diff --git a/CHANGELOG.md b/CHANGELOG.md
index 76825c76d..7398d8418 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,19 @@ of current runtime behavior, see [`docs/CURRENT-STATE.md`](docs/CURRENT-STATE.md
 
 ## [Unreleased]
 
+### Fixed
+
+- The Overpass proxy now rotates to the next mirror on any non-2xx upstream
+  response, not only on 5xx. `overpass-api.de` and its `lz4` alias answer 406 to
+  the proxy's User-Agent while two of the configured mirrors answer 200 to the
+  identical request, so the fan-out stopped at the first refusal with healthy
+  mirrors untried. The refusal was also cached to memory and disk and served as
+  data — boundary-class queries hold a month-long TTL — which affected every
+  Overpass-backed feature: road geometry, annotation outlines and place lookup.
+- Existing cached refusals are now ignored immediately, including during
+  stale-data fallback. Concurrent identical requests share the same last-good
+  fallback when all mirrors refuse, without duplicating upstream requests.
+
 ## [0.1.1] — 2026-09-01 — Installation and live-data fixes
 
 ### Changed
diff --git a/docs/CURRENT-STATE.md b/docs/CURRENT-STATE.md
index f86f627aa..a2bb9dad1 100644
--- a/docs/CURRENT-STATE.md
+++ b/docs/CURRENT-STATE.md
@@ -2006,6 +2006,12 @@ silently demoting every later lookup for the session.
 - **Track trails**: server accumulates per-MMSI ring buffers (`/api/ais-live/track?mmsi=`, Float32+Uint32, 64 samples, 30s/25m thinning); aircraft backfill proxies `/api/opensky-track` (OAuth, own credit bucket) and `/api/adsblol/trace` (tar1090 readsb, ~24h history, ODbL — credit adsb.lol).
 - Shared `src/data/pickRegistry.js` stops the two flight layers' click handlers from fighting over the camera.
 
+### Overpass proxy mirror rotation (September 2026)
+
+- `/api/overpass` fans out across four public mirrors. `overpassPayloadIsData()` governs cache reads, writes, and stale fallback: only a 2xx that is neither rate-limited nor a body-level runtime error qualifies. Previously stored refusals are ignored on both fresh and stale reads, so upgrading does not require manually clearing the disk cache.
+- HTTP refusals such as 406 now rotate alongside the existing network, rate-limit, and runtime-error cases. A refusal from one mirror no longer prevents reaching healthy alternatives or persists under the seven-day road/month-long boundary cache TTLs. Concurrent identical queries share one mirror sequence; if it fails, both the initiating and joined callers can use the same last-good data.
+- A refusal every mirror agrees on is still reported with the first mirror's status and body, so a genuinely malformed query says what upstream said — but only after every mirror has had the chance to answer it. `fetchOverpassPayload` takes injectable endpoints and fetch so the rotation is tested without a live mirror (`src/overpassProxy.test.mjs`).
+
 ### Share-link v2 layer state (August 2026)
 
 - Generated share links use a deterministic v2 hash. Existing camera, visual,
diff --git a/src/data/overpassProxy.test.mjs b/src/data/overpassProxy.test.mjs
index af8b3a0af..f161eb72e 100644
--- a/src/data/overpassProxy.test.mjs
+++ b/src/data/overpassProxy.test.mjs
@@ -16,9 +16,9 @@ import {
 
 test('preflight checks memory, in-flight, then disk before consuming limiter quota', async () => {
   const key = 'normalized query';
-  const fresh = { id: 'memory', cachedAt: 900 };
-  const joined = { id: 'inflight', cachedAt: 950 };
-  const disk = { id: 'disk', cachedAt: 975 };
+  const fresh = { id: 'memory', status: 200, cachedAt: 900 };
+  const joined = { id: 'inflight', status: 200, cachedAt: 950 };
+  const disk = { id: 'disk', status: 200, cachedAt: 975 };
   let diskReads = 0;
   let limiterCalls = 0;
   const allowUpstream = () => { limiterCalls += 1; return true; };
@@ -39,7 +39,7 @@ test('preflight checks memory, in-flight, then disk before consuming limiter quo
 
   const inFlightHit = await resolveOverpassPreflight({
     cacheKey: key,
-    memoryCache: new Map([[key, { id: 'stale', cachedAt: 0 }]]),
+    memoryCache: new Map([[key, { id: 'stale', status: 200, cachedAt: 0 }]]),
     inFlight: new Map([[key, Promise.resolve(joined)]]),
     readDisk: async () => { diskReads += 1; return disk; },
     allowUpstream,
@@ -84,6 +84,21 @@ test('preflight checks memory, in-flight, then disk before consuming limiter quo
   assert.equal(denied.source, 'RATE_LIMITED');
 });
 
