I'm a co-founder of Calcapp, an app builder for formula-driven apps using Excel-like formulas. I spent a couple of days using Claude Code to build 20 new templates for us, and I was blown away. It was able to one-shot most apps, generating competent, intricate apps from having looked at a sample JSON file I put together. I briefly told it about extensions we had made to Excel functions (including lambdas for FILTER, named sort type enums for XMATCH, etc), and it picked those up immediately.

At one point, it generated a verbose formula and mentioned, off-handedly, that it would have been prettier had Calcapp supported LET. "It does!", I replied, "and as an extension, you can use := instead of , to separate names and values!") and it promptly rewrote it using our extended syntax, producing a sleek formula.

These templates were for various verticals, like real estate, financial planning and retail, and I would have been hard-pressed to produce them without Claude's domain knowledge. And I did it in a weekend! Well, "we" did it in a weekend.

So this development doesn't really surprise me. I'm sure that Claude will be right at home in Excel, and I have already thought about how great it would be if Claude Code found a permanent home in our app designer. I'm concerned about the cost, though, so I'm holding off for now. But it does seem unfair that I get to use Claude to write apps with Calcapp, while our customers don't get that privilege.

(I wrote more about integrating Claude Code here: https://news.ycombinator.com/item?id=45662229)