Implement checkpoint blob handling in forwarder service

- Added support for checkpointing phases and blob management in the forwarder.
- Introduced new types and methods for handling checkpoint blobs, including queuing and publishing checkpoints.
- Enhanced the projector to build checkpoint projections with content-addressed blobs.
- Implemented tests to ensure proper checkpoint blob synchronization and handling of cancellation scenarios.

internal/backend/forwarder/actor.go

@@ -23,6 +23,7 @@ const (
 	TurnPhaseWaitingExternal TurnPhase = "waiting_external"
 	TurnPhaseAwaitingUser    TurnPhase = "awaiting_user"
 	TurnPhaseCompacting      TurnPhase = "compacting"
+	TurnPhaseCheckpointing   TurnPhase = "checkpointing"
 	TurnPhaseCompleted       TurnPhase = "completed"
 	TurnPhaseFailed          TurnPhase = "failed"
 	TurnPhaseCanceled        TurnPhase = "canceled"
@@ -66,6 +67,7 @@ const (
 	streamTimerNonStreamingRecovery streamTimerKind = "non_streaming_recovery"
 	streamTimerShellForeground      streamTimerKind = "shell_foreground"
 	streamTimerShellTransportClose  streamTimerKind = "shell_transport_close"
+	streamTimerCheckpointBlobs      streamTimerKind = "checkpoint_blobs"
 	streamTimerOrphanCancel         streamTimerKind = "orphan_cancel"
 )
 
@@ -318,6 +320,9 @@ func (service *Service) handleStreamCommand(stream *ActiveStream, command stream
 	case streamCommandCancel:
 		return service.handleCancelIntent(command.Intent)
 	case streamCommandMetadata:
+		if strings.TrimSpace(command.Intent.Kind) == "kv_result" {
+			return service.handleCheckpointBlobResult(stream, command.Intent.KVClientMessage)
+		}
 		return service.handleMetadataIntent(command.Intent)
 	case streamCommandExecResult:
 		return service.handleExecResult(command.Intent)
@@ -1003,6 +1008,8 @@ func (service *Service) handleTimerEvent(stream *ActiveStream, payload *streamTi
 			return nil
 		}
 		return service.recoverShellWithoutTerminal(stream, current, shellRecoveryReasonTransportClosed)
+	case streamTimerCheckpointBlobs:
+		return service.handleCheckpointBlobTimeout(stream)
 	case streamTimerOrphanCancel:
 		stream.mu.Lock()
 		subscriberCount := len(stream.Subscribers)

internal/backend/forwarder/broker.go

@@ -76,6 +76,12 @@ func (broker *StreamBroker) OpenStream(requestID string, conversationID string,
 		if existing.BackgroundShellActions == nil {
 			existing.BackgroundShellActions = make(map[string]time.Time)
 		}
+		if existing.PendingCheckpointBlobWrites == nil {
+			existing.PendingCheckpointBlobWrites = make(map[uint32]string)
+		}
+		if existing.ConfirmedCheckpointBlobs == nil {
+			existing.ConfirmedCheckpointBlobs = make(map[string]struct{})
+		}
 		existing.UpdatedAt = time.Now().UTC()
 		existing.mu.Unlock()
 		return existing, nil
@@ -102,6 +108,8 @@ func (broker *StreamBroker) OpenStream(requestID string, conversationID string,
 		BackgroundShellsByMessageID: make(map[uint32]string),
 		BackgroundShellsByExecID:    make(map[string]string),
 		BackgroundShellActions:      make(map[string]time.Time),
+		PendingCheckpointBlobWrites: make(map[uint32]string),
+		ConfirmedCheckpointBlobs:    make(map[string]struct{}),
 		CreatedAt:                   now,
 		UpdatedAt:                   now,
 	}

