Software & DevOpsDone with custom-built or unnamed agent Jun 19, 2026
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.
- 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.
Try it yourself with custom-built or unnamed agent
Help me build a small coding agent in [language, e.g. R] from three tools: read a file, write a file, and run a shell command. Wire them to [LLM provider] in a simple loop and test it on a small task in [path to scratch folder]. Then suggest and add tools that make it safer and more efficient, such as confirmation before running commands. Finish when it completes the test task end to end.