Add structured content to tool responses when possible

Author: rhernandez35

mcp/mcp-schemas/model/main.smithy

@@ -171,6 +171,8 @@ structure CallToolResult {
     // TODO add others
     content: TextContentList
 
+    structuredContent: Document
+
     @default(false)
     isError: Boolean
 }

mcp/mcp-server/src/main/java/software/amazon/smithy/java/mcp/server/McpServer.java

@@ -170,8 +170,7 @@ private void handleRequest(JsonRpcRequest req) {
                     writeResponse(req.getId(), result);
                 }
                 case "tools/list" -> {
-                    boolean supportsOutputSchema = protocolVersion != null
-                            && protocolVersion.compareTo(ProtocolVersion.v2025_06_18.INSTANCE) >= 0;
+                    var supportsOutputSchema = supportsOutputSchema();
                     writeResponse(req.getId(),
                             ListToolsResult.builder()
                                     .tools(tools.values()
@@ -223,11 +222,7 @@ private void handleRequest(JsonRpcRequest req) {
                         var adaptedDoc = adaptDocument(argumentsDoc, operation.getApiOperation().inputSchema());
                         var input = adaptedDoc.asShape(operation.getApiOperation().inputBuilder());
                         var output = operation.function().apply(input, null);
-                        var result = CallToolResult.builder()
-                                .content(List.of(TextContent.builder()
-                                        .text(CODEC.serializeToString((SerializableShape) output))
-                                        .build()))
-                                .build();
+                        var result = formatStructuredContent(tool, (SerializableShape) output);
                         writeResponse(req.getId(), result);
                     }
                 }
@@ -240,6 +235,26 @@ private void handleRequest(JsonRpcRequest req) {
         }
     }
 
+    private boolean supportsOutputSchema() {
+        return protocolVersion != null && protocolVersion.compareTo(ProtocolVersion.v2025_06_18.INSTANCE) >= 0;
+    }
+
+    private CallToolResult formatStructuredContent(Tool tool, SerializableShape output) {
+        var result = CallToolResult.builder()
+                .content(List.of(TextContent.builder()
+                        .text(CODEC.serializeToString(output))
+                        .build()));
+
+        if (supportsOutputSchema()) {
+            var outputSchema = tool.toolInfo().getOutputSchema();
+            if (outputSchema != null) {
+                result.structuredContent(Document.of(output));
+            }
+        }
+
+        return result.build();
+    }
+
     private ToolInfo extractToolInfo(Tool tool, boolean supportsOutput) {
         var toolInfo = tool.toolInfo();
         if (supportsOutput || toolInfo.getOutputSchema() == null) {