internal/backend/forwarder/checkpoint_blobs.go

@@ -0,0 +1,238 @@
+package forwarder
+
+import (
+	"encoding/hex"
+	"fmt"
+	"log"
+	"strings"
+	"time"
+
+	"google.golang.org/protobuf/proto"
+
+	"cursor/gen/agentv1"
+)
+
+const checkpointBlobWriteTimeout = 5 * time.Second
+
+type pendingCheckpointBlobWrite struct {
+	requestID uint32
+	blob      CheckpointBlob
+}
+
+func clonePendingTurnCompletion(completion *pendingTurnCompletion) *pendingTurnCompletion {
+	if completion == nil {
+		return nil
+	}
+	cloned := *completion
+	return &cloned
+}
+
+func (service *Service) queueCheckpointProjection(stream *ActiveStream, projection *CheckpointProjection, completion *pendingTurnCompletion) error {
+	if service == nil || stream == nil || projection == nil || projection.State == nil {
+		return nil
+	}
+	state, ok := proto.Clone(projection.State).(*agentv1.ConversationStateStructure)
+	if !ok || state == nil {
+		return fmt.Errorf("clone checkpoint state")
+	}
+
+	stream.mu.Lock()
+	if stream.PendingCheckpointBlobWrites == nil {
+		stream.PendingCheckpointBlobWrites = make(map[uint32]string)
+	}
+	if stream.ConfirmedCheckpointBlobs == nil {
+		stream.ConfirmedCheckpointBlobs = make(map[string]struct{})
+	}
+	if completion == nil && stream.PendingCheckpoint != nil {
+		completion = stream.PendingCheckpoint.Completion
+	}
+	required := make(map[string]struct{}, len(projection.Blobs))
+	pendingKeys := make(map[string]struct{}, len(stream.PendingCheckpointBlobWrites))
+	for _, key := range stream.PendingCheckpointBlobWrites {
+		pendingKeys[key] = struct{}{}
+	}
+	toWrite := make([]pendingCheckpointBlobWrite, 0, len(projection.Blobs))
+	for _, blob := range projection.Blobs {
+		key := string(blob.ID)
+		if key == "" {
+			continue
+		}
+		required[key] = struct{}{}
+		if _, confirmed := stream.ConfirmedCheckpointBlobs[key]; confirmed {
+			continue
+		}
+		if _, pending := pendingKeys[key]; pending {
+			continue
+		}
+		stream.NextCheckpointBlobRequestID++
+		if stream.NextCheckpointBlobRequestID == 0 {
+			stream.NextCheckpointBlobRequestID++
+		}
+		requestID := stream.NextCheckpointBlobRequestID
+		stream.PendingCheckpointBlobWrites[requestID] = key
+		pendingKeys[key] = struct{}{}
+		toWrite = append(toWrite, pendingCheckpointBlobWrite{requestID: requestID, blob: blob})
+	}
+	stream.PendingCheckpoint = &pendingCheckpointPublish{
+		State:      state,
+		Required:   required,
+		Completion: clonePendingTurnCompletion(completion),
+	}
+	if completion != nil {
+		stream.Phase = TurnPhaseCheckpointing
+	}
+	stream.UpdatedAt = time.Now().UTC()
+	stream.mu.Unlock()
+
+	for _, write := range toWrite {
+		if err := service.broker.Publish(stream.RequestID, StreamEvent{
+			Message: buildSetCheckpointBlobMessage(write.requestID, write.blob),
+		}); err != nil {
+			return service.finishAfterCheckpointSyncFailure(stream, fmt.Errorf("publish checkpoint blob: %w", err))
+		}
+	}
+	if service.checkpointProjectionReady(stream) {
+		return service.publishReadyCheckpoint(stream)
+	}
+	service.scheduleStreamTimer(
+		stream,
+		providerTimerKey(streamTimerCheckpointBlobs, ""),
+		checkpointBlobWriteTimeout,
+		streamTimerCheckpointBlobs,
+		"",
+		0,
+		"checkpoint blob write timeout",
+	)
+	return nil
+}
+
+func (service *Service) checkpointProjectionReady(stream *ActiveStream) bool {
+	if stream == nil {
+		return false
+	}
+	stream.mu.Lock()
+	defer stream.mu.Unlock()
+	if stream.PendingCheckpoint == nil {
+		return false
+	}
+	for key := range stream.PendingCheckpoint.Required {
+		if _, confirmed := stream.ConfirmedCheckpointBlobs[key]; !confirmed {
+			return false
+		}
+	}
+	return true
+}
+
+func (service *Service) handleCheckpointBlobResult(stream *ActiveStream, message *agentv1.KvClientMessage) error {
+	if service == nil || stream == nil || message == nil || message.GetSetBlobResult() == nil {
+		return nil
+	}
+	stream.mu.Lock()
+	key, ok := stream.PendingCheckpointBlobWrites[message.GetId()]
+	if ok {
+		delete(stream.PendingCheckpointBlobWrites, message.GetId())
+	}
+	required := false
+	if ok && stream.PendingCheckpoint != nil {
+		_, required = stream.PendingCheckpoint.Required[key]
+	}
+	if ok && message.GetSetBlobResult().GetError() == nil {
+		stream.ConfirmedCheckpointBlobs[key] = struct{}{}
+	}
+	stream.UpdatedAt = time.Now().UTC()
+	stream.mu.Unlock()
+	if !ok {
+		return nil
+	}
+	if blobErr := message.GetSetBlobResult().GetError(); blobErr != nil && required {
+		return service.finishAfterCheckpointSyncFailure(stream, fmt.Errorf(
+			"client rejected checkpoint blob %s: %s",
+			hex.EncodeToString([]byte(key)),
+			firstNonEmpty(strings.TrimSpace(blobErr.GetMessage()), "unknown error"),
+		))
+	}
+	if service.checkpointProjectionReady(stream) {
+		return service.publishReadyCheckpoint(stream)
+	}
+	return nil
+}
+
+func (service *Service) publishReadyCheckpoint(stream *ActiveStream) error {
+	if service == nil || stream == nil {
+		return nil
+	}
+	stream.mu.Lock()
+	pending := stream.PendingCheckpoint
+	if pending == nil {
+		stream.mu.Unlock()
+		return nil
+	}
+	for key := range pending.Required {
+		if _, confirmed := stream.ConfirmedCheckpointBlobs[key]; !confirmed {
+			stream.mu.Unlock()
+			return nil
+		}
+	}
+	stream.PendingCheckpoint = nil
+	state := pending.State
+	completion := clonePendingTurnCompletion(pending.Completion)
+	stream.UpdatedAt = time.Now().UTC()
+	stream.mu.Unlock()
+	clearStreamTimer(stream, providerTimerKey(streamTimerCheckpointBlobs, ""))
+	if err := service.broker.Publish(stream.RequestID, StreamEvent{Message: buildCheckpointMessage(state)}); err != nil {
+		if completion != nil {
+			log.Printf("forwarder checkpoint publish skipped before successful terminal request_id=%s err=%v", stream.RequestID, err)
+			return service.finishSuccessfulTurnAfterCheckpoint(stream, *completion)
+		}
+		return err
+	}
+	if completion != nil {
+		return service.finishSuccessfulTurnAfterCheckpoint(stream, *completion)
+	}
+	return nil
+}
+
+func (service *Service) handleCheckpointBlobTimeout(stream *ActiveStream) error {
+	if stream == nil {
+		return nil
+	}
+	stream.mu.Lock()
+	pendingCount := len(stream.PendingCheckpointBlobWrites)
+	stream.mu.Unlock()
+	return service.finishAfterCheckpointSyncFailure(stream, fmt.Errorf("%d checkpoint blob writes timed out", pendingCount))
+}
+
+func (service *Service) finishAfterCheckpointSyncFailure(stream *ActiveStream, cause error) error {
+	if stream == nil {
+		return nil
+	}
+	stream.mu.Lock()
+	pending := stream.PendingCheckpoint
+	stream.PendingCheckpoint = nil
+	stream.PendingCheckpointBlobWrites = make(map[uint32]string)
+	stream.UpdatedAt = time.Now().UTC()
+	stream.mu.Unlock()
+	clearStreamTimer(stream, providerTimerKey(streamTimerCheckpointBlobs, ""))
+	if cause != nil {
+		log.Printf("forwarder checkpoint blob sync skipped request_id=%s conversation_id=%s err=%v", stream.RequestID, stream.ConversationID, cause)
+	}
+	if pending != nil && pending.Completion != nil {
+		return service.finishSuccessfulTurnAfterCheckpoint(stream, *pending.Completion)
+	}
+	return nil
+}
+
+func (service *Service) discardPendingCheckpoint(stream *ActiveStream, reason string) {
+	if stream == nil {
+		return
+	}
+	stream.mu.Lock()
+	stream.PendingCheckpoint = nil
+	stream.PendingCheckpointBlobWrites = make(map[uint32]string)
+	stream.UpdatedAt = time.Now().UTC()
+	stream.mu.Unlock()
+	clearStreamTimer(stream, providerTimerKey(streamTimerCheckpointBlobs, ""))
+	if strings.TrimSpace(reason) != "" {
+		log.Printf("forwarder pending checkpoint discarded request_id=%s conversation_id=%s reason=%s", stream.RequestID, stream.ConversationID, strings.TrimSpace(reason))
+	}
+}

