diff --git a/internal/ambient/sentinel.go b/internal/ambient/sentinel.go
index bdfb467..af4eaf8 100644
--- a/internal/ambient/sentinel.go
+++ b/internal/ambient/sentinel.go
@@ -150,6 +150,164 @@ func (s *Sentinel) recentForTest() []string {
 	return append([]string(nil), s.recent...)
 }
 
-// silence: keep log imported until evaluate (Task 6) uses it.
-var _ = log.Printf
-var _ = frame.MaxBriefBytes
+// Run drives the cadence loop: every cadence it evaluates, until ctx is
+// cancelled. It re-reads the cadence each cycle so SetConfig takes effect on the
+// next tick.
+func (s *Sentinel) Run(ctx context.Context) {
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-time.After(s.currentCadence()):
+			s.evaluate(ctx)
+		}
+	}
+}
+
+func (s *Sentinel) currentCadence() time.Duration {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.cadence
+}
+
+// SetConfig updates the cadence and mode live (next tick).
+func (s *Sentinel) SetConfig(cadence time.Duration, mode string) {
+	s.mu.Lock()
+	if cadence > 0 {
+		s.cadence = cadence
+	}
+	s.mode = normalizeMode(mode)
+	s.mu.Unlock()
+}
+
+// Snooze mutes the sentinel for d: it clears the current line and episode and
+// suppresses evaluation until the window elapses.
+func (s *Sentinel) Snooze(d time.Duration) {
+	s.mu.Lock()
+	s.snoozeUntil = s.deps.Clock().Add(d)
+	changed := s.line != ""
+	s.line = ""
+	s.drifting = false
+	s.committed = false
+	s.mu.Unlock()
+	if changed {
+		s.fireOnChange()
+	}
+}
+
+// evaluate runs one decision cycle. See the spec's "Trigger discipline."
+func (s *Sentinel) evaluate(ctx context.Context) {
+	s.mu.Lock()
+	if s.mode == modeOff {
+		s.mu.Unlock()
+		return
+	}
+	if s.deps.Clock().Before(s.snoozeUntil) {
+		s.mu.Unlock()
+		return
+	}
+	if s.activeModeLocked() != "" {
+		// A mode is coaching; stay silent and clear any stale line.
+		changed := s.clearLocked()
+		s.mu.Unlock()
+		if changed {
+			s.fireOnChange()
+		}
+		return
+	}
+	titles := append([]string(nil), s.recent...)
+	joined := strings.Join(titles, "\n")
+	// Cheap gate: nothing new to judge and we are on-track -> skip the brain call.
+	if joined == s.lastEvalTitles && s.line == "" {
+		s.mu.Unlock()
+		return
+	}
+	win := s.lastWindow
+	mode := s.mode
+	judge := s.deps.AI
+	s.mu.Unlock()
+
+	if judge == nil {
+		return
+	}
+	cctx, cancel := context.WithTimeout(ctx, evalTimeout)
+	fr := frame.Assemble(cctx, s.deps.Knowledge, s.deps.Tasks)
+	msg, err := judge.AmbientDrift(cctx, fr, titles)
+	cancel()
+
+	s.mu.Lock()
+	s.lastEvalTitles = joined
+	if err != nil {
+		s.mu.Unlock()
+		log.Printf("ambient: drift judge failed: %v", err)
+		return // never fabricate drift; leave the prior line intact
+	}
+
+	if msg == "" { // on-track
+		changed := s.clearLocked()
+		s.mu.Unlock()
+		if changed {
+			s.fireOnChange()
+		}
+		return
+	}
+
+	if !s.drifting { // new episode: show the line, no toast yet
+		s.line = msg
+		s.drifting = true
+		s.committed = false
+		s.mu.Unlock()
+		s.fireOnChange()
+		return
+	}
+
+	// Sustained drift. Commit once: record memory, and toast in notify mode.
+	changed := s.line != msg
+	s.line = msg
+	if s.committed {
+		s.mu.Unlock()
+		if changed {
+			s.fireOnChange()
+		}
+		return
+	}
+	s.committed = true
+	now := s.deps.Clock()
+	mem := s.deps.Memory
+	notifier := s.deps.Notifier
+	s.mu.Unlock()
+
+	if mem != nil {
+		ev := memory.Event{Kind: kindAmbientNudge, At: now, Data: map[string]any{
+			"message": msg, "title": win.Title, "class": win.Class,
+		}}
+		if e := mem.Record(ctx, ev); e != nil {
+			log.Printf("ambient: memory record: %v", e)
+		}
+	}
+	if mode == modeNotify && notifier != nil {
+		if e := notifier.Notify(ctx, "Keel — drift", msg); e != nil {
+			log.Printf("ambient: notify: %v", e)
+		}
+	}
+	s.fireOnChange()
+}
+
+// clearLocked resets line + episode state and reports whether the line changed.
