diff --git a/engineering/understand/snapshot.py b/engineering/understand/snapshot.py
index 2cf93ab..76cab08 100644
--- a/engineering/understand/snapshot.py
+++ b/engineering/understand/snapshot.py
@@ -297,11 +297,16 @@ def records_from_doc(doc, *, source: str) -> Snapshot:
 
 
 def _read(path: str, *, source: str) -> Snapshot:
+    # Every parse or walk failure is one refusal. A truncated file makes ezdxf's
+    # tag reader raise StopIteration, which asyncio.to_thread cannot hand back to
+    # an awaiting tool - uncaught, the tool would never answer.
     try:
-        doc = ezdxf.readfile(path)
-    except (OSError, ezdxf.DXFError) as exc:
-        raise ValueError(f"{path} is not a readable DXF file: {exc}") from exc
-    return records_from_doc(doc, source=source)
+        return records_from_doc(ezdxf.readfile(path), source=source)
+    except MemoryError:
+        raise
+    except Exception as exc:  # noqa: BLE001 - see above
+        reason = str(exc) or type(exc).__name__
+        raise ValueError(f"{path} is not a readable DXF file: {reason}") from exc
 
 
 def read_snapshot(path: str) -> Snapshot:
diff --git a/tests/test_understand_snapshot.py b/tests/test_understand_snapshot.py
index a136b07..2150d62 100644
--- a/tests/test_understand_snapshot.py
+++ b/tests/test_understand_snapshot.py
@@ -8,6 +8,7 @@ behaves the way AutoCAD 2026 was measured to behave (the extension must be
 
 from __future__ import annotations
 
+import asyncio
 import math
 from pathlib import Path
 
@@ -173,6 +174,23 @@ def test_read_snapshot_refuses_missing_and_unreadable_files(tmp_path):
     assert "not a readable DXF" in str(excinfo.value)
 
 
+def test_a_truncated_dxf_is_refused_not_left_hanging(tmp_path):
+    # An interrupted save: the header opens and the file ends. ezdxf's tag reader
+    # raises StopIteration here, which asyncio.to_thread cannot hand back to an
+    # awaiting tool - the call would never answer. It must be the same refusal
+    # as any other unreadable file, also through a worker thread.
+    half = tmp_path / "half.dxf"
+    half.write_text("  0\nSECTION\n  2\nHEADER\n  9\n$ACADVER\n  1\nAC1015\n", encoding="utf-8")
+    with pytest.raises(ValueError, match="not a readable DXF"):
+        read_snapshot(str(half))
+
+    async def through_a_thread():
+        return await asyncio.wait_for(asyncio.to_thread(read_snapshot, str(half)), timeout=10)
+
+    with pytest.raises(ValueError, match="not a readable DXF"):
+        asyncio.run(through_a_thread())
+
+
 # -- take_snapshot on the headless engine ---------------------------------------