34 lines
1.7 KiB
Markdown
34 lines
1.7 KiB
Markdown
# A Simple MCP Weather Server written in Go
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A minimal MCP server that exposes US weather data from the [National Weather Service API](https://www.weather.gov/documentation/services-web-api) as two tools:
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- `get_forecast` – get the forecast for a location (takes `latitude` and `longitude`)
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- `get_alerts` – get active weather alerts for a US state (takes a two-letter state code)
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This example accompanies the [Build an MCP server](https://modelcontextprotocol.io/docs/develop/build-server) tutorial.
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## Prerequisites
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- Go 1.25+
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## Build and run
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```bash
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go build -o weather
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./weather
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```
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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:
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```bash
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cd ../mcp-client-go
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go run main.go ../weather-server-go/weather
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```
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## Structured content
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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](https://modelcontextprotocol.io/specification/2026-07-28/server/tools#structured-content) in the spec.
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`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.
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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.
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