Fix edit tool failing on files with UTF-8 BOM

Strip BOM before matching (LLM won't include invisible BOM in oldText),
restore on write.

Based on #394 by @prathamdby

packages/coding-agent/CHANGELOG.md

@@ -236,6 +236,7 @@ Total color count increased from 46 to 50. See [docs/theme.md](docs/theme.md) fo
 - **Resuming session resets thinking level to off**: Initial model and thinking level were not saved to session file, causing `--resume`/`--continue` to default to `off`. ([#342](https://github.com/badlogic/pi-mono/issues/342) by [@aliou](https://github.com/aliou))
 - **Hook `tool_result` event ignores errors from custom tools**: The `tool_result` hook event was never emitted when tools threw errors, and always had `isError: false` for successful executions. Now emits the event with correct `isError` value in both success and error cases. ([#374](https://github.com/badlogic/pi-mono/issues/374) by [@nicobailon](https://github.com/nicobailon))
 - **Edit tool fails on Windows due to CRLF line endings**: Files with CRLF line endings now match correctly when LLMs send LF-only text. Line endings are normalized before matching and restored to original style on write. ([#355](https://github.com/badlogic/pi-mono/issues/355) by [@Pratham-Dubey](https://github.com/Pratham-Dubey))
+- **Edit tool fails on files with UTF-8 BOM**: Files with UTF-8 BOM marker could cause "text not found" errors since the LLM doesn't include the invisible BOM character. BOM is now stripped before matching and restored on write. ([#394](https://github.com/badlogic/pi-mono/pull/394) by [@prathamdby](https://github.com/prathamdby))
 - **Use bash instead of sh on Unix**: Fixed shell commands using `/bin/sh` instead of `/bin/bash` on Unix systems. ([#328](https://github.com/badlogic/pi-mono/pull/328) by [@dnouri](https://github.com/dnouri))
 - **OAuth login URL clickable**: Made OAuth login URLs clickable in terminal. ([#349](https://github.com/badlogic/pi-mono/pull/349) by [@Cursivez](https://github.com/Cursivez))
 - **Improved error messages**: Better error messages when `apiKey` or `model` are missing. ([#346](https://github.com/badlogic/pi-mono/pull/346) by [@ronyrus](https://github.com/ronyrus))

packages/coding-agent/src/core/tools/edit-diff.ts

@@ -24,6 +24,11 @@ export function restoreLineEndings(text: string, ending: "\r\n" | "\n"): string
 	return ending === "\r\n" ? text.replace(/\n/g, "\r\n") : text;
 }
 
+/** Strip UTF-8 BOM if present, return both the BOM (if any) and the text without it */
+export function stripBom(content: string): { bom: string; text: string } {
+	return content.startsWith("\uFEFF") ? { bom: "\uFEFF", text: content.slice(1) } : { bom: "", text: content };
+}
+
 /**
  * Generate a unified diff string with line numbers and context.
  * Returns both the diff string and the first changed line number (in the new file).
@@ -160,7 +165,10 @@ export async function computeEditDiff(
 		}
 
 		// Read the file
-		const content = await readFile(absolutePath, "utf-8");
+		const rawContent = await readFile(absolutePath, "utf-8");
+
+		// Strip BOM before matching (LLM won't include invisible BOM in oldText)
+		const { text: content } = stripBom(rawContent);
 
 		const normalizedContent = normalizeToLF(content);
 		const normalizedOldText = normalizeToLF(oldText);

packages/coding-agent/src/core/tools/edit.ts

@@ -2,7 +2,7 @@ import type { AgentTool } from "@mariozechner/pi-agent-core";
 import { Type } from "@sinclair/typebox";
 import { constants } from "fs";
 import { access, readFile, writeFile } from "fs/promises";
-import { detectLineEnding, generateDiffString, normalizeToLF, restoreLineEndings } from "./edit-diff.js";
+import { detectLineEnding, generateDiffString, normalizeToLF, restoreLineEndings, stripBom } from "./edit-diff.js";
 import { resolveToCwd } from "./path-utils.js";
 
 const editSchema = Type.Object({
@@ -74,13 +74,16 @@ export function createEditTool(cwd: string): AgentTool<typeof editSchema> {
 						}
 
 						// Read the file
-						const content = await readFile(absolutePath, "utf-8");
+						const rawContent = await readFile(absolutePath, "utf-8");
 
 						// Check if aborted after reading
 						if (aborted) {
 							return;
 						}
 
+						// Strip BOM before matching (LLM won't include invisible BOM in oldText)
+						const { bom, text: content } = stripBom(rawContent);
+
 						const originalEnding = detectLineEnding(content);
 						const normalizedContent = normalizeToLF(content);
 						const normalizedOldText = normalizeToLF(oldText);
@@ -140,7 +143,7 @@ export function createEditTool(cwd: string): AgentTool<typeof editSchema> {
 							return;
 						}
 
-						const finalContent = restoreLineEndings(normalizedNewContent, originalEnding);
+						const finalContent = bom + restoreLineEndings(normalizedNewContent, originalEnding);
 						await writeFile(absolutePath, finalContent, "utf-8");
 
 						// Check if aborted after writing

packages/coding-agent/test/tools.test.ts

@@ -433,4 +433,18 @@ describe("edit tool CRLF handling", () => {
 			}),
 		).rejects.toThrow(/Found 2 occurrences/);
 	});
+
+	it("should preserve UTF-8 BOM after edit", async () => {
+		const testFile = join(testDir, "bom-test.txt");
+		writeFileSync(testFile, "\uFEFFfirst\r\nsecond\r\nthird\r\n");
+
+		await editTool.execute("test-bom", {
+			path: testFile,
+			oldText: "second\n",
+			newText: "REPLACED\n",
+		});
+
+		const content = readFileSync(testFile, "utf-8");
+		expect(content).toBe("\uFEFFfirst\r\nREPLACED\r\nthird\r\n");
+	});
 });