JSK9999

Posted on Feb 25

I built a CLI that writes AI rules once for Claude Code, Cursor, and Codex

#claudecode #cursor #ai #opensource

Every AI coding tool has its own rule format.

Claude Code uses .claude/rules/*.md.
Cursor uses .cursor/rules/*.mdc.
Codex uses .codex/AGENTS.md.

If you use more than one tool, you end up copy-pasting rules between formats, and they inevitably drift apart. On top of that, every prompt loads ALL your rules — even when most of them are irrelevant — wasting tokens and degrading response
quality.

I built ai-nexus to fix both problems.

What is ai-nexus?

ai-nexus is a CLI tool that lets you write AI coding rules once in plain Markdown
and deploy them across Claude Code, Cursor, and Codex automatically.

Claude Code → .claude/rules/ with semantic routing (only relevant rules loaded per prompt)
Cursor → .cursor/rules/*.mdc with auto-converted frontmatter
Codex → .codex/AGENTS.md aggregated from all your rules

One source of truth. Every tool stays in sync.

Quick start

The interactive wizard asks which tools you use, which rule categories you want,
and sets everything up — hooks, settings, symlinks, format conversion. Done in 30
seconds.

npx ai-nexus install

Or quick install with defaults:

npx ai-nexus install -q

Or use your team's rules:

npx ai-nexus install --rules github.com/your-org/team-rules

Key features

Semantic Router — load only what you need

Most setups load every rule file on every prompt. If you have 30+ rules, that's a
lot of wasted context.

ai-nexus includes a Semantic Router that runs as a Claude Code hook on each
prompt. It analyzes your prompt and picks only the 2-3 rules you actually need.

With AI routing (recommended): GPT-4o-mini or Claude Haiku selects rules. Cost: ~$0.50/month.
Without AI (free fallback): Keyword matching activates rules based on words in your prompt.

Result: better AI responses + lower token costs.

Example output:

$ npx ai-nexus test "write a react component with hooks"

Selected rules (3):

rules/essential.md
rules/react.md
skills/react.md

Write once, deploy everywhere

Write a single .md rule file with a description in the frontmatter. ai-nexus
automatically converts it to:

.mdc format for Cursor (with description and alwaysApply frontmatter)
Aggregated AGENTS.md for Codex
Semantic-routed .md for Claude Code

No more copy-pasting between formats.

Team rules via Git

Share rules across your entire team with a Git repository:

npx ai-nexus install --rules github.com/acme/team-rules

When rules are updated:

npx ai-nexus update

Updates are non-destructive — your local customizations are never overwritten.
Only new files from the source are added.

Community marketplace

Browse, search, install, and remove community-contributed rules — from your
browser or CLI:

npx ai-nexus browse

Search and download from the CLI:

npx ai-nexus search react

npx ai-nexus get react.md

Anyone can contribute rules via PR. They become available immediately after merge
— no npm publish needed.

How each tool integration works

Claude Code — Semantic Router
A hook runs on every prompt. It moves relevant rules into .claude/rules/
(active) and parks the rest in rules-inactive/. Only active rules are loaded
into context.

Cursor — Rule Converter
ai-nexus converts your .md rules to Cursor's .mdc format, automatically
adding frontmatter. After that, Cursor's own built-in semantic search handles the
filtering.

Codex — Aggregated Rules
All individual rule files are merged into a single AGENTS.md file, loaded at
session start.

All commands

install — Interactive setup wizard
install -q — Quick install with defaults
init — Install in current project
update — Sync latest rules (respects local changes)
list — Show installed rules
test — Preview which rules would load
search — Search community registry
get — Download a community rule
add — Add rules from a Git repo
remove — Remove a rule source
browse — Open marketplace in browser
doctor — Diagnose installation issues
uninstall — Remove ai-nexus

Links

If you use multiple AI coding tools and are tired of managing rules separately,
give it a try: