diff --git a/src/github/comments.ts b/src/github/comments.ts
index eeb8227713..f12ffd8e6f 100644
--- a/src/github/comments.ts
+++ b/src/github/comments.ts
@@ -11,6 +11,14 @@ const LEGACY_AGENT_COMMAND_COMMENT_MARKER = "<!-- gittensory-agent-command -->";
  *  thread from the sticky PR panel above (that comment stops being useful the moment the PR closes), so this
  *  must never collapse into `PR_PANEL_COMMENT_MARKER`'s aliases the way the two constants above do. */
 export const VISUAL_FOLLOWUP_COMMENT_MARKER = "<!-- loopover:visual-unrelated-followup -->";
+// #8803: the close-explanation comment's idempotency marker, parameterized by closeKind so distinct close
+// reasons on one PR (rare, but e.g. a hard-rule close after an earlier heuristic close attempt) keep their
+// own canonical comments. Routing the executor's closeComment through the marker helper means a
+// comment-succeeded-close-failed retry PATCHes/skips the canonical comment instead of stacking an
+// identical duplicate every failed cycle.
+export function closeExplanationMarker(closeKind: string | undefined): string {
+  return `<!-- loopover:close-explanation:${closeKind ?? "close"} -->`;
+}
 // Bound the marker-comment search at 10 pages (up to 1,000 comments), matching src/github's other pagination
 // caps (app.ts's MAX_WORKFLOW_RUN_LIST_PAGES, pr-actions.ts's REVIEW_PAGE_LIMIT). The old cap of 3 (300 comments)
 // let a PR/issue that accrued >300 comments before LoopOver's own marker comment hide it from this search, so
@@ -45,6 +53,24 @@ export async function createOrUpdatePrIntelligenceComment(
  *  updates the same comment instead of posting a confusing duplicate; a same-body repeat (a retried `closed`
  *  webhook delivery) is a genuine no-op via the same byte-identical-body skip every other marker comment here
  *  already gets. */
+/** #8803: idempotent close-explanation comment — the executor's close path routes here so a retry after a
+ *  failed close never re-posts the identical "why we closed you" body (byte-identical → skip; changed →
+ *  PATCH the canonical comment). createIfMissing stays true: the first attempt must always post. */
+export async function createOrUpdateCloseExplanationComment(
+  env: Env,
+  installationId: number,
+  repoFullName: string,
+  pullNumber: number,
+  body: string,
+  closeKind: string | undefined,
+): Promise<{ id: number; html_url?: string; changed: boolean } | null> {
+  const marker = closeExplanationMarker(closeKind);
+  // The marker MUST live in the posted body — the search-side helper only finds comments whose body contains
+  // it (the intelligence/visual callers embed theirs the same way). An HTML comment renders invisibly, so the
+  // contributor-facing text is unchanged; a byte-identical replan skips, a reworded one PATCHes.
+  return createOrUpdateIssueCommentWithMarker(env, installationId, repoFullName, pullNumber, `${marker}\n${body}`, marker);
+}
+
 export async function createOrUpdateVisualFollowupComment(
   env: Env,
   installationId: number,
diff --git a/src/services/agent-action-executor.ts b/src/services/agent-action-executor.ts
index 12135b15d0..10b20f9b8c 100644
--- a/src/services/agent-action-executor.ts
+++ b/src/services/agent-action-executor.ts
@@ -24,6 +24,7 @@ import { githubRateLimitAdmissionKeyForToken } from "../github/client";
 import { ensurePullRequestAssignee } from "../github/assignees";
 import { ensurePullRequestLabel, removePullRequestLabel } from "../github/labels";
 import { closeIssue, closePullRequest, createIssueComment, createPullRequestReview, dismissLatestBotApproval, mergePullRequest, updatePullRequestBranch } from "../github/pr-actions";
+import { createOrUpdateCloseExplanationComment } from "../github/comments";
 import { fetchPullRequestFreshness, pullRequestFreshnessDetail } from "../github/pr-freshness";
 import { isActingAutonomyLevel, resolveAutonomy } from "../settings/autonomy";
 import { boundStructuredCloseReasonsForPersistence, buildAgentActionAudit, formatAgentPermissionDenial, isGlobalAgentPause, resolveAgentActionMode, resolveAgentPermissionReadiness, type AgentActionMode } from "../settings/agent-execution";
@@ -1051,7 +1052,10 @@ async function performAction(env: Env, ctx: AgentActionExecutionContext, action:
       return;
     }
     case "close":
-      if (action.closeComment) await createIssueComment(env, ctx.installationId, ctx.repoFullName, ctx.pullNumber, action.closeComment);
+      // #8803: marker-idempotent — when the comment lands but the close call fails transiently, the retry's
+      // replan produces the identical closeComment; the marker helper skips/PATCHes the canonical comment
+      // instead of stacking a duplicate "why we closed you" every failed cycle.
