diff --git a/actions/setup/js/add_comment.cjs b/actions/setup/js/add_comment.cjs
index 7f85c86c74d..90888bbd240 100644
--- a/actions/setup/js/add_comment.cjs
+++ b/actions/setup/js/add_comment.cjs
@@ -20,7 +20,7 @@ const { getMessages } = require("./messages_core.cjs");
 const { getBodyHeader } = require("./messages_header.cjs");
 const { sanitizeContent } = require("./sanitize_content.cjs");
 const { MAX_COMMENT_LENGTH, MAX_MENTIONS, MAX_LINKS, enforceCommentLimits } = require("./comment_limit_helpers.cjs");
-const { resolveTopLevelDiscussionCommentId } = require("./github_api_helpers.cjs");
+const { createDiscussionComment, resolveTopLevelDiscussionCommentId } = require("./github_api_helpers.cjs");
 const { logStagedPreviewInfo } = require("./staged_preview.cjs");
 const { ERR_NOT_FOUND } = require("./error_codes.cjs");
 const { isPayloadUserBot } = require("./resolve_mentions.cjs");
@@ -379,33 +379,7 @@ async function commentOnDiscussion(github, owner, repo, discussionNumber, messag
   const discussionUrl = repository.discussion.url;
 
   // 2. Add comment (with optional replyToId for threading)
-  const mutation = replyToId
-    ? /* GraphQL */ `
-        mutation ($dId: ID!, $body: String!, $replyToId: ID!) {
-          addDiscussionComment(input: { discussionId: $dId, body: $body, replyToId: $replyToId }) {
-            comment {
-              id
-              url
-            }
-          }
-        }
-      `
-    : /* GraphQL */ `
-        mutation ($dId: ID!, $body: String!) {
-          addDiscussionComment(input: { discussionId: $dId, body: $body }) {
-            comment {
-              id
-              url
-            }
-          }
-        }
-      `;
-
-  const variables = { dId: discussionId, body: message, ...(replyToId ? { replyToId } : {}) };
-
-  const result = await github.graphql(mutation, variables);
-
-  const comment = result.addDiscussionComment.comment;
+  const comment = await createDiscussionComment(github, discussionId, message, replyToId);
 
   return {
     id: comment.id,
diff --git a/actions/setup/js/add_reaction_and_edit_comment.cjs b/actions/setup/js/add_reaction_and_edit_comment.cjs
index fa00fedee60..34a08807637 100644
--- a/actions/setup/js/add_reaction_and_edit_comment.cjs
+++ b/actions/setup/js/add_reaction_and_edit_comment.cjs
@@ -7,7 +7,7 @@ const { generateWorkflowIdMarker } = require("./generate_footer.cjs");
 const { sanitizeContent } = require("./sanitize_content.cjs");
 const { ERR_API, ERR_NOT_FOUND, ERR_VALIDATION } = require("./error_codes.cjs");
 const { buildWorkflowRunUrl } = require("./workflow_metadata_helpers.cjs");
-const { resolveTopLevelDiscussionCommentId } = require("./github_api_helpers.cjs");
+const { createDiscussionComment, resolveTopLevelDiscussionCommentId } = require("./github_api_helpers.cjs");
 const { resolveInvocationContext } = require("./invocation_context_helpers.cjs");
 const { addReaction, addDiscussionReaction, getDiscussionNodeId } = require("./add_reaction.cjs");
 
@@ -250,19 +250,7 @@ async function addCommentWithWorkflowLink(endpoint, runUrl, eventName, invocatio
       // For discussion_comment events, thread the reply under the triggering comment.
       // GitHub Discussions only supports two nesting levels, so resolve the top-level parent node ID.
       const replyToId = eventName === "discussion_comment" ? await resolveTopLevelDiscussionCommentId(github, eventPayload?.comment?.node_id) : null;
-      const mutation = replyToId
-        ? `mutation($dId: ID!, $body: String!, $replyToId: ID!) {
-            addDiscussionComment(input: { discussionId: $dId, body: $body, replyToId: $replyToId }) {
-              comment { id url }
-            }
-          }`
-        : `mutation($dId: ID!, $body: String!) {
-            addDiscussionComment(input: { discussionId: $dId, body: $body }) {
-              comment { id url }
-            }
-          }`;
-      const result = await github.graphql(mutation, { dId: discussionId, body: commentBody, ...(replyToId ? { replyToId } : {}) });
-      const comment = result.addDiscussionComment.comment;
+      const comment = await createDiscussionComment(github, discussionId, commentBody, replyToId);
       setCommentOutputs(comment.id, comment.url, eventRepo);
       return;
     }
diff --git a/actions/setup/js/github_api_helpers.cjs b/actions/setup/js/github_api_helpers.cjs
index af324c5ea3e..17fb7a16cd9 100644
--- a/actions/setup/js/github_api_helpers.cjs
+++ b/actions/setup/js/github_api_helpers.cjs
@@ -175,7 +175,44 @@ async function resolveTopLevelDiscussionCommentId(github, commentNodeId) {
   }
 }
 
