FASTMCP_ env var prefixes no longer respected since 1.12.3

[cbcoutinho]

EDIT: There appears to be an open PR that addresses this issue already: #1244

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

PR #1198 modified how the MCP SSE Server is instantiated, namely by unpacking all the server settings explicitly and providing a number of defaults. The previous solution used **kwargs and provided defaults after the Settings object had been . This has resulted in various downstream effects, one of which is it's no longer possible to override the binding host:

$ FASTMCP_HOST=0.0.0.0 uv run mcp run -t sse app.py
INFO:     Started server process [87207]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit) <<<<<< NOTE: host=127.0.0.1

Related issues:

• MCP servers can not be accessed via Docker and docker-compose anymore. #790

Open PRs

• Update FastMCP constructor to respect Pydantic settings configuration #1244

Example Code

Python & MCP Python SDK

<1.12.3

[ajeyone]

Please have a good test. The recent releases broke default behavior.
For users, don't upgrade, use 1.12.2 version.

[maxisbey]

Closing this in favour of the discussion happening here: #1244