diff --git a/scripts/generate_pack.py b/scripts/generate_pack.py
index 733d4bd8..a88958c6 100644
--- a/scripts/generate_pack.py
+++ b/scripts/generate_pack.py
@@ -4207,41 +4207,45 @@ def _manifest_core_index(
 
 
 def inject_manifest(zip_path: str, manifest: dict) -> None:
-    """Inject manifest.json into an existing ZIP pack."""
+    """Inject manifest.json into an existing ZIP pack.
+
+    The pack is rewritten beside its final name and swapped in, as
+    _writing_pack builds it: appending in place rewrites the central
+    directory last, so a process killed in between left an unreadable ZIP
+    where a valid pack had been.
+    """
     manifest_json = json.dumps(manifest, indent=2, ensure_ascii=False)
 
-    # Check if manifest already exists
     with zipfile.ZipFile(zip_path, "r") as zf:
         has_manifest = "manifest.json" in zf.namelist()
 
-    if not has_manifest:
-        # Fast path: append directly. Append mode defaults to ZIP_STORED,
-        # which would leave the manifest uncompressed and give the same pack
-        # two different byte layouts depending on the path taken here.
-        with zipfile.ZipFile(zip_path, "a", zipfile.ZIP_DEFLATED) as zf:
-            _write_generated_member(zf, "manifest.json", manifest_json)
-    else:
-        # Rebuild to replace existing manifest
-        import tempfile as _tempfile
-
-        tmp_fd, tmp_path = _tempfile.mkstemp(
-            suffix=".zip", dir=os.path.dirname(zip_path)
-        )
-        os.close(tmp_fd)
-        try:
+    directory = os.path.dirname(zip_path) or "."
+    fd, scratch = tempfile.mkstemp(
+        prefix=f".{os.path.basename(zip_path)}.", suffix=".partial", dir=directory
+    )
+    os.close(fd)
+    try:
+        if not has_manifest:
+            # Append to a copy. Append mode defaults to ZIP_STORED, which
+            # would leave the manifest uncompressed and give the same pack
+            # two different byte layouts depending on the path taken here.
+            shutil.copyfile(zip_path, scratch)
+            with zipfile.ZipFile(scratch, "a", zipfile.ZIP_DEFLATED) as zf:
+                _write_generated_member(zf, "manifest.json", manifest_json)
+        else:
             with (
                 zipfile.ZipFile(zip_path, "r") as src,
-                zipfile.ZipFile(tmp_path, "w", zipfile.ZIP_DEFLATED) as dst,
+                zipfile.ZipFile(scratch, "w", zipfile.ZIP_DEFLATED) as dst,
             ):
                 for item in src.infolist():
                     if item.filename == "manifest.json":
                         continue
                     dst.writestr(item, src.read(item.filename))
                 _write_generated_member(dst, "manifest.json", manifest_json)
-            os.replace(tmp_path, zip_path)
-        except (OSError, zipfile.BadZipFile):
-            os.unlink(tmp_path)
-            raise
+        os.replace(scratch, zip_path)
+    except BaseException:
+        Path(scratch).unlink(missing_ok=True)
+        raise
 
 
 
diff --git a/tests/test_deterministic_zip.py b/tests/test_deterministic_zip.py
index a2eb91a6..78653f49 100644
--- a/tests/test_deterministic_zip.py
+++ b/tests/test_deterministic_zip.py
@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import hashlib
+import os
 import tempfile
 import time
 import unittest
@@ -377,5 +378,40 @@ class TestPackDeterminism(unittest.TestCase):
         self.assertEqual(info.date_time, _FIXED_DATE_TIME)
 
 
+
+class ManifestInjectionIsAtomic(unittest.TestCase):
+    """inject_manifest appended to the final pack in place: zipfile writes
+    the member over the old central directory and the new one only on
+    close, so a process killed in between left a BadZipFile where a valid
+    pack had been."""
+
+    def test_a_kill_mid_write_leaves_the_pack_readable(self):
+        import tempfile
+        from unittest import mock
+
+        from scripts import generate_pack as gp
+
+        with tempfile.TemporaryDirectory(dir=Path(__file__).resolve().parents[1] / "tmp") as d:
+            pack = Path(d) / "Pack.zip"
+            with zipfile.ZipFile(pack, "w", zipfile.ZIP_DEFLATED) as zf:
+                for i in range(20):
+                    zf.writestr(f"bios/{i}.bin", os.urandom(4096))
+            before = pack.read_bytes()
+
+            def die(zf, name, text):
+                zf.writestr(name, text)
+                zf.fp.flush()
+                raise KeyboardInterrupt
+
+            with mock.patch.object(gp, "_write_generated_member", die), \
+                    self.assertRaises(KeyboardInterrupt):
+                gp.inject_manifest(str(pack), {"files": ["x"] * 2000})
+            self.assertEqual(pack.read_bytes(), before)
+            self.assertEqual(
+                [p.name for p in Path(d).iterdir()], ["Pack.zip"],
+                "the scratch copy is removed",
+            )
+
+
 if __name__ == "__main__":
     unittest.main()