internal/backend/forwarder/checkpoint_blobs_test.go

@@ -0,0 +1,208 @@
+package forwarder
+
+import (
+	"testing"
+
+	"google.golang.org/protobuf/encoding/protojson"
+
+	"cursor/gen/agentv1"
+)
+
+func TestCheckpointBlobSyncPublishesCheckpointAfterAcknowledgements(t *testing.T) {
+	service, stream, projection := testCheckpointBlobProjection(t)
+	if err := service.queueCheckpointProjection(stream, projection, nil); err != nil {
+		t.Fatalf("queueCheckpointProjection() error = %v", err)
+	}
+	events := readCheckpointTestEvents(t, service, stream)
+	if len(events) != len(projection.Blobs) {
+		t.Fatalf("events before ACK = %d, want %d Blob writes", len(events), len(projection.Blobs))
+	}
+	for _, event := range events {
+		if event.Message.GetKvServerMessage().GetSetBlobArgs() == nil {
+			t.Fatalf("event before ACK = %#v, want set_blob_args", event.Message)
+		}
+	}
+
+	acknowledgeCheckpointBlobs(t, service, stream)
+	events = readCheckpointTestEvents(t, service, stream)
+	checkpoint := events[len(events)-1].Message.GetConversationCheckpointUpdate()
+	if checkpoint == nil || len(checkpoint.GetTurns()) != 1 {
+		t.Fatalf("last event checkpoint = %#v, want one Blob-backed turn", checkpoint)
+	}
+}
+
+func TestCheckpointBlobSyncPublishesCheckpointBeforeSuccessfulTerminal(t *testing.T) {
+	service, stream, projection := testCheckpointBlobProjection(t)
+	completion := &pendingTurnCompletion{
+		RequestID: stream.RequestID,
+		Usage:     turnUsageSnapshot{InputTokens: 11, OutputTokens: 7},
+	}
+	if err := service.queueCheckpointProjection(stream, projection, completion); err != nil {
+		t.Fatalf("queueCheckpointProjection() error = %v", err)
+	}
+	acknowledgeCheckpointBlobs(t, service, stream)
+
+	events := readCheckpointTestEvents(t, service, stream)
+	checkpointIndex, turnEndedIndex, endIndex := -1, -1, -1
+	for index, event := range events {
+		switch {
+		case event.Message.GetConversationCheckpointUpdate() != nil:
+			checkpointIndex = index
+		case event.Message.GetInteractionUpdate().GetTurnEnded() != nil:
+			turnEndedIndex = index
+		case event.End:
+			endIndex = index
+		}
+	}
+	if checkpointIndex < 0 || turnEndedIndex <= checkpointIndex || endIndex <= turnEndedIndex {
+		t.Fatalf("terminal order checkpoint=%d turn_ended=%d end=%d", checkpointIndex, turnEndedIndex, endIndex)
+	}
+}
+
+func TestCheckpointBlobTimeoutDoesNotFailSuccessfulTurn(t *testing.T) {
+	service, stream, projection := testCheckpointBlobProjection(t)
+	completion := &pendingTurnCompletion{
+		RequestID: stream.RequestID,
+		Usage:     turnUsageSnapshot{InputTokens: 11, OutputTokens: 7},
+	}
+	if err := service.queueCheckpointProjection(stream, projection, completion); err != nil {
+		t.Fatalf("queueCheckpointProjection() error = %v", err)
+	}
+	if err := service.handleCheckpointBlobTimeout(stream); err != nil {
+		t.Fatalf("handleCheckpointBlobTimeout() error = %v", err)
+	}
+
+	events := readCheckpointTestEvents(t, service, stream)
+	var checkpoint, turnEnded, successfulEnd bool
+	for _, event := range events {
+		checkpoint = checkpoint || event.Message.GetConversationCheckpointUpdate() != nil
+		turnEnded = turnEnded || event.Message.GetInteractionUpdate().GetTurnEnded() != nil
+		successfulEnd = successfulEnd || event.End && event.TerminalErrorCode == ""
+	}
+	if checkpoint || !turnEnded || !successfulEnd {
+		t.Fatalf("timeout events checkpoint=%v turn_ended=%v successful_end=%v", checkpoint, turnEnded, successfulEnd)
+	}
+}
+
+func TestCancellationDiscardsPendingCheckpointAndIgnoresLateAcknowledgements(t *testing.T) {
+	service, stream, projection := testCheckpointBlobProjection(t)
+	if err := service.queueCheckpointProjection(stream, projection, nil); err != nil {
+		t.Fatalf("queueCheckpointProjection() error = %v", err)
+	}
+	stream.mu.Lock()
+	requestIDs := make([]uint32, 0, len(stream.PendingCheckpointBlobWrites))
+	for requestID := range stream.PendingCheckpointBlobWrites {
+		requestIDs = append(requestIDs, requestID)
+	}
+	stream.mu.Unlock()
+	if err := service.handleCancelIntent(InboundIntent{
+		Kind:         "cancel",
+		RequestID:    stream.RequestID,
+		CancelReason: "user stopped",
+	}); err != nil {
+		t.Fatalf("handleCancelIntent() error = %v", err)
+	}
+	for _, requestID := range requestIDs {
+		if err := service.handleCheckpointBlobResult(stream, &agentv1.KvClientMessage{
+			Id: requestID,
+			Message: &agentv1.KvClientMessage_SetBlobResult{
+				SetBlobResult: &agentv1.SetBlobResult{},
+			},
+		}); err != nil {
+			t.Fatalf("late ACK %d error = %v", requestID, err)
+		}
+	}
+
+	events := readCheckpointTestEvents(t, service, stream)
+	var checkpoint, canceledEnd bool
+	for _, event := range events {
+		checkpoint = checkpoint || event.Message.GetConversationCheckpointUpdate() != nil
+		canceledEnd = canceledEnd || event.End && event.TerminalErrorCode == "canceled"
+	}
+	stream.mu.Lock()
+	pending := stream.PendingCheckpoint
+	stream.mu.Unlock()
+	if checkpoint || !canceledEnd || pending != nil {
+		t.Fatalf("cancel events checkpoint=%v canceled_end=%v pending=%v", checkpoint, canceledEnd, pending != nil)
+	}
+}
+
+func testCheckpointBlobProjection(t *testing.T) (*Service, *ActiveStream, *CheckpointProjection) {
+	t.Helper()
+	broker := NewStreamBroker()
+	service := &Service{
+		store:     NewConversationFileStore(t.TempDir()),
+		projector: NewHistoryProjector(),
+		broker:    broker,
+	}
+	stream, err := broker.OpenStream(
+		"request-1", "conversation-1", 1, "default", "default",
+		agentv1.AgentMode_AGENT_MODE_AGENT, "hello",
+	)
+	if err != nil {
+		t.Fatalf("OpenStream() error = %v", err)
+	}
+	conversation := &ConversationFile{
+		ConversationID:        "conversation-1",
+		RootConversationID:    "conversation-1",
+		Mode:                  "agent",
+		NextTurnSeq:           2,
+		NextEntrySeq:          3,
+		TokenDetailsMaxTokens: projectedConversationMaxTokens,
+		Entries: []HistoryEntry{
+			testCheckpointUserEntry(t),
+			newAssistantTextEntry(1, "request-1", "hi", "", ""),
+		},
+	}
+	projection, err := service.projector.ProjectCheckpointProjection(conversation)
+	if err != nil {
+		t.Fatalf("ProjectCheckpointProjection() error = %v", err)
+	}
+	if err := service.replaceCheckpointConversation(stream, conversation); err != nil {
+		t.Fatalf("replaceCheckpointConversation() error = %v", err)
+	}
+	return service, stream, projection
+}
+
+func testCheckpointUserEntry(t *testing.T) HistoryEntry {
+	t.Helper()
+	payload, err := protojson.Marshal(&agentv1.UserMessage{Text: "hello", MessageId: "message-1"})
+	if err != nil {
+		t.Fatalf("marshal user message: %v", err)
+	}
+	return HistoryEntry{Seq: 1, TurnSeq: 1, RequestID: "request-1", Role: "user", Kind: "user_message", Payload: payload}
+}
+
+func acknowledgeCheckpointBlobs(t *testing.T, service *Service, stream *ActiveStream) {
+	t.Helper()
+	for {
+		stream.mu.Lock()
+		requestIDs := make([]uint32, 0, len(stream.PendingCheckpointBlobWrites))
+		for requestID := range stream.PendingCheckpointBlobWrites {
+			requestIDs = append(requestIDs, requestID)
+		}
+		stream.mu.Unlock()
+		if len(requestIDs) == 0 {
+			return
+		}
+		for _, requestID := range requestIDs {
+			if err := service.handleCheckpointBlobResult(stream, &agentv1.KvClientMessage{
+				Id: requestID,
+				Message: &agentv1.KvClientMessage_SetBlobResult{
+					SetBlobResult: &agentv1.SetBlobResult{},
+				},
+			}); err != nil {
+				t.Fatalf("handleCheckpointBlobResult(%d) error = %v", requestID, err)
+			}
+		}
+	}
+}
+
+func readCheckpointTestEvents(t *testing.T, service *Service, stream *ActiveStream) []StreamEvent {
+	t.Helper()
+	events, err := service.broker.ReadFromCursor(stream.RequestID, 0)
+	if err != nil {
+		t.Fatalf("ReadFromCursor() error = %v", err)
+	}
+	return events
+}

internal/backend/forwarder/events.go

@@ -245,6 +245,22 @@ func buildCheckpointMessage(state *agentv1.ConversationStateStructure) *agentv1.
 	}
 }
 
