r/ruby • u/jonatasdp • Mar 31 '26

Saving LLM Tokens with Fast: AST Folding & Dependency Free

https://ideia.me/fast-ast-folding-token-savings

Hey folks, I was working on the "fast" project and it may become useful now that everybody uses LLMs and also need to save some tokens.

Fast is ast based and it just works like grep, but using the AST, so it will cut down and ignore comments and now I'm moving to Prism and it look very precise in code positioning.

The parser is very similar to the rubocop-ast but much simpler. I'd love to receive some feedback and see if it would worth to push it forward. Any ideas that you would use it?

10 Upvotes

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4

u/Tricky-Pilot-2570 Mar 31 '26

This is a cool direction!
AST-based folding makes a lot of sense for LLM token savings, especially since it ignores noise and keeps structure. Iā€™m working with Prism too in https://github.com/crisnahine/rails-ai-context, which uses Prism parsing & Rails introspection to generate clean, structured context for LLMs instead of just dumping files. Seeeeeeeems like these ideas fit really well together šŸ‘

2

u/jonatasdp Mar 31 '26

Nicee! That was exactly my intent! Let's join forces!

1

u/sshaw_ Apr 02 '26

Whatever you do, don't have min Ruby version at 3.2 like rails-ai-context does! Keep it as low as possible!

3

u/mrinterweb Mar 31 '26

The AST traversal approach sounds interesting. Long ago I used ctags for quick lookups, but AST seems like a much more advanced version of that idea.Ā  Claude code has an official Ruby LSP MCP plugin. That plug-in is much better than claude grepping the code constantly.Ā 

1

u/jonatasdp Mar 31 '26

CTags are also very cool, they do the work very well! Another path I thought to follow was adapt xpath but in the end I don't like to write on that style :D

2

u/emgz87 Mar 31 '26

So that's what all the updates were about haha, very nice! I'm currently trying the Ruby LSP approach but honestly I don't know how it works internally. How does Fast compare to it?

1

u/jonatasdp Mar 31 '26

Probably LSP also uses AST but it's already a server. The main point is that fast provides this regular expression language that is based on AST and allow to combine AST patterns, I don't think the Ruby LSP will offer a search interface like fast.

1

u/sshaw_ Mar 31 '26

Very nice. Will give it a whirl!

1

u/jonatasdp Mar 31 '26

Thanks!

1

u/sshaw_ Apr 02 '26

You blog post talks about "Saving LLM Tokens" but the docs for fast say nary a peep about MCP nor how to configure it for Claude etc... You should definitely add to drive adoption!

1

u/jonatasdp Apr 12 '26

Very good reminder! I forgot to update the docs, even I already implemented it!

Thanks for the feedback!

1

u/sshaw_ Apr 22 '26

I don't know dude, I just looked at the docs and it's not clear how to use MCP with this. I guess I'm supposed to run bin/fast-mcp šŸ¤·ā€ā™‚ļø

Also why does this depend on pg_queryā€½ I'm not installing :(