diff --git a/src/auto-reply/fallback-state.test.ts b/src/auto-reply/fallback-state.test.ts
index 67dbfa9728b..cc60b18b050 100644
--- a/src/auto-reply/fallback-state.test.ts
+++ b/src/auto-reply/fallback-state.test.ts
@@ -113,6 +113,15 @@ describe("fallback-state", () => {
     expect(resolved.reasonSummary).toContain("Claude Max usage limit reached");
   });
 
+  it("keeps truncated transient error details UTF-16 safe", () => {
+    const detail = "x".repeat(68);
+    const resolved = resolveDemoFallbackTransition({
+      attempts: [{ ...baseAttempt, error: `429 ${detail}😀tail` }],
+    });
+
+    expect(resolved.reasonSummary).toBe(`HTTP 429: ${detail}…`);
+  });
+
   it("refreshes reason when fallback remains active with same model pair", () => {
     const resolved = resolveDemoFallbackTransition({
       attempts: [{ ...baseAttempt, reason: "timeout" }],
diff --git a/src/auto-reply/fallback-state.ts b/src/auto-reply/fallback-state.ts
index 9f3146b210a..e443c243f4d 100644
--- a/src/auto-reply/fallback-state.ts
+++ b/src/auto-reply/fallback-state.ts
@@ -1,5 +1,6 @@
 /** Formats model-fallback notice state for UI/status messages and persisted transition tracking. */
 import { normalizeOptionalString } from "@openclaw/normalization-core/string-coerce";
+import { truncateUtf16Safe } from "@openclaw/normalization-core/utf16-slice";
 import { formatRawAssistantErrorForUi } from "../agents/embedded-agent-helpers.js";
 import { areRuntimeModelRefsEquivalent } from "../agents/model-runtime-aliases.js";
 import type { OpenClawConfig } from "../config/types.openclaw.js";
@@ -29,7 +30,7 @@ function truncateFallbackReasonPart(value: string, max = FALLBACK_REASON_PART_MA
   if (text.length <= max) {
     return text;
   }
-  return `${text.slice(0, Math.max(0, max - 1)).trimEnd()}…`;
+  return `${truncateUtf16Safe(text, max - 1).trimEnd()}…`;
 }
 
 function formatFallbackAttemptErrorPreview(attempt: RuntimeFallbackAttempt): string | undefined {