diff --git a/.gitignore b/.gitignore
index 61db99f..1beba21 100644
--- a/.gitignore
+++ b/.gitignore
@@ -18,6 +18,7 @@ frontend/coverage/
 frontend/test-results/
 frontend/playwright-report/
 frontend/.cortex-*
+/.cortex-frontend-build-*/
 
 # Local application data
 *.sqlite
diff --git a/backend/cortex_backend/launcher/frontend.py b/backend/cortex_backend/launcher/frontend.py
index d406df6..e85bb67 100644
--- a/backend/cortex_backend/launcher/frontend.py
+++ b/backend/cortex_backend/launcher/frontend.py
@@ -149,6 +149,30 @@ def _run(command: list[str], *, cwd: Path) -> None:
         ) from exc
 
 
+def _stage_frontend_source(frontend_root: Path) -> Path:
+    """Copy build inputs beside the source tree so live installs stay untouched."""
+    staging = frontend_root.parent / f".cortex-frontend-build-{uuid.uuid4().hex}"
+    try:
+        shutil.copytree(
+            frontend_root,
+            staging,
+            ignore=shutil.ignore_patterns(
+                "node_modules",
+                "dist",
+                ".cortex-*",
+                "coverage",
+                "test-results",
+                "playwright-report",
+            ),
+        )
+    except OSError as exc:
+        shutil.rmtree(staging, ignore_errors=True)
+        raise FrontendBuildError(
+            "Could not stage frontend sources for an isolated build."
+        ) from exc
+    return staging
+
+
 def _install_if_needed(frontend_root: Path, expected_lock_digest: str) -> None:
     node_modules = frontend_root / "node_modules"
     marker = node_modules / INSTALL_MANIFEST_NAME
@@ -179,15 +203,15 @@ def build_frontend(
     lock = lock_digest(frontend_root)
     node_major = _major_version("node")
     npm_major = _major_version("npm")
-    _install_if_needed(frontend_root, lock)
-
-    staging = frontend_root / f".cortex-dist-staging-{uuid.uuid4().hex}"
+    build_root = _stage_frontend_source(frontend_root)
+    staging = build_root / f".cortex-dist-staging-{uuid.uuid4().hex}"
     dist = frontend_root / "dist"
     backup = frontend_root / f".cortex-dist-backup-{uuid.uuid4().hex}"
     try:
+        _install_if_needed(build_root, lock)
         _run(
             [_tool_name("npm"), "run", "build", "--", "--outDir", str(staging)],
-            cwd=frontend_root,
+            cwd=build_root,
         )
         if not (staging / "index.html").is_file():
             raise FrontendBuildError("Frontend build completed without index.html.")
@@ -221,6 +245,8 @@ def build_frontend(
     finally:
         if staging.exists():
             shutil.rmtree(staging, ignore_errors=True)
+        if build_root.exists():
+            shutil.rmtree(build_root, ignore_errors=True)
         if backup.exists() and not dist.exists():
             os.replace(backup, dist)
 
diff --git a/tests/test_launcher.py b/tests/test_launcher.py
index ac07845..603fa59 100644
--- a/tests/test_launcher.py
+++ b/tests/test_launcher.py
@@ -431,6 +431,57 @@ def fake_run(command: list[str], *, cwd: Path) -> None:
     assert not list(root.glob(".cortex-dist-*"))
 
 
+def test_frontend_build_stages_sources_outside_live_node_modules(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+):
+    root = _frontend_fixture(tmp_path)
+    installed_roots: list[Path] = []
+    monkeypatch.setattr(frontend_module, "_major_version", lambda _: 24)
+
+    def fake_install(frontend_root: Path, _expected_lock_digest: str) -> None:
+        installed_roots.append(frontend_root)
+
+    def fake_run(command: list[str], *, cwd: Path) -> None:
+        assert installed_roots == [cwd]
+        assert cwd != root
+        assert (cwd / "src" / "App.tsx").is_file()
+        output = Path(command[-1])
+        output.mkdir(parents=True)
+        (output / "index.html").write_text("isolated", encoding="utf-8")
+
+    monkeypatch.setattr(frontend_module, "_install_if_needed", fake_install)
+    monkeypatch.setattr(frontend_module, "_run", fake_run)
+
+    dist = frontend_module.build_frontend(root)
+
+    assert dist == root / "dist"
+    assert (dist / "index.html").read_text(encoding="utf-8") == "isolated"
+    assert not list(tmp_path.glob(".cortex-frontend-build-*"))
+
+
+def test_frontend_install_failure_leaves_live_node_modules_untouched(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+):
+    root = _frontend_fixture(tmp_path)
+    live_install = root / "node_modules"
+    live_install.mkdir()
+    sentinel = live_install / "still-running.node"
+    sentinel.write_bytes(b"live")
+    monkeypatch.setattr(frontend_module, "_major_version", lambda _: 24)
+
+    def fail_run(_command: list[str], *, cwd: Path) -> None:
+        assert cwd != root
+        raise FrontendBuildError("synthetic npm failure")
+
+    monkeypatch.setattr(frontend_module, "_run", fail_run)
+
+    with pytest.raises(FrontendBuildError, match="synthetic npm failure"):
+        frontend_module.build_frontend(root)
+
+    assert sentinel.read_bytes() == b"live"
+    assert not list(tmp_path.glob(".cortex-frontend-build-*"))
+
+
 def test_frontend_build_failure_preserves_existing_bundle(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ):