internal/herdr/tmux.go

@@ -133,7 +133,15 @@ func (b *TmuxBackend) hasSession(ctx context.Context, session string) (bool, err
 		return true, nil
 	}
 	message := strings.ToLower(string(out) + " " + err.Error())
-	if strings.Contains(message, "can't find session") || strings.Contains(message, "no server running") || strings.Contains(message, "no sessions") || (strings.Contains(message, "error connecting to") && strings.Contains(message, "no such file")) {
+	// "no current target" is what a *running but empty* server answers: the
+	// target does not resolve and there is no current session to fall back to.
+	// It only started appearing once the runtime became its own unit running
+	// `tmux -D`, which keeps the server alive past its last pane (F32). Before
+	// that the server exited with its last session and answered "no server
+	// running", so this branch was never reached. Without it Kill returns an
+	// error for a pane that is already gone, quarantine never clears, and the
+	// worker's only session slot stays pinned.
+	if strings.Contains(message, "can't find session") || strings.Contains(message, "no server running") || strings.Contains(message, "no sessions") || strings.Contains(message, "no current target") || (strings.Contains(message, "error connecting to") && strings.Contains(message, "no such file")) {
 		return false, nil
 	}
 	return false, err

internal/herdr/tmux_test.go

@@ -421,3 +421,27 @@ func TestPaneProgressIgnoresInputLines(t *testing.T) {
 		t.Fatal("real harness output left the progress digest unchanged")
 	}
 }
+
+// TestMissingSessionIsRecognisedOnAnEmptyRunningServer guards F32. A running
+// but empty tmux server answers "no current target", not "no server running".
+// The runtime only stays alive past its last pane since it became its own unit
+// running `tmux -D`, so this reply had never been seen before. Treating it as
+// a real error made Kill fail for a pane that was already gone, which left the
+// quarantine set and pinned the worker's only session slot.
+func TestMissingSessionIsRecognisedOnAnEmptyRunningServer(t *testing.T) {
+	for _, message := range []string{"no current target", "can't find session: x", "no server running"} {
+		bin := filepath.Join(t.TempDir(), "tmux")
+		script := "#!/bin/sh\necho \"" + message + "\" >&2\nexit 1\n"
+		if err := os.WriteFile(bin, []byte(script), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		b := &TmuxBackend{Binary: bin}
+		if err := b.Kill(context.Background(), Session{PaneID: "s:1.0"}); err != nil {
+			t.Fatalf("%q: killing an already-gone session failed: %v", message, err)
+		}
+		status, err := b.AgentStatus(context.Background(), Session{PaneID: "s:1.0"})
+		if err != nil || status != "exited" {
+			t.Fatalf("%q: status=%q err=%v, want exited", message, status, err)
+		}
+	}
+}