diff --git a/Docs/Tasks.md b/Docs/Tasks.md
index ba42580..36808a8 100644
--- a/Docs/Tasks.md
+++ b/Docs/Tasks.md
@@ -66,6 +66,8 @@ Reference: `Docs/Refactoring.md` for full analysis of each issue.
 
 ### 4. Add Path Validation to `_geoip_reader` Initialization
 
+**Status:** Completed.
+
 **Where:** `backend/app/services/geo_service.py` — the `init_geoip()` function (around line 249) sets the module-level `_geoip_reader` without acquiring `_cache_lock`.
 
 **Goal:** Add a clear code comment documenting the startup-only assumption: `init_geoip()` must only be called during application startup (from `startup.py`) before request handling begins. Optionally, add an assertion or guard that prevents double-initialization.
diff --git a/backend/app/services/geo_service.py b/backend/app/services/geo_service.py
index 89df8ab..82621a5 100644
--- a/backend/app/services/geo_service.py
+++ b/backend/app/services/geo_service.py
@@ -109,6 +109,11 @@ _dirty: set[str] = set()
 #: Optional MaxMind GeoLite2 reader initialised by :func:`init_geoip`.
 _geoip_reader: geoip2.database.Reader | None = None
 
+#: Indicates whether :func:`init_geoip` has already been called.
+#: This function is startup-only and must not be invoked again while the
+#: process is handling requests.
+_geoip_initialized: bool = False
+
 #: Lock protecting mutations to the in-memory geo caches.
 _cache_lock: asyncio.Lock = asyncio.Lock()
 
@@ -230,13 +235,19 @@ async def re_resolve_all(
 def init_geoip(mmdb_path: str | None) -> None:
     """Initialise the MaxMind GeoLite2-Country database reader.
 
+    This function is startup-only and must be called before request handling
+    begins. A second initialization attempt is considered a programming error
+    and raises ``RuntimeError``.
+
     If *mmdb_path* is ``None``, empty, or the file does not exist the
     fallback is silently disabled — ip-api.com remains the sole resolver.
 
     Args:
         mmdb_path: Absolute path to a ``GeoLite2-Country.mmdb`` file.
     """
-    global _geoip_reader  # noqa: PLW0603
+    global _geoip_reader, _geoip_initialized  # noqa: PLW0603
+    if _geoip_initialized:
+        raise RuntimeError("GeoIP reader already initialised")
     if not mmdb_path:
         return
     from pathlib import Path  # noqa: PLC0415
@@ -247,6 +258,7 @@ def init_geoip(mmdb_path: str | None) -> None:
         log.warning("geoip_mmdb_not_found", path=mmdb_path)
         return
     _geoip_reader = geoip2.database.Reader(mmdb_path)
+    _geoip_initialized = True
     log.info("geoip_mmdb_loaded", path=mmdb_path)
 
 
diff --git a/backend/tests/test_services/test_geo_service.py b/backend/tests/test_services/test_geo_service.py
index c58bf19..8359b3e 100644
--- a/backend/tests/test_services/test_geo_service.py
+++ b/backend/tests/test_services/test_geo_service.py
@@ -48,6 +48,31 @@ def _make_session(response_json: dict[str, object], status: int = 200) -> MagicM
 async def clear_geo_cache() -> None:
     """Flush the module-level geo cache before every test."""
     await geo_service.clear_cache()
+    geo_service._geoip_reader = None
+    geo_service._geoip_initialized = False
+
+
+def test_init_geoip_is_startup_only(tmp_path) -> None:
+    """A second init_geoip() call raises when the reader was already loaded."""
+    path = tmp_path / "GeoLite2-Country.mmdb"
+    path.write_text("dummy")
+
+    with patch("geoip2.database.Reader", MagicMock(name="Reader")) as mock_reader:
+        geo_service.init_geoip(str(path))
+        assert geo_service._geoip_reader is not None
+        assert geo_service._geoip_initialized is True
+
+        with pytest.raises(RuntimeError, match="already initialised"):
+            geo_service.init_geoip(str(path))
+
+    assert mock_reader.call_count == 1
+
+
+def test_init_geoip_no_path_leaves_reader_uninitialised() -> None:
+    """No active reader is created when no path is supplied."""
+    geo_service.init_geoip("")
+    assert geo_service._geoip_reader is None
+    assert geo_service._geoip_initialized is False
 
 
 # ---------------------------------------------------------------------------