diff --git a/app/model_downloader/verify/structural.py b/app/model_downloader/verify/structural.py
index 7b403a31a..2d8fca924 100644
--- a/app/model_downloader/verify/structural.py
+++ b/app/model_downloader/verify/structural.py
@@ -1,4 +1,4 @@
-"""Cheap structural validation, no full read (PRD section 8.2).
+"""Cheap structural validation, no full read.
 
 For ``.safetensors``/``.sft`` we parse the header (first few KB): it carries
 the tensor table and the byte length of the data region. We assert
@@ -53,6 +53,9 @@ def _validate_safetensors(path: str) -> None:
         except (UnicodeDecodeError, json.JSONDecodeError) as e:
             raise StructuralError(f"safetensors header is not valid JSON: {e}") from e
 
+    if not isinstance(header, dict):
+        raise StructuralError("safetensors header is not a JSON object")
+
     data_len = 0
     for name, entry in header.items():
         if name == "__metadata__":
@@ -62,7 +65,18 @@ def _validate_safetensors(path: str) -> None:
         offsets = entry["data_offsets"]
         if not (isinstance(offsets, list) and len(offsets) == 2):
             raise StructuralError(f"tensor {name!r} has malformed data_offsets")
-        data_len = max(data_len, int(offsets[1]))
+        begin, end = offsets
+        # bool is an int subclass; reject it explicitly to avoid True/False offsets.
+        if (
+            not isinstance(begin, int)
+            or not isinstance(end, int)
+            or isinstance(begin, bool)
+            or isinstance(end, bool)
+            or begin < 0
+            or end < begin
+        ):
+            raise StructuralError(f"tensor {name!r} has malformed data_offsets")
+        data_len = max(data_len, end)
 
     expected = 8 + header_len + data_len
     if file_size != expected: