Here's a starting point in 93 lines of Ruby, but that one is already bigger than necessary:

https://radan.dev/articles/coding-agent-in-ruby

Really, of the tools that one implements, you only need the ability to run a shell command - all of the agents know full well how to use cat to read, and sed to edit.

(The main reason to implement more is that it can make it easier to implement optimizations and safeguards, e.g. limit the file reading tool to return a certain length instead of having the agent cat a MB of data into context, or force it to read a file before overwriting it)