diff --git a/apps/desktop/src/main/todo-agent/trpc-router.ts b/apps/desktop/src/main/todo-agent/trpc-router.ts
index 05e679a8929..d288606741e 100644
--- a/apps/desktop/src/main/todo-agent/trpc-router.ts
+++ b/apps/desktop/src/main/todo-agent/trpc-router.ts
@@ -220,11 +220,12 @@ export const createTodoAgentRouter = () => {
 
 		/**
 		 * Edit the user-authored fields (description / goal) of a TODO
-		 * session that has not started yet. Allowed only in pre-start
-		 * states (queued / failed / aborted / escalated) so a running
-		 * worker's prompt can never mutate under its feet. When the
-		 * description / goal changes we rewrite the session's goal.md
-		 * so the next run picks up the edit.
+		 * session. Allowed in queued / preparing / failed / aborted /
+		 * escalated. `preparing` is safe because the supervisor has
+		 * not spawned Claude yet and `prepareArtifacts` will rewrite
+		 * goal.md before it is read. Refused once the session is
+		 * running / verifying so the worker's prompt never mutates
+		 * under its feet.
 		 */
 		updateFields: publicProcedure
 			.input(
@@ -251,6 +252,7 @@ export const createTodoAgentRouter = () => {
 				}
 				if (
 					session.status !== "queued" &&
+					session.status !== "preparing" &&
 					session.status !== "failed" &&
 					session.status !== "aborted" &&
 					session.status !== "escalated"
@@ -258,7 +260,7 @@ export const createTodoAgentRouter = () => {
 					throw new TRPCError({
 						code: "PRECONDITION_FAILED",
 						message:
-							"実行中またはキュー済みでないセッションは編集できません。中断してから再度お試しください。",
+							"実行中のセッションは編集できません。中断してから再度お試しください。",
 					});
 				}
 				const patch: {
diff --git a/apps/desktop/src/renderer/features/todo-agent/TodoManager/TodoManager.tsx b/apps/desktop/src/renderer/features/todo-agent/TodoManager/TodoManager.tsx
index b807cb53a15..aa1d349f95b 100644
--- a/apps/desktop/src/renderer/features/todo-agent/TodoManager/TodoManager.tsx
+++ b/apps/desktop/src/renderer/features/todo-agent/TodoManager/TodoManager.tsx
@@ -1030,7 +1030,25 @@ function SessionDetail({ session, onDeleted }: SessionDetailProps) {
 		}
 	}, [invalidate, rerunMut, session.id]);
 
-	const canEditFields = canStart && !isRunning;
+	// `preparing` is still editable: the supervisor has not spawned
+	// Claude yet, and prepareArtifacts rewrites goal.md before Claude
+	// reads it, so an edit during preparing still takes effect.
+	const canEditFields = canStart || session.status === "preparing";
+
+	// Bail out of any in-flight edit the moment the session starts
+	// actually running — e.g. a queued session whose turn arrived
+	// mid-edit — so the user doesn't hit Save only to get rejected by
+	// the backend guard. Only cancel on running/verifying; terminal
+	// states stay editable (rerun path).
+	useEffect(() => {
+		if (!editingField) return;
+		if (session.status !== "running" && session.status !== "verifying") return;
+		setEditingField(null);
+		setEditDraft("");
+		toast.warning(
+			"タスクの実行が開始されたため編集を中止しました。中断してから再度編集してください。",
+		);
+	}, [editingField, session.status]);
 
 	const startEditField = useCallback(
 		(field: "description" | "goal") => {