Run image decoding and resizing off the event loop

process_image takes ~560ms on a 12MP photo and image_content_block
decodes HEIC/JPEG with PIL; both ran synchronously inside async
handlers, freezing every concurrent chat stream. Run them via
asyncio.to_thread in the upload handlers, the chat stream, and iMessage.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

blueprints/conversation/__init__.py

@@ -257,7 +257,9 @@ async def upload_image():
     content_type = file.content_type or "image/jpeg"
 
     try:
-        processed_bytes, output_content_type = process_image(file_bytes, content_type)
+        processed_bytes, output_content_type = await asyncio.to_thread(
+            process_image, file_bytes, content_type
+        )
     except ImageValidationError as e:
         return jsonify({"error": str(e)}), 400
 
@@ -311,8 +313,8 @@ async def upload_attachment():
 
     if content_type in IMAGE_TYPES:
         try:
-            processed_bytes, output_content_type = process_image(
-                file_bytes, content_type
+            processed_bytes, output_content_type = await asyncio.to_thread(
+                process_image, file_bytes, content_type
             )
         except Exception as e:
             return jsonify({"error": f"Could not process image: {e}"}), 400
@@ -402,7 +404,7 @@ async def stream_query():
     if image_key:
         try:
             image_bytes, _ = await s3_get_image(image_key)
-            image_block = image_content_block(image_bytes)
+            image_block = await asyncio.to_thread(image_content_block, image_bytes)
         except Exception as e:
             logging.error(f"Failed to load image {image_key}: {e}")
             query_text = f"{query_text or ''}\n\n[An image was sent but could not be loaded.]".strip()

blueprints/imessage/__init__.py

@@ -327,7 +327,7 @@ async def _process_and_reply(
             async with httpx.AsyncClient() as client:
                 resp = await client.get(media_url, timeout=30)
                 resp.raise_for_status()
-            image_block = image_content_block(resp.content)
+            image_block = await asyncio.to_thread(image_content_block, resp.content)
         except Exception as e:
             logger.error(f"Failed to load iMessage image: {e}")
             content = f"{content or ''}\n\n[An image was sent but could not be loaded.]".strip()

tests/unit/test_file_upload.py

@@ -1,6 +1,7 @@
 """Tests for conversation file attachments: extraction, /upload and stream-query file_key."""
 
 import io
+import threading
 from datetime import timedelta
 from types import SimpleNamespace
 from unittest.mock import AsyncMock, patch
@@ -190,6 +191,21 @@ class TestUploadRoute:
         s3.upload_image.assert_awaited_once()
         s3.upload_file.assert_not_awaited()
 
+    async def test_image_processing_runs_off_the_event_loop(
+        self, app, headers, s3, monkeypatch
+    ):
+        threads = []
+        real = conv.process_image
+
+        def recording(*args):
+            threads.append(threading.current_thread())
+            return real(*args)
+
+        monkeypatch.setattr(conv, "process_image", recording)
+        resp = await _upload(app, headers, _png_bytes(), "cat.png", "image/png")
+        assert resp.status_code == 200
+        assert threads and threads[0] is not threading.main_thread()
+
     async def test_unsupported_type_is_415(self, app, headers, s3):
         resp = await _upload(app, headers, b"PK\x03\x04", "a.zip", "application/zip")
         assert resp.status_code == 415