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Olivia Craft
Olivia Craft

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Your AI Coding Rules Are Split Across Three Tools — And None of Them Talk to Each Other

#cursor #claudecode #aicoding #tutorial

Your cursor rules file is invisible to Claude. Your CLAUDE.md is invisible to Cursor. Your Codex instructions live in a third place. And none of these tools share a single character of context.

This is the rules fragmentation problem — and it is quietly breaking your AI coding setup.

How It Happens

You start with Cursor. You create .cursor/rules/ and write a few .mdc files. It works. The agent follows them.

Then you add Claude Code or Codex to the mix. Immediately, it ignores your .cursor/rules/. Why would it read them? It has its own config: CLAUDE.md at the root.

So you create a CLAUDE.md. Now Claude Code has context. But Cursor still does not see it (unless you explicitly reference it somewhere).

Then you add another tool — Copilot, Continue.dev, whatever — and the cycle repeats.

You end up with:

  • .cursor/rules/*.mdc — read by Cursor, invisible to everything else
  • CLAUDE.md — read by Claude Code, maybe skipped by agents
  • Vendor-specific configs — read by one tool, noise to all others

Three tools, three config surfaces, zero shared understanding.

Why This Is More Dangerous Than It Looks

The obvious failure is duplication: you say the same thing in three places, they drift, and you stop trusting any of them.

But the subtle failure is compliance collapse.

When your rules live in one place and your agent runs from another, the agent does not fail loudly. It does not say "I cannot find your rules." It just... proceeds without them. Confidently. Into your codebase.

You think the agent has your context. It does not. You approve the diff. The rule you wrote specifically to prevent that pattern gets broken anyway.

This is why the problem deserves a structural fix, not a workaround: a rules file owned by a single tool will always be invisible to the rest of your stack.

The Fix: Canonical Rules First, Tool Configs as Bridges

The mental model that actually works:

CLAUDE.md is your canonical rules document. It is the source of truth for how AI agents should behave in your project. It lives at the root. It travels with the repo.

Tool-specific configs reference it, not replace it.

In practice:

  1. Write your real rules in CLAUDE.md — scope, patterns, what to never do, deployment steps, the things that matter.

  2. In your .cursor/rules/ file, add one block: "Read CLAUDE.md at the project root before acting. These rules take precedence."

  3. For Codex or other agents, same approach: their config points back to CLAUDE.md or sources the same rules.

  4. If a tool cannot reference another file, copy the essential rules into it — but mark them as a copy and keep CLAUDE.md as the master.

One source of truth. Multiple entry points.

What Makes a CLAUDE.md Actually Work

Fragmentation is problem one. But even a single canonical file fails if it is written wrong.

The most common mistake: writing rules as preferences instead of constraints.

# Bad — preference framing
Try to keep functions under 50 lines.
Prefer descriptive variable names.

# Better — constraint framing
Functions over 100 lines require a comment explaining why.
Variable names must match the domain language in /docs/glossary.md.
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Preferences get weighed. Constraints get followed.

The second mistake: burying the rules that matter. Agents read from the top. If your most critical rule is at line 80, it will be deprioritized in long contexts. Put critical constraints at the top. The rest is reference.

The third mistake: no version control discipline. CLAUDE.md should be treated like code. Commit changes with a reason. Review what changed when the agent starts behaving differently.

Getting There Without Starting From Scratch

If you already have rules scattered across Cursor configs, agent prompts, and random markdown files, the path forward is:

  1. Audit what you actually have — list every place you have written instructions for any AI tool
  2. Identify the 10–20 rules that actually matter
  3. Consolidate them into CLAUDE.md at the project root
  4. Update tool configs to reference or mirror from there
  5. Delete the duplicates

It takes an afternoon. The payoff is an AI coding stack where every tool starts from the same ground truth.

If you want a head start, the CLAUDE.md Rules Pack includes a ready-made template with 40 rules covering the most common failure modes — scoped by project type, with constraint framing built in.

→ Get the CLAUDE.md Rules Pack ($27)

Want to see the format before buying? Grab the free starter (includes a sample CLAUDE.md + cursor rules):

→ Free starter — no email required

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