+test('preflight treats cached refusals as misses without spending extra quota', async () => {
+  for (const invalid of [{ status: 406 }, { status: 200, runtimeError: true }]) {
+    let admissions = 0;
+    const result = await resolveOverpassPreflight({
+      cacheKey: 'refused',
+      memoryCache: new Map([['refused', { ...invalid, cachedAt: Date.now() }]]),
+      inFlight: new Map(),
+      readDisk: async () => ({ ...invalid, cachedAt: Date.now() }),
+      allowUpstream: () => { admissions++; return true; },
+    });
+    assert.equal(result.source, 'UPSTREAM');
+    assert.equal(admissions, 1);
+  }
+});
+
 /** Synthetic dense ring: N points on a circle with sub-tolerance jitter. */
 function denseRing(n, { latC = 37.5, lonC = 14.2, radiusDeg = 0.5 } = {}) {
   const pts = [];
diff --git a/src/overpassProxy.test.mjs b/src/overpassProxy.test.mjs
new file mode 100644
index 000000000..106946129
--- /dev/null
+++ b/src/overpassProxy.test.mjs
@@ -0,0 +1,225 @@
+// OVERPASS PROXY — which upstream answers count as an answer.
+//
+// One predicate governs cache reads, writes, and stale fallback. A mirror's
+// refusal must neither end the search for healthy alternatives nor persist as
+// data under the week/month-long cache TTLs. These cases use no live providers.
+//
+// Run with: npm test
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { mkdir, readFile, writeFile, unlink } from 'node:fs/promises';
+import { createHash, randomUUID } from 'node:crypto';
+import path from 'node:path';
+import { Readable } from 'node:stream';
+import createViteConfig, { fetchOverpassPayload, overpassPayloadIsData, readOverpassDisk } from '../vite.config.js';
+
+const ENDPOINTS = ['https://a.example/api', 'https://b.example/api', 'https://c.example/api'];
+
+/** Answer each endpoint from a map of url → {status, body}; record the order. */
+function mirrors(byUrl) {
+  const tried = [];
+  const fetchImpl = async (url) => {
+    tried.push(url);
+    const answer = byUrl[url];
+    if (answer instanceof Error) throw answer;
+    return { status: answer.status, headers: { get: () => answer.contentType || 'application/json' } };
+  };
+  return { fetchImpl, tried, readBody: async (_, __) => byUrl[tried[tried.length - 1]]?.body ?? '' };
+}
+
+const run = (byUrl) => {
+  const m = mirrors(byUrl);
+  return fetchOverpassPayload('data=x', 1e6, {
+    endpoints: ENDPOINTS,
+    fetchImpl: m.fetchImpl,
+    readBody: m.readBody,
+    simplify: (body) => body,
+  }).then((payload) => ({ payload, tried: m.tried }), (error) => ({ error, tried: m.tried }));
+};
+
+const DATA = { status: 200, body: '{"elements":[]}' };
+
+test('disk cache rejects old refusals for fresh and stale reads but preserves last-good data', async () => {
+  const key = `overpass-cache-regression-${randomUUID()}`;
+  const directory = path.join(process.cwd(), '.gev-cache', 'overpass');
+  const file = path.join(directory, `${createHash('sha1').update(key).digest('hex')}.json`);
+  await mkdir(directory, { recursive: true });
+  try {
+    for (const refusal of [
+      { status: 406 }, { status: 429 }, { status: 503 },
+      { status: 200, rateLimited: true }, { status: 200, runtimeError: true },
+    ]) {
+      await writeFile(file, JSON.stringify({ ...DATA, cachedAt: Date.now(), ...refusal }));
+      assert.equal(await readOverpassDisk(key, 60000), null, `fresh ${JSON.stringify(refusal)}`);
+      assert.equal(await readOverpassDisk(key, Infinity), null, `stale ${JSON.stringify(refusal)}`);
+    }
+    const good = { ...DATA, cachedAt: Date.now() - 120000 };
+    await writeFile(file, JSON.stringify(good));
+    assert.equal(await readOverpassDisk(key, 60000), null, 'expired good data misses normal TTL');
+    assert.deepEqual(await readOverpassDisk(key, Infinity), good, 'last-good data survives an outage');
+    await writeFile(file, '{invalid');
+    assert.equal(await readOverpassDisk(key, Infinity), null, 'corrupt cache is ignored');
+  } finally {
+    await unlink(file);
+  }
+});
+
