fix(agents): keep web search error serialization surrogate-safe (#111327)

* fix(agents): keep web search error serialization surrogate-safe

* test(agents): pin web search truncation prefix

Preserve the pre-existing provider_error fallback while proving the exact UTF-16-safe retained prefix.\n\nCo-authored-by: zenglingbiao <zeng.lingbiao@xydigit.com>

---------

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

src/agents/tools/web-search-output.ts

@@ -8,6 +8,7 @@
  * spoof the trust marker and transport-specific extras never reach the model.
  */
 import { isRecord } from "@openclaw/normalization-core/record-coerce";
+import { truncateUtf16Safe } from "@openclaw/normalization-core/utf16-slice";
 import type { Static } from "typebox";
 import { Type } from "typebox";
 import { wrapWebContent } from "../../security/external-content.js";
@@ -211,7 +212,7 @@ export function normalizeWebSearchOutput(params: {
     const rawError =
       typeof result.error === "string"
         ? result.error
-        : (JSON.stringify(result.error)?.slice(0, 2_000) ?? "provider_error");
+        : truncateUtf16Safe(JSON.stringify(result.error) ?? "provider_error", 2_000);
     const rawMessage = typeof result.message === "string" ? result.message : rawError;
     const docs = typeof result.docs === "string" ? toHttpUrl(result.docs) : undefined;
     return {

src/agents/tools/web-search.test.ts

@@ -721,6 +721,23 @@ describe("web_search normalized output contract", () => {
     expect(normalized.message).toContain("quota exceeded");
   });
 
+  it("keeps structured provider error serialization surrogate-safe at the cap", () => {
+    const normalized = normalizeWebSearchOutput({
+      provider: "external-demo",
+      query: "emoji error",
+      // The emoji straddles the 2000-unit cut: a plain slice would keep only
+      // its high surrogate half.
+      result: { error: { message: `${"x".repeat(1_987)}🤖` } },
+    });
+
+    if (normalized.kind !== "error") {
+      throw new Error("expected error branch");
+    }
+    const expectedPrefix = `{"message":"${"x".repeat(1_987)}`;
+    expect(stripWrapMarkers(normalized.message)).toBe(expectedPrefix);
+    expect(expectedPrefix).toHaveLength(1_999);
+  });
+
   it("keeps ordinary Source attribution lines in answer content", () => {
     const normalized = normalizeWebSearchOutput({
       provider: "external-answer",