Function previewFileChange

src/tools/executors/files.ts:140–196

(
	toolName: string,
	args: Record<string, unknown>,
	cwd: string,
)

 * fail to match); the executor surfaces the real error in that case.
 */
export function previewFileChange(
	toolName: string,
	args: Record<string, unknown>,
	cwd: string,
): string | undefined {
	try {
		if (toolName === "file_write") {
			const relPath = String(args.file_path ?? "")
			const filePath = resolveWorkspacePath(cwd, relPath)
			let oldContent = ""
			try {
				oldContent = fs.readFileSync(filePath, "utf8")
			} catch {
				// new file
			}
			const diff = unifiedDiff(oldContent, String(args.content ?? ""))
			return diff ? `${relPath}\n${diff}` : undefined
		}

		const edits: EditSpec[] =
			toolName === "file_edit"
				? [
						{
							file_path: String(args.file_path ?? ""),
							old_string: String(args.old_string ?? ""),
							new_string: String(args.new_string ?? ""),
							replace_all: parseReplaceAll(args.replace_all),
						},
					]
				: ((Array.isArray(args.edits) ? args.edits : []) as EditSpec[])
		if (edits.length === 0) return undefined

		const byFile = new Map<string, EditSpec[]>()
		for (const edit of edits) {
			const key = edit.file_path
			if (!byFile.has(key)) byFile.set(key, [])
			byFile.get(key)!.push(edit)
		}

		const parts: string[] = []
		for (const [relPath, fileEdits] of byFile) {
			const filePath = resolveWorkspacePath(cwd, relPath)
			const oldContent = fs.readFileSync(filePath, "utf8")
			let content = oldContent
			for (const edit of fileEdits) {
				const { content: next, error } = applyEdit(content, edit)
				if (error) return undefined
				content = next
			}
			const diff = unifiedDiff(oldContent, content)
			if (diff) parts.push(`${relPath}\n${diff}`)
		}
		return parts.length > 0 ? parts.join("\n") : undefined
	} catch {
		return undefined
	}
}