diff --git a/src/github/context.ts b/src/github/context.ts
index d8b1581..f0e81b5 100644
--- a/src/github/context.ts
+++ b/src/github/context.ts
@@ -1,6 +1,7 @@
 import * as github from "@actions/github";
 import type {
   IssuesEvent,
+  IssuesAssignedEvent,
   IssueCommentEvent,
   PullRequestEvent,
   PullRequestReviewEvent,
@@ -147,3 +148,9 @@ export function isPullRequestReviewCommentEvent(
 ): context is ParsedGitHubContext & { payload: PullRequestReviewCommentEvent } {
   return context.eventName === "pull_request_review_comment";
 }
+
+export function isIssuesAssignedEvent(
+  context: ParsedGitHubContext,
+): context is ParsedGitHubContext & { payload: IssuesAssignedEvent } {
+  return isIssuesEvent(context) && context.eventAction === "assigned";
+}
diff --git a/src/github/validation/trigger.ts b/src/github/validation/trigger.ts
index 6a06153..40ee933 100644
--- a/src/github/validation/trigger.ts
+++ b/src/github/validation/trigger.ts
@@ -3,6 +3,7 @@
 import * as core from "@actions/core";
 import {
   isIssuesEvent,
+  isIssuesAssignedEvent,
   isIssueCommentEvent,
   isPullRequestEvent,
   isPullRequestReviewEvent,
@@ -22,10 +23,10 @@ export function checkContainsTrigger(context: ParsedGitHubContext): boolean {
   }
 
   // Check for assignee trigger
-  if (isIssuesEvent(context) && context.eventAction === "assigned") {
+  if (isIssuesAssignedEvent(context)) {
     // Remove @ symbol from assignee_trigger if present
     let triggerUser = assigneeTrigger.replace(/^@/, "");
-    const assigneeUsername = context.payload.issue.assignee?.login || "";
+    const assigneeUsername = context.payload.assignee?.login || "";
 
     if (triggerUser && assigneeUsername === triggerUser) {
       console.log(`Issue assigned to trigger user '${triggerUser}'`);
diff --git a/test/mockContext.ts b/test/mockContext.ts
index 692137c..65250c1 100644
--- a/test/mockContext.ts
+++ b/test/mockContext.ts
@@ -91,6 +91,12 @@ export const mockIssueAssignedContext: ParsedGitHubContext = {
   actor: "admin-user",
   payload: {
     action: "assigned",
+    assignee: {
+      login: "claude-bot",
+      id: 11111,
+      avatar_url: "https://avatars.githubusercontent.com/u/11111",
+      html_url: "https://github.com/claude-bot",
+    },
     issue: {
       number: 123,
       title: "Feature: Add dark mode support",
diff --git a/test/trigger-validation.test.ts b/test/trigger-validation.test.ts
index bbe40bd..6c368b0 100644
--- a/test/trigger-validation.test.ts
+++ b/test/trigger-validation.test.ts
@@ -87,6 +87,11 @@ describe("checkContainsTrigger", () => {
         ...mockIssueAssignedContext,
         payload: {
           ...mockIssueAssignedContext.payload,
+          assignee: {
+            ...(mockIssueAssignedContext.payload as IssuesAssignedEvent)
+              .assignee,
+            login: "otherUser",
+          },
           issue: {
             ...(mockIssueAssignedContext.payload as IssuesAssignedEvent).issue,
             assignee: {