diff --git a/internal/app/app.go b/internal/app/app.go
index 22d6ee19..9eb16531 100644
--- a/internal/app/app.go
+++ b/internal/app/app.go
@@ -161,11 +161,17 @@ func Assemble(cfg *config.Config, svc *bizdev.Service) (*Runtime, error) {
 		if pool != nil {
 			instSvc.SetActiveMemberLookup(projectinstance.NewPGMemberLookup(pool))
 		}
+		if parts.Gov != nil {
+			instSvc.SetDocumentDutyProjector(&documentDutyProjectorAdapter{gov: parts.Gov})
+		}
 	}
 	taskMgr := taskrun.NewManager(pwSvc, parts.Ctx)
 	if pool != nil {
 		taskMgr.SetStore(taskrun.NewPGStore(pool))
 	}
+	if instSvc != nil && parts.Gov != nil {
+		instSvc.SetDocTaskBusyChecker(&docTaskBusyCheckerAdapter{gov: parts.Gov, taskRuns: taskMgr})
+	}
 	wsCap := workspace.NewCapability(cfg.DataDir, workspace.NewLeaseTable())
 	cmds := &taskrun.Commands{
 		Ctx:             parts.Ctx,
diff --git a/internal/app/duty_reassign_adapters.go b/internal/app/duty_reassign_adapters.go
new file mode 100644
index 00000000..4783a8df
--- /dev/null
+++ b/internal/app/duty_reassign_adapters.go
@@ -0,0 +1,61 @@
+package app
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"chat-agent-system/internal/docgov"
+	"chat-agent-system/internal/projectinstance"
+	"chat-agent-system/internal/taskrun"
+)
+
+type documentDutyProjectorAdapter struct {
+	gov *docgov.GovernanceEngine
+}
+
+func (a *documentDutyProjectorAdapter) ProjectDocTaskDuties(
+	ctx context.Context,
+	projectID int64,
+	taskKey string,
+	duties projectinstance.DocDuty,
+	dutyPolicy string,
+	names projectinstance.DutyDisplayNames,
+) error {
+	if a == nil || a.gov == nil {
+		return nil
+	}
+	return a.gov.ProjectDocTaskDuties(
+		ctx, projectID, taskKey,
+		duties.AuthorUserID, duties.ReviewerUserID, duties.ApproverUserID,
+		dutyPolicy, names.AuthorName, names.ReviewerName, names.ApproverName,
+	)
+}
+
+type docTaskBusyCheckerAdapter struct {
+	gov      *docgov.GovernanceEngine
+	taskRuns *taskrun.Manager
+}
+
+func (a *docTaskBusyCheckerAdapter) DocTaskSquadBusy(ctx context.Context, projectID int64, taskKey string) (bool, string, error) {
+	if a == nil || a.gov == nil || projectID <= 0 || taskKey == "" {
+		return false, "", nil
+	}
+	doc, err := a.gov.FindByProjectTaskKey(ctx, projectID, taskKey)
+	if err != nil {
+		if err == docgov.ErrDocumentNotFound {
+			return false, "", nil
+		}
+		return false, "", err
+	}
+	if doc == nil || !doc.HasActiveLease(time.Now()) {
+		return false, "", nil
+	}
+	if doc.LeaseHolderType == docgov.LeaseHolderSquad && a.taskRuns != nil {
+		if run, runErr := a.taskRuns.GetRun(doc.LeaseHolderID); runErr == nil && run.Status != "running" {
+			return false, "", nil
+		}
+	}
+	reason := fmt.Sprintf("该文档小队任务进行中（%s/%s），完成后可更换负责人", doc.LeaseHolderType, doc.LeaseHolderID)
+	return true, reason, nil
+}
diff --git a/internal/docgov/duty_project.go b/internal/docgov/duty_project.go
new file mode 100644
index 00000000..66e1d471
--- /dev/null
+++ b/internal/docgov/duty_project.go
@@ -0,0 +1,111 @@
+package docgov
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"time"
+)
+
+// FindByProjectTaskKey 按项目 + task_key 查找文档（无则 ErrDocumentNotFound）。
+func (e *GovernanceEngine) FindByProjectTaskKey(ctx context.Context, projectID int64, taskKey string) (*DocumentRecord, error) {
