diff --git a/ocr-service/main.py b/ocr-service/main.py
index 754ed75b..0f656aaa 100644
--- a/ocr-service/main.py
+++ b/ocr-service/main.py
@@ -259,8 +259,10 @@ async def run_ocr_stream(request: OcrRequest):
 
 
 def _check_training_token(x_training_token: str | None) -> None:
-    """Validate training token if TRAINING_TOKEN env var is set."""
-    if TRAINING_TOKEN and x_training_token != TRAINING_TOKEN:
+    """Validate training token — fails closed when TRAINING_TOKEN is not configured."""
+    if not TRAINING_TOKEN:
+        raise HTTPException(status_code=503, detail="Training not configured on this node")
+    if x_training_token != TRAINING_TOKEN:
         raise HTTPException(status_code=403, detail="Invalid or missing X-Training-Token")
 
 
diff --git a/ocr-service/test_training_auth.py b/ocr-service/test_training_auth.py
new file mode 100644
index 00000000..e8ad23ad
--- /dev/null
+++ b/ocr-service/test_training_auth.py
@@ -0,0 +1,69 @@
+"""Tests for /train and /segtrain endpoint authentication."""
+
+import io
+import zipfile
+from unittest.mock import AsyncMock, patch
+
+import pytest
+from httpx import ASGITransport, AsyncClient
+
+from main import app
+
+
+def _minimal_zip() -> bytes:
+    """Return a ZIP with one .xml file so endpoint validation passes."""
+    buf = io.BytesIO()
+    with zipfile.ZipFile(buf, "w") as zf:
+        zf.writestr("page_01.xml", "<PcGts/>")
+    return buf.getvalue()
+
+
+# ─── Missing TRAINING_TOKEN → fail closed ─────────────────────────────────────
+
+
+@pytest.mark.asyncio
+async def test_train_returns_503_when_training_token_not_configured():
+    """POST /train must return 503 when TRAINING_TOKEN env var is empty.
+
+    An empty token means the service was started without training configured.
+    Allowing requests through would grant unauthenticated access to the
+    training endpoint, contradicting the principle of failing closed.
+    """
+    with patch("main.TRAINING_TOKEN", ""), \
+         patch("main._models_ready", True):
+        async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as client:
+            response = await client.post(
+                "/train",
+                files={"file": ("training.zip", _minimal_zip(), "application/zip")},
+            )
+
+    assert response.status_code == 503
+
+
+@pytest.mark.asyncio
+async def test_segtrain_returns_503_when_training_token_not_configured():
+    """POST /segtrain must return 503 when TRAINING_TOKEN env var is empty."""
+    with patch("main.TRAINING_TOKEN", ""), \
+         patch("main._models_ready", True):
+        async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as client:
+            response = await client.post(
+                "/segtrain",
+                files={"file": ("training.zip", _minimal_zip(), "application/zip")},
+            )
+
+    assert response.status_code == 503
+
+
+@pytest.mark.asyncio
+async def test_train_returns_403_when_token_wrong():
+    """POST /train must return 403 when TRAINING_TOKEN is set but header is wrong."""
+    with patch("main.TRAINING_TOKEN", "secret-token"), \
+         patch("main._models_ready", True):
+        async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as client:
+            response = await client.post(
+                "/train",
+                files={"file": ("training.zip", _minimal_zip(), "application/zip")},
+                headers={"X-Training-Token": "wrong-token"},
+            )
+
+    assert response.status_code == 403