diff --git a/src/gateway/chat-display-projection.canvas.ts b/src/gateway/chat-display-projection.canvas.ts
index 641e70e5c50f..5a62618dc997 100644
--- a/src/gateway/chat-display-projection.canvas.ts
+++ b/src/gateway/chat-display-projection.canvas.ts
@@ -65,19 +65,24 @@ export function isToolResultHistoryBlockType(type: unknown): boolean {
 export function projectToolResultDetails(
   details: unknown,
   maxChars: number,
-): Record<string, unknown> | undefined {
+): { details: Record<string, unknown> | undefined; truncated: boolean } {
   const record = readRecord(details);
   if (!record) {
-    return undefined;
+    return { details: undefined, truncated: false };
   }
   const projected: Record<string, unknown> = {};
+  // The diff is the one display-capped field here; surface the fact so the
+  // message-level marker covers capped tool-result details too.
+  let truncated = false;
   for (const key of ["changed", "created"] as const) {
     if (typeof record[key] === "boolean") {
       projected[key] = record[key];
     }
   }
   if (typeof record.diff === "string" && record.diff.trim()) {
-    projected.diff = truncateChatHistoryText(record.diff, maxChars).text;
+    const diff = truncateChatHistoryText(record.diff, maxChars);
+    projected.diff = diff.text;
+    truncated = diff.truncated;
   }
   if (Array.isArray(record.approvalReviews)) {
     const reviews = record.approvalReviews
@@ -109,7 +114,7 @@ export function projectToolResultDetails(
       mcpApp: preview.mcpApp,
     };
   }
-  return Object.keys(projected).length > 0 ? projected : undefined;
+  return { details: Object.keys(projected).length > 0 ? projected : undefined, truncated };
 }
 
 export function messageHasToolResultShape(message: Record<string, unknown>): boolean {
diff --git a/src/gateway/chat-display-projection.sanitize.ts b/src/gateway/chat-display-projection.sanitize.ts
index db7fa5b4efde..7a95521d5d11 100644
--- a/src/gateway/chat-display-projection.sanitize.ts
+++ b/src/gateway/chat-display-projection.sanitize.ts
@@ -142,29 +142,34 @@ function projectChatHistoryMediaFacts(value: unknown): unknown[] | undefined {
 export function sanitizeChatHistoryContentBlock(
   block: unknown,
   opts?: { preserveExactToolPayload?: boolean; maxChars?: number },
-): { block: unknown; changed: boolean } {
+): { block: unknown; changed: boolean; truncated: boolean } {
   if (!block || typeof block !== "object") {
-    return { block, changed: false };
+    return { block, changed: false, truncated: false };
   }
   const entry = { ...(block as Record<string, unknown>) };
   let changed = false;
+  // Display-cap truncation is a fact consumers need (to fetch the full row), so
+  // it is tracked apart from `changed`, which also covers metadata stripping.
+  let truncated = false;
   const preserveExactToolPayload =
     opts?.preserveExactToolPayload === true || isToolHistoryBlockType(entry.type);
   const maxChars = opts?.maxChars ?? DEFAULT_CHAT_HISTORY_TEXT_MAX_CHARS;
   if (isToolResultHistoryBlockType(entry.type) && "details" in entry) {
     const projectedDetails = projectToolResultDetails(entry.details, maxChars);
-    if (projectedDetails) {
-      entry.details = projectedDetails;
+    if (projectedDetails.details) {
+      entry.details = projectedDetails.details;
     } else {
       delete entry.details;
     }
     changed = true;
+    truncated ||= projectedDetails.truncated;
   }
   if (typeof entry.text === "string") {
     if (!preserveExactToolPayload) {
       const res = truncateChatHistoryText(entry.text, maxChars);
       entry.text = res.text;
       changed ||= res.truncated;
+      truncated ||= res.truncated;
     }
   }
   if (typeof entry.content === "string") {
@@ -172,22 +177,26 @@ export function sanitizeChatHistoryContentBlock(
       const res = truncateChatHistoryText(entry.content, maxChars);
       entry.content = res.text;
       changed ||= res.truncated;
+      truncated ||= res.truncated;
     }
   }
   if (typeof entry.partialJson === "string" && !preserveExactToolPayload) {
     const res = truncateChatHistoryText(entry.partialJson, maxChars);
     entry.partialJson = res.text;
     changed ||= res.truncated;
+    truncated ||= res.truncated;
   }
   if (typeof entry.arguments === "string" && !preserveExactToolPayload) {
     const res = truncateChatHistoryText(entry.arguments, maxChars);
     entry.arguments = res.text;
     changed ||= res.truncated;
+    truncated ||= res.truncated;
   }
   if (typeof entry.thinking === "string") {
     const res = truncateChatHistoryText(entry.thinking, maxChars);
     entry.thinking = res.text;
     changed ||= res.truncated;
+    truncated ||= res.truncated;
   }
   if ("thinkingSignature" in entry) {
     delete entry.thinkingSignature;
@@ -199,7 +208,7 @@ export function sanitizeChatHistoryContentBlock(
   }
   const mediaChanged = projectChatHistoryMediaBlock(entry);
   changed ||= mediaChanged;
-  return { block: changed ? entry : block, changed };
+  return { block: changed ? entry : block, changed, truncated };
 }
 
