I'm not sure what do you mean by "whole file format", but if it refers to the write_file tool that overwrites the whole file, there is also the replace tool which is apparently inspired by a blog post [1] by Anthropic. It seems that Claude Code also supports the roughly identical tool (inferred from error messages), so editing tools can't be the reason why Claude Code is good.
The replace tool is a form of diff (although it's rudimentary), and the read_file tool can be called with line ranges. I do wish robust patching but it is not the "whole" file reading/writing. Maybe you wanted to say about subagent file handling? I can agree then.
(Also I think Gemini is significantly better when it comes to the context rot, in my experience 100K--300K tokens were required for symptoms to appear. So burning tokens is less problematic with Gemini.)
[1] https://www.anthropic.com/engineering/swe-bench-sonnet