diff --git a/ocr-service/main.py b/ocr-service/main.py
index 3545f6a4..370dd999 100644
--- a/ocr-service/main.py
+++ b/ocr-service/main.py
@@ -25,6 +25,7 @@ from confidence import apply_confidence_markers, get_threshold
 from engines import kraken as kraken_engine
 from engines import surya as surya_engine
 from models import OcrBlock, OcrRequest
+from preprocessing import preprocess_page
 
 TRAINING_TOKEN = os.environ.get("TRAINING_TOKEN", "")
 KRAKEN_MODEL_PATH = os.environ.get("KRAKEN_MODEL_PATH", "/app/models/german_kurrent.mlmodel")
@@ -86,6 +87,10 @@ async def run_ocr(request: OcrRequest):
 
     images = await _download_and_convert_pdf(request.pdfUrl)
 
+    for i, img in enumerate(images):
+        images[i] = await asyncio.to_thread(preprocess_page, img)
+        del img
+
     script_type = request.scriptType.upper()
 
     if script_type == "HANDWRITING_KURRENT":
@@ -157,6 +162,8 @@ async def run_ocr_stream(request: OcrRequest):
                     continue
 
                 try:
+                    yield json.dumps({"type": "preprocessing", "pageNumber": page_idx}) + "\n"
+                    image = await asyncio.to_thread(preprocess_page, image)
                     blocks = []
                     for region in page_regions:
                         text = await asyncio.to_thread(
@@ -214,6 +221,8 @@ async def run_ocr_stream(request: OcrRequest):
 
         for page_idx, image in enumerate(images, start=1):
             try:
+                yield json.dumps({"type": "preprocessing", "pageNumber": page_idx}) + "\n"
+                image = await asyncio.to_thread(preprocess_page, image)
                 blocks = await asyncio.to_thread(
                     engine.extract_page_blocks, image, page_idx, request.language
                 )
diff --git a/ocr-service/test_stream.py b/ocr-service/test_stream.py
index 73164602..ab0f9ef5 100644
--- a/ocr-service/test_stream.py
+++ b/ocr-service/test_stream.py
@@ -301,3 +301,83 @@ async def test_ocr_stream_applies_confidence_markers(mock_images):
     page = [l for l in lines if l["type"] == "page"][0]
     assert page["blocks"][0]["text"] == "Lieber [unleserlich]"
     assert "words" not in page["blocks"][0]
+
+
+# ─── Preprocessing integration ───────────────────────────────────────────────
+
+
+@pytest.mark.asyncio
+async def test_stream_emits_preprocessing_event_per_page_before_page_event(mock_images):
+    """generate() must emit a preprocessing event for each page before its page event."""
+    with patch("main._download_and_convert_pdf", new_callable=AsyncMock, return_value=mock_images), \
+         patch("main._models_ready", True), \
+         patch("main.surya_engine") as mock_surya, \
+         patch("main.preprocess_page", side_effect=lambda img: img) as mock_preprocess:
+        mock_surya.extract_page_blocks.return_value = []
+
+        async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as client:
+            response = await client.post("/ocr/stream", json={
+                "pdfUrl": "http://minio/test.pdf",
+                "scriptType": "TYPEWRITER",
+            })
+
+    lines = [json.loads(line) for line in response.text.strip().split("\n") if line.strip()]
+    preprocessing_events = [l for l in lines if l["type"] == "preprocessing"]
+    page_events = [l for l in lines if l["type"] == "page"]
+
+    assert len(preprocessing_events) == 3
+    assert preprocessing_events[0]["pageNumber"] == 1
+    assert preprocessing_events[1]["pageNumber"] == 2
+    assert preprocessing_events[2]["pageNumber"] == 3
+
+    # Each preprocessing event must come immediately before the corresponding page event
+    for pre, page in zip(preprocessing_events, page_events):
+        assert pre["pageNumber"] == page["pageNumber"]
+
+    assert mock_preprocess.call_count == 3
+
+
+@pytest.mark.asyncio
+async def test_guided_stream_preprocesses_once_per_page_not_per_region(mock_images):
+    """generate_guided() must call preprocess_page once per page, not once per region."""
+    regions = [
+        {"pageNumber": 1, "x": 0.0, "y": 0.0, "width": 0.5, "height": 0.5, "annotationId": "a1"},
+        {"pageNumber": 1, "x": 0.5, "y": 0.0, "width": 0.5, "height": 0.5, "annotationId": "a2"},
+        {"pageNumber": 2, "x": 0.0, "y": 0.0, "width": 1.0, "height": 1.0, "annotationId": "a3"},
+    ]
+
+    with patch("main._download_and_convert_pdf", new_callable=AsyncMock, return_value=mock_images[:2]), \
+         patch("main._models_ready", True), \
+         patch("main.surya_engine") as mock_surya, \
+         patch("main.preprocess_page", side_effect=lambda img: img) as mock_preprocess:
+        mock_surya.extract_region_text.return_value = "text"
+
+        async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as client:
+            response = await client.post("/ocr/stream", json={
+                "pdfUrl": "http://minio/test.pdf",
+                "scriptType": "TYPEWRITER",
+                "regions": regions,
+            })
+
+    assert response.status_code == 200
+    # 2 pages, each preprocessed once — not 3 (once per region)
+    assert mock_preprocess.call_count == 2
+
+
+@pytest.mark.asyncio
+async def test_ocr_endpoint_preprocesses_silently_without_emitting_events(mock_images):
+    """/ocr endpoint preprocesses each page without emitting preprocessing events."""
+    with patch("main._download_and_convert_pdf", new_callable=AsyncMock, return_value=mock_images), \
+         patch("main._models_ready", True), \
+         patch("main.surya_engine") as mock_surya, \
+         patch("main.preprocess_page", side_effect=lambda img: img) as mock_preprocess:
+        mock_surya.extract_blocks.return_value = []
+
+        async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as client:
+            response = await client.post("/ocr", json={
+                "pdfUrl": "http://minio/test.pdf",
+                "scriptType": "TYPEWRITER",
+            })
+
+    assert response.status_code == 200
+    assert mock_preprocess.call_count == 3