 function sanitizeAssistantPhasedContentBlocks(content: unknown[]): {
@@ -372,6 +381,7 @@ export function sanitizeChatHistoryMessage(
   }
   const entry = { ...(message as Record<string, unknown>) };
   let changed = false;
+  let truncated = false;
   if ("providerReplay" in entry) {
     delete entry.providerReplay;
     changed = true;
@@ -407,17 +417,19 @@ export function sanitizeChatHistoryMessage(
     typeof entry.tool_call_id === "string";
 
   if ("details" in entry) {
-    const projectedDetails =
-      projectWorkspaceConflictDetails(entry) ??
-      (messageHasToolResultShape(entry)
+    const conflictDetails = projectWorkspaceConflictDetails(entry);
+    const toolResultDetails =
+      !conflictDetails && messageHasToolResultShape(entry)
         ? projectToolResultDetails(entry.details, maxChars)
-        : undefined);
+        : undefined;
+    const projectedDetails = conflictDetails ?? toolResultDetails?.details;
     if (projectedDetails) {
       entry.details = projectedDetails;
     } else {
       delete entry.details;
     }
     changed = true;
+    truncated ||= toolResultDetails?.truncated === true;
   }
 
   if (entry.role !== "assistant") {
@@ -464,6 +476,7 @@ export function sanitizeChatHistoryMessage(
       const res = truncateChatHistoryText(controlStripped, maxChars);
       entry.content = res.text;
       changed ||= res.truncated;
+      truncated ||= res.truncated;
     }
   } else if (Array.isArray(entry.content)) {
     const updated = entry.content.map((block) => {
@@ -486,12 +499,13 @@ export function sanitizeChatHistoryMessage(
       const text = stripSuppressedControlReplyToken(contentBlock.text);
       return text === contentBlock.text
         ? sanitized
-        : { block: { ...contentBlock, text }, changed: true };
+        : { block: { ...contentBlock, text }, changed: true, truncated: sanitized.truncated };
     });
     if (updated.some((item) => item.changed)) {
       entry.content = updated.map((item) => item.block);
       changed = true;
     }
+    truncated ||= updated.some((item) => item.truncated);
     if (entry.role === "assistant" && Array.isArray(entry.content)) {
       const mixedToolContent = projectAssistantMixedToolContent(entry.content, maxChars);
       if (mixedToolContent) {
@@ -521,9 +535,24 @@ export function sanitizeChatHistoryMessage(
       const res = truncateChatHistoryText(controlStripped, maxChars);
       entry.text = res.text;
       changed ||= res.truncated;
+      truncated ||= res.truncated;
     }
   }
 