+func buildSetCheckpointBlobMessage(id uint32, blob CheckpointBlob) *agentv1.AgentServerMessage {
+	return &agentv1.AgentServerMessage{
+		Message: &agentv1.AgentServerMessage_KvServerMessage{
+			KvServerMessage: &agentv1.KvServerMessage{
+				Id: id,
+				Message: &agentv1.KvServerMessage_SetBlobArgs{
+					SetBlobArgs: &agentv1.SetBlobArgs{
+						BlobId:   append([]byte(nil), blob.ID...),
+						BlobData: append([]byte(nil), blob.Data...),
+					},
+				},
+			},
+		},
+	}
+}
+
 // buildExecAbortMessage 构造对客户端执行桥的 abort 控制消息。
 func buildExecAbortMessage(pending runtimecore.PendingExec) *agentv1.AgentServerMessage {
 	return &agentv1.AgentServerMessage{

internal/backend/forwarder/projector.go

@@ -2,6 +2,7 @@
 package forwarder
 
 import (
+	"crypto/sha256"
 	"encoding/json"
 	"fmt"
 	"strings"
@@ -19,6 +20,51 @@ const projectedConversationMaxTokens = 130000
 type HistoryProjector struct {
 }
 
+type CheckpointBlob struct {
+	ID   []byte
+	Data []byte
+}
+
+type CheckpointProjection struct {
+	State *agentv1.ConversationStateStructure
+	Blobs []CheckpointBlob
+}
+
+type checkpointBlobGraph struct {
+	blobs map[[sha256.Size]byte][]byte
+	order [][sha256.Size]byte
+}
+
+func newCheckpointBlobGraph() *checkpointBlobGraph {
+	return &checkpointBlobGraph{blobs: make(map[[sha256.Size]byte][]byte)}
+}
+
+func (graph *checkpointBlobGraph) add(data []byte) []byte {
+	if graph == nil || len(data) == 0 {
+		return nil
+	}
+	id := sha256.Sum256(data)
+	if _, exists := graph.blobs[id]; !exists {
+		graph.blobs[id] = append([]byte(nil), data...)
+		graph.order = append(graph.order, id)
+	}
+	return append([]byte(nil), id[:]...)
+}
+
+func (graph *checkpointBlobGraph) list() []CheckpointBlob {
+	if graph == nil || len(graph.order) == 0 {
+		return nil
+	}
+	blobs := make([]CheckpointBlob, 0, len(graph.order))
+	for _, id := range graph.order {
+		blobs = append(blobs, CheckpointBlob{
+			ID:   append([]byte(nil), id[:]...),
+			Data: append([]byte(nil), graph.blobs[id]...),
+		})
+	}
+	return blobs
+}
+
 // NewHistoryProjector 创建 history 投影器。
 func NewHistoryProjector() *HistoryProjector {
 	return &HistoryProjector{}
@@ -475,6 +521,16 @@ func isHistoricalReplayToolResult(conversation *ConversationFile, entry HistoryE
 
