diff --git a/CHANGELOG.md b/CHANGELOG.md
index f2d27b49866..fb04c6b100c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -151,6 +151,7 @@ Docs: https://docs.openclaw.ai
 - Telegram/groups: in single-account setups, treat an explicit empty `accounts.<id>.groups: {}` map the same as undefined so the root `channels.telegram.groups` allowlist still applies, instead of silently dropping every group update under the default `groupPolicy: "allowlist"`. Multi-account semantics are unchanged so per-account explicit-empty groups still scope-disable a single account without affecting siblings; the explicit way to block all groups for any account remains `groupPolicy: "disabled"`. Fixes #79427. (#81030) Thanks @kinjitakabe.
 - Codex (app-server): project user-configured `mcp.servers` into new Codex thread configs, matching the codex-cli runtime's existing `-c mcp_servers=...` behavior so app-server-runtime agents see the same user MCP servers the CLI runtime already exposes. Plugin-curated apps remain attached via the separate `apps` config patch. Fixes #80814. Thanks @kinjitakabe.
 - Enforce Slack plugin approval button authorization [AI]. (#80899) Thanks @pgondhi987.
+- Mattermost: log a structured `mattermost no-visible-reply` diagnostic when a substantive (non-reasoning) final reply payload reaches `deliverMattermostReplyPayload` but the underlying `deliverTextOrMediaReply` returns `"empty"` — previously the run completed with a misleading `delivered reply to <channel>` log even though no Mattermost API send happened, masking silent completions in channel/thread contexts. No behavior change; the diagnostic surfaces the failure so operators can detect it instead of seeing the agent appear to go silent. Fixes #80501. Thanks @robbyproc87.
 - Recognize PowerShell -ec inline commands [AI]. (#80893) Thanks @pgondhi987.
 - fix(qqbot): authorize approval button callbacks [AI]. (#80892) Thanks @pgondhi987.
 - Telegram: render supported HTML tags in streamed and durable replies instead of showing literal markup. (#80977)
diff --git a/extensions/mattermost/src/mattermost/monitor.ts b/extensions/mattermost/src/mattermost/monitor.ts
index 14ff0eaebd8..c6f85814f0c 100644
--- a/extensions/mattermost/src/mattermost/monitor.ts
+++ b/extensions/mattermost/src/mattermost/monitor.ts
@@ -66,6 +66,10 @@ import {
   type MattermostEventPayload,
   type MattermostWebSocketFactory,
 } from "./monitor-websocket.js";
+import {
+  evaluateMattermostNoVisibleReply,
+  formatMattermostNoVisibleReplyLog,
+} from "./no-visible-reply-diagnostic.js";
 import { runWithReconnect } from "./reconnect.js";
 import { deliverMattermostReplyPayload } from "./reply-delivery.js";
 import type {
@@ -1680,7 +1684,7 @@ export async function monitorMattermostProvider(opts: MonitorMattermostOpts = {}
                 previewState,
                 logVerboseMessage,
                 deliverFinal: async () => {
-                  await deliverMattermostReplyPayload({
+                  const outcome = await deliverMattermostReplyPayload({
                     core,
                     cfg,
                     payload,
@@ -1696,7 +1700,19 @@ export async function monitorMattermostProvider(opts: MonitorMattermostOpts = {}
                     tableMode,
                     sendMessage: sendMessageMattermost,
                   });
-                  runtime.log?.(`delivered reply to ${to}`);
+                  const violation = evaluateMattermostNoVisibleReply({ outcome, payload });
+                  if (violation) {
+                    runtime.log?.(
+                      formatMattermostNoVisibleReplyLog({
+                        violation,
+                        to,
+                        accountId: account.accountId,
+                        agentId: route.agentId,
+                      }),
+                    );
+                  } else if (outcome !== "reasoning_skipped") {
+                    runtime.log?.(`delivered reply to ${to}`);
+                  }
                 },
               });
             },
diff --git a/extensions/mattermost/src/mattermost/no-visible-reply-diagnostic.test.ts b/extensions/mattermost/src/mattermost/no-visible-reply-diagnostic.test.ts
new file mode 100644
index 00000000000..d516a54e291
--- /dev/null
+++ b/extensions/mattermost/src/mattermost/no-visible-reply-diagnostic.test.ts
