diff --git a/app/assets/javascripts/discourse/app/widgets/post-small-action.js b/app/assets/javascripts/discourse/app/widgets/post-small-action.js
index bc23ebf97ab..cf502373f6c 100644
--- a/app/assets/javascripts/discourse/app/widgets/post-small-action.js
+++ b/app/assets/javascripts/discourse/app/widgets/post-small-action.js
@@ -87,6 +87,10 @@ export function addPostSmallActionClassesCallback(callback) {
   addPostSmallActionClassesCallbacks.push(callback);
 }
 
+export function resetPostSmallActionClassesCallbacks() {
+  addPostSmallActionClassesCallbacks.length = 0;
+}
+
 export default createWidget("post-small-action", {
   buildKey: (attrs) => `post-small-act-${attrs.id}`,
   tagName: "div.small-action.onscreen-post",
@@ -97,13 +101,15 @@ export default createWidget("post-small-action", {
 
   buildClasses(attrs) {
     let classNames = [];
+
     if (attrs.deleted) {
       classNames.push("deleted");
     }
 
-    if (addPostSmallActionClassesCallbacks) {
+    if (addPostSmallActionClassesCallbacks.length > 0) {
       addPostSmallActionClassesCallbacks.forEach((callback) => {
         const additionalClasses = callback.call(this, attrs);
+
         if (additionalClasses) {
           classNames.push(...additionalClasses);
         }
diff --git a/app/assets/javascripts/discourse/tests/integration/components/widgets/post-small-action-test.js b/app/assets/javascripts/discourse/tests/integration/components/widgets/post-small-action-test.js
index 90a7658f136..2de44cca90d 100644
--- a/app/assets/javascripts/discourse/tests/integration/components/widgets/post-small-action-test.js
+++ b/app/assets/javascripts/discourse/tests/integration/components/widgets/post-small-action-test.js
@@ -3,6 +3,8 @@ import { setupRenderingTest } from "discourse/tests/helpers/component-test";
 import { render } from "@ember/test-helpers";
 import { exists } from "discourse/tests/helpers/qunit-helpers";
 import { hbs } from "ember-cli-htmlbars";
+import { withPluginApi } from "discourse/lib/plugin-api";
+import { resetPostSmallActionClassesCallbacks } from "discourse/widgets/post-small-action";
 
 module(
   "Integration | Component | Widget | post-small-action",
@@ -89,5 +91,41 @@ module(
         "it adds the recover small action button"
       );
     });
+
+    test("`addPostSmallActionClassesCallback` plugin api", async function (assert) {
+      try {
+        withPluginApi("1.6.0", (api) => {
+          api.addPostSmallActionClassesCallback((postAttrs) => {
+            if (postAttrs.canRecover) {
+              return ["abcde"];
+            }
+          });
+        });
+
+        this.set("args", { id: 123, canRecover: false });
+
+        await render(
+          hbs`<MountWidget @widget="post-small-action" @args={{this.args}} />`
+        );
+
+        assert.notOk(
+          exists(".abcde"),
+          "custom CSS class is not added when condition is not met"
+        );
+
+        this.set("args", { id: 123, canRecover: true });
+
+        await render(
+          hbs`<MountWidget @widget="post-small-action" @args={{this.args}} />`
+        );
+
+        assert.ok(
+          exists(".abcde"),
+          "it adds custom CSS class as registered from the plugin API"
+        );
+      } finally {
+        resetPostSmallActionClassesCallbacks();
+      }
+    });
   }
 );