diff --git a/.ralph/prd.json b/.ralph/prd.json
index f0bf0d4..e6451a1 100644
--- a/.ralph/prd.json
+++ b/.ralph/prd.json
@@ -137,7 +137,7 @@
         "go test ./pkg/state -run TestAtomicSave passes"
       ],
       "priority": 9,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {
diff --git a/.ralph/progress.txt b/.ralph/progress.txt
index ea466ed..44297b4 100644
--- a/.ralph/progress.txt
+++ b/.ralph/progress.txt
@@ -6,7 +6,7 @@ Tool 返回值结构化重构 - 将 Tool 接口返回值从 (string, error) 改
 
 ## Progress
 
-### Completed (7/21)
+### Completed (8/21)
 
 - US-001: Add ToolResult struct and helper functions
 - US-002: Modify Tool interface to return *ToolResult
@@ -15,6 +15,7 @@ Tool 返回值结构化重构 - 将 Tool 接口返回值从 (string, error) 改
 - US-006: Add AsyncCallback type and AsyncTool interface
 - US-007: Heartbeat async task execution support
 - US-008: Inject callback into async tools in AgentLoop
+- US-009: State save atomicity - SetLastChannel
 
 ### In Progress
 
@@ -30,7 +31,7 @@ Tool 返回值结构化重构 - 将 Tool 接口返回值从 (string, error) 改
 | US-006 | Add AsyncCallback type and AsyncTool interface | Completed | |
 | US-007 | Heartbeat async task execution support | Completed | |
 | US-008 | Inject callback into async tools in AgentLoop | Completed | |
-| US-009 | State save atomicity - SetLastChannel | Pending | |
+| US-009 | State save atomicity - SetLastChannel | Completed | |
 | US-010 | Update RecordLastChannel to use atomic save | Pending | |
 | US-011 | Refactor MessageTool to use ToolResult | Completed | |
 | US-012 | Refactor ShellTool to use ToolResult | Completed | |
@@ -149,4 +150,29 @@ Tool 返回值结构化重构 - 将 Tool 接口返回值从 (string, error) 改
   - **Gotchas encountered:** 更新方法签名时需要同时更新所有调用点。我修改了 `ExecuteWithContext` 的签名，所以也更新了 `Execute` 方法的调用。
   - **Useful context:** 异步工具完成时会调用回调，回调将 `ForUser` 内容发送给用户。这允许长时间运行的操作（如子代理）在后台完成并通知用户，而不阻塞主循环。
 
+---
+
+## [2026-02-12] - US-009
+- What was implemented:
+  - 创建新的 `pkg/state` 包，包含状态管理和原子保存功能
+  - 定义 `State` 结构体，包含 `LastChannel`、`LastChatID` 和 `Timestamp` 字段
+  - 定义 `Manager` 结构体，使用 `sync.RWMutex` 保护并发访问
+  - 实现 `NewManager(workspace string)` 构造函数，创建状态目录并加载现有状态
+  - 实现 `SetLastChannel(workspace, channel string)` 方法，使用临时文件 + 重命名模式实现原子保存
+  - 实现 `SetLastChatID(workspace, chatID string)` 方法
+  - 实现 `GetLastChannel()` 和 `GetLastChatID()` getter 方法
+  - 实现 `saveAtomic()` 内部方法，使用 `os.WriteFile` 写入临时文件，然后用 `os.Rename` 原子性地重命名
+  - 如果重命名失败，清理临时文件
+  - 实现 `load()` 方法，从磁盘加载状态
+  - 添加完整的测试：`TestAtomicSave`、`TestSetLastChatID`、`TestAtomicity_NoCorruptionOnInterrupt`、`TestConcurrentAccess`、`TestNewManager_ExistingState`、`TestNewManager_EmptyWorkspace`
+
+- Files changed:
+  - `pkg/state/state.go` (新增)
+  - `pkg/state/state_test.go` (新增)
+
+- **Learnings for future iterations:**
+  - **Patterns discovered:** 临时文件 + 重命名模式是实现原子写入的标准方法。在 POSIX 系统上，`os.Rename` 是原子操作。
+  - **Gotchas encountered:** 临时文件必须与目标文件在同一文件系统中，否则 `os.Rename` 会失败。
+  - **Useful context:** 这个模式将在 US-010 中用于 `RecordLastChannel`，确保状态更新的原子性。
+
 ---
\ No newline at end of file
diff --git a/pkg/state/state.go b/pkg/state/state.go
new file mode 100644
index 0000000..280aafd
--- /dev/null
+++ b/pkg/state/state.go
@@ -0,0 +1,160 @@
+package state
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+)
+
+// State represents the persistent state for a workspace.
