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name: parallel-resource-isolation description: Use when about to start a dev server, run e2e tests, or connect to a database — proactively detects resource conflicts from other running processes and silently adapts to avoid collisions

Parallel Resource Isolation

Overview

Multiple Claude instances working on the same project will collide on ports, databases, and browser instances. This skill teaches proactive detection and silent adaptation — check what's in use, pick something else, report what you chose.

Core principle: Observe system state, adapt silently, report in terminal. No coordination files, no lock files, no shared state.

When to Use

Before any of these actions, run the detection checks:

• Starting a dev server (next dev, npm run dev)
• Running Playwright e2e tests
• Running tests that write to or assert against the database

Port Isolation (Dev Server)

Before starting a dev server:

lsof -i :3000 -t 2>/dev/null

If occupied, scan upward (3001, 3002, ...) until a free port is found. Start with:

PORT=<free_port> next dev

Playwright with webServer config:

• Override port via env var, pass same port as baseURL
• If env-based port override isn't supported, flag the user — do NOT modify config files

Playwright without webServer (external server):

• Start dev server on the free port
• Run Playwright with overridden baseURL:

BASE_URL=http://localhost:<free_port> npx playwright test

Hardcoded baseURL in Playwright config (common case): Most configs hardcode baseURL: 'http://localhost:3000' instead of reading from env. If so, BASE_URL=... won't work. Flag the user with this exact fix:

Your playwright.config.ts has a hardcoded baseURL. Please update it to: baseURL: process.env.BASE_URL || 'http://localhost:3000' This lets me override the port via env var without modifying config files.

Database Isolation (Supabase)

Before running tests that write to or assert against the DB:

1. Determine if isolation is needed:
   • Reading only → share the DB, skip isolation
   • Writing / mutating / asserting on DB state → isolate
2. Generate a timestamp-based schema prefix: parallel_<epoch>_ (e.g., parallel_1741234567_)
3. Create the isolated schema and run migrations:

psql "$DATABASE_URL" -c "CREATE SCHEMA parallel_1741234567_;"
PGOPTIONS="--search_path=parallel_1741234567_" npx supabase db push

4. Point the app at the isolated schema via env var:

SUPABASE_SCHEMA=parallel_1741234567_ npx playwright test

5. After tests complete: drop the schema:

psql "$DATABASE_URL" -c "DROP SCHEMA parallel_1741234567_ CASCADE;"

Playwright Browser Isolation

Before running Playwright tests:

1. Check for running Playwright browser processes that could conflict
2. Isolate browser state to avoid lock contention — use either approach:

TMPDIR=/tmp/playwright_<epoch> npx playwright test

Or more targeted — isolate only the browser path:

PLAYWRIGHT_BROWSERS_PATH=/tmp/pw_browsers_<epoch> npx playwright install chromium
PLAYWRIGHT_BROWSERS_PATH=/tmp/pw_browsers_<epoch> npx playwright test

3. Ensure the Playwright instance targets the same port assigned in Port Isolation above

Reporting

After adapting, report once and move on:

⚡ Parallel isolation:
  Dev server: port 3005 (3000 was in use)
  DB schema: parallel_1741234567_
  Playwright: /tmp/playwright_1741234567

Only report resources that were actually adapted. If nothing conflicted, say nothing.

Quick Reference

Resource	Detection	Adaptation	Cleanup
Dev server port	lsof -i :<port>	PORT=<next_free>	None needed
Supabase DB	Check if tests write/assert	parallel_<epoch>_ schema	Drop schema after tests
Playwright browser	Check for lock contention	TMPDIR or PLAYWRIGHT_BROWSERS_PATH	Temp dir auto-cleaned

Common Mistakes

Killing the other Claude's process

• Problem: Port 3000 is in use, so you kill the process to free it
• Fix: Never kill processes you didn't start. Pick a different port.

Modifying config files for port overrides

• Problem: Editing playwright.config.ts or next.config.js creates git conflicts
• Fix: Use environment variables. If env vars don't work, flag the user.

Forgetting to pass the adapted port to Playwright

• Problem: Dev server on 3005, Playwright still hits 3000
• Fix: Always coordinate — the port assigned to the dev server must be passed as BASE_URL to Playwright.

Isolating the DB when not needed

• Problem: Overhead of schema creation for read-only tests
• Fix: Only isolate when tests write or assert on DB state.

Red Flags

Never:

• Kill another process to free a resource
• Modify config files to change ports
• Ignore port conflicts and hope for the best
• Skip DB cleanup after tests

Always:

• Check before starting
• Adapt silently via env vars
• Report what you chose
• Clean up DB schemas after tests