Python version: 3.13.9
Platform: Windows-11-10.0.26200-SP0