fix(edit): skipped notebook bom sniffing

Skipped binary BOM detection for notebook-backed hashline reads so virtual cell text is serialized without a leading U+FEFF marker.

packages/coding-agent/src/edit/hashline/filesystem.ts

@@ -28,6 +28,7 @@ import { invalidateFsScanAfterWrite } from "../../tools/fs-cache-invalidation";
 import { isInternalUrlPath } from "../../tools/path-utils";
 import { enforcePlanModeWrite, resolvePlanPath, targetsLocalSandbox } from "../../tools/plan-mode-guard";
 import { canonicalSnapshotKey } from "../file-snapshot-store";
+import { isNotebookPath } from "../notebook";
 import { readEditFileText, serializeEditFileText } from "../read-file";
 import type { LspBatchRequest } from "../renderer";
 
@@ -123,8 +124,9 @@ export class HashlineFilesystem extends Filesystem {
 		return content;
 	}
 
-	async readBinary(relativePath: string): Promise<Uint8Array> {
+	async readBinary(relativePath: string): Promise<Uint8Array | undefined> {
 		const absolutePath = this.resolveAbsolute(relativePath);
+		if (isNotebookPath(absolutePath)) return undefined;
 		try {
 			return await fs.readFile(absolutePath);
 		} catch (error) {

packages/coding-agent/test/core/hashline.test.ts

@@ -128,6 +128,39 @@ describe("hashline executor", () => {
 		});
 	});
 
+	it("edits BOM-prefixed notebooks through the virtual cell text", async () => {
+		await withTempDir(async tempDir => {
+			const filePath = path.join(tempDir, "notebook.ipynb");
+			const notebook = {
+				cells: [
+					{
+						cell_type: "markdown",
+						metadata: { keep: true },
+						source: ["# Title\n"],
+					},
+				],
+				metadata: {},
+				nbformat: 4,
+				nbformat_minor: 5,
+			};
+			await Bun.write(
+				filePath,
+				new Uint8Array([0xef, 0xbb, 0xbf, ...new TextEncoder().encode(JSON.stringify(notebook))]),
+			);
+			const session = makeHashlineSession(tempDir);
+			const editableText = "# %% [markdown] cell:0\n# Title\n";
+			const sourceTag = recordFullSnapshot(getFileReadCache(session), filePath, editableText);
+			const input = `${header("notebook.ipynb", sourceTag)}\n${sameLineRange(tag(2, "# Title"))}\n${repl("# Updated")}\n`;
+
+			await executeHashlineSingle(hashlineExecuteOptions(tempDir, input, undefined, session));
+
+			const updated = await Bun.file(filePath).json();
+			expect(updated.cells).toHaveLength(1);
+			expect(updated.cells[0].source).toEqual(["# Updated\n"]);
+			expect(updated.cells[0].metadata).toEqual({ keep: true });
+		});
+	});
+
 	it("emits an actionable no-op diagnostic when the payload matches the file byte-for-byte", async () => {
 		await withTempDir(async tempDir => {
 			const filePath = path.join(tempDir, "a.ts");

packages/hashline/src/fs.ts

@@ -65,8 +65,8 @@ export abstract class Filesystem {
 	/** Read the file's full text content. Throw on missing file. */
 	abstract readText(path: string): Promise<string>;
 
-	/** Read the file's raw bytes when text decoding may hide leading bytes such as a UTF-8 BOM. */
-	readBinary?(path: string): Promise<Uint8Array>;
+	/** Read raw bytes for backends whose text is a direct decode of persisted bytes. */
+	readBinary?(path: string): Promise<Uint8Array | undefined>;
 
 	/** Validate that `path` is writable before a prepared batch starts committing. */
 	async preflightWrite(_path: string, _options?: PreflightWriteOptions): Promise<void> {}