+  if (truncated) {
+    // Record the display cap where it is applied so any session.message or
+    // chat.history consumer can tell a bounded preview from the full row and
+    // fetch it via chat.message.get. An upstream "oversized" transcript
+    // marker already explains the truncation; never overwrite its reason.
+    const meta = readRecord(entry["__openclaw"]);
+    entry["__openclaw"] = {
+      ...meta,
+      truncated: true,
+      reason: typeof meta?.reason === "string" ? meta.reason : "display-cap",
+    };
+    changed = true;
+  }
+
   return { message: changed ? entry : message, changed };
 }
 
diff --git a/src/gateway/chat-display-projection.test.ts b/src/gateway/chat-display-projection.test.ts
index 246ea168dfcd..6494bedb9407 100644
--- a/src/gateway/chat-display-projection.test.ts
+++ b/src/gateway/chat-display-projection.test.ts
@@ -366,6 +366,86 @@ describe("transcript metadata projection", () => {
       );
     }
   });
+
+  it("records a display-cap marker on every history transport when text is truncated", () => {
+    const message = { role: "assistant", content: "x".repeat(9_000), timestamp: 1 };
+    for (const messages of projectHistoryTransports(message)) {
+      const projected = messages[0] as Record<string, unknown>;
+      expect(JSON.stringify(projected.content)).toContain("...(truncated)...");
+      // Structured fact, so consumers fetch the full row via chat.message.get
+      // instead of sniffing the in-band sentinel.
+      expect(projected["__openclaw"]).toEqual({ truncated: true, reason: "display-cap" });
+    }
+  });
+
+  it("marks display-cap truncation inside content blocks and keeps existing metadata", () => {
+    const [projected] = sanitizeChatHistoryMessages(
+      [
+        {
+          role: "assistant",
+          content: [{ type: "text", text: "block text ".repeat(20) }],
+          __openclaw: { id: "message-9", senderId: "assistant-1" },
+        },
+      ],
+      16,
+    ) as Record<string, unknown>[];
+    expect(projected?.["__openclaw"]).toEqual({
+      id: "message-9",
+      senderId: "assistant-1",
+      truncated: true,
+      reason: "display-cap",
+    });
+  });
+
+  it("leaves untruncated messages without a truncation marker", () => {
+    const [projected] = sanitizeChatHistoryMessages(
+      [{ role: "assistant", content: "short", timestamp: 1 }],
+      16,
+    ) as Record<string, unknown>[];
+    expect(projected?.["__openclaw"]).toBeUndefined();
+  });
+
+  it("marks display-cap truncation of a tool-result diff on both tool-result shapes", () => {
+    const longDiff = "+line\n".repeat(40);
+    const [blockShaped, messageShaped] = sanitizeChatHistoryMessages(
+      [
+        {
+          role: "assistant",
+          content: [
+            { type: "toolResult", toolName: "edit", details: { changed: true, diff: longDiff } },
+          ],
+        },
+        { role: "toolResult", toolName: "edit", details: { changed: true, diff: longDiff } },
+      ],
+      32,
+    ) as Record<string, unknown>[];
+    for (const projected of [blockShaped, messageShaped]) {
+      expect(JSON.stringify(projected)).toContain("...(truncated)...");
+      expect(projected?.["__openclaw"]).toMatchObject({ truncated: true, reason: "display-cap" });
+    }
+  });
+
+  it("leaves a tool-result diff within the cap unmarked", () => {
+    const [projected] = sanitizeChatHistoryMessages(
+      [{ role: "toolResult", toolName: "edit", details: { changed: true, diff: "+ok" } }],
+      32,
+    ) as Record<string, unknown>[];
+    expect(projected?.["__openclaw"]).toBeUndefined();
+  });
+
+  it("does not overwrite an upstream oversized reason with display-cap", () => {
+    const [projected] = sanitizeChatHistoryMessages(
+      [
+        {
+          role: "assistant",
+          content: "still long enough to cap ".repeat(4),
+          __openclaw: { truncated: true, reason: "oversized" },
+        },
+      ],
+      16,
+    ) as Record<string, unknown>[];
+    expect(projected?.["__openclaw"]).toEqual({ truncated: true, reason: "oversized" });
+  });
 });
 
 describe("managed inbound media fact projection", () => {
diff --git a/src/gateway/server-methods/server-methods.test.ts b/src/gateway/server-methods/server-methods.test.ts
index be721368b98c..a9c926547970 100644
--- a/src/gateway/server-methods/server-methods.test.ts
+++ b/src/gateway/server-methods/server-methods.test.ts
@@ -888,7 +888,12 @@ describe("sanitizeChatHistoryMessages", () => {
     );
 
     expect(result).toEqual([
-      assistantHistoryMessage(`${prefix}\n...(truncated)...`, { timestamp: 1 }),
+      assistantHistoryMessage(`${prefix}\n...(truncated)...`, {
+        timestamp: 1,
+        // The display cap is recorded structurally so consumers need not sniff
+        // the in-band sentinel to know the row is a bounded preview.
+        __openclaw: { truncated: true, reason: "display-cap" },
+      }),
     ]);
   });
 
