A Simple MCP Weather Server written in TypeScript

A minimal Model Context Protocol server that exposes US weather data from the National Weather Service API as two read-only tools.

The NWS API is free, keyless and US-only, so this server needs no credentials and no account to run.

Tool	Input	Returns
get-alerts	state — two-letter code, e.g. CA	Active weather alerts for that state
get-forecast	latitude, longitude	The next five forecast periods for that point

Both tools are annotated readOnlyHint: true and destructiveHint: false, so a host that respects tool annotations can invoke them without asking first. Both reach out to api.weather.gov and are marked openWorldHint: true for that reason.

Use it hosted, paid per call

Running it yourself costs nothing. There is also a hosted version, paid per call in USDC on Base over x402, for agents that would rather not carry a Node process and a data layer:

https://agent-evidence-api.thx93.workers.dev/weather/mcp

It serves the same two tools, with the same output schemas and the same null fallbacks, plus a free health tool and free tools/list so the service can be discovered and probed before paying. Discovery is free; a tools/call for get-alerts or get-forecast returns 402 with the price. Both are declared read-only, so a host that respects annotations will not ask twice.

The server.json in this repository advertises that remote, so an MCP client can install it by registry name rather than by cloning anything.

Prerequisites

• Node.js 24+
• npm

Build, test and run

npm install
npm test          # builds, then runs the suite
npm run build     # compile only
node build/index.js

The suite drives the real server over InMemoryTransport with the NWS API stubbed, so npm test touches no network and needs no credentials. It covers tool discovery and annotations, the get-alerts and get-forecast data paths, the fallbacks for fields the NWS sends as null, the failure contract, and input validation.

Running node build/index.js speaks the MCP protocol over stdio: it prints to stderr and waits for a client on stdin, so it is meant to be launched by a host rather than by hand.

Adding it to a client

Most MCP hosts use the same three-line shape:

{
  "mcpServers": {
    "weather": {
      "command": "node",
      "args": ["/absolute/path/to/mcp-weather-server/build/index.js"]
    }
  }
}

Or with npx, once the package is published:

{
  "mcpServers": {
    "weather": { "command": "npx", "args": ["-y", "@thx93/mcp-weather-server"] }
  }
}

Behaviour worth knowing

Failures are tool results, not protocol errors. A request the NWS cannot answer — a non-US location, an outage, a timeout — comes back as { isError: true, content: [{ type: "text", text: "<what failed>" }] }. That is the MCP contract for "this call was well formed and the work failed", as distinct from a JSON-RPC error, which means the call itself was broken. A client can show the text to a user and retry.

The NWS is slow to give up and we are not. Every request carries a 10-second timeout, so a stalled upstream surfaces as a failed tool call instead of a hung one.

Missing fields fall back rather than printing null. The NWS returns null for fields it does not have. A missing event name renders as Unknown, a missing temperature as Unknown — never the literal string null.

get-forecast returns at most five periods. Enough to answer "what is the weather like", short enough for a context window.

Structured content

Both tools declare 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. serveStdio serves both protocol eras from one factory, and the SDK projects the array-rooted schema down to the {"result": [...]} form for a 2025-11-25 client, so adopting it costs older clients nothing. The tests assert the data either way.

Provenance

Started from the Build an MCP server tutorial and then went past it: typed output schemas, read-only tool annotations, a bounded fetch with an explicit failure contract, and a test suite. Licensed ISC.