@@ -0,0 +1,140 @@
+import { describe, expect, it } from "vitest";
+import {
+  evaluateMattermostNoVisibleReply,
+  formatMattermostNoVisibleReplyLog,
+} from "./no-visible-reply-diagnostic.js";
+
+describe("evaluateMattermostNoVisibleReply", () => {
+  it("flags substantive text payloads that delivered empty (regression: #80501)", () => {
+    const violation = evaluateMattermostNoVisibleReply({
+      outcome: "empty",
+      payload: { text: "Here is the result of the work I did..." },
+    });
+    expect(violation).toStrictEqual({
+      reason: "no-visible-reply-after-final-delivery",
+      outcome: "empty",
+      finalTextLength: "Here is the result of the work I did...".length,
+      mediaUrlCount: 0,
+    });
+  });
+
+  it("flags payloads with media URLs that delivered empty (regression: #80501)", () => {
+    const violation = evaluateMattermostNoVisibleReply({
+      outcome: "empty",
+      payload: { mediaUrl: "https://example.org/a.png" },
+    });
+    expect(violation).toStrictEqual({
+      reason: "no-visible-reply-after-final-delivery",
+      outcome: "empty",
+      finalTextLength: 0,
+      mediaUrlCount: 1,
+    });
+  });
+
+  it("counts entries from mediaUrls[] alongside the singular mediaUrl", () => {
+    const violation = evaluateMattermostNoVisibleReply({
+      outcome: "empty",
+      payload: {
+        mediaUrl: "https://example.org/a.png",
+        mediaUrls: ["https://example.org/b.png", "https://example.org/c.png"],
+      },
+    });
+    expect(violation?.mediaUrlCount).toBe(3);
+  });
+
+  it("does not flag reasoning_skipped outcome (intentional suppression)", () => {
+    expect(
+      evaluateMattermostNoVisibleReply({
+        outcome: "reasoning_skipped",
+        payload: { text: "Reasoning: hidden" },
+      }),
+    ).toBeNull();
+  });
+
+  it("does not flag text outcome (visible delivery happened)", () => {
+    expect(
+      evaluateMattermostNoVisibleReply({
+        outcome: "text",
+        payload: { text: "hello" },
+      }),
+    ).toBeNull();
+  });
+
+  it("does not flag media outcome (visible delivery happened)", () => {
+    expect(
+      evaluateMattermostNoVisibleReply({
+        outcome: "media",
+        payload: { mediaUrl: "https://example.org/a.png" },
+      }),
+    ).toBeNull();
+  });
+
+  it("does not flag empty outcome when the payload was nominally empty (no text or media at all)", () => {
+    expect(
+      evaluateMattermostNoVisibleReply({
+        outcome: "empty",
+        payload: {},
+      }),
+    ).toBeNull();
+    expect(
+      evaluateMattermostNoVisibleReply({
+        outcome: "empty",
+        payload: { text: "" },
+      }),
+    ).toBeNull();
+    expect(
+      evaluateMattermostNoVisibleReply({
+        outcome: "empty",
+        payload: { text: "   \n\t  " },
+      }),
+    ).toBeNull();
+  });
+
+  it("trims whitespace when measuring finalTextLength", () => {
+    const violation = evaluateMattermostNoVisibleReply({
+      outcome: "empty",
+      payload: { text: "   hello   " },
+    });
+    expect(violation?.finalTextLength).toBe("hello".length);
+  });
+});
+
+describe("formatMattermostNoVisibleReplyLog", () => {
+  it("emits a grep-friendly single-line diagnostic with the expected key/value pairs", () => {
+    const line = formatMattermostNoVisibleReplyLog({
+      violation: {
+        reason: "no-visible-reply-after-final-delivery",
+        outcome: "empty",
+        finalTextLength: 137,
+        mediaUrlCount: 0,
+      },
+      to: "channel:town-square",
+      accountId: "default",
+      agentId: "main",
+    });
+    expect(line).toBe(
+      "mattermost no-visible-reply: no-visible-reply-after-final-delivery" +
+        " to=channel:town-square" +
+        " accountId=default" +
+        " agentId=main" +
+        " outcome=empty" +
+        " finalTextLength=137" +
+        " mediaUrlCount=0",
+    );
+  });
+
+  it("falls back to unknown when agentId is undefined", () => {
+    const line = formatMattermostNoVisibleReplyLog({
+      violation: {
+        reason: "no-visible-reply-after-final-delivery",
+        outcome: "empty",
+        finalTextLength: 1,
+        mediaUrlCount: 0,
+      },
+      to: "channel:x",
+      accountId: "y",
+      agentId: undefined,
+    });
+    expect(line).toContain("agentId=unknown");
+  });
+});