+	if e == nil || projectID <= 0 {
+		return nil, ErrDocumentNotFound
+	}
+	taskKey = strings.TrimSpace(taskKey)
+	if taskKey == "" {
+		return nil, ErrDocumentNotFound
+	}
+
+	e.mu.Lock()
+	defer e.mu.Unlock()
+
+	if e.pool != nil {
+		var docID string
+		err := e.pool.QueryRow(ctx, `
+			SELECT doc_id FROM document_records
+			WHERE project_id = $1 AND task_key = $2
+			ORDER BY updated_at DESC
+			LIMIT 1
+		`, projectID, taskKey).Scan(&docID)
+		if err != nil {
+			// fall through to mem
+		} else if docID != "" {
+			return e.ensureDocumentLocked(ctx, docID)
+		}
+	}
+
+	for _, doc := range e.memDocs {
+		if doc != nil && doc.ProjectID == projectID && doc.TaskKey == taskKey {
+			cp := *doc
+			return &cp, nil
+		}
+	}
+	return nil, ErrDocumentNotFound
+}
+
+// ProjectDocTaskDuties 受控改派：更新 document_records 三职投影（含 duty_locked）。
+func (e *GovernanceEngine) ProjectDocTaskDuties(
+	ctx context.Context,
+	projectID int64,
+	taskKey string,
+	authorID, reviewerID, approverID int64,
+	dutyPolicy string,
+	authorName, reviewerName, approverName string,
+) error {
+	if e == nil || projectID <= 0 {
+		return nil
+	}
+	taskKey = strings.TrimSpace(taskKey)
+	if taskKey == "" {
+		return nil
+	}
+	if authorName == "" {
+		authorName = fmt.Sprintf("user-%d", authorID)
+	}
+	if reviewerName == "" {
+		reviewerName = fmt.Sprintf("user-%d", reviewerID)
+	}
+	if approverName == "" {
+		approverName = fmt.Sprintf("user-%d", approverID)
+	}
+
+	e.mu.Lock()
+	defer e.mu.Unlock()
+
+	now := time.Now()
+	if e.pool != nil {
+		tag, err := e.pool.Exec(ctx, `
+			UPDATE document_records SET
+				author_id = NULLIF($3, 0), author_name = $4,
+				reviewer_id = NULLIF($5, 0), reviewer_name = $6,
+				approver_id = NULLIF($7, 0), approver_name = $8,
+				duty_policy = $9,
+				updated_at = $10
+			WHERE project_id = $1 AND task_key = $2
+		`, projectID, taskKey,
+			authorID, authorName, reviewerID, reviewerName, approverID, approverName,
+			strings.TrimSpace(dutyPolicy), now)
+		if err != nil {
+			return fmt.Errorf("docgov: project doc task duties: %w", err)
+		}
+		if tag.RowsAffected() == 0 {
+			return nil // 文档尚未生成，仅实例侧已更新
+		}
+	}
+
+	for _, doc := range e.memDocs {
+		if doc == nil || doc.ProjectID != projectID || doc.TaskKey != taskKey {
+			continue
+		}
+		doc.AuthorID = authorID
+		doc.AuthorName = authorName
+		doc.ReviewerID = reviewerID
+		doc.ReviewerName = reviewerName
+		doc.ApproverID = approverID
+		doc.ApproverName = approverName
+		doc.DutyPolicy = strings.TrimSpace(dutyPolicy)
+		doc.UpdatedAt = now
+	}
+	return nil
+}
diff --git a/internal/projectinstance/duty_reassign.go b/internal/projectinstance/duty_reassign.go
new file mode 100644
index 00000000..9394666e
--- /dev/null
+++ b/internal/projectinstance/duty_reassign.go
@@ -0,0 +1,170 @@
+package projectinstance
+
+import (
+	"context"
+	"fmt"
+
+	"chat-agent-system/internal/workflowdef"
+)
+
+// ReassignDocTaskDutiesInput 激活后受控改派单文档三职。
+type ReassignDocTaskDutiesInput struct {
+	TaskKey    string  `json:"task_key"`
+	Duties     DocDuty `json:"duties"`
+	DutyPolicy string  `json:"duty_policy,omitempty"`
+}
+
