If you're giving GPT-5 a findAndReplace tool, this helps work around its tendency to mix up spaces/tabs

findAndReplace.ts

/**
 * Replaces old_string with new_string in the given code.
 * Validates that old_string appears exactly once to avoid ambiguous replacements.
 */
export function findAndReplace(
    code: string,
    old_string: string,
    new_string: string,
): { success: true; result: string } | { success: false; error: string } {
    let pattern = old_string.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
    let occurrences = code.match(new RegExp(pattern, "g"))?.length ?? 0;
    let searchValue: string | RegExp = old_string;

    if (occurrences === 0) {
        // Try one more time but with flexible indentation. If our more general search pattern matches exactly one place
        // in the code, we will use it to replace the matching code.
        //
        // Note: We replace with [\t ] instead of \s here because otherwise it would also match the preceding newline.
        pattern = pattern.replace(/^\s+/gm, "([\\t ]+)");
        searchValue = new RegExp(pattern, "g");
        occurrences = code.match(searchValue)?.length ?? 0;
        if (occurrences === 0) {
            return {
                success: false,
                error: `The string "${old_string}" was not found in the code.`,
            };
        }
        if (occurrences > 1) {
            return {
                success: false,
                error: `The string "${old_string}" does not appear in the code. However, a similar string with different indentation exists. Please add more context and use exact indentation to avoid ambiguous replacements.`,
            };
        }
    }

    if (occurrences > 1) {
        return {
            success: false,
            error: `The string "${old_string}" appears ${occurrences} times in the code. Please be more specific to avoid ambiguous replacements.`,
        };
    }

    return {
        success: true,
        // Note: Using a function here also avoids the default behavior of replace changing "$$" to "$".
        result: code.replace(searchValue, (_, indent) => `${typeof indent === "string" ? indent : ""}${new_string}`),
    };
}