diff --git a/extensions/mattermost/src/mattermost/no-visible-reply-diagnostic.ts b/extensions/mattermost/src/mattermost/no-visible-reply-diagnostic.ts
new file mode 100644
index 00000000000..b3eca6417cd
--- /dev/null
+++ b/extensions/mattermost/src/mattermost/no-visible-reply-diagnostic.ts
@@ -0,0 +1,70 @@
+import type { ReplyPayload } from "openclaw/plugin-sdk/reply-runtime";
+import type { MattermostReplyDeliveryOutcome } from "./reply-delivery.js";
+
+export type MattermostNoVisibleReplyViolation = {
+  reason: "no-visible-reply-after-final-delivery";
+  outcome: MattermostReplyDeliveryOutcome;
+  finalTextLength: number;
+  mediaUrlCount: number;
+};
+
+function countMediaUrls(payload: ReplyPayload): number {
+  const single = typeof payload.mediaUrl === "string" && payload.mediaUrl.length > 0 ? 1 : 0;
+  const list = Array.isArray(payload.mediaUrls)
+    ? payload.mediaUrls.filter((url) => typeof url === "string" && url.length > 0).length
+    : 0;
+  return single + list;
+}
+
+/**
+ * Detects the #80501 symptom: `deliverMattermostReplyPayload` accepted a
+ * substantive (non-reasoning) payload, called the underlying
+ * `deliverTextOrMediaReply`, and the outcome was `"empty"` — meaning the
+ * payload had no text and no media to send, so no Mattermost API call
+ * happened. The agent's run completes successfully, but no visible
+ * channel/thread reply ever surfaces to the user.
+ *
+ * Returns a structured violation when the outcome is `"empty"` for a payload
+ * that nominally carried user-facing content (text or media bytes that ended
+ * up dropped by `resolveSendableOutboundReplyParts`/`sendMediaWithLeadingCaption`).
+ * Returns `null` for `"reasoning_skipped"` (intentional suppression),
+ * `"text"`, or `"media"` (successful visible sends).
+ */
+export function evaluateMattermostNoVisibleReply(params: {
+  outcome: MattermostReplyDeliveryOutcome;
+  payload: ReplyPayload;
+}): MattermostNoVisibleReplyViolation | null {
+  if (params.outcome !== "empty") {
+    return null;
+  }
+  const finalText = typeof params.payload.text === "string" ? params.payload.text.trim() : "";
+  const mediaUrlCount = countMediaUrls(params.payload);
+  // If the payload had no text and no media even nominally, the run had
+  // nothing to send and "empty" is the correct outcome — do not flag.
+  if (finalText.length === 0 && mediaUrlCount === 0) {
+    return null;
+  }
+  return {
+    reason: "no-visible-reply-after-final-delivery",
+    outcome: params.outcome,
+    finalTextLength: finalText.length,
+    mediaUrlCount,
+  };
+}
+
+export function formatMattermostNoVisibleReplyLog(params: {
+  violation: MattermostNoVisibleReplyViolation;
+  to: string;
+  accountId: string;
+  agentId: string | undefined;
+}): string {
+  return (
+    `mattermost no-visible-reply: ${params.violation.reason}` +
+    ` to=${params.to}` +
+    ` accountId=${params.accountId}` +
+    ` agentId=${params.agentId ?? "unknown"}` +
+    ` outcome=${params.violation.outcome}` +
+    ` finalTextLength=${params.violation.finalTextLength}` +
+    ` mediaUrlCount=${params.violation.mediaUrlCount}`
+  );
+}
diff --git a/extensions/mattermost/src/mattermost/reply-delivery.test.ts b/extensions/mattermost/src/mattermost/reply-delivery.test.ts
index 3699922aa5f..b253df035e1 100644
--- a/extensions/mattermost/src/mattermost/reply-delivery.test.ts
+++ b/extensions/mattermost/src/mattermost/reply-delivery.test.ts
@@ -43,7 +43,7 @@ describe("deliverMattermostReplyPayload", () => {
     const cfg = {} satisfies OpenClawConfig;
     const core = createReplyDeliveryCore();
 
-    await deliverMattermostReplyPayload({
+    const outcome = await deliverMattermostReplyPayload({
       core,
       cfg,
       payload: { text: "hidden", isReasoning: true },
@@ -57,6 +57,33 @@ describe("deliverMattermostReplyPayload", () => {
     });
 
     expect(sendMessage).not.toHaveBeenCalled();
+    expect(outcome).toBe("reasoning_skipped");
+  });
+
+  it("returns 'empty' for substantive text that produced no send (regression: #80501)", async () => {
+    const sendMessage = vi.fn(async () => undefined);
+    const cfg = {} satisfies OpenClawConfig;
+    const core = createReplyDeliveryCore();
+    // Make the markdown table converter strip the text to empty so
+    // deliverTextOrMediaReply sees an empty chunked text and returns "empty".