+// DocumentDutyProjector 将实例职责投影到 document_records（含 duty_locked 行）。
+type DocumentDutyProjector interface {
+	ProjectDocTaskDuties(ctx context.Context, projectID int64, taskKey string, duties DocDuty, dutyPolicy string, names DutyDisplayNames) error
+}
+
+// DutyDisplayNames 投影用显示名；空则保留原名或写占位。
+type DutyDisplayNames struct {
+	AuthorName   string
+	ReviewerName string
+	ApproverName string
+}
+
+// DocTaskBusyChecker 改派前检查小队任务是否占用文档。
+type DocTaskBusyChecker interface {
+	DocTaskSquadBusy(ctx context.Context, projectID int64, taskKey string) (busy bool, reason string, err error)
+}
+
+// ReassignDocTaskDuties 在 live 实例上改派单个文档任务职责。
+// 不放开规划冻结：不修改 lifecycle / selection，仅更新 snapshot 与 assignments。
+func (s *Service) ReassignDocTaskDuties(
+	ctx context.Context,
+	projectID, userID int64,
+	allowed bool,
+	in ReassignDocTaskDutiesInput,
+) (*ProjectSettings, error) {
+	if !allowed {
+		return nil, ErrForbiddenDutyReassign
+	}
+	if projectID <= 0 || in.TaskKey == "" {
+		return nil, ErrInvalidPlan
+	}
+	if !s.Available() {
+		return nil, ErrNotAvailable
+	}
+
+	live, err := s.store.GetLiveInstance(ctx, projectID)
+	if err != nil {
+		return nil, err
+	}
+	if live == nil {
+		return nil, ErrInstanceNotFound
+	}
+
+	plan := cloneWorkflowPlan(live.ConfigSnapshot)
+	dt := findDocTaskPlanMutable(&plan, in.TaskKey)
+	if dt == nil || !dt.Enabled {
+		return nil, ErrDocTaskNotFound
+	}
+	if in.DutyPolicy != "" {
+		dt.DutyPolicy = in.DutyPolicy
+	}
+	dt.Duties = in.Duties
+	*dt = normalizeDocTaskDuties(*dt)
+
+	version, verErr := s.resolvePublishedVersion(ctx, plan)
+	if verErr != nil {
+		return nil, verErr
+	}
+	td := workflowdef.DocTaskDef{Key: dt.TaskKey, Required: true}
+	if def := findDocTaskDefInVersion(version.Definition, dt.TaskKey); def != nil {
+		td = *def
+	}
+	issues := validateDocTaskDuties(*dt, td)
+	if len(issues) > 0 {
+		return nil, newValidationError(workflowdef.ValidationResult{Valid: false, Issues: issues})
+	}
+
+	plan = normalizeWorkflowPlanDuties(plan)
+	if planUsesAuthorOnly(plan) {
+		if _, err := s.requireAuthorOnlyAllowed(ctx); err != nil {
+			return nil, err
+		}
+	}
+	if err := s.checkActiveMembers(ctx, projectID, plan); err != nil {
+		return nil, err
+	}
+
+	if s.busyChecker != nil {
+		busy, reason, busyErr := s.busyChecker.DocTaskSquadBusy(ctx, projectID, in.TaskKey)
+		if busyErr != nil {
+			return nil, busyErr
+		}
+		if busy {
+			if reason == "" {
+				reason = fmt.Sprintf("文档任务 %s 小队执行中，完成后可更换负责人", in.TaskKey)
+			}
+			return nil, &DocTaskBusyError{TaskKey: in.TaskKey, Reason: reason}
+		}
+	}
+
+	updated, err := s.store.UpdateLiveDocTaskDuties(ctx, projectID, in.TaskKey, plan, userID)
+	if err != nil {
+		return nil, err
+	}
+
+	updatedDT := findDocTaskPlan(updated.ConfigSnapshot, in.TaskKey)
+	if updatedDT != nil && s.projector != nil {
+		mode, effective, _ := ResolveDocTaskDuty(*updatedDT)
+		if mode != DutyModeNone {
+			_ = s.projector.ProjectDocTaskDuties(ctx, projectID, in.TaskKey, effective, updatedDT.DutyPolicy, DutyDisplayNames{})
+		}
+	}
+
+	return s.GetSettings(ctx, projectID, userID, true)
+}
+
+func (s *Service) SetDocumentDutyProjector(p DocumentDutyProjector) {
+	if s == nil {
+		return
+	}
