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A Simple MCP Weather Server written in Python

A minimal MCP server that exposes US weather data from the National Weather Service API as two tools:

  • get_forecast – get the forecast for a location (takes latitude and longitude)
  • get_alerts – get active weather alerts for a US state (takes a two-letter state code)

This example accompanies the Build an MCP server tutorial.

Prerequisites

  • Python 3.10+
  • uv

Run the server

uv run weather.py

uv run creates the virtual environment and installs dependencies on first use.

The server communicates over stdio, so running it directly just waits for a client on stdin. To see it in action, launch it from one of the client examples in this repository, for example:

cd ../mcp-client-python
uv run client.py uv --directory ../weather-server-python run weather.py

Tool annotations

Both tools declare the same annotations: readOnlyHint: true, destructiveHint: false, idempotentHint: true and openWorldHint: true. They tell a host that the tools only read data, and that they read it from an outside service (the NWS API), so a host can, for example, run them without asking the user to confirm each call. Annotations are hints, not guarantees: a host should rely on them only for servers it trusts.

Structured content

Both tools declare an output schema and return structured_content, and both return an object: get_alerts returns {"alerts": [...]}, with an empty list when there are none.

Revision 2026-07-28 is the first to allow a top-level array as the root of an output schema (see Structured Content in the spec), but 2025-11-25 and earlier require an object, and that is still the revision most clients negotiate. This SDK does not convert an array schema for an older client: tools/list fails with an internal error, so the client gets no tools at all. So all five servers wrap the list in an object, and a 2025-11-25 client sees the same get_alerts as a 2026-07-28 one. They can return a bare array once every SDK converts it for older clients; see #272.