diff --git a/.jules/bolt.md b/.jules/bolt.md
index 0fb92dce..f996f711 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -2,3 +2,7 @@
 ## 2026-02-19 - [Frontend Download Optimization]
 **Learning:** Using `fetch` + `blob` to download large files (500MB+) in browser JavaScript causes significant memory spikes and potential crashes.
 **Action:** Always prefer direct link downloads (creating a hidden `<a>` tag) for large file downloads. Implemented `triggerDownload` utility in `api.ts` to handle this centrally with backend compatibility checks.
+
+## 2025-05-23 - Avoid Reading Files for Size Validation in Async Python
+**Learning:** Iterating over `UploadFile.file` (a `SpooledTemporaryFile`) to check file size is a synchronous operation that blocks the event loop, causing severe performance degradation (e.g., 1s block for 50MB file).
+**Action:** Always use `file.file.seek(0, 2)` and `file.file.tell()` to determine size in O(1) time without reading content. Ensure to reset the file pointer with `await file.seek(0)` afterwards.
diff --git a/backend/src/api/conversions.py b/backend/src/api/conversions.py
index 54b82298..3651ab70 100644
--- a/backend/src/api/conversions.py
+++ b/backend/src/api/conversions.py
@@ -201,16 +201,17 @@ async def validate_file_size(file: UploadFile) -> tuple[bool, str]:
     Returns:
         Tuple of (is_valid, error_message)
     """
-    # Read file to check size
-    total_size = 0
-    for chunk in file.file:
-        total_size += len(chunk)
-        if total_size > MAX_UPLOAD_SIZE:
-            return False, f"File size exceeds {MAX_UPLOAD_SIZE // (1024 * 1024)}MB limit"
+    # Check size using seek/tell (O(1) instead of O(N) read)
+    # file.file is a SpooledTemporaryFile which supports seek/tell synchronously
+    file.file.seek(0, 2)
+    file_size = file.file.tell()
 
     # Reset file pointer
     await file.seek(0)
 
+    if file_size > MAX_UPLOAD_SIZE:
+        return False, f"File size exceeds {MAX_UPLOAD_SIZE // (1024 * 1024)}MB limit"
+
     return True, ""