diff --git a/src/core/tools/NewTaskTool.ts b/src/core/tools/NewTaskTool.ts
index f36d8e1e37..bc95d9edb6 100644
--- a/src/core/tools/NewTaskTool.ts
+++ b/src/core/tools/NewTaskTool.ts
@@ -109,16 +109,23 @@ export class NewTaskTool extends BaseTool<"new_task"> {
 				return
 			}
 
+			// IMPORTANT: Push the tool_result BEFORE delegation, because delegateParentAndOpenChild
+			// disposes the parent task. If we push after, the tool_result is lost and
+			// flushPendingToolResultsToHistory will generate a placeholder "interrupted" tool_result,
+			// causing duplicate tool_results when the child completes (EXT-665).
+			//
+			// The child taskId isn't known yet, so we use a generic message. The actual completion
+			// result will be injected by reopenParentFromDelegation when the child completes.
+			pushToolResult(`Delegating to subtask...`)
+
 			// Delegate parent and open child as sole active task
-			const child = await (provider as any).delegateParentAndOpenChild({
+			await (provider as any).delegateParentAndOpenChild({
 				parentTaskId: task.taskId,
 				message: unescapedMessage,
 				initialTodos: todoItems,
 				mode,
 			})
 
-			// Reflect delegation in tool result (no pause/unpause, no wait)
-			pushToolResult(`Delegated to child task ${child.taskId}`)
 			return
 		} catch (error) {
 			await handleError("creating new task", error)
diff --git a/src/core/tools/__tests__/newTaskTool.spec.ts b/src/core/tools/__tests__/newTaskTool.spec.ts
index fc383c13ee..5bce727391 100644
--- a/src/core/tools/__tests__/newTaskTool.spec.ts
+++ b/src/core/tools/__tests__/newTaskTool.spec.ts
@@ -175,7 +175,7 @@ describe("newTaskTool", () => {
 		)
 
 		// Verify side effects
-		expect(mockPushToolResult).toHaveBeenCalledWith(expect.stringContaining("Delegated to child task"))
+		expect(mockPushToolResult).toHaveBeenCalledWith("Delegating to subtask...")
 	})
 
 	it("should not un-escape single escaped \@", async () => {
@@ -280,7 +280,7 @@ describe("newTaskTool", () => {
 		expect(mockStartSubtask).toHaveBeenCalledWith("Test message", [], "code")
 
 		// Should complete successfully
-		expect(mockPushToolResult).toHaveBeenCalledWith(expect.stringContaining("Delegated to child task"))
+		expect(mockPushToolResult).toHaveBeenCalledWith("Delegating to subtask...")
 	})
 
 	it("should work with todos parameter when provided", async () => {
@@ -311,7 +311,7 @@ describe("newTaskTool", () => {
 			"code",
 		)
 
-		expect(mockPushToolResult).toHaveBeenCalledWith(expect.stringContaining("Delegated to child task"))
+		expect(mockPushToolResult).toHaveBeenCalledWith("Delegating to subtask...")
 	})
 
 	it("should error when mode parameter is missing", async () => {
@@ -423,7 +423,7 @@ describe("newTaskTool", () => {
 			expect(mockStartSubtask).toHaveBeenCalledWith("Test message", [], "code")
 
 			// Should complete successfully
-			expect(mockPushToolResult).toHaveBeenCalledWith(expect.stringContaining("Delegated to child task"))
+			expect(mockPushToolResult).toHaveBeenCalledWith("Delegating to subtask...")
 		})
 
 		it("should REQUIRE todos when VSCode setting is enabled", async () => {
@@ -501,7 +501,7 @@ describe("newTaskTool", () => {
 			)
 
 			// Should complete successfully
-			expect(mockPushToolResult).toHaveBeenCalledWith(expect.stringContaining("Delegated to child task"))
+			expect(mockPushToolResult).toHaveBeenCalledWith("Delegating to subtask...")
 		})
 
 		it("should work with empty todos string when VSCode setting is enabled", async () => {
@@ -536,7 +536,7 @@ describe("newTaskTool", () => {
 			expect(mockStartSubtask).toHaveBeenCalledWith("Test message", [], "code")
 
 			// Should complete successfully
-			expect(mockPushToolResult).toHaveBeenCalledWith(expect.stringContaining("Delegated to child task"))
+			expect(mockPushToolResult).toHaveBeenCalledWith("Delegating to subtask...")
 		})
 
 		it("should check VSCode setting with Package.name configuration key", async () => {
@@ -671,7 +671,7 @@ describe("newTaskTool delegation flow", () => {
 		)
 		expect(pauseEvents.length).toBe(0)
 
-		// Assert: tool result reflects delegation
-		expect(mockPushToolResult).toHaveBeenCalledWith(expect.stringContaining("Delegated to child task child-1"))
+		// Assert: tool result reflects delegation (pushed BEFORE delegation, so no child ID yet)
+		expect(mockPushToolResult).toHaveBeenCalledWith("Delegating to subtask...")
 	})
 })