CrossRepo - Multi-Repo PR Breaking Change Detector for Full-Stack Teams
Your frontend PR merged. Your backend PR is still open. Nobody noticed the API contract changed and now production is broken at 2am. CrossRepo links PRs across repos, diffs the API surface automatically, and posts a breaking-change warning in Slack before anyone approves either PR.
Difficulty
intermediate
Category
Developer Tools
Market Demand
High
Revenue Score
7/10
Platform
Bot / Integration
Vibe Code Friendly
No
Hackathon Score
6/10
Validated by Real Pain
— seeded from real developer complaints
Developers working across separate frontend and backend Git repos have no automated tooling to detect when linked PRs introduce API contract mismatches, forcing manual coordination and causing production breakages that are only discovered after both PRs are merged.
What is it?
Teams building frontend and backend in separate repos have no native tooling to detect when two linked PRs introduce a contract mismatch — a renamed endpoint, a dropped field, a changed response shape. CrossRepo installs as a GitHub App, lets developers tag PRs with a partner repo PR number, then diffs OpenAPI specs or TypeScript interface exports between the two branches and posts a structured Slack alert listing every breaking change with file and line references. V1 targets TypeScript monorepo-adjacent teams where API types are exported from a shared types file. The GitHub App webhook model means zero polling infrastructure — events are push-based and cheap to run.
Why now?
The April 2026 vibe-coding wave has teams shipping frontend and backend at separate velocities using Cursor and Claude, making cross-repo contract drift a daily incident — and GitHub still has no native solution for it.
- ▸GitHub App that listens to PR open and update events across linked repo pairs (Implementation: GitHub App webhooks, Octokit for PR content fetch).
- ▸TypeScript interface diff engine that compares exported types between base and head branches (Implementation: ts-morph AST traversal, exported interface shape comparison).
- ▸Slack alert template showing breaking changes with file path, old value, and new value side-by-side (Implementation: Slack Block Kit message via Incoming Webhook).
- ▸PR comment bot that posts a breaking-change summary directly on both linked PRs (Implementation: Octokit createComment on both PR threads).
Target Audience
Full-stack engineering teams of 10-40 developers working across frontend and backend repos, primarily at product startups and scale-ups — estimated 80k+ such teams globally.
Example Use Case
A 20-person startup's frontend dev tags their PR with 'partner: backend/pr/412'. CrossRepo diffs the TypeScript API types, finds that a field was renamed from userId to accountId, and posts a Slack alert to the engineering channel — caught before either PR is approved.
User Stories
- ▸As a frontend developer, I want to tag my PR with a partner backend PR and get a Slack alert if the API contract changed, so that I can fix mismatches before either PR is merged.
- ▸As an engineering lead, I want every cross-repo PR pair to be automatically checked for TypeScript type mismatches, so that I stop discovering API breakages in production at 2am.
- ▸As a staff engineer, I want a PR comment on both linked PRs listing every breaking change with file and line context, so that reviewers have full context without switching repos.
Done When
- ✓PR Linking: done when developer adds a CrossRepo tag to their PR body and the bot comments on both PRs within 60 seconds confirming the link.
- ✓TypeScript Diff: done when a renamed interface field triggers a Slack alert listing the old field name, new field name, and the file path where the change occurred.
- ✓Slack Alert: done when the engineering channel receives a Block Kit message with a clear breaking/non-breaking label and a link to both PRs.
- ✓Billing: done when Stripe checkout completes, team plan activates, and unlimited repo pairs are unlocked in the settings UI.
Is it worth building?
$29/month x 50 teams = $1,450 MRR at month 2. $29/month x 300 teams = $8,700 MRR at month 6. Math assumes 4% conversion from GitHub Marketplace listing and dev Twitter cold outreach.
Unit Economics
CAC: $50 via dev Twitter outreach and GitHub Marketplace (organic). LTV: $348 (12 months at $29/month). Payback: 2 months. Gross margin: 93%.
Business Model
SaaS subscription
Monetization Path
Free for 1 repo pair. $29/month per team for up to 5 repo pairs. $99/month for unlimited pairs plus org-wide dashboard.
Revenue Timeline
First dollar: week 4 via beta team upgrade. $1k MRR: month 3. $5k MRR: month 8.