+	s.projector = p
+}
+
+func (s *Service) SetDocTaskBusyChecker(c DocTaskBusyChecker) {
+	if s == nil {
+		return
+	}
+	s.busyChecker = c
+}
+
+func findDocTaskPlanMutable(plan *WorkflowPlan, taskKey string) *DocTaskPlan {
+	if plan == nil {
+		return nil
+	}
+	for i := range plan.Stages {
+		for j := range plan.Stages[i].DocTasks {
+			if plan.Stages[i].DocTasks[j].TaskKey == taskKey {
+				return &plan.Stages[i].DocTasks[j]
+			}
+		}
+	}
+	return nil
+}
+
+func findDocTaskDefInVersion(def workflowdef.VersionDefinition, taskKey string) *workflowdef.DocTaskDef {
+	for _, st := range def.Stages {
+		for i := range st.DocTasks {
+			if st.DocTasks[i].Key == taskKey {
+				return &st.DocTasks[i]
+			}
+		}
+	}
+	return nil
+}
+
+func cloneWorkflowPlan(in WorkflowPlan) WorkflowPlan {
+	out := in
+	out.Stages = make([]StagePlan, len(in.Stages))
+	for i, st := range in.Stages {
+		out.Stages[i] = st
+		out.Stages[i].DocTasks = append([]DocTaskPlan(nil), st.DocTasks...)
+	}
+	return out
+}
diff --git a/internal/projectinstance/duty_reassign_test.go b/internal/projectinstance/duty_reassign_test.go
new file mode 100644
index 00000000..01bb13d3
--- /dev/null
+++ b/internal/projectinstance/duty_reassign_test.go
@@ -0,0 +1,110 @@
+package projectinstance
+
+import (
+	"context"
+	"errors"
+	"testing"
+)
+
+type stubBusyChecker struct {
+	busy   bool
+	reason string
+}
+
+func (s *stubBusyChecker) DocTaskSquadBusy(context.Context, int64, string) (bool, string, error) {
+	return s.busy, s.reason, nil
+}
+
+type stubProjector struct {
+	calls int
+	last  DocDuty
+}
+
+func (s *stubProjector) ProjectDocTaskDuties(_ context.Context, _ int64, _ string, duties DocDuty, _ string, _ DutyDisplayNames) error {
+	s.calls++
+	s.last = duties
+	return nil
+}
+
+func TestReassignDocTaskDuties_Success(t *testing.T) {
+	ctx := context.Background()
+	store := newMemStore()
+	_ = SeedProjectMeta(store, 80, LifecyclePlanning, "", 1, 0)
+	svc := NewService(store, testResolver(), nil, nil)
+	svc.SetActiveMemberLookup(&mapMemberLookup{ids: map[int64]struct{}{1: {}, 2: {}, 3: {}, 4: {}}})
+	mustActivateTestPlan(t, svc, 80, 1)
+
+	proj := &stubProjector{}
+	svc.SetDocumentDutyProjector(proj)
+	svc.SetDocTaskBusyChecker(&stubBusyChecker{busy: false})
+
+	settings, err := svc.ReassignDocTaskDuties(ctx, 80, 1, true, ReassignDocTaskDutiesInput{
+		TaskKey: "intake",
+		Duties:  DocDuty{AuthorUserID: 4, ReviewerUserID: 2, ApproverUserID: 3},
+	})
+	if err != nil {
+		t.Fatalf("reassign: %v", err)
+	}
+	if settings == nil || settings.LiveInstance == nil {
+		t.Fatal("expected live settings")
+	}
+	dt := findDocTaskPlan(settings.LiveInstance.ConfigSnapshot, "intake")
+	if dt == nil || dt.Duties.AuthorUserID != 4 {
+		t.Fatalf("duties not updated: %+v", dt)
+	}
+	if proj.calls != 1 || proj.last.AuthorUserID != 4 {
+		t.Fatalf("projector not called: %+v", proj)
+	}
+}
+
+func TestReassignDocTaskDuties_Busy(t *testing.T) {
+	ctx := context.Background()
+	store := newMemStore()
+	_ = SeedProjectMeta(store, 81, LifecyclePlanning, "", 1, 0)
+	svc := NewService(store, testResolver(), nil, nil)
+	svc.SetActiveMemberLookup(&mapMemberLookup{ids: map[int64]struct{}{1: {}, 2: {}, 3: {}}})
