diff --git a/app/public/app.js b/app/public/app.js
index 41995bc..6343224 100644
--- a/app/public/app.js
+++ b/app/public/app.js
@@ -7,6 +7,7 @@ const TIERS = [[6, 0], [8, 5], [10, 10], [Infinity, 15]];
 const TIER_LABELS = { 0: "0% share", 5: "5% share", 10: "10% share", 15: "15% share" };
 const TIER_VARS = { 0: "--tier-0", 5: "--tier-5", 10: "--tier-10", 15: "--tier-15" };
 const DRAWS = 4000;
+const ASSET_V = "20260809a"; // bump with index.html's app.js?v= on every deploy that changes any asset
 const SCEN_SCHEMA = "snap_qc_sim.model_scenarios.v1";
 
 const fmtM = (v) => {
@@ -329,8 +330,8 @@ async function loadModelArtifacts() {
   if (!ARTIFACTS) {
     ARTIFACTS = (async () => {
       const [mdRes, scRes] = await Promise.all([
-        fetch("model_data.json"),
-        fetch("model_scenarios.json"),
+        fetch("model_data.json?v=" + ASSET_V),
+        fetch("model_scenarios.json?v=" + ASSET_V),
       ]);
       if (!mdRes.ok || !scRes.ok) throw new Error("model artifact fetch failed");
       const mdBytes = await mdRes.arrayBuffer();
@@ -620,7 +621,7 @@ function render() {
 }
 
 async function main() {
-  DATA = await (await fetch("data.json")).json();
+  DATA = await (await fetch("data.json?v=" + ASSET_V)).json();
   const sel = $("state");
   for (const [code, st] of Object.entries(DATA.states)) {
     const o = document.createElement("option");
diff --git a/app/public/index.html b/app/public/index.html
index 0eca0ee..b060747 100644
--- a/app/public/index.html
+++ b/app/public/index.html
@@ -132,6 +132,6 @@ <h2>All states at a glance</h2>
   Built by <a href="https://policyengine.org">PolicyEngine</a> · <a href="/paper/">working paper</a> · estimates are illustrative, from public data · <a href="mailto:hello@policyengine.org">hello@policyengine.org</a>
 </div></footer>
 
-<script type="module" src="app.js"></script>
+<script type="module" src="app.js?v=20260809a"></script>
 </body>
 </html>
diff --git a/tests/test_asset_versioning.py b/tests/test_asset_versioning.py
new file mode 100644
index 0000000..37a0263
--- /dev/null
+++ b/tests/test_asset_versioning.py
@@ -0,0 +1,23 @@
+"""The deploy cache-buster: index.html's script version must match app.js's."""
+
+import re
+from pathlib import Path
+
+PUBLIC = Path(__file__).resolve().parent.parent / "app" / "public"
+
+
+def test_index_script_version_matches_app_constant():
+    html = (PUBLIC / "index.html").read_text()
+    js = (PUBLIC / "app.js").read_text()
+    html_v = re.search(r'src="app\.js\?v=([A-Za-z0-9._-]+)"', html)
+    js_v = re.search(r'const ASSET_V = "([A-Za-z0-9._-]+)"', js)
+    assert html_v, "index.html must load app.js with a ?v= cache-buster"
+    assert js_v, "app.js must define ASSET_V"
+    assert html_v.group(1) == js_v.group(1)
+
+
+def test_all_data_fetches_are_versioned():
+    js = (PUBLIC / "app.js").read_text()
+    for name in ("data.json", "model_data.json", "model_scenarios.json"):
+        assert f'fetch("{name}?v=" + ASSET_V)' in js, f"{name} fetch must carry ASSET_V"
+        assert f'fetch("{name}")' not in js, f"unversioned {name} fetch remains"