Build a working coding agent in R from three tools

Author builds a functional coding agent in R starting from three tools, then adds tools to make it safer and more efficient.

Done with a custom-built or unnamed agent

What they did

The author uses R's ellmer package to build a Claude-backed agent from three plain R functions: read file, write file and run shell command. Each function returns a string to the model, and a short system prompt tells it to read before editing and to run tests. He then adds list and search tools, a path check that keeps access inside the project directory, and a targeted edit tool.

How it went

The three-tool agent can find a function, read the existing tests, write a new test and run the suite, though the author says it falls well short of Claude Code. The shell tool stays risky, and the author defers fuller security coverage to a later post.

Worth knowing

Wrap every file tool in the path check, not just search. The unrestricted shell tool can still bypass it, so don't treat the project-directory check as full protection.

Discussion on Bluesky · Jun 19, 2026