Improve handling of tool responses, adding explicit classes

mcp-server/src/index.ts

@@ -19,7 +19,7 @@
  */
 
 import { Server } from "@modelcontextprotocol/sdk/server/index.js";
-import { CallToolRequestSchema, ListToolsRequestSchema } from "@modelcontextprotocol/sdk/types.js";
+import { CallToolRequestSchema, CallToolResult, ListToolsRequestSchema } from "@modelcontextprotocol/sdk/types.js";
 import { WebSocketServer, WebSocket } from "ws";
 
 import { Tool } from "./interfaces/Tool.js";
@@ -100,7 +100,7 @@ class PenpotMcpServer {
             };
         });
 
-        this.server.setRequestHandler(CallToolRequestSchema, async (request) => {
+        this.server.setRequestHandler(CallToolRequestSchema, async (request): Promise<CallToolResult> => {
             const { name, arguments: args } = request.params;
 
             const tool = this.tools.get(name);

mcp-server/src/interfaces/Tool.ts

@@ -2,6 +2,7 @@ import { Tool as MCPTool } from "@modelcontextprotocol/sdk/types.js";
 import { validate, ValidationError } from "class-validator";
 import { plainToClass } from "class-transformer";
 import "reflect-metadata";
+import { ToolResponse } from "./ToolResponse";
 
 /**
  * Defines the contract for MCP tool implementations.
@@ -21,7 +22,7 @@ export interface Tool {
      * @param args - The arguments passed to the tool (validated by implementation)
      * @returns A promise that resolves to the tool's execution result
      */
-    execute(args: unknown): Promise<{ content: Array<{ type: string; text: string }> }>;
+    execute(args: unknown): Promise<ToolResponse>;
 }
 
 /**
@@ -66,7 +67,7 @@ export abstract class TypeSafeTool<TArgs extends object> implements Tool {
      * This method handles the unknown args from the MCP protocol,
      * validates them, and delegates to the type-safe implementation.
      */
-    async execute(args: unknown): Promise<{ content: Array<{ type: string; text: string }> }> {
+    async execute(args: unknown): Promise<ToolResponse> {
         try {
             // Transform plain object to class instance
             const argsInstance = plainToClass(this.ArgsClass, args as object);
@@ -207,5 +208,5 @@ export abstract class TypeSafeTool<TArgs extends object> implements Tool {
      *
      * @param args - The validated, strongly-typed arguments
      */
-    protected abstract executeTypeSafe(args: TArgs): Promise<{ content: Array<{ type: string; text: string }> }>;
+    protected abstract executeTypeSafe(args: TArgs): Promise<ToolResponse>;
 }

mcp-server/src/interfaces/ToolResponse.ts

@@ -0,0 +1,20 @@
+import type { CallToolResult } from "@modelcontextprotocol/sdk/types.js";
+
+type CallToolContent = CallToolResult["content"][number];
+type TextItem = Extract<CallToolContent, { type: "text" }>;
+
+class TextContent implements TextItem {
+    [x: string]: unknown;
+    readonly type = "text" as const;
+    constructor(public text: string) {}
+}
+
+export class TextResponse implements CallToolResult {
+    [x: string]: unknown;
+    content: CallToolContent[]; // <- IMPORTANT: protocol’s union
+    constructor(text: string) {
+        this.content = [new TextContent(text)];
+    }
+}
+
+export type ToolResponse = TextResponse;

mcp-server/src/tools/HelloWorldTool.ts

@@ -1,6 +1,8 @@
-import { IsString, IsNotEmpty } from "class-validator";
+import { IsNotEmpty, IsString } from "class-validator";
 import { TypeSafeTool } from "../interfaces/Tool.js";
 import "reflect-metadata";
+import type { ToolResponse } from "../interfaces/ToolResponse.js";
+import { TextResponse } from "../interfaces/ToolResponse.js";
 
 /**
  * Arguments class for the HelloWorld tool with validation decorators.
@@ -41,14 +43,9 @@ export class HelloWorldTool extends TypeSafeTool<HelloWorldArgs> {
      *
      * @param args - The validated HelloWorldArgs instance
      */
-    protected async executeTypeSafe(args: HelloWorldArgs): Promise<{ content: Array<{ type: string; text: string }> }> {
-        return {
-            content: [
-                {
-                    type: "text",
-                    text: `Hello, ${args.name}! This greeting was generated with full type safety and automatic validation.`,
-                },
-            ],
-        };
+    protected async executeTypeSafe(args: HelloWorldArgs): Promise<ToolResponse> {
+        return new TextResponse(
+            `Hello, ${args.name}! This greeting was generated with full type safety and automatic validation.`
+        );
     }
 }

mcp-server/src/tools/ToolPrintText.ts

@@ -1,6 +1,8 @@
-import { IsString, IsNotEmpty } from "class-validator";
+import { IsNotEmpty, IsString } from "class-validator";
 import { TypeSafeTool } from "../interfaces/Tool.js";
 import { PluginTaskPrintText, PluginTaskPrintTextParams } from "../interfaces/PluginTask.js";
+import type { ToolResponse } from "../interfaces/ToolResponse.js";
+import { TextResponse } from "../interfaces/ToolResponse.js";
 import "reflect-metadata";
 
 /**
@@ -50,7 +52,7 @@ export class ToolPrintText extends TypeSafeTool<PrintTextArgs> {
      *
      * @param args - The validated PrintTextArgs instance
      */
-    protected async executeTypeSafe(args: PrintTextArgs): Promise<{ content: Array<{ type: string; text: string }> }> {
+    protected async executeTypeSafe(args: PrintTextArgs): Promise<ToolResponse> {
         try {
             // Create the plugin task
             const taskParams = new PluginTaskPrintTextParams(args.text);
@@ -58,14 +60,9 @@ export class ToolPrintText extends TypeSafeTool<PrintTextArgs> {
 
             // Check if there are connected clients
             if (this.connectedClients.size === 0) {
-                return {
-                    content: [
-                        {
-                            type: "text",
-                            text: `No Penpot plugin instances are currently connected. Please ensure the plugin is running and connected.`,
-                        },
-                    ],
-                };
+                return new TextResponse(
+                    `No Penpot plugin instances are currently connected. Please ensure the plugin is running and connected.`
+                );
             }
 
             // Send task to all connected clients
@@ -81,34 +78,17 @@ export class ToolPrintText extends TypeSafeTool<PrintTextArgs> {
             });
 
             if (sentCount === 0) {
-                return {
-                    content: [
-                        {
-                            type: "text",
-                            text: `All connected plugin instances appear to be disconnected. No text was created.`,
-                        },
-                    ],
-                };
+                return new TextResponse(
+                    `All connected plugin instances appear to be disconnected. No text was created.`
+                );
             }
 
-            return {
-                content: [
-                    {
-                        type: "text",
-                        text: `Successfully sent text creation task to ${sentCount} connected plugin instance(s). Text "${args.text}" should now appear in Penpot.`,
-                    },
-                ],
-            };
+            return new TextResponse(
+                `Successfully sent text creation task to ${sentCount} connected plugin instance(s). Text "${args.text}" should now appear in Penpot.`
+            );
         } catch (error) {
             const errorMessage = error instanceof Error ? error.message : "Unknown error";
-            return {
-                content: [
-                    {
-                        type: "text",
-                        text: `Failed to create text in Penpot: ${errorMessage}`,
-                    },
-                ],
-            };
+            return new TextResponse(`Failed to create text in Penpot: ${errorMessage}`);
         }
     }
 }