+// ── The predicate ────────────────────────────────────────────────────────────
+
+test('only a 2xx that is neither rate-limited nor a runtime error is data', () => {
+  assert.equal(overpassPayloadIsData({ status: 200 }), true);
+  assert.equal(overpassPayloadIsData({ status: 204 }), true);
+
+  // The measured refusal, and its neighbours. `< 500` admitted every one.
+  for (const status of [400, 403, 406, 410, 429]) {
+    assert.equal(overpassPayloadIsData({ status }), false, `${status} is not data`);
+  }
+  assert.equal(overpassPayloadIsData({ status: 502 }), false);
+  // A 200 can still not be data: Overpass reports runtime failures in the body.
+  assert.equal(overpassPayloadIsData({ status: 200, runtimeError: true }), false);
+  assert.equal(overpassPayloadIsData({ status: 200, rateLimited: true }), false);
+  assert.equal(overpassPayloadIsData({}), false);
+  assert.equal(overpassPayloadIsData(null), false);
+});
+
+// ── The fan-out ──────────────────────────────────────────────────────────────
+
+test('a refusal moves to the next mirror instead of ending the fan-out', async () => {
+  // The exact shape measured against the live mirrors.
+  const { payload, tried } = await run({
+    [ENDPOINTS[0]]: { status: 406, contentType: 'text/html', body: '<!DOCTYPE HTML><title>406</title>' },
+    [ENDPOINTS[1]]: DATA,
+    [ENDPOINTS[2]]: DATA,
+  });
+
+  assert.equal(payload.status, 200);
+  assert.equal(payload.endpoint, ENDPOINTS[1]);
+  assert.deepEqual(tried, ENDPOINTS.slice(0, 2), 'the healthy mirror must be reached, and no further');
+});
+
+test('the first mirror to answer wins, and the rest are left alone', async () => {
+  const { payload, tried } = await run({
+    [ENDPOINTS[0]]: DATA, [ENDPOINTS[1]]: DATA, [ENDPOINTS[2]]: DATA,
+  });
+
+  assert.equal(payload.endpoint, ENDPOINTS[0]);
+  assert.deepEqual(tried, [ENDPOINTS[0]]);
+});
+
+test('a refusal every mirror agrees on is reported, not swallowed', async () => {
+  // A genuinely bad query must still say what upstream said — but only after
+  // every mirror has had its chance to answer it.
+  const refusal = { status: 400, body: 'line 1: parse error' };
+  const { payload, tried } = await run({
+    [ENDPOINTS[0]]: refusal, [ENDPOINTS[1]]: refusal, [ENDPOINTS[2]]: refusal,
+  });
+
+  assert.equal(payload.status, 400);
+  assert.equal(payload.endpoint, ENDPOINTS[0], 'the FIRST refusal is the one reported');
+  assert.deepEqual(tried, ENDPOINTS);
+  assert.equal(overpassPayloadIsData(payload), false, 'so it is neither cached nor served as data');
+});
+
+test('a mirror that throws is no different from one that refuses', async () => {
+  const { payload, tried } = await run({
+    [ENDPOINTS[0]]: new Error('ECONNRESET'),
+    [ENDPOINTS[1]]: { status: 503, body: 'busy' },
+    [ENDPOINTS[2]]: DATA,
+  });
+
+  assert.equal(payload.endpoint, ENDPOINTS[2]);
+  assert.deepEqual(tried, ENDPOINTS);
+});
+
+test('when every mirror is unreachable the caller gets a throw, not a payload', async () => {
+  const { error, payload } = await run({
+    [ENDPOINTS[0]]: new Error('ECONNRESET'),
+    [ENDPOINTS[1]]: new Error('ETIMEDOUT'),
+    [ENDPOINTS[2]]: new Error('ENOTFOUND'),
+  });
+
+  assert.equal(payload, undefined);
+  assert.match(error.message, /ENOTFOUND/);
+});
+
+test('production reader rotates past oversized, runtime-error and rate-limited bodies', async () => {
+  for (const [status, body] of [
+    [200, 'x'.repeat(200)], [200, '{"remark":"runtime error: timed out","elements":[]}'],
+    [200, 'rate_limited'], [429, 'busy'], [403, 'forbidden'],
+  ]) {
+    const tried = [];
+    const payload = await fetchOverpassPayload('data=x', 100, {
+      endpoints: ENDPOINTS,
+      fetchImpl: async (url) => {
+        tried.push(url);
+        return new Response(tried.length === 1 ? body : DATA.body, {
+          status: tried.length === 1 ? status : 200,
+          headers: { 'content-type': 'application/json' },
+        });
+      },
+    });
+    assert.equal(payload.body, DATA.body);
+    assert.deepEqual(tried, ENDPOINTS.slice(0, 2));
+  }
