fix(workflow): isolate exact model lane entry

apps/backend/internal/backendapp/adapters.go

@@ -920,6 +920,8 @@ func (a *lifecycleAdapter) ResolveAgentProfile(ctx context.Context, profileID st
 		AgentName:                  info.AgentName,
 		Model:                      info.Model,
 		Mode:                       info.Mode,
+		FallbackModel:              info.FallbackModel,
+		AutoFallback:               info.AutoFallback,
 		ConfigOptions:              info.ConfigOptions,
 		AutoApprove:                info.AutoApprove,
 		DangerouslySkipPermissions: info.DangerouslySkipPermissions,

apps/backend/internal/orchestrator/event_handlers_workflow.go

@@ -2781,6 +2781,14 @@ func (s *Service) prepareWorkflowStepSession(
 	effectiveProfile := s.resolveStepAgentProfile(ctx, step)
 	if effectiveProfile == "" || effectiveProfile == session.AgentProfileID {
 		if effectiveProfile != "" {
+			if s.workflowEntryRequiresFreshExactModelSession(ctx, session, step, sourceStep, effectiveProfile) {
+				endPolicy := s.resolveStepProfileSessionEndPolicy(sourceStep)
+				newSession, err := s.createNewSessionForStepWithEndPolicy(ctx, taskID, session, effectiveProfile, endPolicy)
+				if err != nil {
+					return nil, false, err
+				}
+				return newSession, true, nil
+			}
 			s.tagSessionAsWorkflowSwitched(ctx, session.ID)
 		}
 		if !session.IsPrimary {
@@ -2808,6 +2816,38 @@ func (s *Service) prepareWorkflowStepSession(
 	return newSession, true, nil
 }
 
+// workflowEntryRequiresFreshExactModelSession prevents a workflow lane from
+// resuming a parked session whose persisted provider model disagrees with the
+// same exact profile's configured model. Explicit session overrides remain
+// valid within a lane; this boundary applies only while entering a different
+// workflow step and only to profiles that did not authorize a fallback.
+func (s *Service) workflowEntryRequiresFreshExactModelSession(
+	ctx context.Context,
+	session *models.TaskSession,
+	step, sourceStep *wfmodels.WorkflowStep,
+	profileID string,
+) bool {
+	if session == nil || step == nil || sourceStep == nil || sourceStep.ID == step.ID || profileID == "" {
+		return false
+	}
+	profile, err := s.agentManager.ResolveAgentProfile(ctx, profileID)
+	if err != nil || profile == nil || profile.Model == "" || profile.AutoFallback || profile.FallbackModel != "" {
+		return false
+	}
+	effective, ok := models.LoadEffectiveSessionRuntimeConfig(session)
+	if !ok || effective.Model == "" || effective.Model == profile.Model {
+		return false
+	}
+	s.logger.Info("creating fresh workflow session for exact model identity",
+		zap.String("session_id", session.ID),
+		zap.String("profile_id", profileID),
+		zap.String("source_step_id", sourceStep.ID),
+		zap.String("target_step_id", step.ID),
+		zap.String("configured_model", profile.Model),
+		zap.String("persisted_model", effective.Model))
+	return true
+}
+
 func (s *Service) preflightWorkflowStepCredentials(
 	ctx context.Context,
 	taskID string,

apps/backend/internal/orchestrator/event_handlers_workflow_profile_session_policy_test.go

@@ -651,6 +651,72 @@ func TestPrepareWorkflowStepSession_QAToPRDoesNotReuseSolRuntime(t *testing.T) {
 	require.Error(t, err, "the destination must not inherit the QA ACP resume identity before launch")
 }
 
