A coding agent is not magic, it’s a loop. We call this a harness. The harness is a deterministic layer of code that wraps an LLM. Claude Code is a harness. Codex is a harness. Pi is a harness. The harness, on initialization, provides to the LLM a system prompt defining all tools the harness implements for the LLM. Without the harness, you cannot read or modify files on the user’s local filesystem without them having to copy-and-pasting by hand. The harness is the final place where engineers can customize how coding agents do work before the LLM takes over. Think of the LLM as a train and the harness as the rails the train rides on. Below… one full task executed by a harness, traced step by step.
