Importing mcp.types takes ~2s in small containers due to heavy module-level initialization #1508

Initial Checks

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Description

Description

Summary
When running mcp.types inside a constrained container (2 vCPU / 4 GB RAM), importing the top-level package already costs ~2.5 seconds before any server code runs. Profiling shows the time is spent entirely in import mcp.types because the module eagerly imports/open-initializes several heavy dependencies.

Environment

• fastmcp 2.12.5 (PyPI)
• Python 3.12
• Alpine-based container, 2 vCPU / 4 GB RAM
• No custom .env or optional features enabled

What happens
Importing fastmcp alone blocks startup for ~2s. A py-spy flamegraph highlights:

• fastmcp.server.server (~2.2s)
• mcp.types / jsonschema / rfc3987 (combined ~1.8s)
  All of this occurs before FastMCP('...') is even instantiated.

Why this hurts
In serverless-style deployments every cold start pays this tax, delaying HTTP readiness and tool registration. The delays come from JSON schema / OpenAPI generation and optional format checkers that the server might never need.

Requested fix
Please consider deferring or making optional the heavy module-level work. A few ideas:

1. Move jsonschema format checker registration, openapi-core imports, and .env loading behind opt-in flags.
2. Generate OpenAPI/JSON schema lazily (only when FastMCP.build_schema() is called) or allow caching.
3. Provide environment variables to disable optional features so small deployments can skip them entirely.

Thanks for looking into this—shaving even 1s off import time would significantly improve startup latency for containerized deployments.

Example Code

import mcp.types

Version Information

FastMCP version:                                                 2.12.5
MCP version:                                                     1.16.0
Python version:                                                 3.12.11
Platform:          Linux-5.4.250.bsk.9-sign-amd64-x86_64-with-glibc2.35
FastMCP root path:         /opt/python3.12/lib/python3.12/site-packages

Example Code

import mcp.types

Python & MCP Python SDK

1.18.0

Ref: python-jsonschema/jsonschema#853