Running 5 Claude Code Instances in Parallel on One Flutter App

The Setup

Five Claude Code environments running simultaneously on the same Flutter Web project:

Same repo. Same main branch. All five pushing simultaneously.

Collision Patterns and Fixes

1. Concurrent writes to the same file

# Problem: VSCode and PS both update ROADMAP at the same time
VSCode: git push → success
PS:     git push → rejected (non-fast-forward)

Fix: rebase before every push:

git fetch origin main
git log HEAD..origin/main --oneline  # detect parallel commits
git pull --rebase origin main        # absorb before pushing

ROADMAP collisions are common. Having each instance append its own ### InstanceN#X section at the bottom makes auto-rebase resolution work most of the time.

2. cancel-in-progress: true silently drops deploys

With 5 instances pushing within 30 seconds of each other, the deploy queue gets congested:

# ❌ Later push cancels the earlier deploy mid-flight
concurrency:
  group: deploy-production
  cancel-in-progress: true

# ✅ Queue deploys — all commits eventually land
concurrency:
  group: deploy-production
  cancel-in-progress: false  # waits up to 11min × N but nothing is lost

This is the non-obvious one. cancel-in-progress: true is fine for a single developer; with 5 parallel instances it destroys work silently.

3. Ownership boundaries prevent most collisions

lib/pages/**         → VSCode (UI owner)
supabase/functions/  → Windows App (EF owner)
docs/blog-drafts/    → PowerShell (blog owner)
.github/workflows/   → PowerShell (CI owner)
supabase/migrations/ → Windows App (migration owner)

When every instance has a different area of the codebase, parallel work mostly doesn't conflict.

4. Cross-instance PRs for handoffs

When work crosses ownership boundaries:

# docs/cross-instance-prs/20260419_dart_fix.md
from: PowerShell
to: VSCode
---
philosophy_page.dart has a dart:ui_web compile error in tests.
Please fix when VSCode has context ready.

When done, move to docs/cross-instance-prs/done/. The file acts as a message bus between instances that have no shared memory.

A Day's Timeline

06:00 JST  Windows: AI University provider migration pushed
06:05      PowerShell: 3 blog posts dispatched
06:30      VSCode: UI DESIGN.md fixes pushed
06:31      PowerShell: rebase absorbs VSCode commit → push succeeds
07:00      WEB: competitor monitoring → cross-instance-pr created
09:00      Mobile: real iPhone test → GitHub Issue created

Numbers

Token consumption multiplies by 5, but $20/month delivers roughly $200 worth of work.

What Makes This Work

1. Ownership — define who touches what; conflicts drop to near zero
2. Cross-instance PRs — async message passing between instances with no shared memory
3. cancel-in-progress: false — all pushes deploy, nothing is silently discarded
4. Rebase-before-push — absorb parallel commits before adding yours

It's essentially a multi-developer workflow, but the developers are AI instances rather than humans. The coordination patterns translate directly.

Building in public: https://my-web-app-b67f4.web.app/

ClaudeCode #Flutter #buildinpublic #CI/CD