diff --git a/src/client/index.ts b/src/client/index.ts
index 03a6b40b5..f3c6bf97d 100644
--- a/src/client/index.ts
+++ b/src/client/index.ts
@@ -757,7 +757,7 @@ export class Client<
             }
 
             // Only validate structured content if present (not when there's an error)
-            if (result.structuredContent) {
+            if (result.structuredContent && !result.isError) {
                 try {
                     // Validate the structured content against the schema
                     const validationResult = validator(result.structuredContent);
diff --git a/src/experimental/tasks/client.ts b/src/experimental/tasks/client.ts
index f62941dc8..5c70b3b51 100644
--- a/src/experimental/tasks/client.ts
+++ b/src/experimental/tasks/client.ts
@@ -122,7 +122,7 @@ export class ExperimentalClientTasks<
                 }
 
                 // Only validate structured content if present (not when there's an error)
-                if (result.structuredContent) {
+                if (result.structuredContent && !result.isError) {
                     try {
                         // Validate the structured content against the schema
                         const validationResult = validator(result.structuredContent);
diff --git a/test/client/index.test.ts b/test/client/index.test.ts
index f5c6a348d..31e5d2f41 100644
--- a/test/client/index.test.ts
+++ b/test/client/index.test.ts
@@ -1871,6 +1871,102 @@ describe('outputSchema validation', () => {
         expect(result.structuredContent).toEqual({ result: 'success', count: 42 });
     });
 
+    /***
+     * Test: Do Not Validate structuredContent when isError is true
+     */
+    test('should not validate structuredContent when tool returns isError', async () => {
+        const server = new Server(
+            {
+                name: 'test-server',
+                version: '1.0.0'
+            },
+            {
+                capabilities: {
+                    tools: {}
+                }
+            }
+        );
+
+        // Set up server handlers
+        server.setRequestHandler(InitializeRequestSchema, async request => ({
+            protocolVersion: request.params.protocolVersion,
+            capabilities: { tools: {} },
+            serverInfo: {
+                name: 'test-server',
+                version: '1.0.0'
+            }
+        }));
+
+        server.setRequestHandler(ListToolsRequestSchema, async () => ({
+            tools: [
+                {
+                    name: 'test-tool',
+                    description: 'A test tool',
+                    inputSchema: {
+                        type: 'object',
+                        properties: {}
+                    },
+                    outputSchema: {
+                        type: 'object',
+                        properties: {
+                            result: { type: 'string' }
+                        },
+                        required: ['result']
+                    }
+                }
+            ]
+        }));
+
+        server.setRequestHandler(CallToolRequestSchema, async request => {
+            if (request.params.name === 'test-tool') {
+                return {
+                    content: [{ type: 'text', text: 'Tool failed' }],
+                    structuredContent: {},
+                    isError: true
+                };
+            }
+            throw new Error('Unknown tool');
+        });
+
+        const client = new Client(
+            {
+                name: 'test-client',
+                version: '1.0.0'
+            },
+            {
+                capabilities: {
+                    tasks: {
+                        requests: {
+                            tools: {
+                                call: {}
+                            },
+                            tasks: {
+                                get: true,
+                                list: {},
+                                result: true
+                            }
+                        }
+                    }
+                }
+            }
+        );
+
+        const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+
+        await Promise.all([client.connect(clientTransport), server.connect(serverTransport)]);
+
+        // List tools to cache the schemas
+        await client.listTools();
+
+        const result = await client.callTool({ name: 'test-tool' });
+        expect(result.isError).toBe(true);
+        expect(result.structuredContent).toEqual({});
+        expect(result.content).toEqual([{ type: 'text', text: 'Tool failed' }]);
+
+        await client.close();
+        await server.close();
+    });
+
     /***
      * Test: Throw Error when structuredContent Does Not Match Schema
      */