fix(matrix): truncate reply context on code-point boundaries (#97471)

ly-wang19 · claude · web-flow · commit 55d7b5b36ca1 · 2026-06-28T11:06:36.000-07:00
truncateReplyBody used a raw value.slice(0, MAX_REPLY_BODY_LENGTH - 3)
to shorten long reply bodies. When the cut index fell between the two
UTF-16 code units of a surrogate pair (e.g. an emoji), the slice left a
lone high surrogate before the ellipsis, which renders as a broken glyph
in the reply context shown to the agent.

Replace the raw slice with sliceUtf16Safe from the plugin SDK so the
truncation never cuts inside a surrogate pair. A normal (non-astral)
body is unaffected.

Co-authored-by: ly-wang19 &lt;ly-wang19@users.noreply.github.com&gt;
Co-authored-by: Claude Opus 4.8 (1M context) &lt;noreply@anthropic.com&gt;
diff --git a/extensions/matrix/src/matrix/monitor/reply-context.test.ts b/extensions/matrix/src/matrix/monitor/reply-context.test.ts
@@ -39,6 +39,30 @@ describe("matrix reply context", () => {
     expect(result.endsWith("...")).toBe(true);
   });
 
+  it("truncates on a code-point boundary without orphaning a surrogate half", () => {
+    // Body is 496 'a' + 😀 (U+1F600, a surrogate pair at UTF-16 indices 496-497)
+    // + "bcd". Raw `.slice(0, 497)` would split the emoji and leave a lone high
+    // surrogate (\uD83D) before the ellipsis. The fix must drop the half emoji.
+    const body = `${"a".repeat(496)}😀bcd`;
+    expect(body.length).toBe(501);
+    const result = summarizeMatrixReplyEvent({
+      event_id: "$original",
+      sender: "@alice:example.org",
+      type: "m.room.message",
+      origin_server_ts: Date.now(),
+      content: {
+        msgtype: "m.text",
+        body,
+      },
+    } as MatrixRawEvent);
+    if (result === undefined) {
+      throw new Error("expected truncated reply context");
+    }
+    expect(result).toBe(`${"a".repeat(496)}...`);
+    // No dangling high surrogate should survive the truncation.
+    expect(result.includes("\uD83D")).toBe(false);
+  });
+
   it("handles media-only reply events", () => {
     expect(
       summarizeMatrixReplyEvent({
diff --git a/extensions/matrix/src/matrix/monitor/reply-context.ts b/extensions/matrix/src/matrix/monitor/reply-context.ts
@@ -1,4 +1,5 @@
 // Matrix plugin module implements reply context behavior.
+import { sliceUtf16Safe } from "openclaw/plugin-sdk/text-utility-runtime";
 import type { MatrixClient } from "../sdk.js";
 import { summarizeMatrixMessageContextEvent, trimMatrixMaybeString } from "./context-summary.js";
 import type { MatrixRawEvent } from "./types.js";
@@ -16,7 +17,7 @@ function truncateReplyBody(value: string): string {
   if (value.length <= MAX_REPLY_BODY_LENGTH) {
     return value;
   }
-  return `${value.slice(0, MAX_REPLY_BODY_LENGTH - 3)}...`;
+  return `${sliceUtf16Safe(value, 0, MAX_REPLY_BODY_LENGTH - 3)}...`;
 }
 
 export function summarizeMatrixReplyEvent(event: MatrixRawEvent): string | undefined {

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,5 @@`
`1`	`1`	`// Matrix plugin module implements reply context behavior.`
	`2`	`+import { sliceUtf16Safe } from "openclaw/plugin-sdk/text-utility-runtime";`
`2`	`3`	`import type { MatrixClient } from "../sdk.js";`
`3`	`4`	`import { summarizeMatrixMessageContextEvent, trimMatrixMaybeString } from "./context-summary.js";`
`4`	`5`	`import type { MatrixRawEvent } from "./types.js";`
`@@ -16,7 +17,7 @@ function truncateReplyBody(value: string): string {`
`16`	`17`	`if (value.length <= MAX_REPLY_BODY_LENGTH) {`
`17`	`18`	`return value;`
`18`	`19`	`}`
`19`		- return `${value.slice(0, MAX_REPLY_BODY_LENGTH - 3)}...`;
	`20`	+ return `${sliceUtf16Safe(value, 0, MAX_REPLY_BODY_LENGTH - 3)}...`;
`20`	`21`	`}`
`21`	`22`
`22`	`23`	`export function summarizeMatrixReplyEvent(event: MatrixRawEvent): string \| undefined {`