diff --git a/internal/context/prompt_test.go b/internal/context/prompt_test.go
index b442d67e..7356cbc7 100644
--- a/internal/context/prompt_test.go
+++ b/internal/context/prompt_test.go
@@ -125,6 +125,12 @@ func TestDefaultToolUsagePromptIncludesPermissionAndAntiLoopGuidance(t *testing.
 	if !strings.Contains(toolUsage, "`todo_write`") {
 		t.Fatalf("expected Tool Usage to mention todo_write for task state, got %q", toolUsage)
 	}
+	if !strings.Contains(toolUsage, "If the user clearly switches to a different task") {
+		t.Fatalf("expected Tool Usage to describe task-switch todo handling, got %q", toolUsage)
+	}
+	if !strings.Contains(toolUsage, "mark it `canceled` before planning or executing the new task") {
+		t.Fatalf("expected Tool Usage to require canceling stale todos on task switches, got %q", toolUsage)
+	}
 	if !strings.Contains(toolUsage, "Execute todos sequentially in the main loop") {
 		t.Fatalf("expected Tool Usage to enforce sequential todo execution, got %q", toolUsage)
 	}
diff --git a/internal/context/source_todos.go b/internal/context/source_todos.go
index 50f7f55c..cb58cbf2 100644
--- a/internal/context/source_todos.go
+++ b/internal/context/source_todos.go
@@ -82,7 +82,8 @@ func (todosSource) Sections(ctx context.Context, input BuildInput) ([]promptSect
 
 	lines = append(lines, "",
 		"stale_todo_reminder: If any todo above is no longer relevant to the current task,",
-		"cancel it via todo_write set_status=canceled before signaling completion.",
+		"or the user clearly switches to a different task, use todo_write to mark it completed",
+		"only if the work is actually done; otherwise set_status=canceled before moving on.",
 	)
 
 	return []promptSection{
diff --git a/internal/context/source_todos_test.go b/internal/context/source_todos_test.go
index e467ea95..5276b9fe 100644
--- a/internal/context/source_todos_test.go
+++ b/internal/context/source_todos_test.go
@@ -54,13 +54,19 @@ func TestTodosSourceSections(t *testing.T) {
 	if sections[0].Title != "Todo State" {
 		t.Fatalf("title = %q, want %q", sections[0].Title, "Todo State")
 	}
-	if strings.Contains(sections[0].Content, "done") {
+	if strings.Contains(sections[0].Content, `id="done"`) {
 		t.Fatalf("expected terminal todo filtered, got %q", sections[0].Content)
 	}
 	lines := strings.Split(sections[0].Content, "\n")
 	if len(lines) < 2 || !strings.Contains(lines[0], "in-progress") {
 		t.Fatalf("expected in_progress todo first, got %q", sections[0].Content)
 	}
+	if !strings.Contains(sections[0].Content, "user clearly switches to a different task") {
+		t.Fatalf("expected stale todo reminder to mention task switching, got %q", sections[0].Content)
+	}
+	if !strings.Contains(sections[0].Content, "only if the work is actually done") {
+		t.Fatalf("expected stale todo reminder to distinguish completed from canceled, got %q", sections[0].Content)
+	}
 }
 
 func TestTodosSourceSectionsBoundaries(t *testing.T) {
diff --git a/internal/promptasset/templates/core/tool_usage.md b/internal/promptasset/templates/core/tool_usage.md
index e80cf99f..410c4ea2 100644
--- a/internal/promptasset/templates/core/tool_usage.md
+++ b/internal/promptasset/templates/core/tool_usage.md
@@ -24,6 +24,7 @@
 - For multi-step implementation, debugging, refactoring, or long-running work, keep task state explicit via `todo_write` (plan/add/update/set_status/claim/complete/fail) instead of relying on implicit memory.
 - Create todos that map to real acceptance work, not vague activity.
 - Required todos are acceptance-relevant and must converge before finalization.
+- If the user clearly switches to a different task, do not carry unfinished todos forward blindly: mark each old todo `completed` only when the work is actually done, otherwise mark it `canceled` before planning or executing the new task.
 - `todo_write` parameters must match schema strictly: `id` must be a string (for example, `"3"` instead of `3`).
 - `todo_write` does not auto-dispatch subagents. Setting todo metadata does not trigger execution by itself.
 - `todo_write` `set_status` requires: `{"action":"set_status","id":"<todo_id>","status":"pending|in_progress|blocked|completed|failed|canceled"}`.