One rules file for all your parallel Claude Code sessions

Run four Claude Code sessions on the same repository and you will notice they disagree. One writes tests next to the source file, another in tests/. One runs npm, another pnpm. One commits, another doesn't. None of them is wrong in isolation; they just each guessed.

You can fix this one prompt at a time, or once. Claude Code reads a CLAUDE.md file from the project as standing instructions, which makes it the natural place for the rules every session should share.

What belongs in the shared file

Keep it to things that are true for every session, whatever task it has:

• Commands. How to install, run tests, lint and type-check. "Use pnpm. Run pnpm test before reporting done."
• Layout. Where tests, scripts and docs go.
• Hard limits. "Do not commit or push. Do not edit lockfiles or CI config unless the task says so."
• Shared resources. "Use the port and database in .env.local." (Why that matters is in port and database collisions.)
• How to report. "End with: files changed, the command you ran, and its output."

What to leave out

• The task itself. That belongs in each session's brief — allowed area, forbidden area, stop condition, proof of done (the four-part brief). If the task is in the shared file, every session receives it.
• Long explanations. Every line is read by every session. A rule that needs a paragraph of justification can usually be a one-line instruction plus a link to a doc.
• Things that change daily. Status and progress go in a handoff note, not in the rules.

Keep it short enough to be followed

A useful test: could you read the whole file aloud in a minute? If not, some of it is probably being skimmed. When an agent breaks a rule twice, the answer is often to make that rule shorter and more specific, not to add another one next to it.

Changing the rules while sessions are running

Sessions that are already running started with the old file. After you change a rule that matters, tell the running sessions to re-read it — one short message such as "CLAUDE.md changed: re-read it before continuing." With many sessions, sending that to each one by hand is where people miss one; sending one command to many sessions covers that step.

Where Agent Tile fits

Agent Tile is a one-time $10 macOS app that shows each Claude Code session as a tile with its project and folder visible, and relaunches each one with --resume in its original directory after a restart. It does not write your rules file. It makes it easier to see which sessions share a repository — and therefore which ones need to hear that the rules changed.

Takeaway

Put the rules every session shares — commands, layout, hard limits, how to report — in one short CLAUDE.md. Keep tasks, explanations and daily status out of it. When it changes, tell the running sessions to re-read it.