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d913f3a150
The mcp gem 1.1.0 (2026-08-01) is the first to negotiate 2026-07-28, so the Ruby examples can finally show the same thing the other four do: get_alerts declares an array-rooted output_schema and returns a bare top-level JSON array, get_forecast returns an object. Declaring `type:` at the root is what suppresses the SDK object-root default. Failure paths return an error result rather than a bare text one, because a tool declaring an output_schema MUST return conforming structured content. No alerts is an empty array, not an error. The client compiles each declared schema at connect time and validates every non-error result. The SDK client does not do this itself, so the spec client-side SHOULD costs real code here, as it does in Go and Rust. NOT READY TO SHIP. The gem never emits the `resultType` field that the 2026-07-28 schema marks mandatory, and no user-level knob sets it, so a spec-strict client rejects every response including tools/list. Verified end-to-end against the Ruby client, which does not check the field; blocked against the TypeScript SDK client. See the server README. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> |
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| Gemfile | ||
| README.md | ||
| weather.rb | ||
A Simple MCP Weather Server written in Ruby
See the Build an MCP server tutorial for more information.
Structured output
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 output.
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.