+// It includes information about the last active channel/chat.
+type State struct {
+	// LastChannel is the last channel used for communication
+	LastChannel string `json:"last_channel,omitempty"`
+
+	// LastChatID is the last chat ID used for communication
+	LastChatID string `json:"last_chat_id,omitempty"`
+
+	// Timestamp is the last time this state was updated
+	Timestamp time.Time `json:"timestamp"`
+}
+
+// Manager manages persistent state with atomic saves.
+type Manager struct {
+	workspace string
+	state     *State
+	mu        sync.RWMutex
+	stateFile string
+}
+
+// NewManager creates a new state manager for the given workspace.
+func NewManager(workspace string) *Manager {
+	stateDir := filepath.Join(workspace, "state")
+	stateFile := filepath.Join(stateDir, "state.json")
+
+	// Create state directory if it doesn't exist
+	os.MkdirAll(stateDir, 0755)
+
+	sm := &Manager{
+		workspace: workspace,
+		stateFile: stateFile,
+		state:     &State{},
+	}
+
+	// Load existing state if available
+	sm.load()
+
+	return sm
+}
+
+// SetLastChannel atomically updates the last channel and saves the state.
+// This method uses a temp file + rename pattern for atomic writes,
+// ensuring that the state file is never corrupted even if the process crashes.
+//
+// The workspace parameter is used to construct the state file path.
+func (sm *Manager) SetLastChannel(workspace, channel string) error {
+	sm.mu.Lock()
+	defer sm.mu.Unlock()
+
+	// Update state
+	sm.state.LastChannel = channel
+	sm.state.Timestamp = time.Now()
+
+	// Atomic save using temp file + rename
+	if err := sm.saveAtomic(); err != nil {
+		return fmt.Errorf("failed to save state atomically: %w", err)
+	}
+
+	return nil
+}
+
+// SetLastChatID atomically updates the last chat ID and saves the state.
+func (sm *Manager) SetLastChatID(workspace, chatID string) error {
+	sm.mu.Lock()
+	defer sm.mu.Unlock()
+
+	// Update state
+	sm.state.LastChatID = chatID
+	sm.state.Timestamp = time.Now()
+
+	// Atomic save using temp file + rename
+	if err := sm.saveAtomic(); err != nil {
+		return fmt.Errorf("failed to save state atomically: %w", err)
+	}
+
+	return nil
+}
+
+// GetLastChannel returns the last channel from the state.
+func (sm *Manager) GetLastChannel() string {
+	sm.mu.RLock()
+	defer sm.mu.RUnlock()
+	return sm.state.LastChannel
+}
+
+// GetLastChatID returns the last chat ID from the state.
+func (sm *Manager) GetLastChatID() string {
+	sm.mu.RLock()
+	defer sm.mu.RUnlock()
+	return sm.state.LastChatID
+}
+
+// GetTimestamp returns the timestamp of the last state update.
+func (sm *Manager) GetTimestamp() time.Time {
+	sm.mu.RLock()
+	defer sm.mu.RUnlock()
+	return sm.state.Timestamp
+}
+
+// saveAtomic performs an atomic save using temp file + rename.
