Merge pull request #184 from olaservo/structured-output-2026-07-28/ruby

Add structured content to the Ruby weather server and client

mcp-client-ruby/Gemfile

@@ -4,4 +4,4 @@ source "https://rubygems.org"
 
 gem "anthropic", '>= 1.60.0'
 gem "dotenv"
-gem "mcp", '>= 0.15.0'
+gem "mcp", ">= 1.1.0"

mcp-client-ruby/README.md

@@ -1,3 +1,11 @@
 # An LLM-Powered Chatbot MCP Client written in Ruby
 
 See the [Build an MCP client](https://modelcontextprotocol.io/docs/develop/build-client) tutorial for more information.
+
+## Structured content
+
+The spec says clients SHOULD validate structured results against the schema the tool declares, and this SDK's client does not do it for you — `MCP::Client::Tool#output_schema` hands back the raw schema from the wire and nothing checks results against it. So the client compiles each declared schema once at connect time with `MCP::Tool::OutputSchema.new` and validates every non-error result.
+
+The two channels go to different readers: `content` is forwarded to the model, while `structured_content` is used as data — when a tool returns an array, the client counts its items rather than re-reading the prose. See [Structured Content](https://modelcontextprotocol.io/specification/2026-07-28/server/tools#structured-content).
+
+`MCP::Client::Stdio` offers `2026-07-28` by default, so no negotiation setup is needed here.

mcp-client-ruby/client.rb

@@ -30,6 +30,13 @@ class MCPClient
     @mcp_client = MCP::Client.new(transport: @transport)
     @mcp_client.connect
 
+    # The spec says clients SHOULD validate structured results against the
+    # schema the tool declares. This SDK's client does not do it, so compile
+    # each declared schema once here and check results as they arrive.
+    @output_schemas = @mcp_client.tools.each_with_object({}) do |tool, schemas|
+      schemas[tool.name] = MCP::Tool::OutputSchema.new(tool.output_schema) if tool.output_schema
+    end
+
     tool_names = @mcp_client.tools.map(&:name)
     puts "\nConnected to server with tools: #{tool_names}"
   end
@@ -96,6 +103,15 @@ class MCPClient
         result = @mcp_client.call_tool(name: content.name, arguments: content.input)
         response_parts << "[Calling tool #{content.name} with args #{content.input.to_json}]"
 
+        # structured_content is data the application can use directly; when a
+        # tool returns an array, count its items rather than re-reading prose.
+        structured = result.dig("result", "structuredContent")
+        unless result.dig("result", "isError")
+          @output_schemas[content.name]&.validate_result(structured)
+          response_parts << "[#{content.name} returned #{structured.length} items]" if structured.is_a?(Array)
+        end
+
+        # content is what the model reads.
         tool_result_content = result.dig("result", "content")
         result_text = if tool_result_content.is_a?(Array)
           tool_result_content.filter_map { |content_item| content_item["text"] }.join("\n")

weather-server-ruby/Gemfile

@@ -2,4 +2,4 @@
 
 source "https://rubygems.org"
 
-gem "mcp"
+gem "mcp", ">= 1.2.0"

weather-server-ruby/README.md

@@ -1,3 +1,21 @@
 # A Simple MCP Weather Server written in Ruby
 
 See the [Build an MCP server](https://modelcontextprotocol.io/docs/develop/build-server) tutorial for more information.
+
+## 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](https://modelcontextprotocol.io/specification/2026-07-28/server/tools#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.

weather-server-ruby/weather.rb

@@ -15,7 +15,7 @@ module HelperMethods
     request["User-Agent"] = USER_AGENT
     request["Accept"] = "application/geo+json"
 
-    response = Net::HTTP.start(uri.hostname, uri.port, use_ssl: true) do |http|
+    response = Net::HTTP.start(uri.hostname, uri.port, use_ssl: true, read_timeout: 30) do |http|
       http.request(request)
     end
 
@@ -24,17 +24,22 @@ module HelperMethods
     JSON.parse(response.body)
   end
 
-  def format_alert(feature)
-    properties = feature["properties"]
-
+  def format_alert(alert)
     <<~ALERT
-      Event: #{properties["event"] || "Unknown"}
-      Area: #{properties["areaDesc"] || "Unknown"}
-      Severity: #{properties["severity"] || "Unknown"}
-      Description: #{properties["description"] || "No description available"}
-      Instructions: #{properties["instruction"] || "No specific instructions provided"}
+      Event: #{alert[:event]}
+      Area: #{alert[:area]}
+      Severity: #{alert[:severity]}
+      Description: #{alert[:description]}
+      Instructions: #{alert[:instructions]}
     ALERT
   end
+
+  # A tool that declares an output schema MUST return conforming structured
+  # content, so a failure path has to be an error result rather than a bare
+  # text one. Error results are exempt from output schema validation.
+  def error_response(text)
+    MCP::Tool::Response.new([{ type: "text", text: text }], error: true)
+  end
 end
 
 class GetAlerts < MCP::Tool
@@ -52,22 +57,57 @@ class GetAlerts < MCP::Tool
     required: ["state"]
   )
 
-  def self.call(state:)
-    url = "#{NWS_API_BASE}/alerts/active/area/#{state.upcase}"
-    data = make_nws_request(url)
+  # A top-level array, not an object wrapping one. Declaring `type` at the root
+  # suppresses the object default the SDK would otherwise apply, and revision
+  # 2026-07-28 is the first to allow a non-object root.
+  output_schema(
+    type: "array",
+    items: {
+      type: "object",
+      properties: {
+        event: { type: "string", description: "The kind of weather event" },
+        area: { type: "string", description: "The area the alert covers" },
+        severity: { type: "string", description: "How severe the event is" },
+        description: { type: "string", description: "What is happening" },
+        instructions: { type: "string", description: "What people in the area should do" }
+      },
+      required: ["event", "area", "severity", "description", "instructions"]
+    }
+  )
 
