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The License section named MIT. LICENSE is a transition file: new code contributions are Apache-2.0, documentation excluding specifications is CC-BY-4.0, and only earlier contributions without relicensing consent stay MIT. "Rust (stable)" understates the floor. rmcp, darling and time declare rust-version 1.88, and cargo metadata resolves both crates to 1.88. The Ruby aside said 3.3 also satisfies the gems. anthropic requires >= 3.2.0 and mcp >= 2.7.0, so the gem floor was 3.2, and 3.2 reached EOL on 2026-03-31. CI runs 3.4, which is what the READMEs now state. The Go and Rust clients take a command plus arguments, not a path to a server script. Removed the commit-message convention, which is attributed to the MCP contributing guidelines but does not appear in them. Added the AI_POLICY.md link that CONTRIBUTING.md points to. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01GdvHa5MzDw33PEz3DbTaJD
2.9 KiB
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 (takeslatitudeandlongitude)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.