@@ -2074,6 +2079,7 @@ describe("projectRecentChatDisplayMessages", () => {
       assistantAudioAttachmentHistoryMessage(
         `${projectedVisibleText.slice(0, 24)}\n...(truncated)...`,
         1,
+        { __openclaw: { truncated: true, reason: "display-cap" } },
       ),
     ]);
   });
diff --git a/src/gateway/server.chat.gateway-server-chat-b.test.ts b/src/gateway/server.chat.gateway-server-chat-b.test.ts
index 1c5622648e16..5bb74205ee8e 100644
--- a/src/gateway/server.chat.gateway-server-chat-b.test.ts
+++ b/src/gateway/server.chat.gateway-server-chat-b.test.ts
@@ -6363,12 +6363,19 @@ describe("gateway server chat", () => {
 
       const historyMessages = await fetchHistoryMessages(ws, { maxChars: 5 });
       expect(JSON.stringify(historyMessages)).toContain("abcde\\n...(truncated)...");
+      // The capped row is structurally marked so a client can detect the bounded
+      // preview without sniffing the sentinel, then fetch the durable content.
+      expect(
+        (historyMessages[0] as Record<string, unknown> | undefined)?.["__openclaw"],
+      ).toMatchObject({ truncated: true, reason: "display-cap" });
 
       const full = await fetchChatMessage(ws, makeMainMessageParams("msg-full-assistant"));
       expect(full.ok).toBe(true);
       expect(full.unavailableReason).toBeUndefined();
       expect(JSON.stringify(full.message)).toContain("abcdefghij");
       expect(JSON.stringify(full.message)).not.toContain("...(truncated)...");
+      const fullMeta = (full.message as Record<string, unknown> | undefined)?.["__openclaw"];
+      expect((fullMeta as { truncated?: unknown } | undefined)?.truncated).toBeUndefined();
     });
   });
 
diff --git a/src/gateway/session-message-events.test.ts b/src/gateway/session-message-events.test.ts
index c2f1af9b763b..a06f5a7dbbfe 100644
--- a/src/gateway/session-message-events.test.ts
+++ b/src/gateway/session-message-events.test.ts
@@ -1977,6 +1977,39 @@ describe("session.message websocket events", () => {
     });
   });
 
