diff --git a/src/app/formatters/summary-formatter.ts b/src/app/formatters/summary-formatter.ts
index 7d83f2237..8f37e8843 100644
--- a/src/app/formatters/summary-formatter.ts
+++ b/src/app/formatters/summary-formatter.ts
@@ -359,6 +359,10 @@ function formatDiff(diff: string): string {
   return formattedLines.join("\n");
 }
 
+// Unlabelled .txt attachments are guessed at by the reader; without this mark Notepad
+// and Android browsers fall back to the system code page and render UTF-8 as mojibake
+const UTF8_BOM = "\uFEFF";
+
 export function prepareCodeFile(
   content: string,
   filePath: string,
@@ -386,7 +390,7 @@ export function prepareCodeFile(
       : t("tool.file_header.edit", { path: displayPath });
   const fullContent = header + processedContent;
 
-  const buffer = Buffer.from(fullContent, "utf8");
+  const buffer = Buffer.from(UTF8_BOM + fullContent, "utf8");
   const basename = path.basename(filePath);
   const filename = `${operation}_${basename}.txt`;
 
diff --git a/tests/bot/messages/summary-message-formatter.test.ts b/tests/bot/messages/summary-message-formatter.test.ts
index 76154a35a..fc43a25d1 100644
--- a/tests/bot/messages/summary-message-formatter.test.ts
+++ b/tests/bot/messages/summary-message-formatter.test.ts
@@ -349,6 +349,22 @@ describe("bot/messages/summary-message-formatter", () => {
     expect(oversized).toBeNull();
   });
 
+  it("starts file payloads with a UTF-8 BOM so readers do not guess the system code page", () => {
+    const writeFile = prepareCodeFile("const x = 1;", "src/app.ts", "write");
+    expect(writeFile?.buffer.subarray(0, 3).toString("hex")).toBe("efbbbf");
+    expect(writeFile?.buffer.subarray(3).toString("utf8")).toBe(
+      "Write File/Path: src/app.ts\n============================================================\n\nconst x = 1;",
+    );
+
+    const editFile = prepareCodeFile("@@ -1,1 +1,1 @@\n-line1\n+line2", "src/app.ts", "edit");
+    expect(editFile?.buffer.subarray(0, 3).toString("hex")).toBe("efbbbf");
+    expect(editFile?.buffer.subarray(3).toString("utf8")).toContain("Edit File/Path: src/app.ts");
+
+    const cyrillic = prepareCodeFile('const привет = "мир";', "src/app.ts", "write");
+    expect(cyrillic?.buffer.subarray(0, 3).toString("hex")).toBe("efbbbf");
+    expect(cyrillic?.buffer.subarray(3).toString("utf8")).toContain('const привет = "мир";');
+  });
+
   it("normalizes absolute paths to project-relative form", () => {
     const writeText = formatToolInfo({
       sessionId: "s1",