+// Caller holds mu.
+func (s *Sentinel) clearLocked() bool {
+	changed := s.line != ""
+	s.line = ""
+	s.drifting = false
+	s.committed = false
+	return changed
+}
+
+// activeModeLocked reads the active-mode kind, treating a nil hook as idle.
+// Caller holds mu (the hook does no locking on the sentinel).
+func (s *Sentinel) activeModeLocked() string {
+	if s.deps.ActiveMode == nil {
+		return ""
+	}
+	return s.deps.ActiveMode()
+}
diff --git a/internal/ambient/sentinel_test.go b/internal/ambient/sentinel_test.go
index e6c849e..c623819 100644
--- a/internal/ambient/sentinel_test.go
+++ b/internal/ambient/sentinel_test.go
@@ -4,8 +4,10 @@ package ambient
 import (
 	"context"
 	"testing"
+	"time"
 
 	"keel/internal/evidence"
+	"keel/internal/memory"
 )
 
 func snap(title, class string) evidence.WindowSnapshot {
@@ -44,3 +46,201 @@ func TestOnWindowSkipsEmptyTitle(t *testing.T) {
 }
 
 var _ = context.Background
+
+// --- helpers ---
+
+type fakeJudge struct {
+	out   []string // queued returns, consumed front-to-back
+	err   error
+	calls int
+}
+
+func (f *fakeJudge) AmbientDrift(_ context.Context, _ string, _ []string) (string, error) {
+	f.calls++
+	if f.err != nil {
+		return "", f.err
+	}
+	if len(f.out) == 0 {
+		return "", nil
+	}
+	r := f.out[0]
+	f.out = f.out[1:]
+	return r, nil
+}
+
+type fakeNotifier struct {
+	calls int
+	body  string
+}
+
+func (f *fakeNotifier) Notify(_ context.Context, _, body string) error {
+	f.calls++
+	f.body = body
+	return nil
+}
+
+func newTestSentinel(j *fakeJudge, n *fakeNotifier, mem *memoryFake, mode string, active *string) *Sentinel {
+	return New(Deps{
+		AI:         j,
+		Notifier:   n,
+		Memory:     mem,
+		Clock:      func() time.Time { return time.Unix(1_700_000_000, 0) },
+		ActiveMode: func() string { return *active },
+	}, time.Minute, mode)
+}
+
+// memoryFake is a minimal memory.Store recorder.
+type memoryFake struct {
+	events []memory.Event
+}
+
+func (m *memoryFake) Record(_ context.Context, e memory.Event) error {
+	m.events = append(m.events, e)
+	return nil
+}
+func (m *memoryFake) Recent(context.Context, string, int) ([]memory.Event, error) { return nil, nil }
+
+// --- behavioural matrix ---
+
+func TestEvaluateOnTrackIsSilent(t *testing.T) {
+	j := &fakeJudge{out: []string{""}}
+	n := &fakeNotifier{}
+	mem := &memoryFake{}
+	active := ""
+	s := newTestSentinel(j, n, mem, modeNotify, &active)
+	s.OnWindow(snap("main.go - code", "code"))
+	s.evaluate(context.Background())
+	if s.Line() != "" || n.calls != 0 || len(mem.events) != 0 {
+		t.Fatalf("on-track must be silent: line=%q toasts=%d mem=%d", s.Line(), n.calls, len(mem.events))
+	}
+}
+
+func TestEvaluateSingleDriftShowsLineNoToast(t *testing.T) {
+	j := &fakeJudge{out: []string{"deep in YouTube"}}
+	n := &fakeNotifier{}
+	mem := &memoryFake{}
+	active := ""
+	s := newTestSentinel(j, n, mem, modeNotify, &active)
+	s.OnWindow(snap("YouTube - Brave", "Brave"))
+	s.evaluate(context.Background())
+	if s.Line() == "" {
+		t.Fatal("first drift must set the status line")
+	}
+	if n.calls != 0 {
+		t.Fatalf("first drift must NOT toast, got %d", n.calls)
+	}
+	if len(mem.events) != 0 {
+		t.Fatalf("first drift must not record memory yet, got %d", len(mem.events))
+	}
+}
+
+func TestEvaluateSustainedDriftTostsOnceAndRecords(t *testing.T) {
+	j := &fakeJudge{out: []string{"deep in YouTube", "still in YouTube", "yet more YouTube"}}
+	n := &fakeNotifier{}
+	mem := &memoryFake{}
+	active := ""
+	s := newTestSentinel(j, n, mem, modeNotify, &active)
+	s.OnWindow(snap("YouTube - Brave", "Brave"))
+
+	s.evaluate(context.Background()) // first: line, no toast
