AI-Native Formal Specification Language — a bounded model checker (Lark + Z3) built for LLM write/verify/repair loops.
Latest release v4.12.0 · 19 Oct 2026
18 files for Codex, Claude Code and OpenCode: checkpoint, task-start, fsl-codebase-explorer, fsl-coupled-change-reviewer and 14 more — 2,301 tokens loaded in every session.
AGENTS.md A 1,685 tok CLAUDE.md A 616 tok .claude/settings.json A — .claude/settings.json A — .claude/settings.json A — .claude/settings.json A — .agents/skills/checkpoint/SKILL.md A 27 tok .agents/skills/task-start/SKILL.md A 25 tok .claude/skills/add-language-feature/SKILL.md A 39 tok .claude/skills/new-spec/SKILL.md A 36 tok .claude/skills/pr-review/SKILL.md A 58 tok .claude/skills/release/SKILL.md A 58 tok .agents/skills/focused-change/SKILL.md A 48 tok .claude/agents/fsl-codebase-explorer.md A 40 tok .claude/agents/fsl-coupled-change-reviewer.md A 58 tok .claude/agents/fsl-soundness-reviewer.md A 62 tok .claude/agents/fsl-test-diagnostician.md A 48 tok .claude/agents/fsl-vacuity-reviewer.md A 64 tok These files are ymm-oss/fsl's own configuration — they tell Codex, Claude Code and OpenCode how to work on this repository, so they are not mods to install elsewhere. Copy one as a starting point and replace the parts that are about this project.