+  test("marks display-cap truncation structurally on live session.message events", async () => {
+    const storePath = await createSessionStoreFile();
+    await writeSessionStore({
+      entries: { main: { sessionId: "sess-main", updatedAt: Date.now() } },
+      storePath,
+    });
+    const transcriptMessage = {
+      role: "assistant",
+      content: [{ type: "text", text: "x".repeat(9_000) }],
+      timestamp: Date.now(),
+    };
+    await persistSessionTranscriptTurn(
+      { agentId: "main", sessionId: "sess-main", sessionKey: "agent:main:main", storePath },
+      { messages: [{ message: transcriptMessage }], updateMode: "none" },
+    );
+
+    await withOperatorSessionSubscriber(async (ws) => {
+      const { messageEvent } = await emitTranscriptUpdateAndCollectMessageEvent({
+        ws,
+        sessionKey: "agent:main:main",
+        sessionFile: "agent:main:main",
+        message: transcriptMessage,
+        messageId: "msg-capped",
+      });
+      const payload = requireRecord(messageEvent.payload, "capped message payload");
+      const message = requireRecord(payload.message, "capped message");
+      // The preview is bounded by the display cap and says so structurally, so a
+      // non-UI consumer can fetch the full row instead of sniffing the sentinel.
+      expect(JSON.stringify(message.content)).toContain("...(truncated)...");
+      expect(message["__openclaw"]).toMatchObject({ truncated: true, reason: "display-cap" });
+    });
+  });
+
   test("prefers carried transcript sequence for live session events", async () => {
     const storePath = await createSessionStoreFile();
     await writeSessionStore({
diff --git a/ui/src/e2e/chat-message-actions.e2e.test.ts b/ui/src/e2e/chat-message-actions.e2e.test.ts
index b5383d892eb8..0fcd12ef998f 100644
--- a/ui/src/e2e/chat-message-actions.e2e.test.ts
+++ b/ui/src/e2e/chat-message-actions.e2e.test.ts
@@ -222,7 +222,14 @@ describeControlUiE2e("Control UI chat message actions", () => {
           role: "assistant",
           content: [{ type: "text", text: truncatedPreview }],
           timestamp: Date.now() + 4,
-          __openclaw: { id: "assistant-full-message", seq: 5 },
+          // The Gateway records a display-cap structurally; the sentinel alone is
+          // ordinary Markdown to the UI.
+          __openclaw: {
+            id: "assistant-full-message",
+            seq: 5,
+            truncated: true,
+            reason: "display-cap",
+          },
         },
       ],
       methodResponses: {
diff --git a/ui/src/pages/chat/components/chat-message-markdown.test.ts b/ui/src/pages/chat/components/chat-message-markdown.test.ts
new file mode 100644
index 000000000000..bf283f6932a6
--- /dev/null
+++ b/ui/src/pages/chat/components/chat-message-markdown.test.ts
@@ -0,0 +1,53 @@
+/* @vitest-environment jsdom */
+// Contract for the full-message fetch flag: the Gateway marks every display-
+// capped projection (user rows included), but the expander that consumes this
+// flag renders loaded content for assistant rows alone.
+import { describe, expect, it } from "vitest";
+import { resolveMessageActionDetails } from "./chat-message-markdown.ts";
+
+const cappedMeta = { id: "msg-1", truncated: true, reason: "display-cap" };
+
+describe("resolveMessageActionDetails full-message fetch flag", () => {
+  it.each([
+    { role: "assistant", shouldFetch: true },
+    { role: "user", shouldFetch: false },
+  ])(
+    "role=$role capped by metadata -> shouldFetchFullMessage=$shouldFetch",
+    ({ role, shouldFetch }) => {
+      const details = resolveMessageActionDetails({
+        message: { role, content: "Preview\n...(truncated)...", __openclaw: cappedMeta },
+        messageId: "msg-1",
+        canFetchFullMessage: true,
+        onReply: () => {},
+        senderLabel: role,
+      });
+      expect(details?.shouldFetchFullMessage).toBe(shouldFetch);
+    },
+  );
+
+  it("does not fetch an assistant message that merely contains the sentinel text", () => {
+    // The in-band "...(truncated)..." is ordinary Markdown to the UI; without the
+    // Gateway's structural marker it is not evidence of a display cap.
+    const details = resolveMessageActionDetails({
+      message: {
+        role: "assistant",
+        content: "Quoting a log line:\n...(truncated)...\nand continuing normally.",
+        __openclaw: { id: "msg-3" },
+      },
+      messageId: "msg-3",
+      canFetchFullMessage: true,
+      senderLabel: "assistant",
+    });
+    expect(details?.shouldFetchFullMessage).toBe(false);
+  });
+
+  it("does not fetch an untruncated assistant message", () => {
+    const details = resolveMessageActionDetails({