Estimated Monthly Cost
Vercel: $20, Supabase: $25, Stripe fees: ~$5, GitHub API: free for public repos. Total: ~$50/month at launch.
Profit Potential
Full-time viable at $8k MRR. Low infrastructure cost since GitHub webhooks are push-based.
Scalability
High — add OpenAPI spec diffing, support for GraphQL schema changes, Jira ticket auto-creation for breaking changes, and GitHub Enterprise.
Success Metrics
Week 3: 5 teams installed GitHub App. Month 1: 2 paying teams. Month 3: 30 teams installed, 10 paying.
Launch & Validation Plan
Post in r/ExperiencedDevs and engineering Twitter asking 'how do you handle cross-repo API contract drift?' — if 15+ replies complain about the problem, launch immediately with a GitHub App beta.
Customer Acquisition Strategy
First customer: DM 20 CTOs and staff engineers on Twitter who have posted about monorepo vs multi-repo tradeoffs, offer free unlimited access for 3 months in exchange for a case study. Ongoing: GitHub Marketplace listing, dev.to article about TypeScript API contract drift, r/ExperiencedDevs, Hacker News Show HN.
What's the competition?
Competition Level
Low
Similar Products
Dependabot (only dependency updates, no contract diffing), Danger.js (requires per-repo config, no cross-repo awareness), Nx affected (monorepo-only, not multi-repo) — none detect API contract mismatches across separate repos automatically.
Competitive Advantage
GitHub's native Dependabot and PR reviews don't cross repo boundaries. No existing tool combines PR linking with TypeScript AST diffing and Slack alerts in a single GitHub App install.
Regulatory Risks
Low regulatory risk. GitHub App must request minimum necessary scopes (pull_requests read, contents read). Document data handling clearly — code is read temporarily and not stored beyond diff results.
What's the roadmap?
Feature Roadmap
V1 (launch): GitHub App install, TypeScript AST diff, Slack alerts, PR comments, Stripe billing. V2 (month 2-3): OpenAPI spec diffing, configurable severity levels, email digest. V3 (month 4+): GitHub Enterprise, Jira auto-ticket, org-wide breaking change dashboard.
Milestone Plan
Phase 1 (Week 1-2): GitHub App, webhook handler, ts-morph diff engine — done when real PR pair triggers correct diff. Phase 2 (Week 3): Slack alerts, PR comments, Stripe billing — done when first beta team gets alert. Phase 3 (Month 2): GitHub Marketplace listing, Show HN launch — done when 5 paying teams active.
How do you build it?
Tech Stack
Next.js API routes, GitHub App webhooks, TypeScript AST diffing via ts-morph, Slack Webhooks, Supabase, Stripe — build with Cursor for all backend logic, v0 for settings UI
Suggested Frameworks
ts-morph for TypeScript AST analysis, Octokit for GitHub API, Next.js App Router
Time to Ship
3 weeks
Required Skills
GitHub App setup, webhook handling, TypeScript AST diffing with ts-morph, Slack Incoming Webhooks, Stripe billing.
Resources
GitHub Apps documentation, ts-morph docs, Octokit REST client, Slack Webhooks API, Supabase auth guide.
MVP Scope
app/api/github/webhook/route.ts (GitHub App webhook handler), app/api/slack/notify/route.ts (Slack alert dispatcher), lib/diff/typescript-diff.ts (ts-morph AST diff engine), lib/github/pr-linker.ts (PR comment parser for partner PR tag), lib/db/schema.ts (Team, RepoPair, DiffResult tables), app/settings/page.tsx (repo pair config UI), app/api/billing/route.ts (Stripe checkout), .env.example (GitHub App keys, Slack webhook, Stripe, Supabase), seed.ts (demo team with fake diff results)
Core User Journey
Install GitHub App -> configure repo pair in settings -> open PR and tag partner PR -> receive Slack alert with breaking changes -> approve or fix before merge.
Architecture Pattern
GitHub PR event webhook -> Next.js handler parses partner PR tag -> Octokit fetches both PR branch file trees -> ts-morph diffs exported TypeScript interfaces -> breaking changes stored in Supabase -> Slack Block Kit alert fired -> Octokit posts comment on both PRs.
Data Model
Team has many RepoPairs. RepoPair has many DiffResults. DiffResult has many BreakingChanges with field name, old type, new type, and file path.