 // ProjectLegacyCheckpoint 按需从 JSON history 投影出兼容旧客户端的 checkpoint 结构。
 func (projector *HistoryProjector) ProjectLegacyCheckpoint(conversation *ConversationFile) (*agentv1.ConversationStateStructure, error) {
+	projection, err := projector.ProjectCheckpointProjection(conversation)
+	if err != nil || projection == nil {
+		return nil, err
+	}
+	return projection.State, nil
+}
+
+// ProjectCheckpointProjection 同时返回 checkpoint 状态及其引用的内容寻址 Blob。
+func (projector *HistoryProjector) ProjectCheckpointProjection(conversation *ConversationFile) (*CheckpointProjection, error) {
+	blobs := newCheckpointBlobGraph()
 	state := &agentv1.ConversationStateStructure{
 		TokenDetails: &agentv1.ConversationTokenDetails{
 			UsedTokens: conversationTokenDetailsUsedTokens(conversation),
@@ -488,7 +544,7 @@ func (projector *HistoryProjector) ProjectLegacyCheckpoint(conversation *Convers
 	if conversation == nil {
 		mode := agentv1.AgentMode_AGENT_MODE_AGENT
 		state.Mode = &mode
-		return state, nil
+		return &CheckpointProjection{State: state}, nil
 	}
 	mode, err := parseModeAlias(conversation.Mode)
 	if err != nil {
@@ -506,10 +562,11 @@ func (projector *HistoryProjector) ProjectLegacyCheckpoint(conversation *Convers
 	if structuredState.HasTodos {
 		state.Todos = encodeConversationTodoBytes(structuredState.Todos)
 	}
-	// Cursor 3.14 treats ConversationStateStructure.turns as content-addressed
-	// blob IDs. Inline protobuf turns make Fork Chat look up the serialized turn
-	// itself as an ID and fail with "Missing turn blob". Keep model-visible
-	// history in root_prompt_messages_json until checkpoint blob sync is safe.
+	turnIDs, err := projectCheckpointTurnBlobs(conversation, blobs)
+	if err != nil {
+		return nil, err
+	}
+	state.Turns = turnIDs
 	replayMessages, err := projector.ProjectPromptReplay(conversation)
 	if err != nil {
 		return nil, err
@@ -537,7 +594,183 @@ func (projector *HistoryProjector) ProjectLegacyCheckpoint(conversation *Convers
 		return nil, err
 	}
 	state.RootPromptMessagesJson = rootPromptMessages
-	return state, nil
+	return &CheckpointProjection{State: state, Blobs: blobs.list()}, nil
+}
+
+func projectCheckpointTurnBlobs(conversation *ConversationFile, blobs *checkpointBlobGraph) ([][]byte, error) {
+	if conversation == nil || blobs == nil {
+		return nil, nil
+	}
+	grouped := make(map[int64][]HistoryEntry)
+	order := make([]int64, 0, conversation.NextTurnSeq)
+	for _, entry := range checkpointProjectionEntries(conversation.Entries) {
+		if entry.TurnSeq <= 0 {
+			continue
+		}
+		if _, ok := grouped[entry.TurnSeq]; !ok {
+			order = append(order, entry.TurnSeq)
+		}
+		grouped[entry.TurnSeq] = append(grouped[entry.TurnSeq], entry)
+	}
+
+	turnIDs := make([][]byte, 0, len(order))
+	for _, turnSeq := range order {
+		entries := grouped[turnSeq]
+		var userMessageID []byte
+		var turnRequestID string
+		stepIDs := make([][]byte, 0, len(entries))
+		seenToolCalls := make(map[string]struct{})
+		openToolCalls := make(map[string]struct{})
+		for _, entry := range entries {
+			if turnRequestID == "" {
+				turnRequestID = strings.TrimSpace(entry.RequestID)
+			}
+			switch strings.TrimSpace(entry.Kind) {
+			case "user_message":
+				userMessage := &agentv1.UserMessage{}
+				if err := protojson.Unmarshal(entry.Payload, userMessage); err != nil {
+					return nil, fmt.Errorf("decode checkpoint user_message: %w", err)
+				}
+				payload, err := proto.Marshal(userMessage)
+				if err != nil {
+					return nil, err
+				}
+				userMessageID = blobs.add(payload)
+			case "assistant_text":
+				var payload assistantTextPayload
+				if err := json.Unmarshal(entry.Payload, &payload); err != nil {
+					return nil, err
+				}
+				if strings.TrimSpace(payload.Text) == "" && strings.TrimSpace(payload.ReasoningContent) != "" && len(openToolCalls) > 0 {
+					continue
+				}
+				if strings.TrimSpace(payload.ReasoningContent) != "" {
+					stepID, err := addCheckpointStepBlob(blobs, &agentv1.ConversationStep{
+						Message: &agentv1.ConversationStep_ThinkingMessage{
