diff --git a/scripts/generate_pack.py b/scripts/generate_pack.py
index e1589c91..dee880c3 100644
--- a/scripts/generate_pack.py
+++ b/scripts/generate_pack.py
@@ -94,8 +94,15 @@ def _add_pack_member(zf: zipfile.ZipFile, source_path: str, arcname: str) -> Non
     """
     info = zipfile.ZipInfo(filename=arcname, date_time=_FIXED_DATE_TIME)
     info.compress_type = zipfile.ZIP_DEFLATED
-    info.external_attr = 0o100644 << 16
-    size = os.path.getsize(source_path)
+    # The executable bit is the one permission worth carrying: the RetroDECK
+    # pack ships the two Voxatron engine binaries, and a user who extracts
+    # them at 644 cannot run them. Git records the bit, so reading it from
+    # the source keeps the pack the same from any clone. Nothing else about
+    # the source's mode reaches the archive.
+    stat_result = os.stat(source_path)
+    mode = 0o755 if stat_result.st_mode & 0o111 else 0o644
+    info.external_attr = (0o100000 | mode) << 16
+    size = stat_result.st_size
     info.file_size = size
     with open(source_path, "rb") as source, zf.open(
         info, "w", force_zip64=size >= 1 << 31
diff --git a/tests/test_deterministic_zip.py b/tests/test_deterministic_zip.py
index 829a89d6..a2eb91a6 100644
--- a/tests/test_deterministic_zip.py
+++ b/tests/test_deterministic_zip.py
@@ -321,6 +321,49 @@ class TestPackDeterminism(unittest.TestCase):
         with zipfile.ZipFile(pack) as zf:
             self.assertEqual({i.date_time for i in zf.infolist()}, {_FIXED_DATE_TIME})
 
+    def test_an_executable_source_stays_executable(self):
+        """Pinning the mode stripped the bit the RetroDECK pack needs.
+
+        It ships the two Voxatron engine binaries; extracted at 644 they
+        cannot be run. Git records the bit, so reading it from the source
+        leaves the pack the same from any clone.
+        """
+        import os
+        import stat as stat_module
+
+        runnable = self.bios / "engine.bin"
+        runnable.write_bytes(b"NOT REALLY AN ELF")
+        os.chmod(runnable, 0o755)
+        digest = hashlib.sha1(runnable.read_bytes()).hexdigest()
+        self.db["files"][digest] = {
+            "path": str(runnable), "name": "engine.bin",
+            "size": runnable.stat().st_size, "sha1": digest,
+            "md5": hashlib.md5(runnable.read_bytes()).hexdigest(),
+        }
+        self.db["indexes"]["by_name"]["engine.bin"] = [digest]
+        config = (self.platforms / "detplat.yml")
+        config.write_text(
+            config.read_text()
+            + "      - name: engine.bin\n"
+            + "        destination: engine.bin\n"
+            + "        required: true\n"
+        )
+        pack = self._build("exec1")
+        with zipfile.ZipFile(pack) as zf:
+            modes = {
+                i.filename.rsplit("/", 1)[-1]: (i.external_attr >> 16) & 0o777
+                for i in zf.infolist()
+            }
+        self.assertIn("engine.bin", modes, sorted(modes))
+        self.assertEqual(
+            modes["engine.bin"] & stat_module.S_IXUSR, stat_module.S_IXUSR,
+            "a runnable payload must stay runnable after extraction",
+        )
+        self.assertEqual(
+            modes["boot.rom"] & 0o777, 0o644,
+            "a plain ROM keeps the fixed mode, whatever the local filesystem says",
+        )
+
     def test_injected_manifest_is_pinned_to_the_database(self):
         from scripts.generate_pack import inject_manifest, verify_pack
 
diff --git a/wiki/architecture.md b/wiki/architecture.md
index 0f4b5af2..0d78dc3c 100644
--- a/wiki/architecture.md
+++ b/wiki/architecture.md
@@ -155,6 +155,26 @@ with different hardware filters get separate packs. Emulator and system packs
 also retain system identity, `variant_group` and requested region; same-named
 regional files are never collapsed merely because their display label matches.
 
+## Pack reproducibility
+
+A pack is a function of its inputs. Two builds from the same collection and
+the same `database.json` produce the same bytes, so a third party can rebuild
+a published pack and compare it against the checksum in `SHA256SUMS.txt`.
+
+Three things make that hold. Generated members (`README.txt`, `manifest.json`)
+carry a fixed date rather than the wall clock, and the manifest's `generated`
+field is read from `database.json` instead of the build time. MAME and FBNeo
+romsets are rebuilt deterministically from their ROMs, so a pack does not
+inherit whatever metadata the source archive happened to carry. And every
+member is written with the same fixed date: `ZipFile.write` copies the source
+file's mtime, which is the checkout time for a file from the collection and
+the wall clock for one the build just staged in `tmp/`.
+
+`tests/test_deterministic_zip.py` builds the same fixture twice and compares
+the bytes, for both the platform and the emulator pack paths. Its fixture
+holds a romset, because without one the comparison never reaches the rebuild
+path and passes while the real packs still move.
+
 ## Storage tiers
 
 | Tier | Meaning |