+});
+
+function proxyHandler() {
+  const plugin = createViteConfig({ mode: 'test' }).plugins.find(p => p.name === 'overpass-proxy');
+  const routes = new Map();
+  plugin.configureServer({ middlewares: { use: (route, handler) => routes.set(route, handler) } });
+  return routes.get('/api/overpass');
+}
+
+function invoke(handler, body) {
+  const req = Readable.from([Buffer.from(body)]);
+  Object.assign(req, { method: 'POST', headers: {}, socket: { remoteAddress: '127.0.0.1' } });
+  return new Promise((resolve, reject) => {
+    const res = {
+      writeHead(status, headers) { this.status = status; this.headers = headers; },
+      end(body) { resolve({ status: this.status, headers: this.headers, body }); },
+    };
+    Promise.resolve(handler(req, res)).catch(reject);
+  });
+}
+
+test('coalesced outage callers both receive last-good data, never a cached refusal', async (t) => {
+  const handler = proxyHandler();
+  for (const status of [406, 503, 429]) {
+    const query = `[out:json][timeout:12];node(around:10,30.27,-97.74)["name"="${randomUUID()}"];out;`;
+    const body = `data=${encodeURIComponent(query)}`;
+    const directory = path.join(process.cwd(), '.gev-cache', 'overpass');
+    const file = path.join(directory, `${createHash('sha1').update(body).digest('hex')}.json`);
+    await mkdir(directory, { recursive: true });
+    const stale = { ...DATA, cachedAt: Date.now() - 40 * 86400000 };
+    await writeFile(file, JSON.stringify(stale));
+    const entered = Promise.withResolvers();
+    const release = Promise.withResolvers();
+    let fetches = 0;
+    const mock = t.mock.method(globalThis, 'fetch', async () => {
+      fetches++;
+      entered.resolve();
+      await release.promise;
+      return new Response('upstream unavailable', { status });
+    });
+    try {
+      const first = invoke(handler, body);
+      await entered.promise;
+      const second = invoke(handler, body);
+      // The second request consumes its in-memory stream and joins the pending
+      // promise before releasing upstream. No network or elapsed-time sleep.
+      await new Promise(resolve => setImmediate(resolve));
+      release.resolve();
+      for (const response of await Promise.all([first, second])) {
+        assert.equal(response.status, 200, `${status}: both callers use last-good data`);
+        assert.equal(response.body, DATA.body);
+        assert.equal(response.headers['X-Overpass-Cache'], 'STALE');
+      }
+      assert.equal(fetches, 4, 'one shared, bounded mirror sequence');
+      assert.deepEqual(JSON.parse(await readFile(file, 'utf8')), stale);
+    } finally {
+      release.resolve();
+      mock.mock.restore();
+      await unlink(file);
+    }
+  }
+});
diff --git a/vite.config.js b/vite.config.js
index 6edefe8e7..5b2a79ad1 100644
--- a/vite.config.js
+++ b/vite.config.js
@@ -342,11 +342,14 @@ function overpassDiskPath(cacheKey) {
  * serve-stale path when every mirror is down).
  * @returns {Promise<?Object>} Payload with cachedAt, or null.
  */
-async function readOverpassDisk(cacheKey, maxAgeMs) {
+export async function readOverpassDisk(cacheKey, maxAgeMs) {
   try {
     const raw = await fsp.readFile(overpassDiskPath(cacheKey), 'utf8');
     const payload = JSON.parse(raw);
     if (!payload || typeof payload.body !== 'string' || !Number.isFinite(payload.cachedAt)) return null;
+    // Older versions persisted 4xx refusals with normal data TTLs. Ignore
+    // them on both fresh and stale reads so an upgrade can recover immediately.
+    if (!overpassPayloadIsData(payload)) return null;
     if (Date.now() - payload.cachedAt > maxAgeMs) return null;
     return payload;
   } catch {
@@ -387,18 +390,24 @@ export async function resolveOverpassPreflight({
   cacheMs = OVERPASS_CACHE_MS,
 }) {
   const cached = memoryCache.get(cacheKey);
-  if (cached && now - cached.cachedAt <= cacheMs) return { source: 'HIT', payload: cached };
+  if (overpassPayloadIsData(cached) && now - cached.cachedAt <= cacheMs) return { source: 'HIT', payload: cached };
 