+							ThinkingMessage: &agentv1.ThinkingMessage{Text: payload.ReasoningContent},
+						},
+					})
+					if err != nil {
+						return nil, err
+					}
+					stepIDs = append(stepIDs, stepID)
+				}
+				if strings.TrimSpace(payload.Text) == "" {
+					continue
+				}
+				stepID, err := addCheckpointStepBlob(blobs, &agentv1.ConversationStep{
+					Message: &agentv1.ConversationStep_AssistantMessage{
+						AssistantMessage: &agentv1.AssistantMessage{Text: strings.TrimSpace(payload.Text)},
+					},
+				})
+				if err != nil {
+					return nil, err
+				}
+				stepIDs = append(stepIDs, stepID)
+			case "tool_call":
+				var payload toolCallEntryPayload
+				if err := json.Unmarshal(entry.Payload, &payload); err != nil {
+					return nil, err
+				}
+				if strings.TrimSpace(payload.ReasoningContent) != "" {
+					stepID, err := addCheckpointStepBlob(blobs, &agentv1.ConversationStep{
+						Message: &agentv1.ConversationStep_ThinkingMessage{
+							ThinkingMessage: &agentv1.ThinkingMessage{Text: payload.ReasoningContent},
+						},
+					})
+					if err != nil {
+						return nil, err
+					}
+					stepIDs = append(stepIDs, stepID)
+				}
+				toolCall := &agentv1.ToolCall{}
+				if err := protojson.Unmarshal(payload.ToolCall, toolCall); err != nil {
+					return nil, err
+				}
+				if !shouldPersistToolResultName(firstNonEmpty(strings.TrimSpace(payload.ToolName), inferToolName(toolCall))) {
+					continue
+				}
+				stepID, err := addCheckpointStepBlob(blobs, &agentv1.ConversationStep{
+					Message: &agentv1.ConversationStep_ToolCall{ToolCall: toolCall},
+				})
+				if err != nil {
+					return nil, err
+				}
+				stepIDs = append(stepIDs, stepID)
+				if toolCallID := strings.TrimSpace(payload.ToolCallID); toolCallID != "" {
+					seenToolCalls[toolCallID] = struct{}{}
+					openToolCalls[toolCallID] = struct{}{}
+				}
+			case "tool_result":
+				var payload toolResultEntryPayload
+				if err := json.Unmarshal(entry.Payload, &payload); err != nil {
+					return nil, err
+				}
+				if toolCallID := strings.TrimSpace(payload.ToolCallID); toolCallID != "" {
+					if _, ok := seenToolCalls[toolCallID]; ok {
+						delete(openToolCalls, toolCallID)
+						continue
+					}
+				}
+				if strings.TrimSpace(payload.ReasoningContent) != "" {
+					stepID, err := addCheckpointStepBlob(blobs, &agentv1.ConversationStep{
+						Message: &agentv1.ConversationStep_ThinkingMessage{
+							ThinkingMessage: &agentv1.ThinkingMessage{Text: payload.ReasoningContent},
+						},
+					})
+					if err != nil {
+						return nil, err
+					}
+					stepIDs = append(stepIDs, stepID)
+				}
+				if len(payload.ToolCall) == 0 {
+					continue
+				}
+				toolCall := &agentv1.ToolCall{}
+				if err := protojson.Unmarshal(payload.ToolCall, toolCall); err != nil {
+					return nil, err
+				}
+				if !shouldPersistToolResultName(firstNonEmpty(strings.TrimSpace(payload.ToolName), inferToolName(toolCall))) {
+					continue
+				}
+				stepID, err := addCheckpointStepBlob(blobs, &agentv1.ConversationStep{
+					Message: &agentv1.ConversationStep_ToolCall{ToolCall: toolCall},
+				})
+				if err != nil {
+					return nil, err
+				}
+				stepIDs = append(stepIDs, stepID)
+			}
+		}
+		if len(userMessageID) == 0 && len(stepIDs) == 0 {
+			continue
+		}
+		agentTurn := &agentv1.AgentConversationTurnStructure{
+			UserMessage: userMessageID,
+			Steps:       stepIDs,
+		}
+		if turnRequestID != "" {
+			agentTurn.RequestId = &turnRequestID
+		}
+		turnPayload, err := proto.Marshal(&agentv1.ConversationTurnStructure{
+			Turn: &agentv1.ConversationTurnStructure_AgentConversationTurn{
+				AgentConversationTurn: agentTurn,
+			},
+		})
+		if err != nil {
+			return nil, err
+		}
+		turnIDs = append(turnIDs, blobs.add(turnPayload))
+	}
+	return turnIDs, nil
+}
+
+func addCheckpointStepBlob(blobs *checkpointBlobGraph, step *agentv1.ConversationStep) ([]byte, error) {
+	payload, err := proto.Marshal(step)
+	if err != nil {
+		return nil, err
+	}
+	return blobs.add(payload), nil
 }
 
