Spec-Driven Agent Coding: Specs as Source of Truth with forgectl

Takeaways

• He kept hitting the same problem with a coding agent: end-to-end and integration tests exposed code that had to be redone, which doubled the work.
• Spec-driven development, from the Ralph Playbook, treats specs as the source of truth; he also had an agent reverse-engineer an existing codebase into specs, which surfaced a bug he was stuck on.
• forgectl takes a plan through specifying, implementation planning and implementation, much as a compiler works through layers, with specs as the code you edit.
• It runs as a state machine with evaluators at nearly every step; the evaluation rounds, commit behavior and agent type are configurable, and it uses Claude Code sub-agents.
• Specs stay with the codebase, later iterations produce spec diffs, and he says it cannot yet reverse-engineer brownfield projects; plugins are planned.