+    core.channel.text.convertMarkdownTables = vi.fn(() => "");
+    core.channel.text.chunkMarkdownTextWithMode = vi.fn(() => []);
+
+    const outcome = await deliverMattermostReplyPayload({
+      core,
+      cfg,
+      payload: { text: "non-trivial input that the converter strips" },
+      to: "channel:town-square",
+      accountId: "default",
+      agentId: "agent-1",
+      replyToId: "root-post",
+      textLimit: 4000,
+      tableMode: "off",
+      sendMessage,
+    });
+
+    expect(sendMessage).not.toHaveBeenCalled();
+    expect(outcome).toBe("empty");
   });
 
   it("suppresses reasoning-prefixed payloads even without an explicit flag", async () => {
@@ -188,7 +215,7 @@ describe("deliverMattermostReplyPayload", () => {
     const core = createReplyDeliveryCore();
     core.channel.text.chunkMarkdownTextWithMode = vi.fn(() => ["hello"]);
 
-    await deliverMattermostReplyPayload({
+    const outcome = await deliverMattermostReplyPayload({
       core,
       cfg,
       payload: { text: "hello" },
@@ -207,5 +234,6 @@ describe("deliverMattermostReplyPayload", () => {
       accountId: "default",
       replyToId: "root-post",
     });
+    expect(outcome).toBe("text");
   });
 });
diff --git a/extensions/mattermost/src/mattermost/reply-delivery.ts b/extensions/mattermost/src/mattermost/reply-delivery.ts
index b34c9668be1..a7c12a73e22 100644
--- a/extensions/mattermost/src/mattermost/reply-delivery.ts
+++ b/extensions/mattermost/src/mattermost/reply-delivery.ts
@@ -21,6 +21,14 @@ type SendMattermostMessage = (
   },
 ) => Promise<unknown>;
 
+/**
+ * Result of `deliverMattermostReplyPayload`. Callers in `monitor.ts` use this
+ * to distinguish a successful visible send from an intentionally suppressed
+ * reasoning payload from a substantive payload that ended up sending nothing
+ * (the silent-completion symptom in #80501).
+ */
+export type MattermostReplyDeliveryOutcome = "reasoning_skipped" | "empty" | "text" | "media";
+
 export async function deliverMattermostReplyPayload(params: {
   core: PluginRuntime;
   cfg: OpenClawConfig;
@@ -32,9 +40,9 @@ export async function deliverMattermostReplyPayload(params: {
   textLimit: number;
   tableMode: MarkdownTableMode;
   sendMessage: SendMattermostMessage;
-}): Promise<void> {
+}): Promise<MattermostReplyDeliveryOutcome> {
   if (isReasoningReplyPayload(params.payload)) {
-    return;
+    return "reasoning_skipped";
   }
   const reply = resolveSendableOutboundReplyParts(params.payload, {
     text: params.core.channel.text.convertMarkdownTables(
@@ -48,7 +56,7 @@ export async function deliverMattermostReplyPayload(params: {
     "mattermost",
     params.accountId,
   );
-  await deliverTextOrMediaReply({
+  return await deliverTextOrMediaReply({
     payload: params.payload,
     text: reply.text,
     chunkText: (value) =>