diff --git a/extensions/mattermost/src/__traces__/final-then-error.trace.jsonl b/extensions/mattermost/src/__traces__/final-then-error.trace.jsonl
new file mode 100644
index 000000000000..7847c544d922
--- /dev/null
+++ b/extensions/mattermost/src/__traces__/final-then-error.trace.jsonl
@@ -0,0 +1,8 @@
+{"seq":1,"at":0,"dir":"in","kind":"reply-start"}
+{"seq":2,"at":0,"dir":"in","kind":"partial","data":{"text":"Successful assistant"}}
+{"seq":3,"at":0,"dir":"out","kind":"POST /posts","data":{"payload":{"channel_id":"channel-trace","message":"Successful assistant","root_id":"root-trace"},"result":{"id":"post-1"},"target":"channel-trace"}}
+{"seq":4,"at":1200,"dir":"in","kind":"final","data":{"text":"Successful assistant final"}}
+{"seq":5,"at":1200,"dir":"out","kind":"PUT /posts/post-1","data":{"payload":{"id":"post-1","message":"Successful assistant final"},"result":{"id":"post-1"},"target":"post-1"}}
+{"seq":6,"at":1200,"dir":"in","kind":"final","data":{"isError":true,"text":"Tool error warning"}}
+{"seq":7,"at":1200,"dir":"out","kind":"POST /posts","data":{"payload":{"channel_id":"channel-trace","message":"Tool error warning","root_id":"root-trace"},"result":{"id":"post-2"},"target":"channel-trace"}}
+{"seq":8,"at":1200,"dir":"in","kind":"idle"}
diff --git a/extensions/mattermost/src/delivery-trace.test.ts b/extensions/mattermost/src/delivery-trace.test.ts
index b031f9506d55..0f7c89f29c50 100644
--- a/extensions/mattermost/src/delivery-trace.test.ts
+++ b/extensions/mattermost/src/delivery-trace.test.ts
@@ -12,7 +12,7 @@ import {
   expectDeliveryTraceMatchesGolden,
   runDeliveryTraceScenario,
   type DeliveryTraceInStep,
-  type DeliveryTraceScenarioName,
+  type DeliveryTraceScenario,
   type WireRecorder,
 } from "openclaw/plugin-sdk/channel-contract-testing";
 import type { OpenClawConfig, PluginRuntime } from "openclaw/plugin-sdk/core";
@@ -251,21 +251,32 @@ function setupMattermostTrace(recorder: WireRecorder) {
   };
 }
 
-const MATTERMOST_TRACE_SCENARIOS: readonly DeliveryTraceScenarioName[] = [
-  "streaming-happy",
-  "final-only",
-  "cancel-mid-stream",
+const MATTERMOST_TRACE_SCENARIOS: readonly DeliveryTraceScenario[] = [
+  deliveryTraceScenarios["streaming-happy"],
+  deliveryTraceScenarios["final-only"],
+  deliveryTraceScenarios["cancel-mid-stream"],
+  {
+    name: "final-then-error",
+    steps: [
+      { kind: "reply-start" },
+      { kind: "partial", text: "Successful assistant" },
+      { kind: "advance", ms: DRAFT_THROTTLE_MS },
+      { kind: "final", text: "Successful assistant final" },
+      { kind: "final", text: "Tool error warning", isError: true },
+      { kind: "idle" },
+    ],
+  },
 ];
 
