diff --git a/internal/session/lease.go b/internal/session/lease.go
index 7d0257e..dc33d0f 100644
--- a/internal/session/lease.go
+++ b/internal/session/lease.go
@@ -7,6 +7,7 @@ import (
 	"os"
 	"os/user"
 	"path/filepath"
+	"strings"
 	"time"
 )
 
@@ -155,3 +156,76 @@ func CleanupStaleLease(path string, staleAfter time.Duration) (*Lease, error) {
 	}
 	return &l, nil
 }
+
+// FormatActiveWarning renders the human-readable warning printed when a
+// fresh active lease is detected on session start.
+func FormatActiveWarning(l *Lease, now time.Time) string {
+	startedAgo := now.Sub(l.StartedAt)
+	lastSeenAgo := now.Sub(l.LastHeartbeatAt)
+
+	var b strings.Builder
+	b.WriteString("[ctask] This workspace has an active session:\n")
+	fmt.Fprintf(&b, "  Session:   %s\n", l.SessionID)
+	fmt.Fprintf(&b, "  Host:      %s\n", l.Hostname)
+	fmt.Fprintf(&b, "  Agent:     %s\n", l.Agent)
+	fmt.Fprintf(&b, "  Started:   %s (%s ago)\n",
+		l.StartedAt.Local().Format("2006-01-02 15:04"),
+		FormatAgo(startedAgo))
+	fmt.Fprintf(&b, "  Last seen: %s ago\n", FormatAgoShort(lastSeenAgo))
+	b.WriteString("\n")
+	b.WriteString("  Opening a second session may cause conflicts.\n")
+	b.WriteString("  Continue anyway? [y/N] ")
+	return b.String()
+}
+
+// FormatStaleCleanupNotice renders the single-line notice printed when a
+// stale lease has been removed during session start.
+func FormatStaleCleanupNotice(l *Lease, now time.Time) string {
+	return fmt.Sprintf("[ctask] Cleaned up stale session from %s (started %s, last seen %s ago)\n",
+		l.Hostname,
+		l.StartedAt.Local().Format("2006-01-02 15:04"),
+		FormatAgo(now.Sub(l.LastHeartbeatAt)))
+}
+
+// FormatAgo renders a coarse "2h 15m" / "3m" / "45s" for the active warning.
+func FormatAgo(d time.Duration) string {
+	if d < 0 {
+		d = 0
+	}
+	h := int(d.Hours())
+	m := int(d.Minutes()) % 60
+	if h > 0 {
+		return fmt.Sprintf("%dh %dm", h, m)
+	}
+	if m > 0 {
+		return fmt.Sprintf("%dm", m)
+	}
+	return fmt.Sprintf("%ds", int(d.Seconds()))
+}
+
+// FormatAgoShort renders "12 seconds" / "3 minutes" / "2 hours" for the
+// Last seen line, which the spec shows with a noun suffix.
+func FormatAgoShort(d time.Duration) string {
+	if d < 0 {
+		d = 0
+	}
+	s := int(d.Seconds())
+	if s < 60 {
+		if s == 1 {
+			return "1 second"
+		}
+		return fmt.Sprintf("%d seconds", s)
+	}
+	m := int(d.Minutes())
+	if m < 60 {
+		if m == 1 {
+			return "1 minute"
+		}
+		return fmt.Sprintf("%d minutes", m)
+	}
+	h := int(d.Hours())
+	if h == 1 {
+		return "1 hour"
+	}
+	return fmt.Sprintf("%d hours", h)
+}
diff --git a/internal/session/lease_test.go b/internal/session/lease_test.go
index e86927c..3f4dc92 100644
--- a/internal/session/lease_test.go
+++ b/internal/session/lease_test.go
@@ -237,3 +237,56 @@ func TestCleanupStaleLeaseCorruptRemoves(t *testing.T) {
 		t.Error("corrupt lease file should be removed")
 	}
 }
+
+func TestFormatActiveWarning(t *testing.T) {
+	now := time.Date(2026, 4, 21, 16, 45, 10, 0, time.UTC)
+	started := now.Add(-2*time.Hour - 15*time.Minute)
+	lastSeen := now.Add(-12 * time.Second)
+
+	lease := &Lease{
+		SessionID:       "warren-desktop-12345-20260421143022",
+		Hostname:        "warren-desktop",
+		Agent:           "claude",
+		StartedAt:       started,
+		LastHeartbeatAt: lastSeen,
+	}
+
+	got := FormatActiveWarning(lease, now)
+
+	for _, want := range []string{
+		"[ctask] This workspace has an active session:",
+		"warren-desktop-12345-20260421143022",
+		"Host:",
+		"warren-desktop",
+		"Agent:",
+		"claude",
+		"Started:",
+		"2h 15m ago",
+		"Last seen:",
+		"12 seconds ago",
+		"Continue anyway? [y/N]",
+	} {
+		if !strings.Contains(got, want) {
+			t.Errorf("FormatActiveWarning missing %q in:\n%s", want, got)
+		}
+	}
+}
+
+func TestFormatStaleCleanupNoticeRenders(t *testing.T) {
+	now := time.Date(2026, 4, 21, 16, 45, 10, 0, time.UTC)
+	lease := &Lease{
+		Hostname:        "other-host",
+		StartedAt:       now.Add(-2 * time.Hour),
+		LastHeartbeatAt: now.Add(-65 * time.Second),
+	}
+	got := FormatStaleCleanupNotice(lease, now)
+	for _, want := range []string{
+		"[ctask] Cleaned up stale session",
+		"other-host",
+		"started",
+	} {
+		if !strings.Contains(got, want) {
+			t.Errorf("FormatStaleCleanupNotice missing %q in:\n%s", want, got)
+		}
+	}
+}