diff --git a/haiku_rag_slim/haiku/rag/client/rebuild.py b/haiku_rag_slim/haiku/rag/client/rebuild.py
index 6ea1986c..9b6a2c1a 100644
--- a/haiku_rag_slim/haiku/rag/client/rebuild.py
+++ b/haiku_rag_slim/haiku/rag/client/rebuild.py
@@ -1,3 +1,4 @@
+import asyncio
 import json
 import logging
 from collections.abc import AsyncGenerator
@@ -618,6 +619,33 @@ async def _rebuild_rechunk(
         await _flush_rebuild_batch(client, pending_docs, pending_chunks)
 
 
+def _apply_descriptions_sync(docling_doc, doc, descriptions):
+    """Patch picture descriptions into the docling document and re-compress.
+
+    Updates only docling_document — set_docling would also overwrite
+    docling_pages by routing through compress_docling_split, which
+    extracts pages from the in-memory JSON and finds none (the pages
+    blob is stored separately and is not loaded by get_docling_document).
+    That would silently destroy page rasters for every doc with at
+    least one undescribed picture.
+    """
+    from docling_core.types.doc.document import DescriptionMetaField, PictureMeta
+    from haiku.rag.store.compression import compress_docling_split
+
+    for pic in docling_doc.pictures:
+        text = descriptions.get(pic.self_ref)
+        if not text:
+            continue
+        if pic.meta is None:
+            pic.meta = PictureMeta()
+        pic.meta.description = DescriptionMetaField(text=text)
+
+    structure_bytes, _ = compress_docling_split(docling_doc.model_dump_json())
+    doc.docling_document = structure_bytes
+    doc.docling_version = docling_doc.version
+    return len(descriptions)
+
+
 async def _patch_picture_descriptions(client: "HaikuRAG", doc: Document) -> int:
     """Run the VLM against pictures lacking a description, patch the docling
     blob in-place. Returns the number of newly described pictures.
@@ -660,34 +688,10 @@ async def _patch_picture_descriptions(client: "HaikuRAG", doc: Document) -> int:
     if not descriptions:
         return 0
 
-    # Patch the docling document in-place. PictureMeta + DescriptionMetaField
-    # are pydantic models; build them and assign.
-    from docling_core.types.doc.document import (
-        DescriptionMetaField,
-        PictureMeta,
+    return await asyncio.to_thread(
+        _apply_descriptions_sync, docling_doc, doc, descriptions
     )
 
-    for pic in docling_doc.pictures:
-        text = descriptions.get(pic.self_ref)
-        if not text:
-            continue
-        if pic.meta is None:
-            pic.meta = PictureMeta()
-        pic.meta.description = DescriptionMetaField(text=text)
-
-    # Update only docling_document — set_docling would also overwrite
-    # docling_pages by routing through compress_docling_split, which
-    # extracts pages from the in-memory JSON and finds none (the pages
-    # blob is stored separately and is not loaded by get_docling_document).
-    # That would silently destroy page rasters for every doc with at
-    # least one undescribed picture.
-    from haiku.rag.store.compression import compress_docling_split
-
-    structure_bytes, _ = compress_docling_split(docling_doc.model_dump_json())
-    doc.docling_document = structure_bytes
-    doc.docling_version = docling_doc.version
-    return len(descriptions)
-
 
 async def _rebuild_descriptions(
     client: "HaikuRAG", documents: list[Document]