 func conversationTokenDetailsUsedTokens(conversation *ConversationFile) uint32 {

internal/backend/forwarder/projector_fork_test.go

@@ -1,15 +1,17 @@
 package forwarder
 
 import (
+	"crypto/sha256"
 	"strings"
 	"testing"
 
 	"google.golang.org/protobuf/encoding/protojson"
+	"google.golang.org/protobuf/proto"
 
 	"cursor/gen/agentv1"
 )
 
-func TestProjectLegacyCheckpointKeepsForkContextWithoutDanglingTurnBlobs(t *testing.T) {
+func TestProjectCheckpointProjectionBuildsResolvableForkState(t *testing.T) {
 	userPayload, err := protojson.Marshal(&agentv1.UserMessage{
 		Text:      "parent question",
 		MessageId: "message-1",
@@ -30,12 +32,41 @@ func TestProjectLegacyCheckpointKeepsForkContextWithoutDanglingTurnBlobs(t *test
 		},
 	}
 
-	state, err := NewHistoryProjector().ProjectLegacyCheckpoint(conversation)
+	projection, err := NewHistoryProjector().ProjectCheckpointProjection(conversation)
 	if err != nil {
-		t.Fatalf("ProjectLegacyCheckpoint() error = %v", err)
+		t.Fatalf("ProjectCheckpointProjection() error = %v", err)
 	}
-	if len(state.GetTurns()) != 0 {
-		t.Fatalf("ProjectLegacyCheckpoint() turns = %d, want 0 dangling inline blobs", len(state.GetTurns()))
+	state := projection.State
+	if len(state.GetTurns()) != 1 {
+		t.Fatalf("ProjectCheckpointProjection() turns = %d, want 1 Blob-backed turn", len(state.GetTurns()))
+	}
+	blobs := make(map[string][]byte, len(projection.Blobs))
+	for _, blob := range projection.Blobs {
+		digest := sha256.Sum256(blob.Data)
+		if len(blob.ID) != sha256.Size || string(blob.ID) != string(digest[:]) {
+			t.Fatalf("invalid content-addressed Blob id=%x", blob.ID)
+		}
+		blobs[string(blob.ID)] = blob.Data
+	}
+	turnPayload, ok := blobs[string(state.GetTurns()[0])]
+	if !ok {
+		t.Fatal("turn references a missing Blob")
+	}
+	turn := &agentv1.ConversationTurnStructure{}
+	if err := proto.Unmarshal(turnPayload, turn); err != nil {
+		t.Fatalf("decode turn Blob: %v", err)
+	}
+	agentTurn := turn.GetAgentConversationTurn()
+	if agentTurn == nil {
+		t.Fatal("turn Blob does not contain an agent turn")
+	}
+	if _, ok := blobs[string(agentTurn.GetUserMessage())]; !ok {
+		t.Fatal("turn references a missing user message Blob")
+	}
+	for _, stepID := range agentTurn.GetSteps() {
+		if _, ok := blobs[string(stepID)]; !ok {
+			t.Fatal("turn references a missing step Blob")
+		}
 	}
 
 	messages, err := importedConversationStateModelMessages(state)
@@ -52,3 +83,58 @@ func TestProjectLegacyCheckpointKeepsForkContextWithoutDanglingTurnBlobs(t *test
 		t.Fatalf("second imported message = %#v", messages[1])
 	}
 }
+
+func TestProjectCheckpointProjectionKeepsForkPointIsolatedFromLaterHistory(t *testing.T) {
+	firstUser, err := protojson.Marshal(&agentv1.UserMessage{Text: "first question", MessageId: "message-1"})
+	if err != nil {
+		t.Fatalf("marshal first user message: %v", err)
+	}
+	conversation := &ConversationFile{
+		ConversationID:        "conversation-1",
+		RootConversationID:    "conversation-1",
+		Mode:                  "agent",
+		NextTurnSeq:           2,
+		NextEntrySeq:          3,
+		TokenDetailsMaxTokens: projectedConversationMaxTokens,
+		Entries: []HistoryEntry{
+			{Seq: 1, TurnSeq: 1, RequestID: "request-1", Role: "user", Kind: "user_message", Payload: firstUser},
+			newAssistantTextEntry(1, "request-1", "first answer", "", ""),
+		},
+	}
+	projector := NewHistoryProjector()
+	midpoint, err := projector.ProjectCheckpointProjection(conversation)
+	if err != nil {
+		t.Fatalf("midpoint projection: %v", err)
+	}
+
+	secondUser, err := protojson.Marshal(&agentv1.UserMessage{Text: "second question", MessageId: "message-2"})
+	if err != nil {
+		t.Fatalf("marshal second user message: %v", err)
+	}
+	appendEntriesInPlace(conversation, []HistoryEntry{
+		{TurnSeq: 2, RequestID: "request-2", Role: "user", Kind: "user_message", Payload: secondUser},
+		newAssistantTextEntry(2, "request-2", "second answer", "", ""),
+	})
+	latest, err := projector.ProjectCheckpointProjection(conversation)
+	if err != nil {
+		t.Fatalf("latest projection: %v", err)
+	}
+
+	midpointMessages, err := importedConversationStateModelMessages(midpoint.State)
+	if err != nil {
+		t.Fatalf("import midpoint messages: %v", err)
+	}
+	latestMessages, err := importedConversationStateModelMessages(latest.State)
+	if err != nil {
+		t.Fatalf("import latest messages: %v", err)
+	}
+	if len(midpoint.State.GetTurns()) != 1 || len(midpointMessages) != 2 {
+		t.Fatalf("midpoint turns=%d messages=%d, want 1 turn and 2 messages", len(midpoint.State.GetTurns()), len(midpointMessages))
+	}
+	if len(latest.State.GetTurns()) != 2 || len(latestMessages) != 4 {
+		t.Fatalf("latest turns=%d messages=%d, want 2 turns and 4 messages", len(latest.State.GetTurns()), len(latestMessages))
+	}
+	if midpointMessages[1].Content != "first answer" || latestMessages[3].Content != "second answer" {
+		t.Fatalf("fork snapshots are not isolated: midpoint=%#v latest=%#v", midpointMessages, latestMessages)
+	}
+}

