fix(tasks): keep completion failure reason truncation UTF-16 safe

wm0018 · wm0018 · commit 92b3540a3bbc · 2026-07-07T19:37:06.000+08:00
String.prototype.slice at offset 159 can split surrogate pairs in
task completion failure reasons, producing broken U+FFFD in terminal
output and task registry views.

Replace raw slice(0, 159) with truncateUtf16Safe.
diff --git a/src/tasks/task-completion-contract.test.ts b/src/tasks/task-completion-contract.test.ts
@@ -0,0 +1,29 @@
+// Task completion contract tests.
+import { expect, describe, it } from "vitest";
+import { truncateUtf16Safe } from "../utils.js";
+
+describe("normalizeCompletionFailureReason truncation", () => {
+  it("does not cut failure reason with an emoji straddling the 159-char boundary", () => {
+    // "🚀" is a surrogate pair (2 UTF-16 code units). A failure reason with
+    // 158 'z' + the emoji has 160 code units. slice(0,159) splits the pair;
+    // truncateUtf16Safe backs out to 158 code points.
+    const text = "z".repeat(158) + "🚀";
+    // slice splits the surrogate pair at position 159
+    const sliced = text.slice(0, 159);
+    expect(sliced.charCodeAt(158)).toBe(0xd83d); // lone high surrogate
+    // truncateUtf16Safe drops the incomplete pair
+    expect(truncateUtf16Safe(text, 159)).toBe("z".repeat(158));
+  });
+
+  it("preserves failure reason text shorter than the limit unchanged", () => {
+    expect(truncateUtf16Safe("task blocked: no provider", 159)).toBe("task blocked: no provider");
+    expect(truncateUtf16Safe("", 159)).toBe("");
+  });
+
+  it("truncates long failure reasons without broken surrogates", () => {
+    const long = "a".repeat(200);
+    const result = truncateUtf16Safe(long, 159);
+    expect(result).toBe("a".repeat(159));
+    expect(result.length).toBe(159);
+  });
+});
diff --git a/src/tasks/task-completion-contract.ts b/src/tasks/task-completion-contract.ts
@@ -1,3 +1,4 @@
+import { truncateUtf16Safe } from "../utils.js";
 // Defines task terminal outcome contracts used by completion handling.
 import type { TaskTerminalOutcome } from "./task-registry.types.js";
 
@@ -25,7 +26,7 @@ function normalizeCompletionFailureReason(value: string | null | undefined): str
   if (!normalized) {
     return "";
   }
-  return normalized.length <= 160 ? normalized : `${normalized.slice(0, 159)}...`;
+  return normalized.length <= 160 ? normalized : `${truncateUtf16Safe(normalized, 159)}...`;
 }
 
 function matchesProgressOnlyPrefix(value: string): boolean {

Original file line number	Diff line number	Diff line change
`@@ -1,3 +1,4 @@`
	`1`	`+import { truncateUtf16Safe } from "../utils.js";`
`1`	`2`	`// Defines task terminal outcome contracts used by completion handling.`
`2`	`3`	`import type { TaskTerminalOutcome } from "./task-registry.types.js";`
`3`	`4`
`@@ -25,7 +26,7 @@ function normalizeCompletionFailureReason(value: string \| null \| undefined): str`
`25`	`26`	`if (!normalized) {`
`26`	`27`	`return "";`
`27`	`28`	`}`
`28`		- return normalized.length <= 160 ? normalized : `${normalized.slice(0, 159)}...`;
	`29`	+ return normalized.length <= 160 ? normalized : `${truncateUtf16Safe(normalized, 159)}...`;
`29`	`30`	`}`
`30`	`31`
`31`	`32`	`function matchesProgressOnlyPrefix(value: string): boolean {`