diff --git a/internal/session/session.go b/internal/session/session.go
index 124602e..93fa3b0 100644
--- a/internal/session/session.go
+++ b/internal/session/session.go
@@ -17,6 +17,7 @@ import (
 
 	"antidrift/internal/ai"
 	"antidrift/internal/domain"
+	"antidrift/internal/enforce"
 	"antidrift/internal/evidence"
 	"antidrift/internal/knowledge"
 	"antidrift/internal/statemachine"
@@ -75,10 +76,11 @@ const (
 )
 
 const (
-	driftDebounce = 10 * time.Second
-	driftTimeout  = 30 * time.Second
-	nudgeDebounce = 5 * time.Minute
-	nudgeTimeout  = 30 * time.Second
+	driftDebounce  = 10 * time.Second
+	driftTimeout   = 30 * time.Second
+	enforceTimeout = 5 * time.Second
+	nudgeDebounce  = 5 * time.Minute
+	nudgeTimeout   = 30 * time.Second
 )
 
 const recentTitlesMax = 10
@@ -149,6 +151,7 @@ type Controller struct {
 
 	allowedClasses   []string                // durable: the active session's allowed window classes
 	enforcementLevel domain.EnforcementLevel // durable: block enables window-minimize enforcement
+	guard            enforce.Guard
 	judge            ai.DriftJudge
 	driftStatus      string
 	driftReason      string
@@ -184,9 +187,10 @@ type ProposalView struct {
 // Status: it is populated precisely when Status is "ontask" (semantic drift
 // inside an allowed app), so it cannot be folded into the status enum.
 type DriftView struct {
-	Status string `json:"status"`
-	Reason string `json:"reason,omitempty"`
-	Nudge  string `json:"nudge,omitempty"`
+	Status   string `json:"status"`
+	Reason   string `json:"reason,omitempty"`
+	Nudge    string `json:"nudge,omitempty"`
+	Enforced bool   `json:"enforced,omitempty"`
 }
 
 type CoachView struct {
@@ -407,7 +411,8 @@ func (c *Controller) stateLocked() State {
 		if status == "" {
 			status = driftIdle
 		}
-		st.Drift = &DriftView{Status: status, Reason: c.driftReason, Nudge: c.nudgeMessage}
+		enforced := c.enforcementLevel == domain.EnforcementBlock && c.driftStatus == driftDrifting
+		st.Drift = &DriftView{Status: status, Reason: c.driftReason, Nudge: c.nudgeMessage, Enforced: enforced}
 	}
 	return st
 }
@@ -720,6 +725,14 @@ func (c *Controller) SetDriftJudge(j ai.DriftJudge) {
 	c.mu.Unlock()
 }
 
+// SetGuard injects the OS enforcement guard. A nil guard disables window-minimize
+// enforcement; everything else behaves identically.
+func (c *Controller) SetGuard(g enforce.Guard) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.guard = g
+}
+
 // SetNudge injects the AI semantic nudge judge. A nil nudger disables nudging;
 // local matching and the drift judge are unaffected.
 func (c *Controller) SetNudge(n ai.Nudger) {
@@ -1015,13 +1028,35 @@ func (c *Controller) RecordWindow(snap evidence.WindowSnapshot) {
 	c.applyEvent(now, snap)
 	c.recordTitleLocked(snap.Title)
 	launch := c.evaluateDriftLocked(now, snap)
+	enforceAct := c.enforceActionLocked()
 	c.mu.Unlock()
 	if launch != nil {
 		go launch()
 	}
+	if enforceAct != nil {
+		go enforceAct()
+	}
 	c.notify()
 }
 
+// enforceActionLocked returns the minimize thunk when this observation should be
+// enforced — guard wired, level is block, and drift is confirmed — else nil. The
+// returned func performs blocking X11 I/O and MUST run after the caller releases
+// c.mu, following the off-lock async-I/O discipline used by the other roles.
+func (c *Controller) enforceActionLocked() func() {
+	if c.guard == nil || c.enforcementLevel != domain.EnforcementBlock || c.driftStatus != driftDrifting {
+		return nil
+	}
+	guard := c.guard
+	return func() {
+		ctx, cancel := context.WithTimeout(context.Background(), enforceTimeout)
+		defer cancel()
+		if err := guard.MinimizeActive(ctx); err != nil {
+			log.Printf("session: enforce minimize failed: %v", err)
+		}
+	}
+}
+
 // evaluateDriftLocked runs the local-first drift pipeline for one observation
 // and updates synchronous drift state. When an async judgment is warranted it
 // returns the judging closure; the caller runs it in a goroutine after
@@ -1096,10 +1131,15 @@ func (c *Controller) evaluateDriftLocked(now time.Time, snap evidence.WindowSnap
 			return
 		}
 		c.judgedClasses[class] = v
+		var enforceAct func()
 		if c.stats != nil && c.stats.Current.Class == class {
 			c.applyVerdictLocked(v)
+			enforceAct = c.enforceActionLocked()
 		}
 		c.mu.Unlock()
+		if enforceAct != nil {
+			enforceAct()
+		}
 		c.notify()
 	}
 }
diff --git a/internal/session/session_test.go b/internal/session/session_test.go
index 60d6d52..16d1b62 100644
--- a/internal/session/session_test.go
+++ b/internal/session/session_test.go
@@ -558,6 +558,91 @@ func TestRestoredActiveSessionCanBeJudged(t *testing.T) {
 	waitDriftStatus(t, second, "drifting")
 }
 
