diff --git a/runtime/durable_test.go b/runtime/durable_test.go
index 435d893..6bc7fd3 100644
--- a/runtime/durable_test.go
+++ b/runtime/durable_test.go
@@ -208,3 +208,157 @@ func TestSpansUseAgreedNamesWithoutPrompt(t *testing.T) {
 		t.Fatal("prompt leaked into span attrs")
 	}
 }
+
+// ---- N3 WP1：崩溃窗口尾部配对兜底（验收① 库层机器核心）----
+
+// 窗口内死亡（persist(assistant) 之后、任何 intent 之前）：转录只有带
+// tool_calls 的 assistant。恢复后每个调用都有「进程中断」配对，且 run 继续。
+func TestResumePairsOrphanTailAfterCrashWindow(t *testing.T) {
+	dir := t.TempDir()
+	st, err := session.Open(filepath.Join(dir, "s.jsonl"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := st.Append(llm.Message{Role: llm.RoleSystem, Content: "sys"}); err != nil {
+		t.Fatal(err)
+	}
+	if err := st.Append(llm.Message{Role: llm.RoleUser, Content: "干个活"}); err != nil {
+		t.Fatal(err)
+	}
+	// 崩溃窗口形态：assistant 落盘、零意图、零结果。
+	if err := st.Append(llm.Message{
+		Role: llm.RoleAssistant, StopReason: llm.StopToolCalls,
+		ToolCalls: []llm.ToolCall{
+			{ID: "c1", Name: "write_file", Arguments: `{}`},
+			{ID: "c2", Name: "read_file", Arguments: `{}`},
+		},
+	}); err != nil {
+		t.Fatal(err)
+	}
+	reg := tools.NewRegistry(dir)
+	tool := &countingTool{name: "write_file"}
+	reg.Register(tool)
+	reg.Register(&countingTool{name: "read_file"})
+	_, err = Run(context.Background(), Config{
+		Model: "fake", Session: st, Tools: reg, MaxTurns: 2,
+		Stream: script(llm.Message{Role: llm.RoleAssistant, Content: "继续", StopReason: llm.StopEnd}),
+	}, llm.Message{Role: llm.RoleUser, Content: "还在吗"})
+	if err != nil {
+		t.Fatalf("恢复后的 Run 不得失败（配对修复应让提供方接受历史）：%v", err)
+	}
+	msgs, err := st.Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	// 双向配对断言：每个调用有结果、每个结果有调用。
+	calls, results := map[string]string{}, map[string]bool{}
+	for _, m := range msgs {
+		for _, c := range m.ToolCalls {
+			calls[c.ID] = c.Name
+		}
+		if m.Role == llm.RoleTool && m.ToolCallID != "" {
+			results[m.ToolCallID] = true
+			if m.Content != crashInterrupted && m.Content != unknownSideEffect {
+				t.Fatalf("兜底文案不对：%q", m.Content)
+			}
+			if !m.IsError {
+				t.Fatal("兜底结果必须 IsError")
+			}
+		}
+	}
+	for id := range calls {
+		if !results[id] {
+			t.Fatalf("孤立 tool_call：%s", id)
+		}
+	}
+	if tool.calls != 0 {
+		t.Fatalf("兜底不得重放写工具：%d", tool.calls)
+	}
+}
+
+// 执行中死亡（c1 意图+结果、c2 意图无结果）+ 窗口死亡（c3 无意图）混合：
+// 意图路径配 c2、尾部兜底配 c3，互不重复。
+func TestResumeMixedIntentAndOrphanTail(t *testing.T) {
+	dir := t.TempDir()
+	st, err := session.Open(filepath.Join(dir, "s.jsonl"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := st.Append(llm.Message{
+		Role: llm.RoleAssistant, StopReason: llm.StopToolCalls,
+		ToolCalls: []llm.ToolCall{
+			{ID: "c1", Name: "read_file", Arguments: `{}`},
+			{ID: "c2", Name: "read_file", Arguments: `{}`},
+			{ID: "c3", Name: "write_file", Arguments: `{}`},
+		},
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if err := st.AppendToolIntent(session.ToolIntent{CallID: "c1", Name: "read_file", Arguments: `{}`, Replay: tools.ReplaySafe}); err != nil {
+		t.Fatal(err)
+	}
+	if err := st.Append(llm.Message{Role: llm.RoleTool, ToolCallID: "c1", Name: "read_file", Content: "真实结果"}); err != nil {
