fix(gateway): keep chat history display text truncation surrogate-safe (#102470)

* fix(gateway): keep chat history display text truncation surrogate-safe

truncateChatHistoryText uses slice(0, N) to truncate chat history text
for the Control UI. When the truncation boundary falls inside a UTF-16
surrogate pair (emoji, CJK extended), the resulting string contains a
dangling surrogate that browsers render as U+FFFD (�).

Replace slice(0, maxChars) with truncateUtf16Safe(text, maxChars) so
the truncation point always falls on a complete code-point boundary.

* test(gateway): cover chat display UTF-16 boundary

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/gateway/chat-display-projection.ts

@@ -4,6 +4,7 @@ import { createHash } from "node:crypto";
 import { asFiniteNumber } from "@openclaw/normalization-core/number-coercion";
 import { asOptionalRecord as readRecord } from "@openclaw/normalization-core/record-coerce";
 import { normalizeOptionalString } from "@openclaw/normalization-core/string-coerce";
+import { truncateUtf16Safe } from "@openclaw/normalization-core/utf16-slice";
 import { OPENCLAW_RUNTIME_CONTEXT_CUSTOM_TYPE } from "../agents/internal-runtime-context.js";
 import { STREAM_ERROR_FALLBACK_TEXT } from "../agents/stream-message-shared.js";
 import { isHeartbeatOkResponse, isHeartbeatUserMessage } from "../auto-reply/heartbeat-filter.js";
@@ -57,7 +58,7 @@ function truncateChatHistoryText(
     return { text, truncated: false };
   }
   return {
-    text: `${text.slice(0, maxChars)}\n...(truncated)...`,
+    text: `${truncateUtf16Safe(text, maxChars)}\n...(truncated)...`,
     truncated: true,
   };
 }

src/gateway/server-methods/server-methods.test.ts

@@ -660,7 +660,6 @@ describe("waitForAgentJob", () => {
     expect(agentJobTesting.getAgentRunCacheSize()).toBe(max);
     agentJobTesting.resetAgentRunCache();
   });
-
 });
 
 describe("augmentChatHistoryWithCanvasBlocks", () => {
@@ -824,6 +823,28 @@ describe("injectTimestamp", () => {
 });
 
 describe("sanitizeChatHistoryMessages", () => {
+  it("truncates display text without splitting surrogate pairs", () => {
+    const prefix = "a".repeat(7);
+    const result = sanitizeChatHistoryMessages(
+      [
+        {
+          role: "assistant",
+          content: [{ type: "text", text: `${prefix}😀tail` }],
+          timestamp: 1,
+        },
+      ],
+      8,
+    );
+
+    expect(result).toEqual([
+      {
+        role: "assistant",
+        content: [{ type: "text", text: `${prefix}\n...(truncated)...` }],
+        timestamp: 1,
+      },
+    ]);
+  });
+
   it("redacts base64 audio content blocks from chat history", () => {
     const data = Buffer.from("voice-bytes").toString("base64");
     const result = sanitizeChatHistoryMessages([