 describe("mattermost delivery trace goldens", () => {
-  for (const scenarioName of MATTERMOST_TRACE_SCENARIOS) {
-    it(`records ${scenarioName}`, async () => {
+  for (const scenario of MATTERMOST_TRACE_SCENARIOS) {
+    it(`records ${scenario.name}`, async () => {
       const events = await runDeliveryTraceScenario({
-        scenario: deliveryTraceScenarios[scenarioName],
+        scenario,
         setup: setupMattermostTrace,
       });
       expectDeliveryTraceMatchesGolden({
-        goldenUrl: new URL(`./__traces__/${scenarioName}.trace.jsonl`, import.meta.url),
+        goldenUrl: new URL(`./__traces__/${scenario.name}.trace.jsonl`, import.meta.url),
         events,
       });
     });
diff --git a/extensions/mattermost/src/mattermost/monitor-draft-delivery.ts b/extensions/mattermost/src/mattermost/monitor-draft-delivery.ts
index bea876734f15..cdd86fd681c0 100644
--- a/extensions/mattermost/src/mattermost/monitor-draft-delivery.ts
+++ b/extensions/mattermost/src/mattermost/monitor-draft-delivery.ts
@@ -14,6 +14,7 @@ import { canFinalizeMattermostPreviewInPlace } from "./monitor-context.js";
 import type { ChatType, ReplyPayload } from "./runtime-api.js";
 
 export type MattermostDraftPreviewState = {
+  /** True once the preview is the durable final post and must not be reused as a draft. */
   finalizedViaPreviewPost: boolean;
 };
 
@@ -48,13 +49,19 @@ export async function deliverMattermostReplyWithDraftPreview(
     kind: params.info.kind,
     payload: params.payload,
     adapter: defineFinalizableLivePreviewAdapter<ReplyPayload, string, { message: string }>({
-      draft: {
-        flush: params.draftStream.flush,
-        clear: params.draftStream.clear,
-        discardPending: params.draftStream.discardPending,
-        seal: params.draftStream.seal,
-        id: params.draftStream.postId,
-      },
+      // Once the preview is finalized, later payloads must use durable sends.
+      // Reusing the sealed draft would clear and delete the successful final post.
+      ...(params.previewState.finalizedViaPreviewPost
+        ? {}
+        : {
+            draft: {
+              flush: params.draftStream.flush,
+              clear: params.draftStream.clear,
+              discardPending: params.draftStream.discardPending,
+              seal: params.draftStream.seal,
+              id: params.draftStream.postId,
+            },
+          }),
       buildFinalEdit: (payload) => {
         const hasMedia = Boolean(payload.mediaUrl) || (payload.mediaUrls?.length ?? 0) > 0;
         const ttsSupplement = getReplyPayloadTtsSupplement(payload);
diff --git a/extensions/mattermost/src/mattermost/monitor.test.ts b/extensions/mattermost/src/mattermost/monitor.test.ts
index 5515c386a2d4..e6fea71a70f0 100644
--- a/extensions/mattermost/src/mattermost/monitor.test.ts
+++ b/extensions/mattermost/src/mattermost/monitor.test.ts
@@ -348,6 +348,40 @@ describe("deliverMattermostReplyWithDraftPreview", () => {
     expect(recordThreadParticipation).toHaveBeenCalledTimes(1);
   });
 
+  it("keeps a finalized preview when a later tool warning is delivered", async () => {
+    const draftStream = createDraftStreamMock();
+    const deliverFinal = vi.fn(async () => {});
+    const previewState = { finalizedViaPreviewPost: false };
+    const params = {
+      kind: "direct" as const,
+      client: createMattermostClientMock(),
+      draftStream,
+      resolvePreviewFinalText: (text?: string) => text?.trim(),
+      previewState,
+      logVerboseMessage: vi.fn(),
+      deliverPayload: deliverFinal,
+    };
+
+    await deliverMattermostReplyWithDraftPreview({
+      ...params,
+      payload: { text: "Successful assistant final" } as never,
+      info: { kind: "final" },
+    });
+    await deliverMattermostReplyWithDraftPreview({
+      ...params,
+      payload: { text: "Tool error warning", isError: true } as never,
+      info: { kind: "final" },
+    });
+
+    expect(previewState.finalizedViaPreviewPost).toBe(true);
+    expect(deliverFinal).toHaveBeenCalledExactlyOnceWith({
+      text: "Tool error warning",
+      isError: true,
+    });
+    expect(draftStream.discardPending).not.toHaveBeenCalled();
+    expect(draftStream.clear).not.toHaveBeenCalled();
+  });
+
   it("deletes the preview after a successful normal final send", async () => {
     const draftStream = createDraftStreamMock();
     const deliverFinal = vi.fn(async () => {});