+	mustActivateTestPlan(t, svc, 81, 1)
+	svc.SetDocTaskBusyChecker(&stubBusyChecker{busy: true, reason: "squad running"})
+
+	_, err := svc.ReassignDocTaskDuties(ctx, 81, 1, true, ReassignDocTaskDutiesInput{
+		TaskKey: "intake",
+		Duties:  DocDuty{AuthorUserID: 1, ReviewerUserID: 2, ApproverUserID: 3},
+	})
+	var busy *DocTaskBusyError
+	if !errors.As(err, &busy) {
+		t.Fatalf("expected DocTaskBusyError, got %v", err)
+	}
+}
+
+func TestReassignDocTaskDuties_Forbidden(t *testing.T) {
+	ctx := context.Background()
+	store := newMemStore()
+	_ = SeedProjectMeta(store, 82, LifecyclePlanning, "", 1, 0)
+	svc := NewService(store, testResolver(), nil, nil)
+	mustActivateTestPlan(t, svc, 82, 1)
+
+	_, err := svc.ReassignDocTaskDuties(ctx, 82, 1, false, ReassignDocTaskDutiesInput{
+		TaskKey: "intake",
+		Duties:  DocDuty{AuthorUserID: 1, ReviewerUserID: 2, ApproverUserID: 3},
+	})
+	if !errors.Is(err, ErrForbiddenDutyReassign) {
+		t.Fatalf("expected forbidden, got %v", err)
+	}
+}
+
+func TestReassignDocTaskDuties_PlanStillFrozen(t *testing.T) {
+	ctx := context.Background()
+	store := newMemStore()
+	_ = SeedProjectMeta(store, 83, LifecyclePlanning, "", 1, 0)
+	svc := NewService(store, testResolver(), nil, nil)
+	mustActivateTestPlan(t, svc, 83, 1)
+
+	_, err := svc.SaveWorkflowPlan(ctx, 83, 1, true, SavePlanInput{
+		Plan:                     validPlan(),
+		ExpectedPlanRevision:     1,
+		ExpectedSelectionVersion: 2,
+	})
+	if !errors.Is(err, ErrPlanFrozen) {
+		t.Fatalf("expected plan frozen after activate, got %v", err)
+	}
+}
diff --git a/internal/projectinstance/errors.go b/internal/projectinstance/errors.go
index 5f2b69ac..c82981b1 100644
--- a/internal/projectinstance/errors.go
+++ b/internal/projectinstance/errors.go
@@ -30,8 +30,33 @@ var (
 	ErrAuthorOnlyPolicyDisabled  = errors.New("author-only approval policy disabled")
 	ErrAuthorOnlyPolicyUnavailable = errors.New("author-only approval policy store unavailable")
 	ErrAuthorOnlyPolicyChanged   = errors.New("author-only approval policy changed since preview")
+	ErrForbiddenDutyReassign     = errors.New("forbidden: not allowed to reassign document duties")
+	ErrDocTaskNotFound           = errors.New("document task not found or disabled")
 )
 
+// DocTaskBusyError 文档小队任务运行中，禁止改派。
+type DocTaskBusyError struct {
+	TaskKey string
+	Reason  string
+}
+
+func (e *DocTaskBusyError) Error() string {
+	if e == nil {
+		return "document task busy"
+	}
+	if e.Reason != "" {
+		return e.Reason
+	}
+	return "document task busy: squad run in progress"
+}
+
+func (e *DocTaskBusyError) Is(target error) bool {
+	_, ok := target.(*DocTaskBusyError)
+	return ok
+}
+
+var ErrDocTaskBusy = &DocTaskBusyError{}
+
 // ValidationError 保留具体校验问题，供 HTTP 层返回可操作的错误信息。
 type ValidationError struct {
 	Issues []workflowdef.ValidationIssue
diff --git a/internal/projectinstance/service.go b/internal/projectinstance/service.go
index 2433ca6a..1fbd2942 100644
--- a/internal/projectinstance/service.go
+++ b/internal/projectinstance/service.go
@@ -26,6 +26,8 @@ type Service struct {
 	docs             DocumentReadinessChecker
 	members          ActiveMemberLookup
 	authorOnlyPolicy AuthorOnlyPolicySource