+      if (action.closeComment) await createOrUpdateCloseExplanationComment(env, ctx.installationId, ctx.repoFullName, ctx.pullNumber, action.closeComment, action.closeKind);
       await closePullRequest(env, ctx.installationId, ctx.repoFullName, ctx.pullNumber);
       return;
     case "update_branch": {
diff --git a/test/unit/agent-action-executor.test.ts b/test/unit/agent-action-executor.test.ts
index 802146a03f..31978bec33 100644
--- a/test/unit/agent-action-executor.test.ts
+++ b/test/unit/agent-action-executor.test.ts
@@ -10,6 +10,12 @@ vi.mock("../../src/github/pr-actions", () => ({
   updatePullRequestBranch: vi.fn(async () => undefined),
   dismissLatestBotApproval: vi.fn(async () => ({ dismissed: true })),
 }));
+vi.mock("../../src/github/comments", async (importOriginal) => ({
+  ...(await importOriginal<typeof import("../../src/github/comments")>()),
+  // #8803: the close-explanation comment now routes through the marker helper; mock it like the sibling
+  // pr-actions primitives so no real fetch happens and call sites can assert on it.
+  createOrUpdateCloseExplanationComment: vi.fn(async () => ({ id: 2, changed: true })),
+}));
 vi.mock("../../src/github/labels", () => ({
   ensurePullRequestLabel: vi.fn(async () => ({ applied: true, created: false })),
   removePullRequestLabel: vi.fn(async () => undefined),
@@ -58,6 +64,7 @@ import { ensurePullRequestAssignee } from "../../src/github/assignees";
 import { fetchPullRequestFreshness } from "../../src/github/pr-freshness";
 import { createInstallationToken } from "../../src/github/app";
 import { fetchLiveCiAggregate, fetchLivePullRequestMergeState, fetchLivePullRequestState, fetchLiveReviewThreadBlockers, refreshInstallationHealthForInstallation } from "../../src/github/backfill";
+import { createOrUpdateCloseExplanationComment } from "../../src/github/comments";
 import {
   actionParams,
   applyModerationEscalationForRule,
@@ -192,7 +199,8 @@ describe("executeAgentMaintenanceActions (#778 gate stack)", () => {
     // its own — a live sweep's approve plans no explicit pin, so this is the unpinned/live-sweep case (#2262).
     expect(createPullRequestReview).toHaveBeenCalledWith(env, 123, "owner/repo", 7, "APPROVE", "lgtm", "sha7");
     expect(mergePullRequest).toHaveBeenCalledWith(env, 123, "owner/repo", 7, { mergeMethod: "squash", sha: "sha7" });
-    expect(createIssueComment).toHaveBeenCalledWith(env, 123, "owner/repo", 7, "closing");
+    // #8803: the close explanation routes through the idempotent marker helper (closeKind-scoped marker).
+    expect(createOrUpdateCloseExplanationComment).toHaveBeenCalledWith(env, 123, "owner/repo", 7, "closing", undefined);
     expect(closePullRequest).toHaveBeenCalledWith(env, 123, "owner/repo", 7);
     expect(updatePullRequestBranch).toHaveBeenCalledWith(env, 123, "owner/repo", 7, "sha7");
     expect(fetchPullRequestFreshness).toHaveBeenCalledTimes(6);
diff --git a/test/unit/agent-approval-queue.test.ts b/test/unit/agent-approval-queue.test.ts
index 35f87d4616..adef8786c0 100644
--- a/test/unit/agent-approval-queue.test.ts
+++ b/test/unit/agent-approval-queue.test.ts
@@ -1,5 +1,11 @@
 import { beforeEach, describe, expect, it, vi } from "vitest";
 
+vi.mock("../../src/github/comments", async (importOriginal) => ({
+  ...(await importOriginal<typeof import("../../src/github/comments")>()),
+  // #8803: the close-explanation comment routes through the marker helper; mock it like the pr-actions
+  // primitives below so no real fetch (marker search) happens in these suites.
