diff --git a/scripts/build_release.py b/scripts/build_release.py
index 9821bd8..b07006f 100755
--- a/scripts/build_release.py
+++ b/scripts/build_release.py
@@ -9,6 +9,7 @@ import os
 from pathlib import Path
 import re
 import shutil
+import socket
 import subprocess
 import sys
 import tarfile
@@ -76,6 +77,29 @@ def sha256(path: Path) -> str:
     return digest.hexdigest()
 
 
+def assert_port_free(host: str, port: int) -> None:
+    """Fail closed when something is already listening where the acceptance server must bind."""
+    with socket.socket() as sock:
+        sock.settimeout(1.0)
+        if sock.connect_ex((host, port)) == 0:
+            raise SystemExit(
+                f"Port {port} is already served by another process; refusing to gate or record this "
+                "release against a tree that is not the one under test."
+            )
+
+
+def assert_served_tree(tree: Path, host: str, port: int) -> None:
+    """Prove the responding server is the clone under test, not a stale checkout on the same port."""
+    for relative in ("src/domain.js", "app.js"):
+        served = urllib.request.urlopen(f"http://{host}:{port}/{relative}", timeout=5).read()
+        expected = (tree / relative).read_bytes()
+        if hashlib.sha256(served).hexdigest() != hashlib.sha256(expected).hexdigest():
+            raise SystemExit(
+                f"The acceptance server is serving {relative} from a different tree; "
+                "every gate and the demo would describe a release that is not this commit."
+            )
+
+
 def tracked_files(tree: Path) -> list[str]:
     names = run(["git", "ls-files"], tree, capture=True).splitlines()
     return sorted(name for name in names if name and not name.startswith(EXCLUDED_PREFIXES)
@@ -133,6 +157,10 @@ def main() -> int:
         run(["python3", "scripts/build_vision_corpus.py", "--verify"], tree)
         server_env = dict(os.environ)
         server_env.update({"HOST": "127.0.0.1", "PORT": "4173", "TIMMY_VISION_ENABLED": "false"})
+        # A leftover server from another checkout answers on this port and the readiness probe cannot
+        # tell it apart from ours, so every gate and the recorded demo would silently describe a
+        # different tree. Refuse to build over a live listener, and verify the served bytes afterwards.
+        assert_port_free("127.0.0.1", 4173)
         server_log = (release_dir / "acceptance-server.log").open("w", encoding="utf-8")
         server = subprocess.Popen(["node", "server.mjs"], cwd=tree, env=server_env,
                                   stdout=server_log, stderr=subprocess.STDOUT, text=True)
@@ -147,6 +175,9 @@ def main() -> int:
                     time.sleep(0.1)
             else:
                 raise SystemExit("Acceptance server did not become ready")
+            if server.poll() is not None:
+                raise SystemExit("Acceptance server exited after reporting ready")
+            assert_served_tree(tree, "127.0.0.1", 4173)
             run(["npm", "run", "test:ui"], tree)
             run(["npm", "run", "test:photo"], tree)
             run(["npm", "run", "test:mobile-capture"], tree)
diff --git a/tests/release-demo.test.js b/tests/release-demo.test.js
index 5dad54f..e584e92 100644
--- a/tests/release-demo.test.js
+++ b/tests/release-demo.test.js
@@ -373,3 +373,19 @@ test('release demo shows a versioned backup, an upgraded older file, and a refus
   assert.match(builder, /"portable_backup_restore": "passed"/);
   assert.match(builder, /"test:portability"/);
 });
+
+test('the builder refuses to gate or record a release against a foreign server', async () => {
+  const builder = await readFile(builderPath, 'utf8');
+
+  // A leftover checkout answering on the acceptance port once satisfied every flow and the demo,
+  // so the release described a tree that was not the commit under test. The builder must refuse a
+  // live listener before it starts, then prove the served bytes are this tree's before any gate.
+  assert.match(builder, /def assert_port_free/);
+  assert.match(builder, /assert_port_free\("127\.0\.0\.1", 4173\)/);
+  assert.match(builder, /def assert_served_tree/);
+  assert.match(builder, /assert_served_tree\(tree, "127\.0\.0\.1", 4173\)/);
+  const identityCheck = builder.indexOf('assert_served_tree(tree, "127.0.0.1", 4173)');
+  const firstFlow = builder.indexOf('run(["npm", "run", "test:ui"]');
+  assert.ok(identityCheck > 0, 'the served-tree check must be present');
+  assert.ok(firstFlow > identityCheck, 'the served-tree check must run before the first acceptance flow');
+});