+	projector        DocumentDutyProjector
+	busyChecker      DocTaskBusyChecker
 }
 
 func NewService(store Store, wfStore VersionResolver, projects projectworkflow.Store, docs DocumentReadinessChecker) *Service {
@@ -79,6 +81,9 @@ func (s *Service) GetSettings(ctx context.Context, projectID int64, userID int64
 		settings.LiveInstance = liveInst
 		settings.CanEditPlan = false
 		settings.CanActivate = false
+		// 激活后权威规划在 live snapshot（planning 行已转为 active）。
+		snap := liveInst.ConfigSnapshot
+		settings.Plan = &snap
 		if detail, detailErr := s.store.GetInstanceByID(ctx, liveInst.ID); detailErr == nil && detail != nil {
 			settings.ActiveStageKey = detail.ActiveStageKey
 			if detail.ActiveStageKey != "" && s.docs != nil {
diff --git a/internal/projectinstance/store.go b/internal/projectinstance/store.go
index b43ba424..c532211e 100644
--- a/internal/projectinstance/store.go
+++ b/internal/projectinstance/store.go
@@ -58,6 +58,8 @@ type Store interface {
 	GetInstanceByID(ctx context.Context, instanceID int64) (*InstanceDetail, error)
 	AdvanceStage(ctx context.Context, instanceID int64, stageKey string, expectedRevision int, operatorID int64) (*InstanceDetail, error)
 	MarkDocTaskApproved(ctx context.Context, projectID int64, taskKey string) error
+	// UpdateLiveDocTaskDuties 更新 live 实例 config_snapshot 与 document_duty_assignments（单任务）。
+	UpdateLiveDocTaskDuties(ctx context.Context, projectID int64, taskKey string, plan WorkflowPlan, assignedBy int64) (*WorkflowInstance, error)
 }
 
 // NewStore 构建 PG 或内存 Store。
@@ -684,6 +686,89 @@ func (s *pgStore) ConsumeActivationToken(ctx context.Context, token string, proj
 	return &plan, nil
 }
 
+func (s *pgStore) UpdateLiveDocTaskDuties(ctx context.Context, projectID int64, taskKey string, plan WorkflowPlan, assignedBy int64) (*WorkflowInstance, error) {
+	tx, err := s.pool.Begin(ctx)
+	if err != nil {
+		return nil, err
+	}
+	defer tx.Rollback(ctx)
+
+	live, err := s.GetLiveInstance(ctx, projectID)
+	if err != nil {
+		return nil, err
+	}
+	if live == nil {
+		return nil, ErrInstanceNotFound
+	}
+
+	planHash, err := computePlanHash(plan)
+	if err != nil {
+		return nil, err
+	}
+	snapRaw, err := json.Marshal(plan)
+	if err != nil {
+		return nil, err
+	}
+	now := time.Now()
+	row := tx.QueryRow(ctx, `
+		UPDATE project_workflow_instances SET
+			config_snapshot = $2, plan_hash = $3, config_revision = config_revision + 1, updated_at = $4
+		WHERE id = $1 AND status IN ('active', 'awaiting_stage_confirmation')
+		RETURNING id, project_id, workflow_version_id, instance_no, status, config_revision, plan_hash,
+		          config_snapshot, activated_by, activated_at, created_by, created_at, updated_at
+	`, live.ID, snapRaw, planHash, now)
+	inst, err := scanInstance(row)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return nil, ErrInstanceNotFound
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	var docTaskID int64
+	err = tx.QueryRow(ctx, `
+		SELECT dti.id FROM project_doc_task_instances dti
+		JOIN project_stage_instances si ON si.id = dti.stage_instance_id
+		WHERE si.workflow_instance_id = $1 AND dti.task_key = $2
+	`, inst.ID, taskKey).Scan(&docTaskID)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return nil, ErrDocTaskNotFound
