diff --git a/server/providers/shipwrecks.js b/server/providers/shipwrecks.js
index e680b18a0..8a5fe275f 100644
--- a/server/providers/shipwrecks.js
+++ b/server/providers/shipwrecks.js
@@ -15,6 +15,8 @@ const REFRESH_MS = 24 * 60 * 60 * 1000;
 const STALE_MAX_MS = 7 * REFRESH_MS;
 /** After a source fails, it is not asked again for this long. */
 const RETRY_MS = 15 * 60 * 1000;
+/** A refresh with fewer than this share of the last good records is refused. */
+const MIN_KEEP_RATIO = 0.5;
 // Only the columns the card uses; ship_char/obj_desc histories can name people
 // and are not read beyond a short vessel type.
 const EMODNET_PROPERTIES = [
@@ -109,7 +111,13 @@ export function shipwrecksProxy({
     const done = now();
     results.forEach((result, i) => {
       const state = sources[names[i]];
-      if (result.status === 'fulfilled')
+      // A valid answer far smaller than the last good set (a truncated or
+      // half-built upstream) is a failure too, until the old set expires.
+      const shrunk =
+        result.status === 'fulfilled' &&
+        usable(state, done) &&
+        result.value.length < state.good.records.length * MIN_KEEP_RATIO;
+      if (result.status === 'fulfilled' && !shrunk)
         state.good = { records: result.value, fetchedAt: done };
       else state.retryAt = done + RETRY_MS;
     });
diff --git a/src/data/shipwrecksProxy.test.mjs b/src/data/shipwrecksProxy.test.mjs
index 7793ef145..e0bdc8216 100644
--- a/src/data/shipwrecksProxy.test.mjs
+++ b/src/data/shipwrecksProxy.test.mjs
@@ -292,3 +292,39 @@ test('an upstream body over its cap is a failed source, read only to the cap', a
   assert.ok(cancelled, 'the oversized body is cancelled');
   assert.ok(sent <= 34 * MIB, `read ${sent / MIB} MiB of a 32 MiB cap`);
 });
+
+test('a refresh far smaller than the last good set is refused until it expires', async () => {
+  let clock = 0;
+  const many = {
+    results: {
+      bindings: Array.from({ length: 10 }, (_, i) => ({
+        item: { value: `http://www.wikidata.org/entity/Q${100 + i}` },
+        itemLabel: { value: `Wreck ${i}` },
+        coord: { value: `Point(${i} 40)` },
+      })),
+    },
+  };
+  let wikidata = many;
+  const handler = handlerOf(
+    shipwrecksProxy({
+      fetchImpl: async (url) =>
+        Response.json(
+          String(url).includes('query.wikidata.org') ? wikidata : EMODNET,
+        ),
+      now: () => clock,
+    }),
+  );
+  assert.equal((await request(handler)).body.records.length, 11);
+  wikidata = WIKIDATA;
+  clock = 25 * HOUR;
+  const refused = await request(handler);
+  assert.equal(refused.body.records.length, 11, 'one wreck is not ten');
+  assert.equal(refused.body.stale, true);
+  clock = 8 * 24 * HOUR;
+  const accepted = await request(handler);
+  assert.equal(
+    accepted.body.records.length,
+    2,
+    'taken once the old set expired',
+  );
+});