fix(cli): dispatch /background inline instead of queuing it behind the turn

/background (/bg, /btw) exists to start independent work while the current
turn keeps running. Typed while the agent was busy it went into
_pending_input like ordinary input, and process_loop is blocked inside
self.chat() for the whole run, so the background task only started once the
foreground turn had finished. That is the one moment it was not needed.

/steer had the identical problem and was fixed the same way, by dispatching
inline on the UI thread. The command's own CommandDef already declares
busy_policy="dispatch"; the gateway honours that, the classic CLI never
consulted it.

The foreground turn is untouched: no interrupt, no steer, and ordinary
non-slash input keeps following the configured busy-input behaviour.

cli.py

@@ -9595,6 +9595,37 @@ class HermesCLI(CLIAgentSetupMixin, CLICommandsMixin, CLIBillingMixin):
         except Exception:
             return False
 
+    def _should_handle_background_command_inline(
+        self, text: str, has_images: bool = False
+    ) -> bool:
+        """Return True when /background should be dispatched while the agent runs.
+
+        Same queue problem /steer had. ``/background`` (``/bg``, ``/btw``)
+        exists to start independent work *without* waiting for the current
+        turn, but a slash command typed while the agent is busy goes into
+        ``_pending_input``, and ``process_loop`` is blocked inside
+        ``self.chat()`` for the whole run. The background task therefore only
+        starts once the foreground turn has finished, which is the one moment
+        it was not needed.
+
+        The command's own ``CommandDef`` already declares
+        ``busy_policy="dispatch"``; the gateway honours that, the classic CLI
+        never consulted it. Dispatching inline on the UI thread starts the
+        background session immediately and leaves the foreground turn running
+        untouched: no interrupt, no steer.
+        """
+        if not text or has_images or not _looks_like_slash_command(text):
+            return False
+        if not getattr(self, "_agent_running", False):
+            return False
+        try:
+            from hermes_cli.commands import resolve_command
+            base = text.split(None, 1)[0].lower().lstrip('/')
+            cmd = resolve_command(base)
+            return bool(cmd and cmd.name == "background")
+        except Exception:
+            return False
+
     def _output_console(self):
         """Use prompt_toolkit-safe Rich rendering once the TUI is live."""
         if getattr(self, "_app", None):
@@ -15202,6 +15233,24 @@ class HermesCLI(CLIAgentSetupMixin, CLICommandsMixin, CLIBillingMixin):
                     event.app.invalidate()
                     return
 
+                # Same treatment for /background (/bg, /btw) while the agent is
+                # running.  Queuing it defeats the entire point of the command:
+                # process_loop is blocked inside self.chat(), so the background
+                # task would only start once the foreground turn it was meant to
+                # run alongside has already finished (#75221).  The foreground
+                # turn is left alone: no interrupt, no steer.
+                if self._should_handle_background_command_inline(
+                    text, has_images=has_images
+                ):
+                    self.process_command(text)
+                    event.app.current_buffer.reset(append_to_history=True)
+                    # Repaint for the same reason as the /steer branch above:
+                    # process_command() prints through patch_stdout and never
+                    # invalidates the app, so the submitted text can linger in
+                    # the input area looking unsent.
+                    event.app.invalidate()
+                    return
+
                 # Snapshot and clear attached images
                 images = list(self._attached_images)
                 self._attached_images.clear()