Integration Points
GitHub App webhooks for PR events, Octokit for GitHub REST API, ts-morph for TypeScript AST diffing, Slack Incoming Webhooks for alerts, Stripe for billing, Supabase for team and config storage.
V1 Scope Boundaries
V1 excludes: OpenAPI spec diffing, GraphQL schema diffing, GitHub Enterprise support, Jira integration, org-wide dashboard, mobile notifications.
Success Definition
An engineering team at a company the founder has never contacted installs CrossRepo via GitHub Marketplace, configures a repo pair, receives a real breaking-change alert on their next PR cycle, and upgrades to paid without any support interaction.
Challenges
Distribution is the core challenge — GitHub Marketplace has low organic discovery, and you need to be in the right dev Twitter threads and engineering Slack communities to get initial traction. TypeScript AST diffing with ts-morph covers 80% of teams, but teams using plain JSON contracts or GraphQL require separate parsers that expand v1 scope dangerously.
Avoid These Pitfalls
Don't try to support OpenAPI and GraphQL diffing in v1 — TypeScript interface diffing alone is a full sprint. Don't store full source code in your database — only store diff summaries to avoid legal and storage issues. Finding your first 10 paying teams will take longer than building the product — budget 3x more time for GitHub Marketplace SEO and dev community outreach than development.
Security Requirements
GitHub App uses minimum scopes (pull_requests read, contents read). Webhook signatures validated via crypto.timingSafeEqual. Supabase RLS on all Team tables. Code content never stored — only diff summaries. GDPR: diff results deletable on team offboard request.
Infrastructure Plan
Vercel for Next.js webhook handlers and settings UI, Supabase for team config and diff results, GitHub App for event delivery, Sentry for error tracking, GitHub Actions for CI — total ~$50/month.
Performance Targets
50 teams at launch, ~200 webhook events/day. Webhook handler response under 200ms (GitHub requires quick 200 ACK). Diff computation under 5 seconds per PR pair. No caching needed at this scale.
Go-Live Checklist
- ☐Security audit complete.
- ☐GitHub App webhook signature validation tested.
- ☐Payment flow tested end-to-end.
- ☐Sentry error tracking live.
- ☐Custom domain with SSL active.
- ☐Privacy policy and terms published.
- ☐3 beta engineering teams confirmed alert accuracy.
- ☐Rollback plan documented.
- ☐Show HN and dev Twitter launch post drafted.
First Run Experience
On first run: settings page shows a pre-configured demo repo pair (acme/frontend + acme/backend) with a seeded DiffResult showing 2 breaking changes from a fake PR. User can immediately explore what a breaking-change alert looks like before connecting any real repos. GitHub App install button is front and center.
How to build it, step by step
1. Define lib/db/schema.ts with Team, RepoPair, DiffResult, BreakingChange tables using Drizzle ORM. 2. Register GitHub App with permissions: pull_requests read, contents read, and subscribe to pull_request events. 3. Build webhook handler in app/api/github/webhook/route.ts that validates GitHub signature and routes PR open/sync events. 4. Build lib/github/pr-linker.ts that parses PR body for a 'CrossRepo: owner/repo#123' tag and fetches the partner PR via Octokit. 5. Build lib/diff/typescript-diff.ts using ts-morph to fetch both branch versions of a types file and diff exported interface shapes. 6. Store BreakingChange records in Supabase with field name, old type, new type, file path. 7. Build Slack Block Kit message template in app/api/slack/notify/route.ts listing each breaking change with before/after. 8. Add Octokit createComment to post the same breaking-change summary on both linked PRs. 9. Build settings UI in app/settings/page.tsx for repo pair config and Slack webhook URL input, with Stripe billing for team plan. 10. Verify: open two test PRs in linked repos with a renamed TypeScript interface field, confirm Slack alert and PR comment both appear with correct field diff within 30 seconds.
Generated
April 28, 2026
Model
claude-sonnet-4-6
Disclaimer: Ideas on this site are AI-generated and may contain inaccuracies. Revenue estimates, market demand figures, and financial projections are illustrative assumptions only — not financial advice. Do your own research before making any business or investment decisions. Technology availability, pricing, and market conditions change rapidly; always verify details independently.