weather-server-go

A Simple MCP Weather Server written in Go

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

• Go 1.25+

Build and run

go build -o weather
./weather

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-go
go run main.go ../weather-server-go/weather

Structured content

Both tools advertise an outputSchema and return StructuredContent. get_forecast returns an object; get_alerts returns a top-level JSON array, which protocol revision 2026-07-28 is the first to allow — see Structured Content in the spec.

get_forecast sets no OutputSchema on the Tool: the SDK infers one from the handler's return type, which is the idiomatic Go spelling. get_alerts overrides it, because inference widens a slice to ["null", "array"] — a nil slice marshals to null — and the tool never returns nil.

Note that an array-rooted schema requires a 2026-07-28 client: the SDK sends it as written on every connection rather than projecting it down, so an older client rejects the tool list.