+/**
+ * Create a discussion comment with optional threaded reply target.
+ * @param {Object} github - GitHub API client (must support graphql)
+ * @param {string} discussionId - Discussion GraphQL node ID
+ * @param {string} body - Comment body
+ * @param {string|null|undefined} [replyToId] - Optional top-level discussion comment node ID for threading
+ * @returns {Promise<{id: string, url: string}>}
+ */
+async function createDiscussionComment(github, discussionId, body, replyToId) {
+  const mutation = replyToId
+    ? /* GraphQL */ `
+        mutation ($dId: ID!, $body: String!, $replyToId: ID!) {
+          addDiscussionComment(input: { discussionId: $dId, body: $body, replyToId: $replyToId }) {
+            comment {
+              id
+              url
+            }
+          }
+        }
+      `
+    : /* GraphQL */ `
+        mutation ($dId: ID!, $body: String!) {
+          addDiscussionComment(input: { discussionId: $dId, body: $body }) {
+            comment {
+              id
+              url
+            }
+          }
+        }
+      `;
+
+  const variables = { dId: discussionId, body, ...(replyToId ? { replyToId } : {}) };
+  const result = await github.graphql(mutation, variables);
+  return result.addDiscussionComment.comment;
+}
+
 module.exports = {
+  createDiscussionComment,
   fetchAllRepoLabels,
   getFileContent,
   logGraphQLError,
diff --git a/actions/setup/js/github_api_helpers.test.cjs b/actions/setup/js/github_api_helpers.test.cjs
index ac1f571f3af..0e4932d0f10 100644
--- a/actions/setup/js/github_api_helpers.test.cjs
+++ b/actions/setup/js/github_api_helpers.test.cjs
@@ -11,6 +11,7 @@ describe("github_api_helpers.cjs", () => {
   let logGraphQLError;
   let fetchAllRepoLabels;
   let resolveTopLevelDiscussionCommentId;
+  let createDiscussionComment;
   let mockGithub;
 
   beforeEach(async () => {
@@ -30,6 +31,7 @@ describe("github_api_helpers.cjs", () => {
     logGraphQLError = module.logGraphQLError;
     fetchAllRepoLabels = module.fetchAllRepoLabels;
     resolveTopLevelDiscussionCommentId = module.resolveTopLevelDiscussionCommentId;
+    createDiscussionComment = module.createDiscussionComment;
   });
 
   describe("getFileContent", () => {
@@ -418,4 +420,56 @@ describe("github_api_helpers.cjs", () => {
       expect(mockCore.info).toHaveBeenCalledWith(expect.stringContaining("resolving top-level discussion comment"));
     });
   });
+
+  describe("createDiscussionComment", () => {
+    it("should create a top-level discussion comment when replyToId is omitted", async () => {
+      const mockGraphql = vi.fn().mockResolvedValueOnce({
+        addDiscussionComment: { comment: { id: "DC_1", url: "https://github.com/o/r/discussions/1#discussioncomment-1" } },
+      });
+
+      const result = await createDiscussionComment({ graphql: mockGraphql }, "D_1", "hello");
+
+      expect(result).toEqual({ id: "DC_1", url: "https://github.com/o/r/discussions/1#discussioncomment-1" });
+      expect(mockGraphql).toHaveBeenCalledWith(expect.stringContaining("addDiscussionComment"), {
+        dId: "D_1",
+        body: "hello",
+      });
+      expect(String(mockGraphql.mock.calls[0][0])).not.toContain("$replyToId");
+    });
+
+    it("should create a threaded discussion comment when replyToId is provided", async () => {
+      const mockGraphql = vi.fn().mockResolvedValueOnce({
+        addDiscussionComment: { comment: { id: "DC_2", url: "https://github.com/o/r/discussions/1#discussioncomment-2" } },
+      });
+
+      await createDiscussionComment({ graphql: mockGraphql }, "D_1", "reply", "DC_parent");
+
+      expect(mockGraphql).toHaveBeenCalledWith(expect.stringContaining("$replyToId"), {
+        dId: "D_1",
+        body: "reply",
+        replyToId: "DC_parent",
+      });
+    });
+
+    it("should treat null replyToId as omitted", async () => {
+      const mockGraphql = vi.fn().mockResolvedValueOnce({
+        addDiscussionComment: { comment: { id: "DC_3", url: "https://github.com/o/r/discussions/1#discussioncomment-3" } },
+      });
+
+      await createDiscussionComment({ graphql: mockGraphql }, "D_1", "top-level", null);
+
+      expect(mockGraphql).toHaveBeenCalledWith(expect.stringContaining("addDiscussionComment"), {
+        dId: "D_1",
+        body: "top-level",
+      });
+      expect(String(mockGraphql.mock.calls[0][0])).not.toContain("$replyToId");
+    });
+
+    it("should propagate graphql errors", async () => {
+      const error = new Error("GraphQL failed");
+      const mockGraphql = vi.fn().mockRejectedValueOnce(error);
+
+      await expect(createDiscussionComment({ graphql: mockGraphql }, "D_1", "hello")).rejects.toThrow("GraphQL failed");
+    });
+  });
 });