diff --git a/internal/context/prompt_test.go b/internal/context/prompt_test.go
index b442d67e..70517671 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 cover task switches, got %q", toolUsage)
+	}
+	if !strings.Contains(toolUsage, "otherwise mark it `canceled` before starting the new task") {
+		t.Fatalf("expected Tool Usage to explain canceled vs completed 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..5b900603 100644
--- a/internal/context/source_todos.go
+++ b/internal/context/source_todos.go
@@ -81,8 +81,8 @@ 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.",
+		"stale_todo_reminder: If the user clearly switches to a different task, do not carry old unfinished todos forward.",
+		"Mark each old todo completed only if the work is actually done; otherwise cancel it via todo_write before starting the new task.",
 	)
 
 	return []promptSection{
diff --git a/internal/context/source_todos_test.go b/internal/context/source_todos_test.go
index e467ea95..cc80a233 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, "If the user clearly switches to a different task") {
+		t.Fatalf("expected task-switch reminder in english, got %q", sections[0].Content)
+	}
+	if !strings.Contains(sections[0].Content, "completed only if the work is actually done") {
+		t.Fatalf("expected completed-vs-canceled guidance, 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..88d7349e 100644
--- a/internal/promptasset/templates/core/tool_usage.md
+++ b/internal/promptasset/templates/core/tool_usage.md
@@ -29,6 +29,7 @@
 - `todo_write` `set_status` requires: `{"action":"set_status","id":"<todo_id>","status":"pending|in_progress|blocked|completed|failed|canceled"}`.
 - `todo_write` `update` requires: `{"action":"update","id":"<todo_id>","patch":{...}}`; include `expected_revision` when known to prevent concurrent overwrite.
 - Mark todos `completed` only after the relevant artifact or verification exists.
+- If the user clearly switches to a different task, do not carry old unfinished todos forward. Mark each old todo `completed` only if the work is actually done; otherwise mark it `canceled` before starting the new task.
 - Mark todos `blocked` with a concrete reason when waiting on permission, user input, external resources, or an internal dependency.
 - Execute todos sequentially in the main loop unless the user explicitly asks for another strategy.
 - `spawn_subagent` only supports `mode=inline`: the subagent runs now and returns structured output in the same turn.