+type fakeGuard struct{ calls int32 }
+
+func (g *fakeGuard) MinimizeActive(context.Context) error {
+	atomic.AddInt32(&g.calls, 1)
+	return nil
+}
+
+func waitGuardCalls(t *testing.T, g *fakeGuard, min int32) {
+	t.Helper()
+	deadline := time.Now().Add(2 * time.Second)
+	for time.Now().Before(deadline) {
+		if atomic.LoadInt32(&g.calls) >= min {
+			return
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+	t.Fatalf("guard minimize calls never reached %d (got %d)", min, atomic.LoadInt32(&g.calls))
+}
+
+func TestDriftMinimizesAtBlockViaBothPaths(t *testing.T) {
+	c, _ := newTestController(t)
+	c.SetDriftJudge(&fakeJudge{verdict: ai.Verdict{OnTask: false, Reason: "off-task"}})
+	g := &fakeGuard{}
+	c.SetGuard(g)
+	startActiveLevel(t, c, []string{"code"}, domain.EnforcementBlock)
+
+	// Async path: first off-task observation launches the judge; on confirmation
+	// the guard minimizes once.
+	c.RecordWindow(obs("firefox", "YouTube"))
+	st := waitDriftStatus(t, c, "drifting")
+	if !st.Drift.Enforced {
+		t.Fatalf("Enforced should be true while drifting at block: %+v", st.Drift)
+	}
+	waitGuardCalls(t, g, 1)
+
+	// Synchronous cached path: same class again hits the per-class cache, sets
+	// drifting under the lock, and minimizes again without a new judgment.
+	c.RecordWindow(obs("firefox", "Reddit"))
+	waitGuardCalls(t, g, 2)
+	if got := atomic.LoadInt32(&g.calls); got < 2 {
+		t.Fatalf("cached-drift path did not minimize: %d calls", got)
+	}
+}
+
+func TestWarnLevelNeverMinimizes(t *testing.T) {
+	c, _ := newTestController(t)
+	c.SetDriftJudge(&fakeJudge{verdict: ai.Verdict{OnTask: false, Reason: "off-task"}})
+	g := &fakeGuard{}
+	c.SetGuard(g)
+	startActiveLevel(t, c, []string{"code"}, domain.EnforcementWarn)
+
+	c.RecordWindow(obs("firefox", "YouTube"))
+	st := waitDriftStatus(t, c, "drifting")
+	if st.Drift.Enforced {
+		t.Fatalf("Enforced must be false at warn: %+v", st.Drift)
+	}
+	time.Sleep(50 * time.Millisecond) // allow any stray enforcement goroutine to run
+	if got := atomic.LoadInt32(&g.calls); got != 0 {
+		t.Fatalf("warn level must not minimize, got %d calls", got)
+	}
+}
+
+func TestOnTaskNeverMinimizes(t *testing.T) {
+	c, _ := newTestController(t)
+	c.SetDriftJudge(&fakeJudge{verdict: ai.Verdict{OnTask: false, Reason: "should not be called"}})
+	g := &fakeGuard{}
+	c.SetGuard(g)
+	startActiveLevel(t, c, []string{"code"}, domain.EnforcementBlock)
+
+	c.RecordWindow(obs("code", "main.go")) // local on-task match
+	time.Sleep(50 * time.Millisecond)
+	if got := atomic.LoadInt32(&g.calls); got != 0 {
+		t.Fatalf("on-task must not minimize, got %d calls", got)
+	}
+}
+
+func TestBlockWithoutGuardDoesNotPanic(t *testing.T) {
+	c, _ := newTestController(t)
+	c.SetDriftJudge(&fakeJudge{verdict: ai.Verdict{OnTask: false, Reason: "off-task"}})
+	// No guard wired.
+	startActiveLevel(t, c, []string{"code"}, domain.EnforcementBlock)
+	c.RecordWindow(obs("firefox", "YouTube"))
+	waitDriftStatus(t, c, "drifting") // must reach drifting without panicking
+}
+
 func TestLeavingPlanningClearsCoach(t *testing.T) {
 	c, _ := newTestController(t)
 	_ = c.EnterPlanning()