+  createOrUpdateCloseExplanationComment: vi.fn(async () => ({ id: 2, changed: true })),
+}));
 vi.mock("../../src/github/pr-actions", () => ({
   createPullRequestReview: vi.fn(async () => ({ id: 1 })),
   mergePullRequest: vi.fn(async () => ({ merged: true, sha: "merged-sha" })),
diff --git a/test/unit/github-comments.test.ts b/test/unit/github-comments.test.ts
index a3299a6bc2..d21f29dad1 100644
--- a/test/unit/github-comments.test.ts
+++ b/test/unit/github-comments.test.ts
@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
-import { createOrUpdatePrIntelligenceComment, createOrUpdateVisualFollowupComment, PR_INTELLIGENCE_COMMENT_MARKER, VISUAL_FOLLOWUP_COMMENT_MARKER } from "../../src/github/comments";
+import { closeExplanationMarker, createOrUpdateCloseExplanationComment, createOrUpdatePrIntelligenceComment, createOrUpdateVisualFollowupComment, PR_INTELLIGENCE_COMMENT_MARKER, VISUAL_FOLLOWUP_COMMENT_MARKER } from "../../src/github/comments";
 import { createTestEnv } from "../helpers/d1";
 import { generatePrivateKeyPem } from "../helpers/github-app-key";
 
@@ -520,3 +520,65 @@ describe("createOrUpdateIssueCommentWithMarker repoFullName guard (#8311)", () =
     }
   });
 });
+
+describe("createOrUpdateCloseExplanationComment (#8803)", () => {
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it("first attempt POSTs the explanation with the closeKind-scoped marker embedded in the body", async () => {
+    const privateKey = await generatePrivateKeyPem();
+    let postedBody = "";
+    vi.stubGlobal("fetch", async (input: RequestInfo | URL, init?: RequestInit) => {
+      const url = input.toString();
+      const method = init?.method ?? "GET";
+      if (url.includes("/access_tokens")) return Response.json({ token: "t" });
+      if (url.includes("/issues/7/comments") && method === "GET") return Response.json([]);
+      if (url.includes("/issues/7/comments") && method === "POST") {
+        postedBody = (JSON.parse(String(init?.body)) as { body: string }).body;
+        return Response.json({ id: 55, html_url: "https://github.com/comment/55" });
+      }
+      return new Response("not found", { status: 404 });
+    });
+    const result = await createOrUpdateCloseExplanationComment(
+      createTestEnv({ GITHUB_APP_PRIVATE_KEY: privateKey }),
+      123,
+      "JSONbored/gittensory",
+      7,
+      "Closed: duplicate of #5.",
+      "heuristic",
+    );
+    expect(result?.id).toBe(55);
+    expect(postedBody).toContain(closeExplanationMarker("heuristic"));
+    expect(postedBody).toContain("Closed: duplicate of #5.");
+  });
+
+  it("a retry with the identical body SKIPS (no duplicate 'why we closed you' comment) — the #8803 incident shape", async () => {
+    const privateKey = await generatePrivateKeyPem();
+    const marker = closeExplanationMarker("blacklist");
+    const posts: string[] = [];
+    vi.stubGlobal("fetch", async (input: RequestInfo | URL, init?: RequestInit) => {
+      const url = input.toString();
+      const method = init?.method ?? "GET";
+      if (url.includes("/access_tokens")) return Response.json({ token: "t" });
+      if (url.includes("/issues/7/comments") && method === "GET") {
+        // The first attempt's comment already landed; the close then failed and this is the replan's retry.
+        return Response.json([{ id: 90, user: { login: "loopover-orb[bot]", type: "Bot" }, body: `${marker}\nClosed: policy.` }]);
+      }
+      if (method === "POST" || method === "PATCH") {
+        posts.push(method);
+        return Response.json({ id: 91 });
+      }
+      return new Response("not found", { status: 404 });
+    });
+    const env = createTestEnv({ GITHUB_APP_PRIVATE_KEY: privateKey, GITHUB_APP_SLUG: "loopover-orb" });
+    const result = await createOrUpdateCloseExplanationComment(env, 123, "JSONbored/gittensory", 7, "Closed: policy.", "blacklist");
+    expect(result?.changed).toBe(false); // byte-identical -> skip
+    expect(posts).toEqual([]); // neither a duplicate POST nor a pointless PATCH
+  });
+
+  it("distinct closeKinds use distinct markers so different close reasons keep separate canonical comments", () => {
+    expect(closeExplanationMarker("blacklist")).not.toBe(closeExplanationMarker("heuristic"));
+    expect(closeExplanationMarker(undefined)).toBe("<!-- loopover:close-explanation:close -->");
+  });
+});
diff --git a/test/unit/routes-agent-approval.test.ts b/test/unit/routes-agent-approval.test.ts
index 6fff60182e..6a66ecaa3e 100644
--- a/test/unit/routes-agent-approval.test.ts
+++ b/test/unit/routes-agent-approval.test.ts
@@ -1,5 +1,11 @@
 import { beforeEach, describe, expect, it, vi } from "vitest";
 
+vi.mock("../../src/github/comments", async (importOriginal) => ({
+  ...(await importOriginal<typeof import("../../src/github/comments")>()),
+  // #8803: the close-explanation comment routes through the marker helper; mock it like the pr-actions
+  // primitives below so no real fetch (marker search) happens in these suites.
+  createOrUpdateCloseExplanationComment: vi.fn(async () => ({ id: 2, changed: true })),
+}));
 vi.mock("../../src/github/pr-actions", () => ({
   createPullRequestReview: vi.fn(async () => ({ id: 1 })),
   mergePullRequest: vi.fn(async () => ({ merged: true, sha: "merged-sha" })),