Weather

MCP Weather Server — US alerts and forecasts for AI agents

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.

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 straight from npm — published 2026-10-19:

npx -y @thx93/mcp-weather-server

Or, equivalently, as a client config entry:

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

Or straight from the repository, with no registry account needed:

npm install github:Thx93/mcp-weather-server

That path works because prepare builds the server during the install; build/ is not committed, so without it the bin would point at a file that does not exist. It was verified end to end — install from GitHub, then speak MCP to the result, and both tools list with object output schemas.

Verified against the source

The tools return the National Weather Service's own data, so correctness is checkable rather than asserted. On 2026-10-19, api.weather.gov reported one active alert for TX and a paid call to the hosted get-alerts returned exactly one - Event: Flood Advisory - for the same state.

That matters more than it sounds: the sibling evidence service carries a documented reliability gap in its verdict (see its docs/evidence-quality.md), while these tools have no inferred judgement to get wrong. They fetch and reshape; the values are the NWS's.

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, and both schemas are objects: get-forecast returns { latitude, longitude, periods }, get-alerts returns { alerts: [...] }.

get-alerts previously declared a top-level array, which protocol revision 2026-07-28 does permit. It was changed because a client on the widely-deployed MCP SDK rejects the entire tools/list response when it meets an array schema — invalid_value: expected "object" — so such a client cannot connect to the server at all. Wrapping the array in an object is what makes these tools reachable from ordinary clients; the textual summary a model reads is unchanged.

FAQ

What is this? An MCP (Model Context Protocol) server that exposes US weather data from the National Weather Service as two read-only tools, get-alerts and get-forecast. It runs over stdio when you install it, and the same code is served over streamable HTTP at https://agent-evidence-api.thx93.workers.dev/weather/mcp where it is paid per call.

Do I need an API key? No. The National Weather Service API needs no key, and neither the local server nor the hosted endpoint asks for one, for an account, or for a signup.

How much does it cost? Running it yourself is free — it is ISC-licensed and published as @thx93/mcp-weather-server on npm. The hosted endpoint charges 0.01 USDC per call on Base through x402, which is how an agent with a wallet but no ability to sign up for anything can still call it.

Which locations are supported? US only. The data is the National Weather Service's, which covers the United States and its territories. A coordinate outside that coverage returns an error rather than a guess.

What do the tools return? get-alerts takes a two-letter state code and returns every active alert: event, area, severity, description and the official instruction. get-forecast takes a latitude and longitude and returns five forecast periods: name, temperature, wind speed and direction, and the plain-language forecast. Both declare an object output schema.

How do I add it to Claude Desktop, Cursor or another MCP client?

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

Why does it exist? It began as the MCP "Build a server" tutorial and was extended: typed output schemas, read-only annotations, a bounded fetch, an explicit failure contract and a test suite.

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.