diff --git a/coordinator.go b/coordinator.go
index 4ec3640..40613fb 100644
--- a/coordinator.go
+++ b/coordinator.go
@@ -293,6 +293,19 @@ func sessionCwd(id string) string {
 	return c.Cwd
 }
 
+// sessionTitle is the name a session was given, "" if none.
+func sessionTitle(id string) string {
+	paths, _ := filepath.Glob(filepath.Join(getProjectsDir(), "*", id+".jsonl"))
+	if len(paths) == 0 {
+		return ""
+	}
+	c, err := parseConversationFile(paths[0], time.Time{}, 0)
+	if err != nil || c == nil || !c.IsCustomTitle {
+		return ""
+	}
+	return c.Title
+}
+
 // mcpRun runs claude's or codex's mcp commands, detached so it can't prompt
 // and with stderr, where claude says a server is missing. Stubbed in tests.
 var mcpRun = func(bin string, args ...string) ([]byte, error) {
@@ -479,14 +492,18 @@ func adoptCoordinator(who string) (string, error) {
 		return "", err
 	}
 	rec := loadCoordinator()
-	id, name := "", ""
+	id, name, matches := "", "", 0
 	for sid, s := range scanSessions(nil) {
 		if !s.codex && (s.stat.name == who || sid == who) {
 			id, name = sid, s.stat.name
+			matches++
 		}
 	}
+	if matches > 1 {
+		return "", fmt.Errorf("%d running sessions are named %q: give the id of the one you mean", matches, who)
+	}
 	if id == "" && resumable(who, false) {
-		id = who
+		id, name = who, sessionTitle(who)
 	}
 	if id == "" {
 		id = namedTranscript(who)
@@ -495,6 +512,11 @@ func adoptCoordinator(who string) (string, error) {
 	if id == "" {
 		return "", fmt.Errorf("no Claude Code session named %q, and no session with that id", who)
 	}
+	// Coming back goes through launch, which takes only names like these.
+	if name != "" && !sessionName.MatchString(name) {
+		return "", fmt.Errorf("ccs couldn't reopen a session named %q: rename it with letters, digits, '.', '_' and '-' first (/rename there, or Ctrl+X E in the list)", name)
+	}
+	replaced := rec.SessionID != "" && rec.SessionID != id
 	rec.SessionID, rec.Offered = id, true
 	rec.Name = ""
 	if name != "" && name != coordinatorName {
@@ -503,7 +525,11 @@ func adoptCoordinator(who string) (string, error) {
 	if err := rec.save(); err != nil {
 		return "", err
 	}
-	return fmt.Sprintf("%s (%s) is now the coordinator", cmp.Or(name, id), id), nil
+	msg := fmt.Sprintf("%s (%s) is now the coordinator", cmp.Or(name, id), id)
+	if replaced {
+		msg += "; the one before it stays as an ordinary session"
+	}
+	return msg, nil
 }
 
 // runCoordinator is `ccs coordinator`.
diff --git a/coordinator_test.go b/coordinator_test.go
index 736d894..00a5955 100644
--- a/coordinator_test.go
+++ b/coordinator_test.go
@@ -493,6 +493,69 @@ func TestCoordinatorAdopt(t *testing.T) {
 	}
 }
 
+func TestCoordinatorAdoptOnlyWhatItCanBringBack(t *testing.T) {
+	session := func(t *testing.T, id, title string) {
+		t.Helper()
+		proj := filepath.Join(getProjectsDir(), "-w")
+		os.MkdirAll(proj, 0o700)
+		os.WriteFile(filepath.Join(proj, id+".jsonl"), []byte(`{"type":"user","cwd":"/tmp","sessionId":"`+id+`","timestamp":"2026-11-04T10:00:00Z","message":{"role":"user","content":"hi"}}`+"\n"+`{"type":"custom-title","customTitle":"`+title+`","sessionId":"`+id+`"}`+"\n"), 0o600)
+	}
+	t.Run("two running with the name: never a guess", func(t *testing.T) {
+		coordinatorFixture(t)
+		for i, id := range []string{"one", "two"} {
+			os.WriteFile(filepath.Join(getSessionsDir(), itoa(i)+".json"), []byte(`{"pid":`+itoa(os.Getpid())+`,"sessionId":"`+id+`","name":"hub"}`), 0o600)
+		}
+		out, code := runCoord("--adopt", "hub")
+		if code != 1 || !strings.Contains(out, "2 running sessions") || loadCoordinator().SessionID != "" {
+			t.Errorf("%d %s %+v", code, out, loadCoordinator())
+		}
+		if out, code := runCoord("--adopt", "two"); code != 0 || loadCoordinator().SessionID != "two" {
+			t.Errorf("by id: %d %s", code, out)
+		}
+	})
+	t.Run("a name launch can't take is refused now, not when reopening", func(t *testing.T) {
+		coordinatorFixture(t)
+		os.WriteFile(filepath.Join(getSessionsDir(), "7.json"), []byte(`{"pid":`+itoa(os.Getpid())+`,"sessionId":"their-id","name":"my hub"}`), 0o600)
+		out, code := runCoord("--adopt", "my hub")
+		if code != 1 || !strings.Contains(out, "rename it") || loadCoordinator().SessionID != "" {
+			t.Errorf("%d %s %+v", code, out, loadCoordinator())
+		}
+	})
+	t.Run("by id, closed: it keeps its name", func(t *testing.T) {
+		coordinatorFixture(t)
+		scripts := startsAs(t, "hub")
+		session(t, "their-id", "hub")
+		if out, code := runCoord("--adopt", "their-id"); code != 0 || !strings.Contains(out, "hub (their-id)") {
+			t.Fatalf("%d %s", code, out)
+		}
+		if r := loadCoordinator(); r.Name != "hub" {
+			t.Errorf("%+v", r)
+		}
+		if out, code := runCoord(); code != 0 || !strings.Contains(out, "Resumed hub") {
+			t.Fatalf("%d %s", code, out)
+		}
+		script, _ := os.ReadFile((*scripts)[0])
+		if !strings.Contains(string(script), "--name 'hub'") {
+			t.Errorf("renamed it:\n%s", script)
+		}
+	})
+	t.Run("by id, closed, with a name launch can't take", func(t *testing.T) {
+		coordinatorFixture(t)
+		session(t, "their-id", "my hub")
+		if out, code := runCoord("--adopt", "their-id"); code != 1 || !strings.Contains(out, "rename it") {
+			t.Errorf("%d %s", code, out)
+		}
+	})
+	t.Run("says when it replaces one", func(t *testing.T) {
+		coordinatorFixture(t)
+		coordinatorRecord{SessionID: "old"}.save()
+		session(t, "their-id", "hub")
+		if out, _ := runCoord("--adopt", "their-id"); !strings.Contains(out, "stays as an ordinary session") {
+			t.Error(out)
+		}
+	})
+}
+
 func TestCoordinatorInPlaceOutsideTabs(t *testing.T) {
 	coordinatorFixture(t)
 	t.Setenv("ITERM_SESSION_ID", "")