+	s.evaluate(context.Background()) // second: sustained -> one toast + one memory
+	s.evaluate(context.Background()) // third: still drift -> NO second toast
+
+	if n.calls != 1 {
+		t.Fatalf("sustained drift must toast exactly once, got %d", n.calls)
+	}
+	if len(mem.events) != 1 {
+		t.Fatalf("sustained drift must record exactly one ambient_nudge, got %d", len(mem.events))
+	}
+	if mem.events[0].Kind != kindAmbientNudge {
+		t.Fatalf("memory kind = %q, want %q", mem.events[0].Kind, kindAmbientNudge)
+	}
+}
+
+func TestEvaluateReturnsToOnTrackClearsLine(t *testing.T) {
+	j := &fakeJudge{out: []string{"drift", "drift", ""}}
+	n := &fakeNotifier{}
+	active := ""
+	s := newTestSentinel(j, n, &memoryFake{}, modeNotify, &active)
+	s.OnWindow(snap("YouTube - Brave", "Brave"))
+	s.evaluate(context.Background())
+	s.evaluate(context.Background())
+	s.evaluate(context.Background()) // on-track now
+	if s.Line() != "" {
+		t.Fatalf("return to on-track must clear the line, got %q", s.Line())
+	}
+}
+
+func TestEvaluateSilentWhenModeActive(t *testing.T) {
+	j := &fakeJudge{out: []string{"drift"}}
+	active := "focus"
+	s := newTestSentinel(j, &fakeNotifier{}, &memoryFake{}, modeNotify, &active)
+	s.OnWindow(snap("YouTube - Brave", "Brave"))
+	s.evaluate(context.Background())
+	if j.calls != 0 {
+		t.Fatalf("an active mode must suppress the brain call, got %d", j.calls)
+	}
+	if s.Line() != "" {
+		t.Fatalf("an active mode must keep the line empty, got %q", s.Line())
+	}
+}
+
+func TestEvaluateStatusModeNeverToasts(t *testing.T) {
+	j := &fakeJudge{out: []string{"drift", "drift"}}
+	n := &fakeNotifier{}
+	mem := &memoryFake{}
+	active := ""
+	s := newTestSentinel(j, n, mem, modeStatus, &active)
+	s.OnWindow(snap("YouTube - Brave", "Brave"))
+	s.evaluate(context.Background())
+	s.evaluate(context.Background())
+	if n.calls != 0 {
+		t.Fatalf("status mode must never toast, got %d", n.calls)
+	}
+	if s.Line() == "" {
+		t.Fatal("status mode must still show the line")
+	}
+	if len(mem.events) != 1 {
+		t.Fatalf("status mode must still record the sustained drift, got %d", len(mem.events))
+	}
+}
+
+func TestEvaluateOffModeDoesNothing(t *testing.T) {
+	j := &fakeJudge{out: []string{"drift"}}
+	active := ""
+	s := newTestSentinel(j, &fakeNotifier{}, &memoryFake{}, modeOff, &active)
+	s.OnWindow(snap("YouTube - Brave", "Brave"))
+	s.evaluate(context.Background())
+	if j.calls != 0 {
+		t.Fatalf("off mode must not call the brain, got %d", j.calls)
+	}
+}
+
+func TestEvaluateCheapGateSkipsUnchangedOnTrack(t *testing.T) {
+	j := &fakeJudge{out: []string{""}} // one on-track answer available
+	active := ""
+	s := newTestSentinel(j, &fakeNotifier{}, &memoryFake{}, modeNotify, &active)
+	s.OnWindow(snap("main.go - code", "code"))
+	s.evaluate(context.Background()) // calls=1, on-track
+	s.evaluate(context.Background()) // unchanged ring + on-track -> skip
+	if j.calls != 1 {
+		t.Fatalf("cheap gate must skip the second call, got %d", j.calls)
+	}
+}
+
+func TestSnoozeMutesAndClears(t *testing.T) {
+	j := &fakeJudge{out: []string{"drift", "drift"}}
+	active := ""
+	s := newTestSentinel(j, &fakeNotifier{}, &memoryFake{}, modeNotify, &active)
+	s.OnWindow(snap("YouTube - Brave", "Brave"))
+	s.evaluate(context.Background()) // line set
+	s.Snooze(time.Hour)
+	if s.Line() != "" {
+		t.Fatalf("snooze must clear the line, got %q", s.Line())
+	}
+	callsBefore := j.calls
+	s.evaluate(context.Background()) // snoozed -> no call
+	if j.calls != callsBefore {
+		t.Fatalf("snooze must suppress evaluation, calls %d -> %d", callsBefore, j.calls)
+	}
+}