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	if _, err := tx.Exec(ctx, `DELETE FROM document_duty_assignments WHERE doc_task_instance_id = $1`, docTaskID); err != nil {
+		return nil, err
+	}
+
+	dt := findDocTaskPlan(plan, taskKey)
+	if dt == nil {
+		return nil, ErrDocTaskNotFound
+	}
+	mode, effective, _ := ResolveDocTaskDuty(*dt)
+	if mode != DutyModeNone {
+		for role, uid := range map[string]int64{
+			DutyAuthor: effective.AuthorUserID, DutyReviewer: effective.ReviewerUserID, DutyApprover: effective.ApproverUserID,
+		} {
+			if uid <= 0 {
+				continue
+			}
+			if _, err := tx.Exec(ctx, `
+				INSERT INTO document_duty_assignments (doc_task_instance_id, duty_role, user_id, assigned_by)
+				VALUES ($1, $2, $3, $4)
+			`, docTaskID, role, uid, assignedBy); err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	if err := tx.Commit(ctx); err != nil {
+		return nil, err
+	}
+	return inst, nil
+}
+
 func (s *pgStore) TerminateInstance(ctx context.Context, instanceID int64, projectID int64, terminatedBy int64, reason string) (*WorkflowInstance, error) {
 	tx, err := s.pool.Begin(ctx)
 	if err != nil {
@@ -1107,6 +1192,29 @@ func (m *memStore) MarkDocTaskApproved(_ context.Context, projectID int64, taskK
 	return nil
 }
 
+func (m *memStore) UpdateLiveDocTaskDuties(_ context.Context, projectID int64, taskKey string, plan WorkflowPlan, _ int64) (*WorkflowInstance, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	live := m.live[projectID]
+	if live == nil {
+		return nil, ErrInstanceNotFound
+	}
+	if findDocTaskPlan(plan, taskKey) == nil {
+		return nil, ErrDocTaskNotFound
+	}
+	planHash, err := computePlanHash(plan)
+	if err != nil {
+		return nil, err
+	}
+	copyInst := *live
+	copyInst.ConfigSnapshot = plan
+	copyInst.PlanHash = planHash
+	copyInst.ConfigRevision++
+	copyInst.UpdatedAt = time.Now()
+	m.live[projectID] = &copyInst
+	return &copyInst, nil
+}
+
 func (m *memStore) TerminateInstance(_ context.Context, instanceID int64, projectID int64, terminatedBy int64, reason string) (*WorkflowInstance, error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
diff --git a/internal/server/duty_reassign_handlers.go b/internal/server/duty_reassign_handlers.go
new file mode 100644
index 00000000..2eeaa154
--- /dev/null
+++ b/internal/server/duty_reassign_handlers.go
@@ -0,0 +1,55 @@
+package server
+
+import (
+	"encoding/json"
+	"net/http"
+	"strconv"
+
+	"chat-agent-system/internal/projectinstance"
+)
+
+func (s *Server) handleReassignDocTaskDuties(w http.ResponseWriter, r *http.Request) {
+	projectID, err := strconv.ParseInt(r.PathValue("id"), 10, 64)
+	if err != nil || projectID <= 0 {
+		writeSafeError(w, r, http.StatusBadRequest, `{"error":"ERR_INVALID_PARAM","message":"invalid project id"}`)
+		return
+	}
+	taskKey := r.PathValue("taskKey")
+	if taskKey == "" {
+		writeSafeError(w, r, http.StatusBadRequest, `{"error":"ERR_INVALID_PARAM","message":"invalid task key"}`)
+		return
+	}
+	user, ok := s.requireProjectAccess(w, r, projectID)
+	if !ok {
+		return
+	}
+	if s.instSvc == nil || !s.instSvc.Available() {
+		writeSafeError(w, r, http.StatusServiceUnavailable, `{"error":"ERR_NOT_AVAILABLE","message":"项目实例服务不可用"}`)
