fix(edit): preserve mismatch hint size bound

steipete · steipete · commit b41df44ac58e · 2026-07-04T10:11:43.000+01:00
diff --git a/src/agents/sessions/tools/edit.test.ts b/src/agents/sessions/tools/edit.test.ts
@@ -46,7 +46,7 @@ describe("edit tool", () => {
     ).rejects.toThrow(/Current file contents:\nactual current content/);
   });
 
-  it("truncates exact-match mismatch hints without splitting unicode characters", async () => {
+  it("truncates exact-match mismatch hints without splitting UTF-16 surrogate pairs", async () => {
     const boundaryEmoji = "🙂";
     const filePath = await createTempFile(`${"a".repeat(799)}${boundaryEmoji}tail`);
     const tool = createEditTool(tmpDir);
@@ -60,7 +60,7 @@ describe("edit tool", () => {
         },
         undefined,
       ),
-    ).rejects.toThrow(`${"a".repeat(799)}${boundaryEmoji}\n... (truncated)`);
+    ).rejects.toThrow(`${"a".repeat(799)}\n... (truncated)`);
   });
 
   it("recovers success after a post-write throw when the edit already applied", async () => {
diff --git a/src/agents/sessions/tools/edit.ts b/src/agents/sessions/tools/edit.ts
@@ -11,6 +11,7 @@ import {
 } from "node:fs/promises";
 import { Box, Container, Spacer, Text } from "@earendil-works/pi-tui";
 import { Type } from "typebox";
+import { truncateUtf16Safe } from "../../../shared/utf16-slice.js";
 import { renderDiff } from "../../modes/interactive/components/diff.js";
 import type { AgentTool } from "../../runtime/index.js";
 import { textResult } from "../../tools/common.js";
@@ -173,23 +174,11 @@ function didEditLikelyApply(params: {
   );
 }
 
-function createMismatchHintSnippet(currentContent: string): string {
-  let snippet = "";
-  let length = 0;
-
-  for (const char of currentContent) {
-    if (length === EDIT_MISMATCH_HINT_LIMIT) {
-      return `${snippet}\n... (truncated)`;
-    }
-    snippet += char;
-    length += 1;
-  }
-
-  return snippet;
-}
-
 function appendMismatchHint(error: Error, currentContent: string): Error {
-  const snippet = createMismatchHintSnippet(currentContent);
+  const snippet =
+    currentContent.length <= EDIT_MISMATCH_HINT_LIMIT
+      ? currentContent
+      : `${truncateUtf16Safe(currentContent, EDIT_MISMATCH_HINT_LIMIT)}\n... (truncated)`;
   const enhanced = new Error(`${error.message}\nCurrent file contents:\n${snippet}`, {
     cause: error,
   });