+// This ensures that the state file is never corrupted:
+// 1. Write to a temp file
+// 2. Rename temp file to target (atomic on POSIX systems)
+// 3. If rename fails, cleanup the temp file
+//
+// Must be called with the lock held.
+func (sm *Manager) saveAtomic() error {
+	// Create temp file in the same directory as the target
+	tempFile := sm.stateFile + ".tmp"
+
+	// Marshal state to JSON
+	data, err := json.MarshalIndent(sm.state, "", "  ")
+	if err != nil {
+		return fmt.Errorf("failed to marshal state: %w", err)
+	}
+
+	// Write to temp file
+	if err := os.WriteFile(tempFile, data, 0644); err != nil {
+		return fmt.Errorf("failed to write temp file: %w", err)
+	}
+
+	// Atomic rename from temp to target
+	if err := os.Rename(tempFile, sm.stateFile); err != nil {
+		// Cleanup temp file if rename fails
+		os.Remove(tempFile)
+		return fmt.Errorf("failed to rename temp file: %w", err)
+	}
+
+	return nil
+}
+
+// load loads the state from disk.
+func (sm *Manager) load() error {
+	data, err := os.ReadFile(sm.stateFile)
+	if err != nil {
+		// File doesn't exist yet, that's OK
+		if os.IsNotExist(err) {
+			return nil
+		}
+		return fmt.Errorf("failed to read state file: %w", err)
+	}
+
+	if err := json.Unmarshal(data, sm.state); err != nil {
+		return fmt.Errorf("failed to unmarshal state: %w", err)
+	}
+
+	return nil
+}
diff --git a/pkg/state/state_test.go b/pkg/state/state_test.go
new file mode 100644
index 0000000..4ee049f
--- /dev/null
+++ b/pkg/state/state_test.go
@@ -0,0 +1,216 @@
+package state
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestAtomicSave(t *testing.T) {
+	// Create temp workspace
+	tmpDir, err := os.MkdirTemp("", "state-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	sm := NewManager(tmpDir)
+
+	// Test SetLastChannel
+	err = sm.SetLastChannel(tmpDir, "test-channel")
+	if err != nil {
+		t.Fatalf("SetLastChannel failed: %v", err)
+	}
+
+	// Verify the channel was saved
+	lastChannel := sm.GetLastChannel()
+	if lastChannel != "test-channel" {
+		t.Errorf("Expected channel 'test-channel', got '%s'", lastChannel)
+	}
+
+	// Verify timestamp was updated
+	if sm.GetTimestamp().IsZero() {
+		t.Error("Expected timestamp to be updated")
+	}
+
+	// Verify state file exists
+	stateFile := filepath.Join(tmpDir, "state", "state.json")
+	if _, err := os.Stat(stateFile); os.IsNotExist(err) {
+		t.Error("Expected state file to exist")
+	}
+
+	// Create a new manager to verify persistence
+	sm2 := NewManager(tmpDir)
+	if sm2.GetLastChannel() != "test-channel" {
+		t.Errorf("Expected persistent channel 'test-channel', got '%s'", sm2.GetLastChannel())
+	}
+}
+
+func TestSetLastChatID(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "state-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	sm := NewManager(tmpDir)
+
+	// Test SetLastChatID
+	err = sm.SetLastChatID(tmpDir, "test-chat-id")
+	if err != nil {
+		t.Fatalf("SetLastChatID failed: %v", err)
+	}
+
+	// Verify the chat ID was saved
+	lastChatID := sm.GetLastChatID()
+	if lastChatID != "test-chat-id" {
+		t.Errorf("Expected chat ID 'test-chat-id', got '%s'", lastChatID)
+	}
+
+	// Verify timestamp was updated
+	if sm.GetTimestamp().IsZero() {
+		t.Error("Expected timestamp to be updated")
+	}
+
+	// Create a new manager to verify persistence
+	sm2 := NewManager(tmpDir)
+	if sm2.GetLastChatID() != "test-chat-id" {
+		t.Errorf("Expected persistent chat ID 'test-chat-id', got '%s'", sm2.GetLastChatID())
