diff --git a/src/autoskillit/cli/app.py b/src/autoskillit/cli/app.py
index cc831fd37e..ed126bc03b 100644
--- a/src/autoskillit/cli/app.py
+++ b/src/autoskillit/cli/app.py
@@ -31,7 +31,6 @@
 from autoskillit.core import (
     RecipeSource,
     atomic_write,
-    claude_code_project_dir,
 )
 from autoskillit.execution import _has_active_execution_marker
 
@@ -152,11 +151,11 @@ def serve(*, verbose: Annotated[bool, Parameter(name=["--verbose", "-v"])] = Fal
     try:
         backend_options = {"use_uvloop": sys.platform != "win32"}
 
-        _marker_dir: Path | None = None
-        try:
-            _marker_dir = claude_code_project_dir(str(ctx.project_dir))
-        except OSError:
-            pass
+        _marker_dir: Path | None = (
+            ctx.backend.session_locator().project_log_dir(str(ctx.project_dir))
+            if ctx.backend is not None
+            else None
+        )
 
         async def _guarded_serve() -> None:
             await serve_with_signal_guard(
diff --git a/tests/cli/test_app_main.py b/tests/cli/test_app_main.py
index 6f3e0ed1e7..8b9b950314 100644
--- a/tests/cli/test_app_main.py
+++ b/tests/cli/test_app_main.py
@@ -2,12 +2,15 @@
 
 from __future__ import annotations
 
+import asyncio
 import importlib
 import sys
+from pathlib import Path
+from unittest.mock import MagicMock
 
 import pytest
 
-pytestmark = [pytest.mark.layer("cli"), pytest.mark.small]
+pytestmark = [pytest.mark.layer("cli"), pytest.mark.medium]
 
 
 def test_main_does_not_call_app_after_update_restart(monkeypatch: pytest.MonkeyPatch) -> None:
@@ -36,3 +39,64 @@ def fake_run_update_checks(**kwargs: object) -> None:
         pass
 
     assert not app_called, "app() must not be called when update path exits the process"
+
+
+def test_serve_activity_check_uses_backend_derived_marker_dir(
+    monkeypatch: pytest.MonkeyPatch,
+    tmp_path: Path,
+) -> None:
+    """serve() must derive _marker_dir via backend.session_locator().project_log_dir()."""
+    app_module = importlib.import_module("autoskillit.cli.app")
+
+    monkeypatch.chdir(tmp_path)
+
+    expected_dir = tmp_path / "backend-marker"
+
+    mock_backend = MagicMock()
+    mock_backend.session_locator.return_value.project_log_dir.return_value = expected_dir
+
+    mock_ctx = MagicMock()
+    mock_ctx.project_dir = tmp_path
+    mock_ctx.backend = mock_backend
+    mock_ctx.fleet_lock = None
+
+    mock_cfg = MagicMock()
+    mock_cfg.logging.level = "INFO"
+    mock_cfg.logging.json_output = None
+    mock_cfg.safety.protected_branches = []
+
+    monkeypatch.setattr("autoskillit.config.load_config", lambda _: mock_cfg)
+    monkeypatch.setattr("autoskillit.core.configure_logging", lambda **kw: None)
+    monkeypatch.setattr("autoskillit.server.make_context", lambda *a, **kw: mock_ctx)
+    monkeypatch.setattr("autoskillit.server._initialize", lambda ctx: None)
+
+    captured_activity_check: list = []
+
+    async def fake_serve_guard(mcp_instance, *, activity_check=None, **kw):
+        captured_activity_check.append(activity_check)
+
+    monkeypatch.setattr(app_module, "serve_with_signal_guard", fake_serve_guard)
+    monkeypatch.setattr("anyio.run", lambda fn, **kw: asyncio.run(fn()))
+
+    from autoskillit.cli.app import serve
+
+    serve()
+
+    mock_backend.session_locator.return_value.project_log_dir.assert_called_once_with(
+        str(tmp_path)
+    )
+
+    assert len(captured_activity_check) == 1, "serve_with_signal_guard was not called exactly once"
+
+    seen_marker_dirs: list = []
+    monkeypatch.setattr(
+        app_module,
+        "is_server_active",
+        lambda md, fl: (seen_marker_dirs.append(md), False)[-1],
+    )
+
+    captured_activity_check[0]()
+    assert seen_marker_dirs == [expected_dir], (
+        f"activity_check should pass backend-derived marker_dir ({expected_dir}), "
+        f"got {seen_marker_dirs}"
+    )