+// TestPrepareWorkflowStepSession_HumanQAToWorkReplacesExactProfileRuntimeOverride
+// covers a parked primary whose profile ID is already the Work profile but
+// whose prior Human-QA turn persisted a different runtime model. Entering an
+// exact Work lane must create a clean session before any auto-start prompt can
+// resume that stale ACP identity.
+func TestPrepareWorkflowStepSession_HumanQAToWorkReplacesExactProfileRuntimeOverride(t *testing.T) {
+	ctx := context.Background()
+	fixture := newProfileSwitchFixture(t, models.WorkflowProfileSessionStartPolicyReuse, models.WorkflowProfileSessionEndPolicyPark)
+	fixture.agentMgr.resolveProfileInfo = &executor.AgentProfileInfo{
+		Model: "gpt-5.6-terra",
+	}
+	fixture.current.AgentProfileID = "terra-work-profile"
+	fixture.current.AgentProfileSnapshot = map[string]interface{}{"model": "gpt-5.6-terra"}
+	fixture.current.Metadata = map[string]interface{}{
+		models.SessionMetaKeyRuntimeConfig: models.SessionRuntimeConfig{Model: "gpt-5.6-luna"},
+	}
+	require.NoError(t, fixture.repo.UpdateTaskSession(ctx, fixture.current))
+
+	workStep := &wfmodels.WorkflowStep{
+		ID:                        "step-work",
+		WorkflowID:                "wf1",
+		AgentProfileID:            "terra-work-profile",
+		ProfileSessionStartPolicy: models.WorkflowProfileSessionStartPolicyReuse,
+	}
+	humanQAStep := &wfmodels.WorkflowStep{
+		ID: "step-human-qa", WorkflowID: "wf1", AgentProfileID: "terra-work-profile",
+		ProfileSessionEndPolicy: models.WorkflowProfileSessionEndPolicyPark,
+	}
+
+	workSession, switched, err := fixture.svc.prepareWorkflowStepSession(ctx, "t1", fixture.current, workStep, humanQAStep)
+	require.NoError(t, err)
+	require.True(t, switched)
+	require.NotNil(t, workSession)
+	require.NotEqual(t, fixture.current.ID, workSession.ID)
+	require.Equal(t, "terra-work-profile", workSession.AgentProfileID)
+	_, inheritedRuntime := models.LoadSessionRuntimeConfig(workSession.Metadata)
+	require.False(t, inheritedRuntime, "a Work launch must not inherit Human-QA's Luna runtime model")
+
+	previous, err := fixture.repo.GetTaskSession(ctx, fixture.current.ID)
+	require.NoError(t, err)
+	require.False(t, previous.IsPrimary)
+}
+
+func TestPrepareWorkflowStepSession_HumanQAToWorkKeepsAuthorizedFallbackSession(t *testing.T) {
+	ctx := context.Background()
+	fixture := newProfileSwitchFixture(t, models.WorkflowProfileSessionStartPolicyReuse, models.WorkflowProfileSessionEndPolicyPark)
+	fixture.agentMgr.resolveProfileInfo = &executor.AgentProfileInfo{
+		Model: "gpt-5.6-terra", FallbackModel: "gpt-5.6-luna",
+	}
+	fixture.current.AgentProfileID = "terra-with-luna-fallback"
+	fixture.current.Metadata = map[string]interface{}{
+		models.SessionMetaKeyRuntimeConfig: models.SessionRuntimeConfig{Model: "gpt-5.6-luna"},
+	}
+	require.NoError(t, fixture.repo.UpdateTaskSession(ctx, fixture.current))
+
+	workStep := &wfmodels.WorkflowStep{
+		ID: "step-work", WorkflowID: "wf1", AgentProfileID: "terra-with-luna-fallback",
+	}
+	humanQAStep := &wfmodels.WorkflowStep{ID: "step-human-qa", WorkflowID: "wf1"}
+
+	workSession, switched, err := fixture.svc.prepareWorkflowStepSession(ctx, "t1", fixture.current, workStep, humanQAStep)
+	require.NoError(t, err)
+	require.False(t, switched)
+	require.Equal(t, fixture.current.ID, workSession.ID)
+}
+
 func TestSwitchSessionForStep_NewOnStartParkOnEndRoundTrip(t *testing.T) {
 	ctx := context.Background()
 	fixture := newProfileSwitchFixture(t, models.WorkflowProfileSessionStartPolicyNew, models.WorkflowProfileSessionEndPolicyPark)

apps/backend/internal/orchestrator/executor/executor.go

@@ -380,6 +380,8 @@ type AgentProfileInfo struct {
 	AgentName                  string
 	Model                      string
 	Mode                       string
+	FallbackModel              string
+	AutoFallback               bool
 	ConfigOptions              map[string]string
 	AutoApprove                bool
 	DangerouslySkipPermissions bool