+		return
+	}
+
+	isCreator, _ := s.pwSvc.IsProjectCreator(r.Context(), projectID, user.ID)
+	canAssign, permErr := s.userHasSystemPermission(r.Context(), user, "project:assign_lead")
+	if permErr != nil {
+		if s.failClosedAuthz() {
+			writeSafeError(w, r, http.StatusForbidden, `{"error":"ERR_FORBIDDEN","message":"无权改派文档职责"}`)
+			return
+		}
+		canAssign = false
+	}
+	allowed := isCreator || canAssign
+
+	var req projectinstance.ReassignDocTaskDutiesInput
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeSafeError(w, r, http.StatusBadRequest, `{"error":"ERR_INVALID_PARAM","message":"无效的 JSON 请求体"}`)
+		return
+	}
+	req.TaskKey = taskKey
+
+	settings, err := s.instSvc.ReassignDocTaskDuties(r.Context(), projectID, user.ID, allowed, req)
+	if err != nil {
+		writeProjectInstanceError(w, err)
+		return
+	}
+	writeJSONResponse(w, http.StatusOK, settings)
+}
diff --git a/internal/server/project_settings_handlers.go b/internal/server/project_settings_handlers.go
index b88c90c0..dbd26293 100644
--- a/internal/server/project_settings_handlers.go
+++ b/internal/server/project_settings_handlers.go
@@ -204,7 +204,23 @@ func writeProjectInstanceError(w http.ResponseWriter, err error) {
 		writeSafeError(w, nil, http.StatusConflict, `{"error":"AUTHOR_ONLY_POLICY_CHANGED","message":"兼任总授权已变更，请重新预览并确认激活"}`)
 	case errors.Is(err, projectinstance.ErrAuthorOnlyPolicyUnavailable):
 		writeSafeError(w, nil, http.StatusServiceUnavailable, `{"error":"AUTHOR_ONLY_POLICY_UNAVAILABLE","message":"兼任总授权存储不可用"}`)
+	case errors.Is(err, projectinstance.ErrForbiddenDutyReassign):
+		writeSafeError(w, nil, http.StatusForbidden, `{"error":"ERR_FORBIDDEN","message":"无权改派文档职责"}`)
+	case errors.Is(err, projectinstance.ErrDocTaskNotFound):
+		writeSafeError(w, nil, http.StatusNotFound, `{"error":"DOC_TASK_NOT_FOUND","message":"文档任务不存在或已禁用"}`)
 	default:
+		var busy *projectinstance.DocTaskBusyError
+		if errors.As(err, &busy) {
+			msg := busy.Reason
+			if msg == "" {
+				msg = "该文档小队任务进行中，完成后可更换负责人"
+			}
+			writeJSONResponse(w, http.StatusConflict, map[string]string{
+				"error":   "DUTY_REASSIGN_BUSY",
+				"message": msg,
+			})
+			return
+		}
 		writeSafeError(w, nil, http.StatusInternalServerError, `{"error":"ERR_INTERNAL","message":"`+err.Error()+`"}`)
 	}
 }
diff --git a/internal/server/server.go b/internal/server/server.go
index c75c3ecf..dfe01d57 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -198,6 +198,7 @@ func (s *Server) Routes() http.Handler {
 	mux.HandleFunc("GET /api/v1/projects/{id}/settings", s.handleGetProjectSettings)
 	mux.HandleFunc("GET /api/v1/projects/{id}/workflow-plan/default", s.handleGetDefaultWorkflowPlan)
 	mux.HandleFunc("PUT /api/v1/projects/{id}/workflow-plan", s.handleSaveWorkflowPlan)
+	mux.HandleFunc("PATCH /api/v1/projects/{id}/doc-tasks/{taskKey}/duties", s.handleReassignDocTaskDuties)
 	mux.HandleFunc("POST /api/v1/projects/{id}/activation-preview", s.handleActivationPreview)
 	mux.HandleFunc("POST /api/v1/projects/{id}/workflow-instances/activate", s.handleActivateWorkflowInstance)
 	mux.HandleFunc("GET /api/v1/workflow-instances/{id}", s.handleGetWorkflowInstance)