+		t.Fatal(err)
+	}
+	if err := st.AppendToolIntent(session.ToolIntent{CallID: "c2", Name: "read_file", Arguments: `{}`, Replay: tools.ReplayNever}); err != nil {
+		t.Fatal(err)
+	}
+	reg := tools.NewRegistry(dir)
+	reg.Register(&countingTool{name: "read_file"})
+	reg.Register(&countingTool{name: "write_file"})
+	_, err = Run(context.Background(), Config{
+		Model: "fake", Session: st, Tools: reg, MaxTurns: 1,
+		Stream: script(llm.Message{Role: llm.RoleAssistant, Content: "x", StopReason: llm.StopEnd}),
+	}, llm.Message{Role: llm.RoleUser, Content: "next"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	msgs, err := st.Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	byID := map[string][]string{}
+	for _, m := range msgs {
+		if m.Role == llm.RoleTool && m.ToolCallID != "" {
+			byID[m.ToolCallID] = append(byID[m.ToolCallID], m.Content)
+		}
+	}
+	if got := byID["c1"]; len(got) != 1 || got[0] != "真实结果" {
+		t.Fatalf("c1 必须保持原结果且不重复：%v", got)
+	}
+	if got := byID["c2"]; len(got) != 1 || got[0] != unknownSideEffect {
+		t.Fatalf("c2 走意图路径：%v", got)
+	}
+	if got := byID["c3"]; len(got) != 1 || got[0] != crashInterrupted {
+		t.Fatalf("c3 走尾部兜底：%v", got)
+	}
+}
+
+// 健康转录（尾部是完整配对或 user 消息）不做任何改写。
+func TestRepairOrphanTailNoopOnHealthyTranscript(t *testing.T) {
+	dir := t.TempDir()
+	st, err := session.Open(filepath.Join(dir, "s.jsonl"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, m := range []llm.Message{
+		{Role: llm.RoleSystem, Content: "sys"},
+		{Role: llm.RoleUser, Content: "u"},
+		{Role: llm.RoleAssistant, StopReason: llm.StopToolCalls, ToolCalls: []llm.ToolCall{{ID: "c1", Name: "read_file", Arguments: `{}`}}},
+		{Role: llm.RoleTool, ToolCallID: "c1", Name: "read_file", Content: "r"},
+		{Role: llm.RoleAssistant, Content: "done", StopReason: llm.StopEnd},
+		{Role: llm.RoleUser, Content: "u2"},
+	} {
+		if err := st.Append(m); err != nil {
+			t.Fatal(err)
+		}
+	}
+	before, _ := st.Load()
+	if err := repairOrphanTail(Config{Session: st}); err != nil {
+		t.Fatal(err)
+	}
+	after, _ := st.Load()
+	if len(after) != len(before) {
+		t.Fatalf("健康转录被改写：%d → %d", len(before), len(after))
+	}
+}
diff --git a/runtime/loop.go b/runtime/loop.go
index 5cda993..64cda1f 100644
--- a/runtime/loop.go
+++ b/runtime/loop.go
@@ -184,6 +184,11 @@ func Run(ctx context.Context, cfg Config, user llm.Message) (Result, error) {
 	if err := recoverPending(ctx, cfg); err != nil {
 		return Result{}, err
 	}
+	// 尾部配对兜底：无论进程死在 persist(assistant) 与 commitToolIntent
+	// 之间的哪个窗口，补全尾部 assistant 的无结果调用，转录配对恒成立。
+	if err := repairOrphanTail(cfg); err != nil {
+		return Result{}, err
+	}
 	if cfg.Session != nil {
 		loaded, err := cfg.Session.Load()
 		if err != nil {
@@ -337,8 +342,8 @@ func Run(ctx context.Context, cfg Config, user llm.Message) (Result, error) {
 			if decision.Continue && stopContinues < maxStopContinuations {
 				stopContinues++
 				if decision.Replacement != "" && len(messages) > 0 {
-					messages[len(messages)-1].Content = decision.Replacement
-					last.Content = decision.Replacement
+					// 替换文只以 user 消息追加（N3 WP1/N6：不再改写内存里的
