From adad262ba9cf4d8d9a112f98e84462d4c7157a36 Mon Sep 17 00:00:00 2001
From: can1357 <me@can.ac>
Date: Wed, 29 Jul 2026 22:37:22 +0200
Subject: [PATCH] fix(xdev): include truncation marker in summary byte cap

(cherry picked from commit aa2067bf7952191beab85b71002aafe812f544bc)
---
 packages/coding-agent/src/tools/xdev.ts         | 14 +++++++++-----
 .../test/write-xdev-dispatch.test.ts            | 17 +++++++++++------
 2 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/packages/coding-agent/src/tools/xdev.ts b/packages/coding-agent/src/tools/xdev.ts
index a65919dcf..073a412a2 100644
--- a/packages/coding-agent/src/tools/xdev.ts
+++ b/packages/coding-agent/src/tools/xdev.ts
@@ -179,8 +179,10 @@ function toolSummary(inst: Tool): string {
 	return firstLine?.trim() ?? inst.label ?? inst.name;
 }
 
-/** C0/C1 control characters; a summary must never smuggle escapes or line breaks into the prompt. */
-const SUMMARY_CONTROL_CHARS = /[\u0000-\u001f\u007f-\u009f]+/g;
+/** C0/C1 controls and Unicode line/paragraph separators; summaries must remain one line. */
+const SUMMARY_CONTROL_CHARS = /[\u0000-\u001f\u007f-\u009f\u2028\u2029]+/g;
+const SUMMARY_ELLIPSIS = "…";
+const SUMMARY_ELLIPSIS_BYTES = Buffer.byteLength(SUMMARY_ELLIPSIS, "utf-8");
 
 /**
  * Bound a catalog summary for prompt rendering. External summaries are
@@ -191,9 +193,11 @@ const SUMMARY_CONTROL_CHARS = /[\u0000-\u001f\u007f-\u009f]+/g;
  */
 function sanitizeCatalogSummary(summary: string, maxBytes?: number): string {
 	const cleaned = summary.replace(SUMMARY_CONTROL_CHARS, " ").trim();
-	if (maxBytes === undefined || maxBytes <= 0) return cleaned;
-	if (Buffer.byteLength(cleaned, "utf-8") <= maxBytes) return cleaned;
-	return `${truncateHeadBytes(cleaned, maxBytes).text.trimEnd()}…`;
+	if (maxBytes === undefined || Buffer.byteLength(cleaned, "utf-8") <= maxBytes) return cleaned;
+	if (maxBytes <= 0) return "";
+	if (maxBytes < SUMMARY_ELLIPSIS_BYTES) return truncateHeadBytes(cleaned, maxBytes).text;
+	const body = truncateHeadBytes(cleaned, maxBytes - SUMMARY_ELLIPSIS_BYTES).text.trimEnd();
+	return `${body}${SUMMARY_ELLIPSIS}`;
 }
 
 function promptCatalogSummary(inst: Tool, maxBytes?: number): string {
diff --git a/packages/coding-agent/test/write-xdev-dispatch.test.ts b/packages/coding-agent/test/write-xdev-dispatch.test.ts
index b7ba17853..fc1198597 100644
--- a/packages/coding-agent/test/write-xdev-dispatch.test.ts
+++ b/packages/coding-agent/test/write-xdev-dispatch.test.ts
@@ -302,7 +302,7 @@ describe("read and write route xd:// device URLs", () => {
 			name: "mcp__weather__forecast",
 			label: "Forecast",
 			description: "Weather forecast for a place.",
-			summary: `Napoved\u0007vremena ${multiByteTail}`,
+			summary: `Napoved\u0007\u2028vremena ${multiByteTail}`,
 			parameters: type({ query: "string" }),
 			async execute() {
 				return { content: [{ type: "text", text: "" }] };
@@ -324,16 +324,21 @@ describe("read and write route xd:// device URLs", () => {
 		const dynamic = entries.get("mcp__weather__forecast");
 		if (!dynamic) throw new Error("expected the dynamic device entry");
 		expect(dynamic.dynamic).toBe(true);
-		// Control characters collapse to a space instead of reaching the prompt.
+		// Control characters and Unicode line separators collapse to a space
+		// instead of reaching the prompt.
 		expect(dynamic.summary.startsWith("Napoved vremena ")).toBe(true);
 		expect(dynamic.summary.endsWith("…")).toBe(true);
 
 		const body = dynamic.summary.slice(0, -1);
 		const bodyBytes = Buffer.byteLength(body, "utf-8");
-		expect(bodyBytes).toBeLessThanOrEqual(XDEV_EXTERNAL_DESCRIPTION_CAP);
-		// The cut backs off at most one code point: the character straddling the
-		// budget is dropped whole rather than split.
-		expect(bodyBytes).toBeGreaterThan(XDEV_EXTERNAL_DESCRIPTION_CAP - 3);
+		const summaryBytes = Buffer.byteLength(dynamic.summary, "utf-8");
+		expect(summaryBytes).toBeLessThanOrEqual(XDEV_EXTERNAL_DESCRIPTION_CAP);
+		// The ellipsis is inside the byte budget, and the cut backs off at most
+		// one code point rather than splitting the character at the boundary.
+		expect(bodyBytes).toBeLessThanOrEqual(
+			XDEV_EXTERNAL_DESCRIPTION_CAP - Buffer.byteLength("…", "utf-8"),
+		);
+		expect(bodyBytes).toBeGreaterThan(XDEV_EXTERNAL_DESCRIPTION_CAP - 6);
 		expect(body.endsWith("あ")).toBe(true);
 		// A split code point would decode to U+FFFD and fail the round trip.
 		expect(Buffer.from(body, "utf-8").toString("utf-8")).toBe(body);