diff --git a/scripts/patch-micropub-category-from-posts.mjs b/scripts/patch-micropub-category-from-posts.mjs
new file mode 100644
index 00000000..63921a5e
--- /dev/null
+++ b/scripts/patch-micropub-category-from-posts.mjs
@@ -0,0 +1,81 @@
+/**
+ * Patch: make q=category return distinct tags from MongoDB posts collection.
+ *
+ * In queryController, line 28 rewrites q === "category" → "categories" before
+ * the switch, so the new case label must be "categories" (not "category").
+ * The default branch returns config.categories (always []) because
+ * publication.categories is not configured. This patch inserts a
+ * case "categories": before default: that queries the posts collection instead.
+ *
+ * postsCollection may be undefined (no MongoDB). The ternary provides a []
+ * fallback consistent with the guard pattern used throughout query.js.
+ */
+
+import { access, readFile, writeFile } from "node:fs/promises";
+
+const MARKER = "// [patch] micropub-category-from-posts";
+
+const candidates = [
+  "node_modules/@indiekit/endpoint-micropub/lib/controllers/query.js",
+  "node_modules/@indiekit/indiekit/node_modules/@indiekit/endpoint-micropub/lib/controllers/query.js",
+];
+
+const OLD_SNIPPET = `        break;
+      }
+
+      default: {
+        // Query configuration value (can be filtered, limited and offset)
+        if (config[q]) {`;
+
+const NEW_SNIPPET = `        break;
+      }
+
+      case "categories": { ${MARKER}
+        const cats = postsCollection
+          ? (await postsCollection.distinct("properties.category")).filter(Boolean).sort()
+          : [];
+        response.json({
+          categories: queryConfig(cats, { filter: filter?.toLowerCase(), limit, offset }),
+        });
+        break;
+      }
+
+      default: {
+        // Query configuration value (can be filtered, limited and offset)
+        if (config[q]) {`;
+
+async function exists(p) {
+  try { await access(p); return true; } catch { return false; }
+}
+
+let totalPatched = 0;
+let totalChecked = 0;
+
+for (const filePath of candidates) {
+  if (!(await exists(filePath))) continue;
+  totalChecked++;
+
+  const source = await readFile(filePath, "utf8");
+  if (source.includes(MARKER)) {
+    console.log(`[postinstall] patch-micropub-category-from-posts: already applied to ${filePath}`);
+    continue;
+  }
+
+  if (!source.includes(OLD_SNIPPET)) {
+    console.warn(`[postinstall] patch-micropub-category-from-posts: snippet not found in ${filePath} (upstream changed?)`);
+    continue;
+  }
+
+  const updated = source.replace(OLD_SNIPPET, NEW_SNIPPET);
+  await writeFile(filePath, updated, "utf8");
+  console.log(`[postinstall] Applied patch-micropub-category-from-posts to ${filePath}`);
+  totalPatched++;
+}
+
+if (totalChecked === 0) {
+  console.log("[postinstall] patch-micropub-category-from-posts: no target files found");
+} else if (totalPatched === 0) {
+  console.log("[postinstall] patch-micropub-category-from-posts: already up to date");
+} else {
+  console.log(`[postinstall] patch-micropub-category-from-posts: patched ${totalPatched} file(s)`);
+}