feat(cli): show ▶ N indicator in status bar when /background tasks are running (#27175)

Surface live background-task count in the prompt_toolkit status bar so users
can see at a glance that a /background task exists and is running — no need
to ask the agent about it (the agent has no visibility into bg sessions by
design).

- _get_status_bar_snapshot now reports active_background_tasks from len()
  of the live _background_tasks dict (entries are removed in the task
  thread's finally block, so this reflects truly-running tasks)
- Indicator shown only on medium (<76) and wide (>=76) tiers; narrow (<52)
  stays minimal since it's already cramped
- No invalidate plumbing needed: status bar fragments are pulled via lambda
  on every redraw, and the bg thread already calls _app.invalidate() on exit

Refs #8568

cli.py

@@ -3113,8 +3113,19 @@ class HermesCLI:
             "session_total_tokens": 0,
             "session_api_calls": 0,
             "compressions": 0,
+            "active_background_tasks": 0,
         }
 
+        # Count live /background tasks. The dict entry is removed in the
+        # task thread's finally block, so len() reflects truly-running tasks.
+        # len() on a CPython dict is atomic; safe to read without a lock.
+        try:
+            bg_tasks = getattr(self, "_background_tasks", None)
+            if bg_tasks:
+                snapshot["active_background_tasks"] = len(bg_tasks)
+        except Exception:
+            pass
+
         if not agent:
             return snapshot
 
@@ -3350,6 +3361,9 @@ class HermesCLI:
                 compressions = snapshot.get("compressions", 0)
                 if compressions:
                     parts.append(f"🗜️ {compressions}")
+                bg_count = snapshot.get("active_background_tasks", 0)
+                if bg_count:
+                    parts.append(f"▶ {bg_count}")
                 parts.append(duration_label)
                 if yolo_active:
                     parts.append("⚠ YOLO")
@@ -3366,6 +3380,9 @@ class HermesCLI:
             parts = [f"⚕ {snapshot['model_short']}", context_label, percent_label]
             if compressions:
                 parts.append(f"🗜️ {compressions}")
+            bg_count = snapshot.get("active_background_tasks", 0)
+            if bg_count:
+                parts.append(f"▶ {bg_count}")
             parts.append(duration_label)
             prompt_elapsed = snapshot.get("prompt_elapsed")
             if prompt_elapsed:
@@ -3406,6 +3423,7 @@ class HermesCLI:
                 percent_label = f"{percent}%" if percent is not None else "--"
                 if width < 76:
                     compressions = snapshot.get("compressions", 0)
+                    bg_count = snapshot.get("active_background_tasks", 0)
                     frags = [
                         ("class:status-bar", " ⚕ "),
                         ("class:status-bar-strong", snapshot["model_short"]),
@@ -3415,6 +3433,9 @@ class HermesCLI:
                     if compressions:
                         frags.append(("class:status-bar-dim", " · "))
                         frags.append((self._compression_count_style(compressions), f"🗜️ {compressions}"))
+                    if bg_count:
+                        frags.append(("class:status-bar-dim", " · "))
+                        frags.append(("class:status-bar-strong", f"▶ {bg_count}"))
                     frags.extend([
                         ("class:status-bar-dim", " · "),
                         ("class:status-bar-dim", duration_label),
@@ -3433,6 +3454,7 @@ class HermesCLI:
 
                     bar_style = self._status_bar_context_style(percent)
                     compressions = snapshot.get("compressions", 0)
+                    bg_count = snapshot.get("active_background_tasks", 0)
                     frags = [
                         ("class:status-bar", " ⚕ "),
                         ("class:status-bar-strong", snapshot["model_short"]),
@@ -3446,6 +3468,9 @@ class HermesCLI:
                     if compressions:
                         frags.append(("class:status-bar-dim", " │ "))
                         frags.append((self._compression_count_style(compressions), f"🗜️ {compressions}"))
+                    if bg_count:
+                        frags.append(("class:status-bar-dim", " │ "))
+                        frags.append(("class:status-bar-strong", f"▶ {bg_count}"))
                     frags.extend([
                         ("class:status-bar-dim", " │ "),
                         ("class:status-bar-dim", duration_label),