   const pending = inFlight.get(cacheKey);
   if (pending) return { source: 'INFLIGHT', payload: await pending };
 
   const disk = await readDisk();
-  if (disk) return { source: 'DISK', payload: disk };
+  if (overpassPayloadIsData(disk)) return { source: 'DISK', payload: disk };
 
   return allowUpstream()
     ? { source: 'UPSTREAM', payload: null }
     : { source: 'RATE_LIMITED', payload: null };
 }
+
+/** Return only last-good Overpass data, regardless of its age. */
+async function readStaleOverpass(cacheKey) {
+  const cached = _overpassCache.get(cacheKey);
+  return overpassPayloadIsData(cached) ? cached : readOverpassDisk(cacheKey, Infinity);
+}
 /** OSM routing (FOSSGIS OSRM) cache: profile|coords -> { payload, cachedAt }. */
 const ROUTE_CACHE_MS = 600000;
 const _routeCache = new Map();
@@ -2548,26 +2557,47 @@ function sendOverpassResponse(res, payload, cacheStatus = 'MISS') {
 }
 
 /**
- * Try each Overpass upstream in order until one succeeds.
+ * True only for an upstream response that is actually Overpass data.
  *
- * Skips rate-limited or 5xx responses and falls through to the next
- * mirror. If all mirrors fail, returns the last rate-limited payload
- * (if any) or throws the last error.
- *
- * @param {string} body - URL-encoded Overpass QL query body.
+ * The proxy caches on this and serves stale on its negation, so the two
+ * decisions cannot drift apart: a payload that is not data must never be
+ * written to the cache and must always be eligible for a stale replacement.
+ * @param {{status: number, rateLimited?: boolean, runtimeError?: boolean}} payload
+ * @returns {boolean}
+ */
+export function overpassPayloadIsData(payload) {
+  const status = Number(payload?.status);
+  return Number.isFinite(status)
+    && status >= 200 && status < 300
+    && !payload.rateLimited
+    && !payload.runtimeError;
+}
+
+/**
+ * Try each mirror once, retaining response-size and per-mirror timeout caps.
+ * Refusals and body-level failures rotate; total failure returns the last
+ * rate-limit payload, otherwise the first refusal, or throws a network error.
+ * @param {string} body URL-encoded Overpass QL query body.
  * @param {number} [maxResponseBytes] Endpoint-specific response cap.
+ * @param {object} [options] Server-only endpoint and I/O overrides for tests.
  * @returns {Promise<{status:number,body:string,contentType:string,endpoint:string,rateLimited:boolean}>}
  */
-async function fetchOverpassPayload(body, maxResponseBytes = OVERPASS_MAX_RESPONSE_BYTES) {
+export async function fetchOverpassPayload(body, maxResponseBytes = OVERPASS_MAX_RESPONSE_BYTES, {
+  endpoints = OVERPASS_UPSTREAMS,
+  fetchImpl = fetch,
+  readBody = readResponseTextCapped,
+  simplify = simplifyOverpassPayloadBody,
+} = {}) {
   let lastError = null;
   let lastRateLimitPayload = null;
+  let lastRefusalPayload = null;
 
-  for (const endpoint of OVERPASS_UPSTREAMS) {
+  for (const endpoint of endpoints) {
     const controller = new AbortController();
     const timeoutId = setTimeout(() => controller.abort(), OVERPASS_TIMEOUT_MS);
 
     try {
-      const upstream = await fetch(endpoint, {
+      const upstream = await fetchImpl(endpoint, {
         method: 'POST',
         headers: {
           'Content-Type': 'application/x-www-form-urlencoded',
@@ -2577,7 +2607,7 @@ async function fetchOverpassPayload(body, maxResponseBytes = OVERPASS_MAX_RESPON
         signal: controller.signal,
       });
 
