diff --git a/docs/engineering/login_organisation_preferences.md b/docs/engineering/login_organisation_preferences.md
index 7bd4e2b90..04047ea08 100644
--- a/docs/engineering/login_organisation_preferences.md
+++ b/docs/engineering/login_organisation_preferences.md
@@ -35,3 +35,8 @@ DGX-local Mongo and Mac/Atlas are separate authority stores. Configure and
 verify each independently; federation does not replicate these authority or
 preference records. A stored default is not proof of a successful Google login:
 validate normal issuance, resulting tab context and actual effect namespace.
+
+Template-generated static URLs carry a release prefix, inherited by relative
+ES-module imports. This prevents a new backend footer/build from accompanying
+old cached login-bootstrap JavaScript. A different release gets different asset
+URLs; an old release prefix is never served with the new release's bytes.
diff --git a/src/backend/server/utils_flask.py b/src/backend/server/utils_flask.py
index 30e16bc69..7e96ef29b 100644
--- a/src/backend/server/utils_flask.py
+++ b/src/backend/server/utils_flask.py
@@ -1544,6 +1544,9 @@ def create_flask_app(
 
     _install_request_timing_middleware(app)
     _configure_flask_app_core(app, list_models_func, generate_func)
+    from .versioned_static import install_versioned_static
+
+    install_versioned_static(app, get_runtime_code_version())
     from ..security.cloudflare_access import install_cloudflare_access
 
     install_cloudflare_access(app)
diff --git a/src/backend/server/versioned_static.py b/src/backend/server/versioned_static.py
new file mode 100644
index 000000000..b8de35f53
--- /dev/null
+++ b/src/backend/server/versioned_static.py
@@ -0,0 +1,22 @@
+"""Release-scoped static paths keep relative ES-module graphs coherent."""
+from hashlib import sha256
+
+from flask import abort
+
+
+def install_versioned_static(app, version: str) -> None:
+    build = sha256(version.encode()).hexdigest()[:20]
+
+    @app.url_defaults
+    def version_static_urls(endpoint, values):
+        if endpoint == "static" and "filename" in values:
+            filename = values["filename"]
+            if not filename.startswith("build/"):
+                values["filename"] = f"build/{build}/{filename}"
+
+    @app.get(f"{app.static_url_path}/build/<build_id>/<path:filename>")
+    def versioned_static(build_id, filename):
+        if build_id != build:
+            # Never return new bytes under an old release's URL.
+            abort(404)
+        return app.send_static_file(filename)
diff --git a/tests/backend/test_versioned_static.py b/tests/backend/test_versioned_static.py
new file mode 100644
index 000000000..cd467eea0
--- /dev/null
+++ b/tests/backend/test_versioned_static.py
@@ -0,0 +1,34 @@
+from pathlib import Path
+from urllib.parse import urljoin
+
+from flask import Flask, url_for
+from src.backend.server.versioned_static import install_versioned_static
+
+
+def test_release_paths_include_relative_modules_and_styles(tmp_path):
+    (tmp_path / 'js').mkdir()
+    (tmp_path / 'js' / 'main.js').write_text("import './apiService.js';")
+    (tmp_path / 'js' / 'apiService.js').write_text('export const release = 2;')
+    (tmp_path / 'styles.css').write_text('body {}')
+    app = Flask(__name__, static_folder=str(tmp_path), static_url_path='/static')
+    install_versioned_static(app, 'release-2')
+    with app.test_request_context():
+        entry = url_for('static', filename='js/main.js')
+        css = url_for('static', filename='styles.css')
+    assert entry.startswith('/static/build/')
+    client = app.test_client()
+    assert client.get(entry).data == b"import './apiService.js';"
+    assert client.get(urljoin(entry, './apiService.js')).data == b'export const release = 2;'
+    assert client.get(css).status_code == 200
+    assert client.get('/static/build/old-release/js/main.js').status_code == 404
+    assert client.get(entry.replace('/js/main.js', '/../../private.env')).status_code == 404
+
+
+def test_build_change_changes_entry_url_without_rewriting_templates(tmp_path):
+    urls=[]
+    for version in ('release-1','release-2'):
+        app=Flask(__name__,static_folder=str(tmp_path),static_url_path='/static')
+        install_versioned_static(app,version)
+        with app.test_request_context():
+            urls.append(url_for('static',filename='js/main.js'))
+    assert urls[0] != urls[1]