A Simple MCP Weather Server written in Ruby

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

• Ruby 3.4+
• Bundler

Setup and run

bundle install
bundle exec ruby weather.rb

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

cd ../mcp-client-ruby
bundle install
bundle exec ruby client.rb ../weather-server-ruby/weather.rb

Structured content

Both tools declare an output_schema and return structured_content. 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.

Declaring type: at the root of an output_schema is what makes the array case work. The SDK applies a type: "object" default only when the schema declares no root keyword, so output_schema(type: "array", items: {...}) is taken as written.

Because a tool that declares an output_schema MUST return conforming structured content, the failure paths return an error result rather than a bare text one — error results are exempt from that requirement. "No alerts" is an empty array, not an error.

The tools also return a human-readable content block. When you pass your own content, the SDK leaves it alone; if you omit it entirely, the SDK serializes non-object structured content into a text block for you.

Known limitation: strict 2026-07-28 clients reject this server

The mcp gem never emits the resultType field on results. The 2026-07-28 schema makes it mandatory — "Servers implementing this protocol version MUST include this field" — so a client that enforces the revision rejects every response, including tools/list, before it ever reaches the structured content.

There is no way to set it from user code: MCP::Server accepts ttl_ms: and cache_scope: for the SEP-2549 cache hints, but nothing for resultType. The gem's own client does not check the field, so the gap is invisible when a Ruby client talks to a Ruby server, and only appears cross-SDK.

Until the gem emits resultType, this example is verified only against the Ruby client. Note also that an array-rooted schema requires a 2026-07-28 client regardless: the gem sends it as written rather than projecting it down, so a 2025-11-25 client rejects the tool list.