Release: llm-coding-agent 0.1a0
Simon Willison’s Weblog
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2nd July 2026
Release
llm-coding-agent 0.1a0 — A coding agent built on LLM
Another Fable 5 experiment. Now that my LLM library has evolved into more of an agent framework it's time to see what a simple coding agent would look like built on it.
I started a new Python library using my python-lib-template-repository GitHub template repository, then ran these two prompts (here's the Claude Code for web transcript):
Write a spec.md for this project - it will depend on the latest “llm” alpha from PyPI and implement a Claude code style coding agent complete with tools for reading and editing files and executing commands
Then:
Commit the spec, then build it using red/green TDD in a series of sensible commits (each with passing tests and updated docs) - occasionally manually test it using the OpenAI API key in your environment
Here's the resulting README file and the sequence of commits.
I've shipped a slop-alpha to PyPI, so you can run the new agent like this:
uvx --prerelease=allow --with llm-coding-agent llm code
It's pretty good for a first attempt! Here's the (Fable-authored) README, which lists recipes like llm code --yolo and llm code --allow "pytest*" --allow "git diff*".
It also presents a Python API based around a CodingAgent(model="gpt-5.5", root="/path", approve=True).run("Fix the failing test in tests/test_parser.py") class which I didn't ask for but I'm delighted to see implemented.
Here's the suite of tools it implemented, listed using uvx ... llm tools:
CodingTools_edit_file(path: str, old_string: str, new_string: str, replace_all: bool = False) -> str
Replace an exact string in a file.
old_string must match the file contents exactly (including whitespace) and must identify a unique location unless replace_all is true. Returns a diff of the change so it can be verified.
CodingTools_execute_command(command: str, timeout: int = 120) -> str
Run a shell command in the session root directory.
Returns combined stdout and stderr followed by an Exit code line. timeout is in seconds (maximum 600); on timeout the whole process tree is killed.
CodingTools_list_files(pattern: str = '**/*', path: str = '.') -> str
List files matching a glob pattern, newest first.
Skips hidden directories, node_modules, pycache and (in a git repository) anything covered by .gitignore. Returns at most 200 paths relative to the searched directory.
CodingTools_read_file(path: str, offset: int = 0, limit: int = 2000) -> str
Read a text file, returning numbered lines like cat -n.
Paths are relative to the session root. Use offset (0-based first line) and limit (max lines) to page through files too large to read in one call.
CodingTools_search_files(pattern: str, path: str = '.', glob: str = None, max_results: int = 100) -> str
Search file contents for a regular expression.
Returns matches as path:line_number:line, capped at max_results. Use glob (e.g. "*.py") to restrict which files are searched.
CodingTools_write_file(path: str, content: str) -> str
Create or overwrite a file with the given content.
Parent directories are created as needed. Prefer edit_file for modifying existing files.
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This is a beat by Simon Willison, posted on 2nd July 2026.
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