tests/cli/test_cli_background_status_indicator.py

@@ -0,0 +1,104 @@
+"""Tests for the /background indicator in the CLI status bar.
+
+The classic prompt_toolkit status bar shows `▶ N` when N tasks launched via
+`/background` are still running. Source of truth is `self._background_tasks`
+(a Dict[str, threading.Thread]); entries are removed in the task thread's
+finally block, so len() reflects truly-running tasks.
+"""
+
+import threading
+from datetime import datetime
+
+from cli import HermesCLI
+
+
+def _stub_thread() -> threading.Thread:
+    """Return a Thread instance that's never started — pure dict-value stand-in."""
+    return threading.Thread(target=lambda: None)
+
+
+def _make_cli():
+    """Bare-metal HermesCLI for snapshot/build tests (no __init__ side effects)."""
+    cli_obj = HermesCLI.__new__(HermesCLI)
+    cli_obj.model = "anthropic/claude-opus-4.6"
+    cli_obj.agent = None
+    cli_obj._background_tasks = {}
+    # The snapshot reads session_start to compute duration; supply a stub.
+    cli_obj.session_start = datetime.now()
+    return cli_obj
+
+
+def test_snapshot_reports_zero_when_no_background_tasks():
+    cli_obj = _make_cli()
+    snap = cli_obj._get_status_bar_snapshot()
+    assert snap["active_background_tasks"] == 0
+
+
+def test_snapshot_counts_live_background_tasks():
+    cli_obj = _make_cli()
+    cli_obj._background_tasks = {"bg_a": _stub_thread(), "bg_b": _stub_thread()}
+    snap = cli_obj._get_status_bar_snapshot()
+    assert snap["active_background_tasks"] == 2
+
+
+def test_snapshot_safe_when_background_tasks_attr_missing():
+    """Older HermesCLI instances (tests with __new__, etc.) may lack the attr."""
+    cli_obj = HermesCLI.__new__(HermesCLI)
+    cli_obj.model = "x"
+    cli_obj.agent = None
+    cli_obj.session_start = datetime.now()
+    # No _background_tasks at all — must not raise.
+    snap = cli_obj._get_status_bar_snapshot()
+    assert snap["active_background_tasks"] == 0
+
+
+def test_plain_text_status_omits_indicator_when_idle():
+    cli_obj = _make_cli()
+    text = cli_obj._build_status_bar_text(width=80)
+    assert "▶" not in text
+
+
+def test_plain_text_status_shows_indicator_when_active():
+    cli_obj = _make_cli()
+    cli_obj._background_tasks = {"bg_a": _stub_thread()}
+    text = cli_obj._build_status_bar_text(width=80)
+    assert "▶ 1" in text
+
+
+def test_plain_text_status_shows_higher_count():
+    cli_obj = _make_cli()
+    cli_obj._background_tasks = {
+        "a": _stub_thread(),
+        "b": _stub_thread(),
+        "c": _stub_thread(),
+    }
+    text = cli_obj._build_status_bar_text(width=80)
+    assert "▶ 3" in text
+
+
+def test_narrow_width_omits_bg_indicator():
+    """The narrow tier (<52) is already cramped — bg is secondary, drop it."""
+    cli_obj = _make_cli()
+    cli_obj._background_tasks = {"bg_a": _stub_thread()}
+    text = cli_obj._build_status_bar_text(width=40)
+    assert "▶" not in text
+
+
+def test_fragments_include_bg_segment_when_active():
+    cli_obj = _make_cli()
+    cli_obj._background_tasks = {"a": _stub_thread(), "b": _stub_thread()}
+    cli_obj._status_bar_visible = True
+    # _get_status_bar_fragments asks _get_tui_terminal_width(); stub it wide.
+    cli_obj._get_tui_terminal_width = lambda: 120  # type: ignore[method-assign]
+    frags = cli_obj._get_status_bar_fragments()
+    rendered = "".join(text for _style, text in frags)
+    assert "▶ 2" in rendered
+
+
+def test_fragments_omit_bg_segment_when_idle():
+    cli_obj = _make_cli()
+    cli_obj._status_bar_visible = True
+    cli_obj._get_tui_terminal_width = lambda: 120  # type: ignore[method-assign]
+    frags = cli_obj._get_status_bar_fragments()
+    rendered = "".join(text for _style, text in frags)
+    assert "▶" not in rendered