internal/backend/forwarder/service.go

@@ -858,9 +858,7 @@ func (service *Service) handleCancelIntent(intent InboundIntent) error {
 		})
 	}
 	if hasCheckpoint {
-		if err := service.publishCheckpoint(stream.RequestID, stream.ConversationID); err != nil {
-			return err
-		}
+		service.discardPendingCheckpoint(stream, "checkpoint superseded by cancellation")
 	}
 	clearPendingProviderCompletion(stream)
 	stream.mu.Lock()
@@ -2122,9 +2120,15 @@ func (service *Service) completeSuccessfulTurn(stream *ActiveStream, completion
 			err,
 		)
 	}
-	if err := service.publishCheckpoint(requestID, conversationID); err != nil {
-		return err
+	return service.publishCheckpointWithCompletion(requestID, conversationID, &completion)
+}
+
+func (service *Service) finishSuccessfulTurnAfterCheckpoint(stream *ActiveStream, completion pendingTurnCompletion) error {
+	if stream == nil {
+		return nil
 	}
+	requestID := firstNonEmpty(strings.TrimSpace(completion.RequestID), strings.TrimSpace(stream.RequestID))
+	usage := completion.Usage
 	if err := service.broker.Publish(requestID, StreamEvent{
 		Message: buildTurnEndedMessage(usage.InputTokens, usage.OutputTokens, usage.CacheReadTokens, usage.CacheWriteTokens),
 	}); err != nil {
@@ -2151,7 +2155,11 @@ func (service *Service) failStreamIfNonTerminal(stream *ActiveStream, terminalCo
 }
 
 // publishCheckpoint 按当前内存会话镜像投影出 checkpoint，并广播给所有 RunSSE 订阅者。
-func (service *Service) publishCheckpoint(requestID string, _ string) error {
+func (service *Service) publishCheckpoint(requestID string, conversationID string) error {
+	return service.publishCheckpointWithCompletion(requestID, conversationID, nil)
+}
+
+func (service *Service) publishCheckpointWithCompletion(requestID string, _ string, completion *pendingTurnCompletion) error {
 	stream, ok := service.broker.Get(requestID)
 	if !ok || stream == nil {
 		return fmt.Errorf("request is not active: %s", requestID)
@@ -2160,15 +2168,16 @@ func (service *Service) publishCheckpoint(requestID string, _ string) error {
 	if err != nil {
 		return err
 	}
-	state, err := service.projector.ProjectLegacyCheckpoint(conversation)
+	projection, err := service.projector.ProjectCheckpointProjection(conversation)
 	if err != nil {
 		return err
 	}
-	state.PendingToolCalls = buildPendingToolCalls(pendingExecs, pendingInteractions)
-	service.rewriteCheckpointTokenDetailsForClient(stream, conversation, state)
-	return service.broker.Publish(requestID, StreamEvent{
-		Message: buildCheckpointMessage(state),
-	})
+	if projection == nil || projection.State == nil {
+		return fmt.Errorf("checkpoint projection is empty")
+	}
+	projection.State.PendingToolCalls = buildPendingToolCalls(pendingExecs, pendingInteractions)
+	service.rewriteCheckpointTokenDetailsForClient(stream, conversation, projection.State)
+	return service.queueCheckpointProjection(stream, projection, completion)
 }
 
 func (service *Service) rewriteCheckpointTokenDetailsForClient(stream *ActiveStream, conversation *ConversationFile, state *agentv1.ConversationStateStructure) {

internal/backend/forwarder/types.go

@@ -163,6 +163,10 @@ type ActiveStream struct {
 	ProviderUsage                               turnUsageSnapshot
 	ProviderTerminalToolInvocation              bool
 	PendingCompaction                           *PendingCompaction
+	PendingCheckpointBlobWrites                 map[uint32]string
+	ConfirmedCheckpointBlobs                    map[string]struct{}
+	NextCheckpointBlobRequestID                 uint32
+	PendingCheckpoint                           *pendingCheckpointPublish
 
 	Backlog                     []StreamEvent
 	Subscribers                 map[string]*StreamSubscriber
@@ -219,6 +223,12 @@ type pendingTurnCompletion struct {
 	Disposition    pendingCompletionDisposition
 }
 
+type pendingCheckpointPublish struct {
+	State      *agentv1.ConversationStateStructure
+	Required   map[string]struct{}
+	Completion *pendingTurnCompletion
+}
+
 type PendingCompaction struct {
 	Trigger                   string
 	ContextTokens             int64