Fix/mcp agent error handling

.changeset/tricky-weeks-reply.md

@@ -0,0 +1,5 @@
+---
+"agents": patch
+---
+
+Fix/mcp agent error handling

examples/mcp/src/server.ts

@@ -47,6 +47,34 @@ export class MyMCP extends McpAgent<Env, State, {}> {
   onStateUpdate(state: State) {
     console.log({ stateUpdate: state });
   }
+
+  onError(error: Error): { status: number; message: string } {
+    console.error("MyMCP initialization error:", error);
+
+    // Provide more specific error messages based on error type
+    if (error.message.includes("counter")) {
+      return {
+        status: 500,
+        message:
+          "Failed to initialize counter resource. Please check the counter configuration."
+      };
+    }
+
+    if (error.message.includes("tool")) {
+      return {
+        status: 500,
+        message:
+          "Failed to register MCP tools. Please verify tool configurations."
+      };
+    }
+
+    // Fall back to default error handling
+    return {
+      status: 500,
+      message:
+        error.message || "An unexpected error occurred during initialization"
+    };
+  }
 }
 
 export default MyMCP.serve("/mcp", {

packages/agents/src/mcp/index.ts

@@ -301,6 +301,21 @@ export abstract class McpAgent<
 
   abstract init(): Promise<void>;
 
+  /**
+   * Handle errors that occur during initialization or operation.
+   * Override this method to provide custom error handling.
+   * @param error - The error that occurred
+   * @returns An error response object with status code and message
+   */
+  onError(error: Error): { status: number; message: string } {
+    console.error("McpAgent error:", error);
+    return {
+      status: 500,
+      message:
+        error.message || "An unexpected error occurred during initialization"
+    };
+  }
+
   async _init(props: Props) {
     await this.ctx.storage.put("props", props ?? {});
     if (!this.ctx.storage.get("transportType")) {
@@ -309,7 +324,12 @@ export abstract class McpAgent<
     this.props = props;
     if (!this.initRun) {
       this.initRun = true;
-      await this.init();
+      try {
+        await this.init();
+      } catch (error) {
+        const errorResponse = this.onError(error as Error);
+        throw new Error(`Initialization failed: ${errorResponse.message}`);
+      }
     }
   }
 
@@ -607,7 +627,17 @@ export abstract class McpAgent<
           const doStub = namespace.get(id);
 
           // Initialize the object
-          await doStub._init(ctx.props);
+          try {
+            await doStub._init(ctx.props);
+          } catch (error) {
+            console.error("Failed to initialize McpAgent:", error);
+            await writer.close();
+            const errorMessage =
+              error instanceof Error ? error.message : String(error);
+            return new Response(`Initialization failed: ${errorMessage}`, {
+              status: 500
+            });
+          }
 
           // Connect to the Durable Object via WebSocket
           const upgradeUrl = new URL(request.url);
@@ -968,8 +998,23 @@ export abstract class McpAgent<
           const isInitialized = await doStub.isInitialized();
 
           if (isInitializationRequest) {
-            await doStub._init(ctx.props);
-            await doStub.setInitialized();
+            try {
+              await doStub._init(ctx.props);
+              await doStub.setInitialized();
+            } catch (error) {
+              console.error("Failed to initialize McpAgent:", error);
+              const errorMessage =
+                error instanceof Error ? error.message : String(error);
+              const body = JSON.stringify({
+                error: {
+                  code: -32001,
+                  message: `Initialization failed: ${errorMessage}`
+                },
+                id: null,
+                jsonrpc: "2.0"
+              });
+              return new Response(body, { status: 500 });
+            }
           } else if (!isInitialized) {
             // if we have gotten here, then a session id that was never initialized
             // was provided