-      const responseBody = await readResponseTextCapped(upstream, maxResponseBytes);
+      const responseBody = await readBody(upstream, maxResponseBytes);
       const contentType = upstream.headers.get('content-type') || 'application/json';
       const status = upstream.status;
       const rateLimited = status === 429 || overpassLooksRateLimited(responseBody);
@@ -2601,14 +2631,23 @@ async function fetchOverpassPayload(body, maxResponseBytes = OVERPASS_MAX_RESPON
         lastError = new Error(`Overpass runtime error (${endpoint})`);
         continue;
       }
-      if (status >= 500) {
+      // Anything but 2xx is this mirror declining, not an answer. Only 5xx used
+      // to rotate, so a 4xx ended the fan-out and was returned — and cached —
+      // as data: overpass-api.de and its lz4 alias answer 406 to this proxy's
+      // User-Agent while kumi.systems and private.coffee answer 200 to the very
+      // same request, so every Overpass-backed layer failed on an Apache error
+      // page with two healthy mirrors untried. The first refusal is kept so a
+      // genuinely bad query still reports what upstream said, but only after
+      // every mirror has had the chance to answer it.
+      if (status < 200 || status >= 300) {
+        if (!lastRefusalPayload) lastRefusalPayload = payload;
         lastError = new Error(`Overpass upstream returned ${status} (${endpoint})`);
         continue;
       }
 
       // Success: decimate giant boundary geometry before it reaches the cache,
       // the disk, or the client (what makes the 32 MB read cap safe to hold).
-      payload.body = simplifyOverpassPayloadBody(payload.body);
+      payload.body = simplify(payload.body);
       return payload;
     } catch (error) {
       lastError = error;
@@ -2618,6 +2657,7 @@ async function fetchOverpassPayload(body, maxResponseBytes = OVERPASS_MAX_RESPON
   }
 
   if (lastRateLimitPayload) return lastRateLimitPayload;
+  if (lastRefusalPayload) return lastRefusalPayload;
   throw lastError || new Error('All Overpass upstreams failed');
 }
 
@@ -2692,6 +2732,15 @@ function overpassProxy() {
             return;
           }
           if (preflight.source !== 'UPSTREAM') {
+            // A coalesced caller sees the same failure as the original request
+            // and must get the same last-good fallback, not the raw refusal.
+            if (!overpassPayloadIsData(preflight.payload)) {
+              const stale = await readStaleOverpass(cacheKey);
+              if (stale) {
+                sendOverpassResponse(res, stale, 'STALE');
+                return;
+              }
+            }
             if (preflight.source === 'DISK') {
               _overpassCache.set(cacheKey, preflight.payload);
               trimOverpassCache();
@@ -2710,7 +2759,11 @@ function overpassProxy() {
           _overpassConcurrent += 1;
           const requestPromise = fetchOverpassPayload(safeBody)
             .then((payload) => {
-              if (payload.status < 500 && !payload.rateLimited && !payload.runtimeError) {
+              // Only a 2xx is data. `< 500` cached every 4xx, so one mirror's
+              // refusal was written to memory AND disk — and boundary-class
+              // queries hold a month-long TTL, so a single 406 outlived the
+              // outage that caused it.
+              if (overpassPayloadIsData(payload)) {
                 const entry = { ...payload, cachedAt: Date.now() };
                 _overpassCache.set(cacheKey, entry);
                 trimOverpassCache();
@@ -2728,8 +2781,8 @@ function overpassProxy() {
           // Degraded upstream (rate-limited on every mirror / 5xx / runtime
           // error): last-good roads beat an empty layer — serve stale from
           // memory or disk at ANY age before surfacing the failure.
-          if (payload.rateLimited || payload.runtimeError || payload.status >= 500) {
-            const stale = _overpassCache.get(cacheKey) || await readOverpassDisk(cacheKey, Infinity);
+          if (!overpassPayloadIsData(payload)) {
+            const stale = await readStaleOverpass(cacheKey);
             if (stale) {
               sendOverpassResponse(res, stale, 'STALE');
               return;
@@ -2739,7 +2792,7 @@ function overpassProxy() {
         } catch (e) {
           // Every mirror threw (network-level). Same serve-stale rule.
           const stale = cacheKey
-            ? (_overpassCache.get(cacheKey) || await readOverpassDisk(cacheKey, Infinity).catch(() => null))
+            ? await readStaleOverpass(cacheKey)
             : null;
           if (stale) {
             sendOverpassResponse(res, stale, 'STALE');