fix(agents): keep tool_search_code stderr tail UTF-16 safe (#104767)

* fix(agents): keep tool_search_code stderr tail UTF-16 safe

* test(agents): add real child-exit UTF-16 safety proof

* test(agents): focus stderr UTF-16 regression

---------

Co-authored-by: chengzhichao-xydt <chengzhichao-xydt@users.noreply.github.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/agents/tool-search.stream-errors.test.ts

@@ -87,4 +87,47 @@ describe("tool-search code-mode stream errors", () => {
     });
     expect(spawnedChild?.kill).toHaveBeenCalledOnce();
   });
+
+  it("keeps stderr tail in exit error messages valid at UTF-16 boundaries", async () => {
+    toolSearch.testing.setToolSearchCodeModeSupportedForTest(true);
+    toolSearch.testing.setToolSearchMinCodeTimeoutMsForTest(1000);
+
+    spawnMock.mockImplementationOnce(
+      (_command: string, _args: readonly string[], _options: SpawnOptions): ChildProcess => {
+        const { child, stderr } = createMockSpawnChild();
+        process.nextTick(() => {
+          stderr?.emit("data", `${"a".repeat(500)}😀${"a".repeat(499)}`);
+          process.nextTick(() => {
+            child.emit("exit", 1, null);
+          });
+        });
+        return child as unknown as ChildProcess;
+      },
+    );
+
+    const runtime = new toolSearch.ToolSearchRuntime(
+      {},
+      toolSearch.testing.resolveToolSearchConfig({}),
+    );
+
+    let caught: Error | undefined;
+    try {
+      await toolSearch.testing.runCodeModeChild({
+        code: "return 1;",
+        config: toolSearch.testing.resolveToolSearchConfig({}),
+        logs: [],
+        parentToolCallId: "call-stderr-utf16",
+        runtime,
+      });
+    } catch (error) {
+      caught = error instanceof Error ? error : new Error(String(error));
+    }
+
+    expect(caught).toBeDefined();
+    expect(caught?.message).toMatch(/tool_search_code child exited with 1/);
+    expect(caught?.message).not.toMatch(
+      /[\uD800-\uDBFF](?![\uDC00-\uDFFF])|(?<![\uD800-\uDBFF])[\uDC00-\uDFFF]/,
+    );
+    expect(caught?.message.endsWith("a".repeat(499))).toBe(true);
+  });
 });

src/agents/tool-search.ts

@@ -11,7 +11,7 @@ import {
   uniqueStrings,
   uniqueValues,
 } from "@openclaw/normalization-core/string-normalization";
-import { truncateUtf16Safe } from "@openclaw/normalization-core/utf16-slice";
+import { sliceUtf16Safe, truncateUtf16Safe } from "@openclaw/normalization-core/utf16-slice";
 import { Type } from "typebox";
 import type { OpenClawConfig } from "../config/types.openclaw.js";
 import { getPluginToolMeta, type PluginToolMcpMeta } from "../plugins/tools.js";
@@ -2199,7 +2199,7 @@ function runCodeModeChild(params: {
       }
       const rejectOnExit = () => {
         const suffix = stderrTail.trim();
-        const detail = suffix ? `: ${suffix.slice(-500)}` : "";
+        const detail = suffix ? `: ${sliceUtf16Safe(suffix, -500)}` : "";
         settle(() =>
           reject(
             new Error(