tests/cli/test_cli_background_busy_path.py

@@ -0,0 +1,132 @@
+"""Regression tests for classic-CLI mid-run /background dispatch.
+
+Background
+----------
+``/background`` (``/bg``, ``/btw``) exists to start independent work while
+the current turn keeps running. Typed while the agent was busy it went into
+``self._pending_input`` like ordinary input, and ``process_loop`` is blocked
+inside ``self.chat()`` for the whole run, so the background task only started
+once the foreground turn had finished. That is the one moment it was not
+needed (#75221).
+
+``/steer`` had the identical problem and was fixed by dispatching inline on
+the UI thread; the command's own ``CommandDef`` already declares
+``busy_policy="dispatch"``, which the gateway honours and the classic CLI
+never consulted.
+
+These tests exercise the detector without starting a prompt_toolkit app,
+mirroring tests/cli/test_cli_steer_busy_path.py.
+"""
+
+from __future__ import annotations
+
+import importlib
+import sys
+from unittest.mock import MagicMock, patch
+
+
+def _make_cli():
+    """Create a HermesCLI instance with prompt_toolkit stubbed out."""
+    _clean_config = {
+        "model": {
+            "default": "anthropic/claude-opus-4.6",
+            "base_url": "https://openrouter.ai/api/v1",
+            "provider": "auto",
+        },
+        "display": {"compact": False, "tool_progress": "all"},
+        "agent": {},
+        "terminal": {"env_type": "local"},
+    }
+    clean_env = {"LLM_MODEL": "", "HERMES_MAX_ITERATIONS": ""}
+    prompt_toolkit_stubs = {
+        "prompt_toolkit": MagicMock(),
+        "prompt_toolkit.history": MagicMock(),
+        "prompt_toolkit.styles": MagicMock(),
+        "prompt_toolkit.patch_stdout": MagicMock(),
+        "prompt_toolkit.application": MagicMock(),
+        "prompt_toolkit.layout": MagicMock(),
+        "prompt_toolkit.layout.processors": MagicMock(),
+        "prompt_toolkit.filters": MagicMock(),
+        "prompt_toolkit.layout.dimension": MagicMock(),
+        "prompt_toolkit.layout.menus": MagicMock(),
+        "prompt_toolkit.widgets": MagicMock(),
+        "prompt_toolkit.key_binding": MagicMock(),
+        "prompt_toolkit.completion": MagicMock(),
+        "prompt_toolkit.formatted_text": MagicMock(),
+        "prompt_toolkit.auto_suggest": MagicMock(),
+    }
+    with patch.dict(sys.modules, prompt_toolkit_stubs), patch.dict(
+        "os.environ", clean_env, clear=False
+    ):
+        import cli as _cli_mod
+
+        _cli_mod = importlib.reload(_cli_mod)
+        with patch.object(_cli_mod, "get_tool_definitions", return_value=[]), patch.dict(
+            _cli_mod.__dict__, {"CLI_CONFIG": _clean_config}
+        ):
+            return _cli_mod.HermesCLI()
+
+
+class TestBackgroundInlineDetector:
+    def test_detects_background_when_agent_running(self):
+        cli = _make_cli()
+        cli._agent_running = True
+        assert cli._should_handle_background_command_inline(
+            "/background inspect the test failures"
+        ) is True
+
+    def test_detects_both_aliases(self):
+        cli = _make_cli()
+        cli._agent_running = True
+        assert cli._should_handle_background_command_inline("/bg do work") is True
+        assert cli._should_handle_background_command_inline("/btw do work") is True
+
+    def test_ignores_background_when_agent_idle(self):
+        """Idle input falls through to the normal process_loop dispatch."""
+        cli = _make_cli()
+        cli._agent_running = False
+        assert cli._should_handle_background_command_inline("/bg do work") is False
+
+    def test_ignores_non_slash_input(self):
+        cli = _make_cli()
+        cli._agent_running = True
+        assert cli._should_handle_background_command_inline("bg without slash") is False
+        assert cli._should_handle_background_command_inline("") is False
+
+    def test_ignores_other_slash_commands(self):
+        cli = _make_cli()
+        cli._agent_running = True
+        assert cli._should_handle_background_command_inline("/steer hello") is False
+        assert cli._should_handle_background_command_inline("/queue hello") is False
+        assert cli._should_handle_background_command_inline("/stop") is False
+
+    def test_ignores_background_with_attached_images(self):
+        """Image payloads take the normal path."""
+        cli = _make_cli()
+        cli._agent_running = True
+        assert cli._should_handle_background_command_inline(
+            "/bg look at this", has_images=True
+        ) is False
+
+    def test_case_and_whitespace_tolerant(self):
+        cli = _make_cli()
+        cli._agent_running = True
+        assert cli._should_handle_background_command_inline("/BG do work") is True
+
+
+class TestBackgroundBusyPolicyContract:
+    """The registry already declares the intent this detector implements."""
+
+    def test_background_declares_dispatch_while_busy(self):
+        from hermes_cli.commands import resolve_command
+
+        cmd = resolve_command("background")
+        assert cmd is not None
+        assert cmd.busy_policy == "dispatch"
+
+    def test_aliases_resolve_to_background(self):
+        from hermes_cli.commands import resolve_command
+
+        for alias in ("bg", "btw"):
+            cmd = resolve_command(alias)
+            assert cmd is not None and cmd.name == "background"