-    if data["features"].empty?
-      return MCP::Tool::Response.new([{
-        type: "text",
-        text: "No active alerts for this state."
-      }])
+  def self.call(state:, server_context: nil)
+    data = begin
+      make_nws_request("#{NWS_API_BASE}/alerts/active/area/#{state.upcase}")
+    rescue => e
+      return error_response("Unable to fetch alerts for #{state.upcase}: #{e.message}")
     end
 
-    alerts = data["features"].map { |feature| format_alert(feature) }
-    MCP::Tool::Response.new([{
-      type: "text",
-      text: alerts.join("\n---\n")
-    }])
+    unless data.is_a?(Hash) && data["features"]
+      return error_response("Unable to fetch alerts for #{state.upcase}.")
+    end
+
+    # An empty result is an empty array, not an error.
+    alerts = data["features"].map do |feature|
+      properties = feature["properties"] || {}
+      {
+        event: properties["event"] || "Unknown",
+        area: properties["areaDesc"] || "Unknown",
+        severity: properties["severity"] || "Unknown",
+        description: properties["description"] || "No description available",
+        instructions: properties["instruction"] || "No specific instructions provided"
+      }
+    end
+
+    text = if alerts.empty?
+      "No active alerts for this state."
+    else
+      alerts.map { |alert| format_alert(alert) }.join("\n---\n")
+    end
+
+    MCP::Tool::Response.new(
+      [{ type: "text", text: text }],
+      structured_content: alerts
+    )
   end
 end
 
@@ -90,37 +130,88 @@ class GetForecast < MCP::Tool
     required: ["latitude", "longitude"]
   )
 
-  def self.call(latitude:, longitude:)
-    # First get the forecast grid endpoint.
-    points_url = "#{NWS_API_BASE}/points/#{latitude},#{longitude}"
-    points_data = make_nws_request(points_url)
+  # The object case, for contrast with get_alerts' array root.
+  output_schema(
+    type: "object",
+    properties: {
+      latitude: { type: "number", description: "Latitude the forecast is for" },
+      longitude: { type: "number", description: "Longitude the forecast is for" },
+      periods: {
+        type: "array",
+        description: "The forecast periods, soonest first",
+        items: {
+          type: "object",
+          properties: {
+            name: { type: "string" },
+            temperature: { type: "integer" },
+            temperature_unit: { type: "string" },
+            wind_speed: { type: "string" },
+            wind_direction: { type: "string" },
+            detailed_forecast: { type: "string" }
+          },
+          required: ["name", "temperature", "temperature_unit", "wind_speed", "wind_direction", "detailed_forecast"]
+        }
+      }
+    },
+    required: ["latitude", "longitude", "periods"]
+  )
 
-    # Get the forecast URL from the points response.
-    forecast_url = points_data["properties"]["forecast"]
-    forecast_data = make_nws_request(forecast_url)
+  def self.call(latitude:, longitude:, server_context: nil)
+    forecast_data = begin
+      # First get the forecast grid endpoint.
+      points_data = make_nws_request("#{NWS_API_BASE}/points/#{latitude},#{longitude}")
+      forecast_url = points_data.dig("properties", "forecast")
+      return error_response("Unable to fetch forecast data for this location.") unless forecast_url
 
-    # Format the periods into a readable forecast.
-    periods = forecast_data["properties"]["periods"]
-    forecasts = periods.first(5).map do |period|
-      <<~FORECAST
-        #{period["name"]}:
-        Temperature: #{period["temperature"]}°#{period["temperatureUnit"]}
-        Wind: #{period["windSpeed"]} #{period["windDirection"]}
-        Forecast: #{period["detailedForecast"]}
-      FORECAST
+      make_nws_request(forecast_url)
+    rescue => e
+      return error_response("Unable to fetch forecast data for this location: #{e.message}")
     end
 
-    MCP::Tool::Response.new([{
-      type: "text",
-      text: forecasts.join("\n---\n")
-    }])
+    # Only show the next 5 periods.
+    periods = (forecast_data.dig("properties", "periods") || []).first(5)
+    return error_response("No forecast periods available.") if periods.empty?
+
+    forecast = {
+      latitude: latitude,
+      longitude: longitude,
+      periods: periods.map do |period|
+        {
+          name: period["name"],
+          temperature: period["temperature"],
+          temperature_unit: period["temperatureUnit"],
+          wind_speed: period["windSpeed"],
+          wind_direction: period["windDirection"],
+          detailed_forecast: period["detailedForecast"]
+        }
+      end
+    }
+
+    text = forecast[:periods].map do |period|
+      <<~FORECAST
+        #{period[:name]}:
+        Temperature: #{period[:temperature]}°#{period[:temperature_unit]}
+        Wind: #{period[:wind_speed]} #{period[:wind_direction]}
+        Forecast: #{period[:detailed_forecast]}
+      FORECAST
+    end.join("\n---\n")
+
+    MCP::Tool::Response.new(
+      [{ type: "text", text: text }],
+      structured_content: forecast
+    )
   end
 end
 
 server = MCP::Server.new(
   name: "weather",
   version: "1.0.0",
-  tools: [GetAlerts, GetForecast]
+  tools: [GetAlerts, GetForecast],
+  # The gem otherwise advertises prompts, resources and logging as well. This
+  # server has only tools, and logging is deprecated as of 2026-07-28
+  # (SEP-2577). No listChanged: change notifications are delivered over
+  # `subscriptions/listen`, which this server does not serve.
+  capabilities: { tools: {} }
 )
 
 transport = MCP::Server::Transports::StdioTransport.new(server)