diff --git a/scripts/patch-ap-syndicator-checkin-guard.mjs b/scripts/patch-ap-syndicator-checkin-guard.mjs
new file mode 100644
index 00000000..a21100a2
--- /dev/null
+++ b/scripts/patch-ap-syndicator-checkin-guard.mjs
@@ -0,0 +1,45 @@
+import { readFile, writeFile } from "node:fs/promises";
+
+// Extend the AP syndicator checkin guard to cover both `location` and
+// `checkin` properties. The upstream package only checks `location`; a
+// checkin coming in without coordinates (no `location` property) would slip
+// through. This patch makes both cases explicit.
+
+const candidates = [
+  "node_modules/@rmdes/indiekit-endpoint-activitypub/lib/syndicator.js",
+];
+
+const oldSnippet = `      // Skip location checkins — they have a JF2 \`location\` property.
+      if (properties.location) {
+        console.info(\`[ActivityPub] Skipping syndication for location checkin: \${properties.url}\`);
+        return undefined;
+      }`;
+
+const newSnippet = `      // Skip location checkins — they have a JF2 \`location\` or \`checkin\` property.
+      if (properties.location || properties.checkin) {
+        console.info(\`[ActivityPub] Skipping syndication for location checkin: \${properties.url}\`);
+        return undefined;
+      }`;
+
+for (const filePath of candidates) {
+  let source;
+  try {
+    source = await readFile(filePath, "utf8");
+  } catch {
+    console.log(`[postinstall] patch-ap-syndicator-checkin-guard: ${filePath} not found, skipping`);
+    continue;
+  }
+
+  if (source.includes("properties.checkin")) {
+    console.log("[postinstall] patch-ap-syndicator-checkin-guard: already patched");
+    continue;
+  }
+
+  if (!source.includes(oldSnippet)) {
+    console.log("[postinstall] patch-ap-syndicator-checkin-guard: target snippet not found, skipping");
+    continue;
+  }
+
+  await writeFile(filePath, source.replace(oldSnippet, newSnippet), "utf8");
+  console.log("[postinstall] patch-ap-syndicator-checkin-guard: patched syndicator.js");
+}