feature: Add a tool with Structured Content and an Output Schema

Tools gained Structured Content in Specification 2025-16-18
https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content

The MCP Inspector is able to handle these outputs.

src/everything/README.md

@@ -80,6 +80,15 @@ This MCP server attempts to exercise all the features of the MCP protocol. It is
       - `pets` (enum): Favorite pet
    - Returns: Confirmation of the elicitation demo with selection summary.
 
+10. `structuredContent`
+   - Demonstrates a tool returning structured content using the example in the specification
+   - Provides an output schema to allow testing of client SHOULD advisory to validate the result using the schema
+   - Inputs:
+     - `location` (string): A location or ZIP code, mock data is returned regardless of value
+   - Returns: a response with
+     - `structuredContent` field conformant to the output schema
+     - A backward compatible Text Content field, a SHOULD advisory in the specification
+
 ### Resources
 
 The server provides 100 test resources in two formats:

src/everything/everything.ts

@@ -31,6 +31,9 @@ const instructions = readFileSync(join(__dirname, "instructions.md"), "utf-8");
 const ToolInputSchema = ToolSchema.shape.inputSchema;
 type ToolInput = z.infer<typeof ToolInputSchema>;
 
+const ToolOutputSchema = ToolSchema.shape.outputSchema;
+type ToolOutput = z.infer<typeof ToolOutputSchema>;
+
 /* Input schemas for tools implemented in this server */
 const EchoSchema = z.object({
   message: z.string().describe("Message to echo"),
@@ -93,6 +96,28 @@ const GetResourceLinksSchema = z.object({
     .describe("Number of resource links to return (1-10)"),
 });
 
+const StructuredContentSchema = {
+  input: z.object({
+    location: z
+      .string()
+      .trim()
+      .min(1)
+      .describe("City name or zip code"),
+  }),
+
+  output: z.object({
+    temperature: z
+      .number()
+      .describe("Temperature in celsius"),
+    conditions: z
+      .string()
+      .describe("Weather conditions description"),
+    humidity: z
+      .number()
+      .describe("Humidity percentage"),
+  })
+};
+
 enum ToolName {
   ECHO = "echo",
   ADD = "add",
@@ -104,6 +129,7 @@ enum ToolName {
   GET_RESOURCE_REFERENCE = "getResourceReference",
   ELICITATION = "startElicitation",
   GET_RESOURCE_LINKS = "getResourceLinks",
+  STRUCTURED_CONTENT = "structuredContent"
 }
 
 enum PromptName {
@@ -503,6 +529,13 @@ export const createServer = () => {
           "Returns multiple resource links that reference different types of resources",
         inputSchema: zodToJsonSchema(GetResourceLinksSchema) as ToolInput,
       },
+      {
+        name: ToolName.STRUCTURED_CONTENT,
+        description:
+          "Returns structured content along with an output schema for client data validation",
+        inputSchema: zodToJsonSchema(StructuredContentSchema.input) as ToolInput,
+        outputSchema: zodToJsonSchema(StructuredContentSchema.output) as ToolOutput,
+      },
     ];
 
     return { tools };
@@ -777,6 +810,27 @@ export const createServer = () => {
       return { content };
     }
 
+    if (name === ToolName.STRUCTURED_CONTENT) {
+      // The same response is returned for every input.
+      const validatedArgs = StructuredContentSchema.input.parse(args);
+
+      const weather = {
+        temperature: 22.5,
+        conditions: "Partly cloudy",
+        humidity: 65
+      }
+
+      const backwardCompatiblecontent = {
+        type: "text",
+        text: JSON.stringify(weather)
+      }
+
+      return {
+        content: [ backwardCompatiblecontent ],
+        structuredContent: weather
+      };
+    }
+
     throw new Error(`Unknown tool: ${name}`);
   });