+	}
+}
+
+func TestAtomicity_NoCorruptionOnInterrupt(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "state-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	sm := NewManager(tmpDir)
+
+	// Write initial state
+	err = sm.SetLastChannel(tmpDir, "initial-channel")
+	if err != nil {
+		t.Fatalf("SetLastChannel failed: %v", err)
+	}
+
+	// Simulate a crash scenario by manually creating a corrupted temp file
+	tempFile := filepath.Join(tmpDir, "state", "state.json.tmp")
+	err = os.WriteFile(tempFile, []byte("corrupted data"), 0644)
+	if err != nil {
+		t.Fatalf("Failed to create temp file: %v", err)
+	}
+
+	// Verify that the original state is still intact
+	lastChannel := sm.GetLastChannel()
+	if lastChannel != "initial-channel" {
+		t.Errorf("Expected channel 'initial-channel' after corrupted temp file, got '%s'", lastChannel)
+	}
+
+	// Clean up the temp file manually
+	os.Remove(tempFile)
+
+	// Now do a proper save
+	err = sm.SetLastChannel(tmpDir, "new-channel")
+	if err != nil {
+		t.Fatalf("SetLastChannel failed: %v", err)
+	}
+
+	// Verify the new state was saved
+	if sm.GetLastChannel() != "new-channel" {
+		t.Errorf("Expected channel 'new-channel', got '%s'", sm.GetLastChannel())
+	}
+}
+
+func TestConcurrentAccess(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "state-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	sm := NewManager(tmpDir)
+
+	// Test concurrent writes
+	done := make(chan bool, 10)
+	for i := 0; i < 10; i++ {
+		go func(idx int) {
+			channel := fmt.Sprintf("channel-%d", idx)
+			sm.SetLastChannel(tmpDir, channel)
+			done <- true
+		}(i)
+	}
+
+	// Wait for all goroutines to complete
+	for i := 0; i < 10; i++ {
+		<-done
+	}
+
+	// Verify the final state is consistent
+	lastChannel := sm.GetLastChannel()
+	if lastChannel == "" {
+		t.Error("Expected non-empty channel after concurrent writes")
+	}
+
+	// Verify state file is valid JSON
+	stateFile := filepath.Join(tmpDir, "state", "state.json")
+	data, err := os.ReadFile(stateFile)
+	if err != nil {
+		t.Fatalf("Failed to read state file: %v", err)
+	}
+
+	var state State
+	if err := json.Unmarshal(data, &state); err != nil {
+		t.Errorf("State file contains invalid JSON: %v", err)
+	}
+}
+
+func TestNewManager_ExistingState(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "state-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	// Create initial state
+	sm1 := NewManager(tmpDir)
+	sm1.SetLastChannel(tmpDir, "existing-channel")
+	sm1.SetLastChatID(tmpDir, "existing-chat-id")
+
+	// Create new manager with same workspace
+	sm2 := NewManager(tmpDir)
+
+	// Verify state was loaded
+	if sm2.GetLastChannel() != "existing-channel" {
+		t.Errorf("Expected channel 'existing-channel', got '%s'", sm2.GetLastChannel())
+	}
+
+	if sm2.GetLastChatID() != "existing-chat-id" {
+		t.Errorf("Expected chat ID 'existing-chat-id', got '%s'", sm2.GetLastChatID())
+	}
+}
+
+func TestNewManager_EmptyWorkspace(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "state-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	sm := NewManager(tmpDir)
+
+	// Verify default state
+	if sm.GetLastChannel() != "" {
+		t.Errorf("Expected empty channel, got '%s'", sm.GetLastChannel())
+	}
+
+	if sm.GetLastChatID() != "" {
+		t.Errorf("Expected empty chat ID, got '%s'", sm.GetLastChatID())
+	}
+
+	if !sm.GetTimestamp().IsZero() {
+		t.Error("Expected zero timestamp for new state")
+	}
+}