fix: guard against patching binary files

src/index.ts

@@ -1194,7 +1194,15 @@ async function prepareOperation(cwd: string, hunk: ParsedPatch): Promise<Prepare
 	if (hunk.type === "delete") return { hunk, absolutePath, destination, fuzz: 0 };
 	const preservedMode = sourceStat.mode & 0o777;
 	if (hunk.movePath !== undefined) await requireAbsent(destination);
-	const currentContent = await readFile(absolutePath, "utf-8");
+	// Sniff for null bytes before decoding: decoding binary as UTF-8 mangles
+	// bytes into replacement characters, so a patch could either match
+	// garbage or silently corrupt the file on write. Reject early with a
+	// distinct code instead of letting apply_patch guess at binary content.
+	const raw = await readFile(absolutePath);
+	if (raw.includes(0)) {
+		throw Object.assign(new Error(`Refusing to patch binary file: ${hunk.filePath}`), { code: "EBINARY" });
+	}
+	const currentContent = raw.toString("utf-8");
 	const result =
 		hunk.chunks.length === 0
 			? { content: currentContent, fuzz: 0 }

test/index.test.ts

@@ -4,6 +4,7 @@ import { afterEach, describe, expect, it } from "vitest";
 import {
 	APPLY_PATCH_DESCRIPTION,
 	APPLY_PATCH_LARK_GRAMMAR,
+	ApplyPatchError,
 	type ApplyPatchExtensionAPI,
 	applyPatch,
 	applyPatchDetailed,
@@ -227,6 +228,42 @@ describe("pi-apply-patch", () => {
 		expect(await readFile(path.join(directory, "sample.txt"), "utf-8")).toBe("after\n");
 	});
 
+	it("#given binary file with null byte #when applying patch #then rejects with EBINARY and preserves file", async () => {
+		// given
+		const directory = await createTempDirectory();
+		const binaryPath = path.join(directory, "binary.bin");
+		await writeFile(binaryPath, Buffer.from([0x00, 0x01, 0x02, 0xff]));
+
+		// when
+		let caught: unknown;
+		try {
+			await applyPatch(
+				directory,
+				`*** Begin Patch
+*** Update File: binary.bin
+@@
+-some
++changed
+*** End Patch`,
+			);
+		} catch (error) {
+			caught = error;
+		}
+
+		// then
+		if (!(caught instanceof ApplyPatchError)) {
+			throw new Error("Expected apply_patch to reject with ApplyPatchError");
+		}
+		const failure = caught.failures[0];
+		expect(failure).toMatchObject({
+			filePath: "binary.bin",
+			operation: "update",
+			code: "EBINARY",
+		});
+		expect(failure?.message ?? "").toMatch(/binary|non-text|not text/);
+		expect(await readFile(binaryPath)).toEqual(Buffer.from([0x00, 0x01, 0x02, 0xff]));
+	});
+
 	it("#given parent traversal path #when applying patch #then rejects outside cwd", async () => {
 		// given
 		const directory = await createTempDirectory();