diff --git a/internal/session/stale.go b/internal/session/stale.go
new file mode 100644
index 0000000..260ad1f
--- /dev/null
+++ b/internal/session/stale.go
@@ -0,0 +1,73 @@
+package session
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// DetectExternalChanges compares the current workspace state against the
+// EndManifest recorded in .ctask/last-session-summary.json.
+//
+// Returns (nil, nil) if no summary exists (pre-v0.4 workspace or first
+// session ever) — callers should treat this as "silently skip the warning".
+//
+// Returns a non-nil diff with empty slices when nothing changed outside a
+// ctask session.
+func DetectExternalChanges(wsDir string) (*ManifestDiff, error) {
+	summary, err := ReadSummary(SummaryPath(wsDir))
+	if err != nil {
+		return nil, err
+	}
+	if summary == nil {
+		return nil, nil
+	}
+
+	previous := &Manifest{
+		CapturedAt: summary.EndedAt,
+		Files:      append([]FileEntry{}, summary.EndManifest...),
+	}
+	current, err := CaptureManifest(wsDir)
+	if err != nil {
+		return nil, err
+	}
+	return DiffManifests(previous, current), nil
+}
+
+// FormatStaleWarning renders the Layer-3 warning shown when the workspace
+// was modified outside a ctask session since the last session ended.
+func FormatStaleWarning(summary *SessionSummary, diff *ManifestDiff) string {
+	var b strings.Builder
+	b.WriteString("[ctask] Workspace modified since last session ended:\n\n")
+	fmt.Fprintf(&b, "  Last session: %s (%s, %s)\n\n",
+		summary.EndedAt.Local().Format("2006-01-02 15:04"),
+		summary.Hostname, summary.Agent)
+	b.WriteString("  Modified since then:\n")
+
+	lines := append([]string{}, diff.Modified...)
+	sort.Strings(lines)
+	for _, f := range lines {
+		fmt.Fprintf(&b, "    %s (modified)\n", f)
+	}
+	added := append([]string{}, diff.Added...)
+	sort.Strings(added)
+	for _, f := range added {
+		fmt.Fprintf(&b, "    %s (new file)\n", f)
+	}
+	deleted := append([]string{}, diff.Deleted...)
+	sort.Strings(deleted)
+	for _, f := range deleted {
+		fmt.Fprintf(&b, "    %s (deleted)\n", f)
+	}
+	b.WriteString("\n  These changes were not made during a ctask session.\n")
+	b.WriteString("  Review before continuing? [Y/n] ")
+	return b.String()
+}
+
+// HasChanges returns true if diff reports any added/modified/deleted entries.
+func HasChanges(d *ManifestDiff) bool {
+	if d == nil {
+		return false
+	}
+	return len(d.Added) > 0 || len(d.Modified) > 0 || len(d.Deleted) > 0
+}
diff --git a/internal/session/stale_test.go b/internal/session/stale_test.go
new file mode 100644
index 0000000..1b068eb
--- /dev/null
+++ b/internal/session/stale_test.go
@@ -0,0 +1,105 @@
+package session
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestDetectExternalChangesNoSummarySilent(t *testing.T) {
+	wsDir := t.TempDir()
+
+	diff, err := DetectExternalChanges(wsDir)
+	if err != nil {
+		t.Fatalf("DetectExternalChanges: %v", err)
+	}
+	if diff != nil {
+		t.Errorf("expected nil diff when no summary exists, got %+v", diff)
+	}
+}
+
+func TestDetectExternalChangesNoopWhenUnchanged(t *testing.T) {
+	wsDir := t.TempDir()
+
+	notes := filepath.Join(wsDir, "notes.md")
+	if err := os.WriteFile(notes, []byte("hello"), 0644); err != nil {
+		t.Fatalf("write notes: %v", err)
+	}
+	info, _ := os.Stat(notes)
+
+	summary := &SessionSummary{
+		EndManifest: []FileEntry{
+			{Path: "notes.md", Size: info.Size(), Mtime: info.ModTime().UTC().Truncate(time.Second)},
+		},
+	}
+	WriteSummary(SummaryPath(wsDir), summary)
+
+	diff, err := DetectExternalChanges(wsDir)
+	if err != nil {
+		t.Fatalf("DetectExternalChanges: %v", err)
+	}
+	if diff == nil {
+		t.Fatal("expected non-nil diff")
+	}
+	if len(diff.Added) != 0 || len(diff.Modified) != 0 || len(diff.Deleted) != 0 {
+		t.Errorf("expected empty diff, got %+v", diff)
+	}
+}
+
+func TestDetectExternalChangesDetectsModifications(t *testing.T) {
+	wsDir := t.TempDir()
+
+	oldTime := time.Date(2026, 4, 21, 10, 0, 0, 0, time.UTC)
+	summary := &SessionSummary{
+		EndManifest: []FileEntry{
+			{Path: "notes.md", Size: 5, Mtime: oldTime},
+		},
+	}
+	WriteSummary(SummaryPath(wsDir), summary)
+
+	os.WriteFile(filepath.Join(wsDir, "notes.md"), []byte("hello world"), 0644)
+	os.MkdirAll(filepath.Join(wsDir, "output"), 0755)
+	os.WriteFile(filepath.Join(wsDir, "output", "report.md"), []byte("x"), 0644)
+
+	diff, err := DetectExternalChanges(wsDir)
+	if err != nil {
+		t.Fatalf("DetectExternalChanges: %v", err)
+	}
+	if diff == nil {
+		t.Fatal("expected non-nil diff")
+	}
+	if len(diff.Added) != 1 || diff.Added[0] != "output/report.md" {
+		t.Errorf("Added: got %v, want [output/report.md]", diff.Added)
+	}
+	if len(diff.Modified) != 1 || diff.Modified[0] != "notes.md" {
+		t.Errorf("Modified: got %v, want [notes.md]", diff.Modified)
+	}
+}
+
+func TestFormatStaleWarning(t *testing.T) {
+	summary := &SessionSummary{
+		EndedAt:  time.Date(2026, 4, 21, 15, 45, 0, 0, time.UTC),
+		Hostname: "warren-desktop",
+		Agent:    "claude",
+	}
+	diff := &ManifestDiff{
+		Added:    []string{"output/report.md"},
+		Modified: []string{"notes.md"},
+	}
+	got := FormatStaleWarning(summary, diff)
+	for _, want := range []string{
+		"[ctask] Workspace modified since last session ended",
+		"Last session:",
+		"warren-desktop",
+		"claude",
+		"notes.md",
+		"output/report.md",
+		"Review before continuing? [Y/n]",
+	} {
+		if !strings.Contains(got, want) {
+			t.Errorf("FormatStaleWarning missing %q in:\n%s", want, got)
+		}
+	}
+}