fix(agents): preserve UTF-16 boundaries in context pruning (#104404)

Co-authored-by: Vincent Koc <vincentkoc@ieee.org>

src/agents/agent-hooks/context-pruning/pruner.test.ts

@@ -357,6 +357,33 @@ describe("pruneContextMessages", () => {
     expect(result).toBe(messages);
   });
 
+  it("keeps soft-trim head and tail valid at UTF-16 boundaries", () => {
+    const largeText = `${"h".repeat(99)}😀${"m".repeat(100)}😀${"t".repeat(49)}`;
+    const result = pruneContextMessages({
+      messages: [
+        makeUser("summarize this"),
+        makeToolResult([{ type: "text", text: largeText }]),
+        makeAssistant([{ type: "text", text: "done" }]),
+      ],
+      settings: {
+        ...DEFAULT_CONTEXT_PRUNING_SETTINGS,
+        keepLastAssistants: 1,
+        softTrimRatio: 0,
+        hardClear: { ...DEFAULT_CONTEXT_PRUNING_SETTINGS.hardClear, enabled: false },
+        softTrim: { maxChars: 200, headChars: 100, tailChars: 50 },
+      },
+      ctx: CONTEXT_WINDOW_1M,
+      isToolPrunable: () => true,
+      contextWindowTokensOverride: 1,
+    });
+
+    const toolResult = result[1] as Extract<AgentMessage, { role: "toolResult" }>;
+    const textBlock = toolResult.content[0] as { type: "text"; text: string };
+    expect(textBlock.text).toBe(
+      `${"h".repeat(99)}\n...\n${"t".repeat(49)}\n\n[Tool result trimmed: kept first 100 chars and last 50 chars of 252 chars.]`,
+    );
+  });
+
   it("soft-trims image-containing tool results by replacing image blocks with placeholders", () => {
     const messages: AgentMessage[] = [
       makeUser("summarize this"),

src/agents/agent-hooks/context-pruning/pruner.ts

@@ -1,4 +1,5 @@
 /** Context-pruning planner that trims old assistant/tool content under token pressure. */
+import { sliceUtf16Safe } from "@openclaw/normalization-core/utf16-slice";
 import type { ImageContent, TextContent, ToolResultMessage } from "../../../llm/types.js";
 import { CHARS_PER_TOKEN_ESTIMATE, estimateStringChars } from "../../../utils/cjk-chars.js";
 import { dropThinkingBlocks } from "../../embedded-agent-runner/thinking.js";
@@ -100,7 +101,7 @@ function takeHeadFromJoinedText(parts: string[], maxChars: number): string {
       out += p;
       remaining -= p.length;
     } else {
-      out += p.slice(0, remaining);
+      out += sliceUtf16Safe(p, 0, remaining);
       remaining = 0;
     }
   }
@@ -119,7 +120,7 @@ function takeTailFromJoinedText(parts: string[], maxChars: number): string {
       out.push(p);
       remaining -= p.length;
     } else {
-      out.push(p.slice(p.length - remaining));
+      out.push(sliceUtf16Safe(p, -remaining));
       break;
     }
     if (remaining > 0 && i > 0) {