diff --git a/__tests__/e2e/mock-llm-reporting.test.ts b/__tests__/e2e/mock-llm-reporting.test.ts
index 4fc9466745..04a4b4adbc 100644
--- a/__tests__/e2e/mock-llm-reporting.test.ts
+++ b/__tests__/e2e/mock-llm-reporting.test.ts
@@ -1,10 +1,11 @@
 // @vitest-environment node
-import { describe, expect, it } from "vitest";
+import { afterEach, describe, expect, it, vi } from "vitest";
 
 import { renderReport } from "../../tests/e2e/mock-llm/scripts/render-mock-llm-report.mjs";
 import {
   buildCommentBody,
   findMatchingJobComments,
+  upsertJobComment,
 } from "../../tests/e2e/mock-llm/scripts/upsert-pr-comment.mjs";
 
 const MOCK_MARKER = "<!-- agent-canvas-mock-llm-e2e-report -->";
@@ -86,4 +87,59 @@ describe("mock-LLM E2E reporting", () => {
       1, 2,
     ]);
   });
+
+  it("edits the existing report in place instead of reposting it", async () => {
+    const requests: { method: string; path: string }[] = [];
+    const fetchMock = vi
+      .spyOn(globalThis, "fetch")
+      .mockImplementation(async (url, init) => {
+        const method = init?.method ?? "GET";
+        const path =
+          new URL(String(url)).pathname + new URL(String(url)).search;
+        requests.push({ method, path });
+
+        if (method === "GET") {
+          return new Response(
+            JSON.stringify(
+              path.includes("page=1")
+                ? [
+                    { id: 11, body: "unrelated", user: { type: "User" } },
+                    {
+                      id: 22,
+                      body: `${MOCK_MARKER}\n## ✅ Mock-LLM E2E Tests`,
+                      user: { login: "github-actions[bot]", type: "Bot" },
+                    },
+                  ]
+                : [],
+            ),
+            { status: 200 },
+          );
+        }
+        return new Response(JSON.stringify({ id: 22 }), { status: 200 });
+      });
+
+    const result = await upsertJobComment({
+      repo: "OpenHands/OpenHands",
+      issueNumber: "16521",
+      token: "t",
+      body: "## ✅ Mock-LLM E2E Tests\n\n**62/62 passed**",
+      marker: MOCK_MARKER,
+      legacyTitle: "Mock-LLM E2E Tests",
+    });
+
+    expect(result).toMatchObject({ updated: true, deleted: 0 });
+    expect(requests.filter((r) => r.method === "PATCH")).toEqual([
+      {
+        method: "PATCH",
+        path: "/repos/OpenHands/OpenHands/issues/comments/22",
+      },
+    ]);
+    expect(requests.some((r) => r.method === "POST")).toBe(false);
+    expect(requests.some((r) => r.method === "DELETE")).toBe(false);
+    fetchMock.mockRestore();
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
 });
diff --git a/tests/e2e/mock-llm/scripts/upsert-pr-comment.mjs b/tests/e2e/mock-llm/scripts/upsert-pr-comment.mjs
index 77ee321ee3..aa690e0ce5 100644
--- a/tests/e2e/mock-llm/scripts/upsert-pr-comment.mjs
+++ b/tests/e2e/mock-llm/scripts/upsert-pr-comment.mjs
@@ -201,7 +201,7 @@ export function findMatchingJobComments(comments, options) {
   return comments.filter((comment) => isMatchingJobComment(comment, options));
 }
 
-export async function replaceJobComment({
+export async function upsertJobComment({
   repo,
   issueNumber,
   token,
@@ -210,24 +210,41 @@ export async function replaceJobComment({
   legacyTitle = "",
 }) {
   const comments = await listIssueComments(repo, issueNumber, token);
-  const existing = findMatchingJobComments(comments, { marker, legacyTitle });
+  const [existing, ...duplicates] = findMatchingJobComments(comments, {
+    marker,
+    legacyTitle,
+  });
+  const commentBody = buildCommentBody(body, marker);
 
-  for (const comment of existing) {
+  // Edit the oldest report in place so the comment keeps its position in the
+  // thread and does not notify subscribers on every run. Anything beyond it is
+  // a leftover duplicate from an earlier run.
+  for (const duplicate of duplicates) {
     await githubRequest(
       "DELETE",
-      `/repos/${repo}/issues/comments/${comment.id}`,
+      `/repos/${repo}/issues/comments/${duplicate.id}`,
       token,
     );
   }
 
+  if (existing) {
+    const updated = await githubRequest(
+      "PATCH",
+      `/repos/${repo}/issues/comments/${existing.id}`,
+      token,
+      { body: commentBody },
+    );
+    return { updated: true, deleted: duplicates.length, comment: updated };
+  }
+
   const created = await githubRequest(
     "POST",
     `/repos/${repo}/issues/${issueNumber}/comments`,
     token,
-    { body: buildCommentBody(body, marker) },
+    { body: commentBody },
   );
 
-  return { deleted: existing.length, created };
+  return { updated: false, deleted: duplicates.length, comment: created };
 }
 
 async function main() {
@@ -258,7 +275,7 @@ async function main() {
   );
   const body = readFileSync(bodyFile, "utf8");
 
-  const result = await replaceJobComment({
+  const result = await upsertJobComment({
     repo,
     issueNumber: validatedIssueNumber,
     token,
@@ -267,11 +284,14 @@ async function main() {
     legacyTitle: args.legacy_title ?? "",
   });
 
-  console.log(
-    `Deleted ${result.deleted} existing PR comment${
-      result.deleted === 1 ? "" : "s"
-    }; created PR comment ${result.created.id}.`,
-  );
+  const action = result.updated ? "Updated" : "Created";
+  const cleanup =
+    result.deleted === 0
+      ? ""
+      : ` Deleted ${result.deleted} duplicate PR comment${
+          result.deleted === 1 ? "" : "s"
+        }.`;
+  console.log(`${action} PR comment ${result.comment.id}.${cleanup}`);
 }
 
 if (