+					// assistant——内存与转录保持一致；模型经该消息看到更正）。
 					note := llm.Message{Role: llm.RoleUser, Content: decision.Replacement}
 					if err := persist(cfg, note); err != nil {
 						return Result{}, err
@@ -706,6 +711,10 @@ func persistedProjection(cfg Config, messages []llm.Message) []llm.Message {
 
 const unknownSideEffect = "副作用未知，未重试"
 
+// crashInterrupted 是尾部兜底配对的正文（进程死在 persist(assistant) 与
+// commitToolIntent 之间的窗口时使用）。
+const crashInterrupted = "进程中断，该调用未执行"
+
 func recoverPending(ctx context.Context, cfg Config) error {
 	if cfg.Session == nil {
 		return nil
@@ -735,6 +744,56 @@ func recoverPending(ctx context.Context, cfg Config) error {
 	return nil
 }
 
+// repairOrphanTail 兜底配对：找到转录里最后一个带 tool_calls 的 assistant，
+// 凡没有对应 tool 结果的调用，补一条「进程中断，该调用未执行」的 IsError
+// 结果并落盘。与意图日志正交（按 CallID 判重，已配对的跳过；意图路径
+// 已处理的执行中死亡不受影响）。历史深处、其后已有新消息的孤立调用不在
+// 此处修补（追加只落在尾部，位置语义不破）——那种形态由毒化轮转覆盖。
+func repairOrphanTail(cfg Config) error {
+	if cfg.Session == nil {
+		return nil
+	}
+	msgs, err := cfg.Session.Load()
+	if err != nil {
+		return err
+	}
+	tail := -1
+	for i := len(msgs) - 1; i >= 0; i-- {
+		m := msgs[i]
+		if m.Role == llm.RoleAssistant && len(m.ToolCalls) > 0 {
+			tail = i
+			break
+		}
+		if m.Role == llm.RoleTool {
+			continue // 尾部连续 tool 结果属于同一条 assistant，继续上溯
+		}
+		break
+	}
+	if tail < 0 {
+		return nil
+	}
+	results := map[string]bool{}
+	for _, m := range msgs {
+		if m.Role == llm.RoleTool && m.ToolCallID != "" {
+			results[m.ToolCallID] = true
+		}
+	}
+	for _, call := range msgs[tail].ToolCalls {
+		if results[call.ID] {
+			continue
+		}
+		pair := llm.Message{
+			Role: llm.RoleTool, Name: call.Name, ToolCallID: call.ID,
+			Content: crashInterrupted, IsError: true,
+		}
+		if err := persist(cfg, pair); err != nil {
+			return err
+		}
+		results[call.ID] = true
+	}
+	return nil
+}
+
 func commitToolIntent(cfg Config, call llm.ToolCall) error {
 	if cfg.Session == nil {
 		return nil
diff --git a/runtime/loop_harness_test.go b/runtime/loop_harness_test.go
index 3683693..a4dc152 100644
--- a/runtime/loop_harness_test.go
+++ b/runtime/loop_harness_test.go
@@ -42,20 +42,19 @@ func TestLoopStopContinuesUntilCap(t *testing.T) {
 	if res.Err != nil || res.Notice != "到顶了" {
 		t.Fatalf("notice %q err %v", res.Notice, res.Err)
 	}
-	replaced, forged := 0, 0
+	// N6（N3 WP1）：assistant 不再被改写——替换文只以 user 消息出现，
+	// 内存与转录一致。
+	forged, notes := 0, 0
 	for _, m := range res.Messages {
-		if m.Role != llm.RoleAssistant {
-			continue
-		}
-		if m.Content == "已作废" {
-			replaced++
-		}
-		if m.Content == "forged-claim" {
+		if m.Role == llm.RoleAssistant && m.Content == "forged-claim" {
 			forged++
 		}
+		if m.Role == llm.RoleUser && m.Content == "已作废" {
+			notes++
+		}
 	}
-	if replaced != 3 || forged != 1 {
-		t.Fatalf("replaced=%d forged=%d transcript=%s", replaced, forged, transcriptText(res.Messages))
+	if forged != 4 || notes != 3 {
+		t.Fatalf("forged=%d notes=%d transcript=%s", forged, notes, transcriptText(res.Messages))
 	}
 	if strings.Count(transcriptText(res.Messages), "再试") != 3 {
 		t.Fatalf("reasons = %s", transcriptText(res.Messages))