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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
46 lines
2.9 KiB
Markdown
46 lines
2.9 KiB
Markdown
# A Simple MCP Weather Server written in Ruby
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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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- Ruby 3.4+
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- Bundler
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## Setup and run
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```bash
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bundle install
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bundle exec ruby weather.rb
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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 the Ruby client example in this repository:
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```bash
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cd ../mcp-client-ruby
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bundle install
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bundle exec ruby client.rb ../weather-server-ruby/weather.rb
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```
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## Structured content
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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](https://modelcontextprotocol.io/specification/2026-07-28/server/tools#structured-content) in the spec.
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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.
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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.
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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.
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## Known limitation: strict `2026-07-28` clients reject this server
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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.
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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.
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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.
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