diff --git a/src/process/supervisor/supervisor.test.ts b/src/process/supervisor/supervisor.test.ts
index bba18d2d0df1..28b32dbbcedb 100644
--- a/src/process/supervisor/supervisor.test.ts
+++ b/src/process/supervisor/supervisor.test.ts
@@ -345,21 +345,25 @@ describe("process supervisor", () => {
     expect(exit.stdout).toBe("");
   });
 
-  it("bounds retained stdout and stderr while streaming full chunks", async () => {
+  it("bounds retained output on UTF-16 boundaries while streaming full chunks", async () => {
     const adapter = createStubChildAdapter();
     createChildAdapterMock.mockResolvedValue(adapter);
 
     const supervisor = createProcessSupervisor();
     let streamedStdout = "";
     let streamedStderr = "";
-    const stdoutChunk = `${"a".repeat(300)}stdout-tail`;
-    const stderrChunk = `${"b".repeat(300)}stderr-tail`;
+    const maxCapturedOutputChars = 256;
+    const stdoutMarker = `[openclaw: captured stdout truncated to last ${maxCapturedOutputChars} chars]\n`;
+    const stderrMarker = `[openclaw: captured stderr truncated to last ${maxCapturedOutputChars} chars]\n`;
+    const retainedChars = maxCapturedOutputChars - stdoutMarker.length - 1;
+    const stdoutChunk = `${"a".repeat(stdoutMarker.length)}😀${"s".repeat(retainedChars)}`;
+    const stderrChunk = `${"b".repeat(stderrMarker.length)}😀${"e".repeat(retainedChars)}`;
     const run = await spawnChild(supervisor, {
       sessionId: "s-capture-cap",
       argv: createWriteStdoutArgv(stdoutChunk),
       timeoutMs: 1_000,
       stdinMode: "pipe-closed",
-      maxCapturedOutputChars: 256,
+      maxCapturedOutputChars,
       onStdout: (chunk) => {
         streamedStdout += chunk;
       },
@@ -375,11 +379,7 @@ describe("process supervisor", () => {
     const exit = await run.wait();
     expect(streamedStdout).toBe(stdoutChunk);
     expect(streamedStderr).toBe(stderrChunk);
-    expect(exit.stdout.length).toBeLessThanOrEqual(256);
-    expect(exit.stderr.length).toBeLessThanOrEqual(256);
-    expect(exit.stdout).toContain("captured stdout truncated");
-    expect(exit.stderr).toContain("captured stderr truncated");
-    expect(exit.stdout.endsWith("stdout-tail")).toBe(true);
-    expect(exit.stderr.endsWith("stderr-tail")).toBe(true);
+    expect(exit.stdout).toBe(`${stdoutMarker}${"s".repeat(retainedChars)}`);
+    expect(exit.stderr).toBe(`${stderrMarker}${"e".repeat(retainedChars)}`);
   });
 });
diff --git a/src/process/supervisor/supervisor.ts b/src/process/supervisor/supervisor.ts
index cacd82d372bc..984c3378b903 100644
--- a/src/process/supervisor/supervisor.ts
+++ b/src/process/supervisor/supervisor.ts
@@ -2,6 +2,7 @@
 import crypto from "node:crypto";
 import { performance } from "node:perf_hooks";
 import { normalizeOptionalString } from "@openclaw/normalization-core/string-coerce";
+import { sliceUtf16Safe } from "@openclaw/normalization-core/utf16-slice";
 import { getShellConfig } from "../../agents/shell-utils.js";
 import { createLazyRuntimeModule } from "../../shared/lazy-runtime.js";
 import { createChildAdapter } from "./adapters/child.js";
@@ -54,7 +55,7 @@ function appendCapturedOutput(
   }
   const marker = `[openclaw: captured ${stream} truncated to last ${maxChars} chars]\n`;
   const tailChars = Math.max(0, maxChars - marker.length);
-  return `${marker}${next.slice(-tailChars)}`;
+  return `${marker}${sliceUtf16Safe(next, -tailChars)}`;
 }
 
 function isTimeoutReason(reason: TerminationReason) {