diff --git a/actions/setup/js/add_reaction_and_edit_comment.cjs b/actions/setup/js/add_reaction_and_edit_comment.cjs
index 4773b8aa7b4..69242c35126 100644
--- a/actions/setup/js/add_reaction_and_edit_comment.cjs
+++ b/actions/setup/js/add_reaction_and_edit_comment.cjs
@@ -31,6 +31,34 @@ function expectRestEndpoint(endpoint, endpointName, eventName) {
   return endpoint;
 }
 
+/**
+ * Build the "create issue comment" REST endpoint used to post the workflow-link comment
+ * on the issue or pull request associated with the triggering event.
+ * @param {string} owner - Repository owner
+ * @param {string} repo - Repository name
+ * @param {number} issueNumber - Issue or pull request number
+ * @returns {RestEndpoint}
+ */
+function issueCommentEndpoint(owner, repo, issueNumber) {
+  return { route: "POST /repos/{owner}/{repo}/issues/{issue_number}/comments", params: { owner, repo, issue_number: issueNumber } };
+}
+
+/**
+ * Verify a required payload field is present, calling core.setFailed with a consistent
+ * message when it is missing.
+ * @param {unknown} value - The extracted field value
+ * @param {string} fieldName - Human-readable field name for the error message
+ * @param {string} errorCode - Error code prefix (ERR_NOT_FOUND or ERR_VALIDATION)
+ * @returns {boolean} true if present, false if missing (setFailed already called)
+ */
+function requirePayloadField(value, fieldName, errorCode) {
+  if (!value) {
+    core.setFailed(`${errorCode}: ${fieldName} not found in event payload`);
+    return false;
+  }
+  return true;
+}
+
 /**
  * Resolve the reaction and comment API endpoints for a given event.
  * Returns null (after calling core.setFailed) when the event or payload is invalid.
@@ -44,71 +72,50 @@ async function resolveEventEndpoints(eventName, owner, repo, payload) {
   switch (eventName) {
     case "issues": {
       const issueNumber = payload?.issue?.number;
-      if (!issueNumber) {
-        core.setFailed(`${ERR_NOT_FOUND}: Issue number not found in event payload`);
-        return null;
-      }
+      if (!requirePayloadField(issueNumber, "Issue number", ERR_NOT_FOUND)) return null;
       return {
         reactionEndpoint: { route: "POST /repos/{owner}/{repo}/issues/{issue_number}/reactions", params: { owner, repo, issue_number: issueNumber } },
-        commentUpdateEndpoint: { route: "POST /repos/{owner}/{repo}/issues/{issue_number}/comments", params: { owner, repo, issue_number: issueNumber } },
+        commentUpdateEndpoint: issueCommentEndpoint(owner, repo, issueNumber),
       };
     }
 
     case "issue_comment": {
       const commentId = payload?.comment?.id;
       const issueNumber = payload?.issue?.number;
-      if (!commentId) {
-        core.setFailed(`${ERR_VALIDATION}: Comment ID not found in event payload`);
-        return null;
-      }
-      if (!issueNumber) {
-        core.setFailed(`${ERR_NOT_FOUND}: Issue number not found in event payload`);
-        return null;
-      }
+      if (!requirePayloadField(commentId, "Comment ID", ERR_VALIDATION)) return null;
+      if (!requirePayloadField(issueNumber, "Issue number", ERR_NOT_FOUND)) return null;
       return {
         reactionEndpoint: { route: "POST /repos/{owner}/{repo}/issues/comments/{comment_id}/reactions", params: { owner, repo, comment_id: commentId } },
         // Create new comment on the issue itself, not on the comment
-        commentUpdateEndpoint: { route: "POST /repos/{owner}/{repo}/issues/{issue_number}/comments", params: { owner, repo, issue_number: issueNumber } },
+        commentUpdateEndpoint: issueCommentEndpoint(owner, repo, issueNumber),
       };
     }
 
     case "pull_request": {
       const prNumber = payload?.pull_request?.number;
-      if (!prNumber) {
-        core.setFailed(`${ERR_NOT_FOUND}: Pull request number not found in event payload`);
-        return null;
-      }
+      if (!requirePayloadField(prNumber, "Pull request number", ERR_NOT_FOUND)) return null;
       // PRs are "issues" for the reactions endpoint
       return {
         reactionEndpoint: { route: "POST /repos/{owner}/{repo}/issues/{issue_number}/reactions", params: { owner, repo, issue_number: prNumber } },
-        commentUpdateEndpoint: { route: "POST /repos/{owner}/{repo}/issues/{issue_number}/comments", params: { owner, repo, issue_number: prNumber } },
+        commentUpdateEndpoint: issueCommentEndpoint(owner, repo, prNumber),
       };
     }
 
     case "pull_request_review_comment": {
       const reviewCommentId = payload?.comment?.id;
       const prNumber = payload?.pull_request?.number;
-      if (!reviewCommentId) {
-        core.setFailed(`${ERR_VALIDATION}: Review comment ID not found in event payload`);
-        return null;
-      }
-      if (!prNumber) {
-        core.setFailed(`${ERR_NOT_FOUND}: Pull request number not found in event payload`);
-        return null;
-      }
+      if (!requirePayloadField(reviewCommentId, "Review comment ID", ERR_VALIDATION)) return null;
+      if (!requirePayloadField(prNumber, "Pull request number", ERR_NOT_FOUND)) return null;
       return {
         reactionEndpoint: { route: "POST /repos/{owner}/{repo}/pulls/comments/{comment_id}/reactions", params: { owner, repo, comment_id: reviewCommentId } },
         // Create new comment on the PR itself (using issues endpoint since PRs are issues)
-        commentUpdateEndpoint: { route: "POST /repos/{owner}/{repo}/issues/{issue_number}/comments", params: { owner, repo, issue_number: prNumber } },
+        commentUpdateEndpoint: issueCommentEndpoint(owner, repo, prNumber),
       };
     }
 
     case "discussion": {
       const discussionNumber = payload?.discussion?.number;
-      if (!discussionNumber) {
-        core.setFailed(`${ERR_NOT_FOUND}: Discussion number not found in event payload`);
-        return null;
-      }
+      if (!requirePayloadField(discussionNumber, "Discussion number", ERR_NOT_FOUND)) return null;
       // Discussions use GraphQL API - get the node ID
       const discussionNodeId = await getDiscussionNodeId(owner, repo, discussionNumber);
       return {
@@ -120,15 +127,9 @@ async function resolveEventEndpoints(eventName, owner, repo, payload) {
     case "discussion_comment": {
       const discussionNumber = payload?.discussion?.number;
       const commentId = payload?.comment?.id;
-      if (!discussionNumber || !commentId) {
-        core.setFailed(`${ERR_NOT_FOUND}: Discussion or comment information not found in event payload`);
-        return null;
-      }
+      if (!requirePayloadField(discussionNumber && commentId, "Discussion or comment information", ERR_NOT_FOUND)) return null;
       const commentNodeId = payload?.comment?.node_id;
-      if (!commentNodeId) {
-        core.setFailed(`${ERR_NOT_FOUND}: Discussion comment node ID not found in event payload`);
-        return null;
-      }
+      if (!requirePayloadField(commentNodeId, "Discussion comment node ID", ERR_NOT_FOUND)) return null;
       return {
         reactionEndpoint: commentNodeId, // Store node ID for GraphQL
         commentUpdateEndpoint: `discussion_comment:${discussionNumber}:${commentId}`, // Special format