+      message: { role: "assistant", content: "Complete.", __openclaw: { id: "msg-2" } },
+      messageId: "msg-2",
+      canFetchFullMessage: true,
+      senderLabel: "assistant",
+    });
+    expect(details?.shouldFetchFullMessage).toBe(false);
+  });
+});
diff --git a/ui/src/pages/chat/components/chat-message-markdown.ts b/ui/src/pages/chat/components/chat-message-markdown.ts
index 483da647a389..6ccf5d3a041d 100644
--- a/ui/src/pages/chat/components/chat-message-markdown.ts
+++ b/ui/src/pages/chat/components/chat-message-markdown.ts
@@ -129,13 +129,16 @@ export function resolveMessageActionDetails(params: {
   const normalizedMessage = normalizeMessage(message);
   const role = normalizeRoleForGrouping(normalizedMessage.role);
   const previewMarkdown = resolveMessageReplyText(message);
-  // Loaded text must not erase the preview's truncation fact or collapse its disclosure.
+  // The Gateway records every display-cap truncation as __openclaw.truncated, so
+  // that marker is the whole contract: sniffing the in-band sentinel would fetch
+  // for any reply that merely contains the text. Assistant-only because the
+  // expander renders loaded content for assistant rows alone.
   const shouldFetchFullMessage = Boolean(
+    role === "assistant" &&
     canFetchFullMessage &&
     messageId &&
     !record.openclawMessageToolMirror &&
-    (transcriptMeta?.truncated === true ||
-      (role === "assistant" && previewMarkdown.includes("\n...(truncated)..."))),
+    transcriptMeta?.truncated === true,
   );
   const expansion =
     role === "assistant" && shouldFetchFullMessage && messageId
diff --git a/ui/src/pages/chat/components/chat-message.test.ts b/ui/src/pages/chat/components/chat-message.test.ts
index 0056136ec045..a85df672a24f 100644
--- a/ui/src/pages/chat/components/chat-message.test.ts
+++ b/ui/src/pages/chat/components/chat-message.test.ts
@@ -5624,7 +5624,7 @@ describe("grouped chat rendering", () => {
       {
         role: "assistant",
         content: [{ type: "text", text: preview }],
-        __openclaw: { id: "assistant-disclosure-actions", seq: 1 },
+        __openclaw: { id: "assistant-disclosure-actions", seq: 1, truncated: true },
       },
       {
         sessionKey: "agent:main:main",
@@ -5757,7 +5757,7 @@ describe("grouped chat rendering", () => {
       message: {
         role: "assistant",
         content: [{ type: "text", text: "abcde\n...(truncated)..." }],
-        __openclaw: { id: "msg-truncated-marker", seq: 1 },
+        __openclaw: { id: "msg-truncated-marker", seq: 1, truncated: true },
       },
       messageId: "msg-truncated-marker",
     },
@@ -5802,7 +5802,7 @@ describe("grouped chat rendering", () => {
         {
           role: "assistant",
           content: [{ type: "text", text: "abcde\n...(truncated)..." }],
-          __openclaw: { id: "msg-retry-error", seq: 1 },
+          __openclaw: { id: "msg-retry-error", seq: 1, truncated: true },
         },
         {
           sessionKey: "global",
@@ -5828,7 +5828,7 @@ describe("grouped chat rendering", () => {
       {
         role: "assistant",
         content: [{ type: "text", text: "abcde\n...(truncated)..." }],
-        __openclaw: { id: "msg-retry-exhausted", seq: 1 },
+        __openclaw: { id: "msg-retry-exhausted", seq: 1, truncated: true },
       },
       {
         sessionKey: "global",
@@ -5872,7 +5872,7 @@ describe("grouped chat rendering", () => {
     renderAssistantMessage(container, {
       role: "assistant",
       content: [{ type: "text", text: "abcde\n...(truncated)..." }],
-      __openclaw: { id: "msg-no-loader", seq: 1 },
+      __openclaw: { id: "msg-no-loader", seq: 1, truncated: true },
     });
 
     expect(container.querySelector(".chat-message-disclosure__toggle")).toBeNull();
diff --git a/ui/src/pages/chat/components/chat-transcript-render.test.ts b/ui/src/pages/chat/components/chat-transcript-render.test.ts
index 643704b417b9..81384c826b32 100644
--- a/ui/src/pages/chat/components/chat-transcript-render.test.ts
+++ b/ui/src/pages/chat/components/chat-transcript-render.test.ts
@@ -256,7 +256,7 @@ describe("chat transcript rendering", () => {
         {
           role: "assistant",
           content: "Preview\n...(truncated)...",
-          __openclaw: { id: "assistant-full-1" },
+          __openclaw: { id: "assistant-full-1", truncated: true },
           timestamp: 1_000,
         },
       ]),
@@ -295,7 +295,7 @@ describe("chat transcript rendering", () => {
         {
           role: "assistant",
           content: "Preview\n...(truncated)...",
-          __openclaw: { id: "assistant-retry-1" },
+          __openclaw: { id: "assistant-retry-1", truncated: true },
           timestamp: 1_000,
         },
       ]),