diff --git a/server/src/router.rs b/server/src/router.rs
index 326ae3a..6916b85 100644
--- a/server/src/router.rs
+++ b/server/src/router.rs
@@ -353,4 +353,58 @@ mod tests {
         assert_eq!(res_list["id"], 1);
         assert!(res_list["result"]["tools"].as_array().unwrap().len() > 10);
     }
+
+    #[tokio::test]
+    async fn test_tool_call_success_and_error_responses() {
+        let dir = tempdir().unwrap();
+        let state = Arc::new(MemoryState::new(dir.path().to_str().unwrap()));
+        let handler = MemoryHandler::new(state);
+
+        // 1. Test successful tool call (e.g. read_graph)
+        let req_success = json!({
+            "jsonrpc": "2.0",
+            "id": 2,
+            "method": "tools/call",
+            "params": {
+                "name": "read_graph",
+                "arguments": {}
+            }
+        });
+        let res_success = handler.handle_request(req_success).await.unwrap();
+        assert_eq!(res_success["jsonrpc"], "2.0");
+        assert_eq!(res_success["id"], 2);
+        // A successful tool call should return a result with isError: false
+        assert_eq!(res_success["result"]["isError"], false);
+        assert!(res_success["result"]["content"].as_array().is_some());
+
+        // 2. Test tool call semantic failure (e.g. updating a task that does not exist)
+        let req_fail = json!({
+            "jsonrpc": "2.0",
+            "id": 3,
+            "method": "tools/call",
+            "params": {
+                "name": "update_task_status",
+                "arguments": {
+                    "id": "nonexistent_task_123",
+                    "status": "in_progress"
+                }
+            }
+        });
+        let res_fail = handler.handle_request(req_fail).await.unwrap();
+        assert_eq!(res_fail["jsonrpc"], "2.0");
+        assert_eq!(res_fail["id"], 3);
+        // Semantic failures must explicitly return isError: true inside the result to halt the LLM
+        assert_eq!(res_fail["result"]["isError"], true);
+        assert!(res_fail["result"]["content"][0]["text"].as_str().unwrap().contains("not found"));
+
+        // 3. Test unknown JSON-RPC method returns JSON-RPC protocol error
+        let req_unknown = json!({
+            "jsonrpc": "2.0",
+            "id": 4,
+            "method": "unknown_method_xyz"
+        });
+        let res_unknown = handler.handle_request(req_unknown).await.unwrap();
+        assert!(res_unknown.get("error").is_some());
+        assert_eq!(res_unknown["error"]["code"], -32601);
+    }
 }