diff --git a/server/providers/overpass/osmFeaturesRoute.js b/server/providers/overpass/osmFeaturesRoute.js
index 1c32fc675..e28ce80a5 100644
--- a/server/providers/overpass/osmFeaturesRoute.js
+++ b/server/providers/overpass/osmFeaturesRoute.js
@@ -6,7 +6,10 @@ import {
   OSM_FEATURE_LIMITS,
   summarizeOsmFeatures,
 } from './osmFeatures.js';
-import { resolveOverpassUpstreams } from './constants.js';
+import {
+  resolveOverpassUpstreams,
+  resolveOverpassUpstreamTag,
+} from './constants.js';
 import { fetchOverpassPayload, overpassPayloadIsData } from './transport.js';
 
 /** Places of interest change slowly: answers are kept a day. */
@@ -148,13 +151,16 @@ function sender(res) {
  * substitute for bulk place search.
  *
  * @param {{use: Function}} middlewares
- * @param {{fetchPayload?: Function, featuresCache?: object, cacheRoot?: string|null, configured?: () => boolean}} [deps]
+ * @param {{fetchPayload?: Function, featuresCache?: object, cacheRoot?: string|null, configured?: () => boolean, upstreamTag?: () => string}} [deps]
  */
 export function installOsmFeaturesRoute(
   middlewares,
   {
     fetchPayload = fetchOverpassPayload,
     configured = () => resolveOverpassUpstreams().length > 0,
+    // Keys each answer to the instance that gave it, as the other Overpass
+    // caches do, so repointing OVERPASS_UPSTREAMS never serves the old one's.
+    upstreamTag = resolveOverpassUpstreamTag,
     cacheRoot = path.join(process.cwd(), '.gev-cache'),
     featuresCache = createBoundedCache({
       dir: cacheRoot ? path.join(cacheRoot, 'osm-features') : null,
@@ -203,7 +209,7 @@ export function installOsmFeaturesRoute(
     res.once?.('close', abort);
     try {
       const answer = await features(
-        `${CACHE_VERSION}|features|${compiled.ql}`,
+        `${CACHE_VERSION}|features|${upstreamTag()}|${compiled.ql}`,
         compiled.ql,
         (parsed) => summarizeOsmFeatures(parsed, compiled),
         controller.signal,
diff --git a/src/tooling/osmFeaturesRoute.test.mjs b/src/tooling/osmFeaturesRoute.test.mjs
index 4e95ba54a..040e9903f 100644
--- a/src/tooling/osmFeaturesRoute.test.mjs
+++ b/src/tooling/osmFeaturesRoute.test.mjs
@@ -158,7 +158,10 @@ test('answers are compacted: centres, kept tags, truncation and counts', () => {
   assert.equal(count.count, 5);
 });
 
-function mount(fetchPayload, { featuresCache, configured = () => true } = {}) {
+function mount(
+  fetchPayload,
+  { featuresCache, configured = () => true, upstreamTag } = {},
+) {
   const routes = new Map();
   const writes = [];
   const memoryCache = () => {
@@ -177,6 +180,7 @@ function mount(fetchPayload, { featuresCache, configured = () => true } = {}) {
       fetchPayload,
       cacheRoot: null,
       configured,
+      ...(upstreamTag ? { upstreamTag } : {}),
       featuresCache: featuresCache || memoryCache(),
     },
   );
@@ -254,6 +258,35 @@ test('the route answers, caches, and runs one upstream query at a time', async (
   assert.equal(refused.body.code, 'AREA_TOO_LARGE');
 });
 
+test('a cached answer is served only by the Overpass instance that gave it', async () => {
+  let tag = 'instance-a';
+  let upstream = 0;
+  const { request } = mount(
+    async () => {
+      upstream += 1;
+      return {
+        status: 200,
+        body: JSON.stringify({
+          elements: [{ type: 'node', id: 7, lat: 1, lon: 1, tags: {} }],
+        }),
+      };
+    },
+    { upstreamTag: () => tag },
+  );
+  assert.equal((await request('preset=hospital&bbox=0,0,2,2')).status, 200);
+  const hit = await request('preset=hospital&bbox=0,0,2,2');
+  assert.equal(hit.headers['X-OSM-Cache'], 'HIT');
+  assert.equal(upstream, 1);
+  tag = 'instance-b';
+  const switched = await request('preset=hospital&bbox=0,0,2,2');
+  assert.equal(switched.headers['X-OSM-Cache'], 'MISS');
+  assert.equal(
+    upstream,
+    2,
+    'the new instance is asked, not the old answer served',
+  );
+});
+
 test('an upstream refusal is not an empty answer', async () => {
   const { request } = mount(async () => ({
     status: 429,