Sentinel: Smart Subscription & Churn Guard

AI-powered subscription auditor that kills zombie churn and hidden trial traps with 1-click PayPal pause, cancel, and downgrade actions.

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Inspiration

The average consumer spends upwards of $273/month on recurring digital subscriptions—more than 2.5× what they typically estimate. Between sneaky free-to-paid trial conversions, silent annual price hikes, overlapping streaming libraries, and dark-pattern cancellation mazes, billions of dollars are lost every year to "zombie churn."

We asked ourselves: What if PayPal wasn't just where recurring charges happen, but your intelligent guardian against subscription creep?

We built Sentinel to bridge the gap between AI financial auditing and instant, friction-free payment remediation. With Sentinel, users can audit their entire digital footprint, identify predatory traps, and pause, downgrade, or cancel unwanted recurring billing in a single click through PayPal.

What It Does

Sentinel is an autonomous subscription management and churn-defense platform:

  1. AI Statement Auditor: Upload or paste credit card/bank statements in PDF, CSV, or raw text. An LLM parser extracts recurring billing merchants, frequencies, amounts, and dates.
  2. Predictive Risk & Churn Heuristics: Sentinel flags 4 critical financial traps:
    • 🚨 Trial Ending Alerts: Warns users 48–72 hours before a free trial auto-renews into a paid plan.
    • 🔄 Redundant Overlaps: Detects duplicate subscriptions (e.g., paying for both Spotify and Apple Music).
    • 💤 Unused / Zombie Subscriptions: Detects services with zero activity or logins over 45+ days.
    • 📈 Price Increase Warnings: Alerts users to recent or upcoming plan price adjustments.
  3. 1-Click PayPal Actions: Rather than navigating complex cancellation flows, users can directly Pause, Resume, or Cancel recurring billing profiles via the PayPal Subscriptions v1 API.
  4. Gamified Savings Jar: Eliminating subscriptions deposits cash into a live interactive 3D Savings Jar (built in Three.js), calculating accumulated annual savings in real time.
  5. App Directory: A curated marketplace of verified services (Spotify, Netflix, ChatGPT Plus, GitHub Copilot) where consumers can compare plans and subscribe securely through PayPal.

How We Built It

Sentinel was engineered across three integrated layers:

  • Frontend: React 18, Vite, Tailwind CSS, Lucide Icons, and Three.js (powering our real-time spinning 3D Sentinel logo and animated liquid Savings Jar).
  • Backend API: Node.js and Express with persistent SQLite storage (better-sqlite3 with an automatic synchronous fallback to Node 22 node:sqlite), IP rate limiting, strict CORS protection, and secure webhook verification.
  • PayPal Subscriptions Integration:
    • Direct integration with PayPal Subscriptions REST API v1 (https://api-m.sandbox.paypal.com).
    • OAuth2 client-credentials token management with automatic 60s pre-expiry refreshes.
    • State machine supporting ACTIVE, SUSPENDED, CANCELLED, and REVISED subscription lifecycles.
    • Dual-Mode Engine: Features a seamless DEMO_MODE=true offline simulator for zero-friction hackathon demos, alongside DEMO_MODE=false for live PayPal Sandbox checkout flows.
    • Webhook listener processing subscription events (BILLING.SUBSCRIPTION.CANCELLED, SUSPENDED, ACTIVATED) with deduplication.

Challenges We Ran Into

  • PayPal Sandbox Lifecycle & Origin Validation: Navigating the exact state transitions of PayPal's Subscriptions v1 API required strict adherence to HTTPS redirect URLs, application_context parameters, and buyer account session isolation to prevent checkout authentication collisions.
  • Separating External Audits from PayPal Profiles: When statements are scanned from external bank cards, those services cannot be paused via PayPal's API. We implemented strict data guards (returning 409 Conflict with clear "Bank Audit Only" badges) to prevent invalid API calls while preserving user visibility.
  • Parallel Team Synchronization: Coordinating three independent tracks (Frontend UI, PayPal Engine, and AI Parsing) under tight hackathon timelines. We defined strict JSON schema contracts (contract.md and OpenAPI 3.0 specs) early on so all components integrated seamlessly.

Accomplishments That We're Proud Of

  • True 1-Click Actionability: Transforming passive financial dashboards into an active tool that actually resolves the problem directly through PayPal.
  • Resilient Dual-Mode Design: Hackathon judges can test the full end-to-end application instantly without needing sandbox credentials, while the underlying architecture is fully wired for live PayPal Sandbox execution.
  • Modern Fintech Aesthetics: Crafting a sleek, dark-mode glassmorphic interface with interactive 3D Three.js visual elements and sound effects that make saving money satisfying.

What We Learned

  • In-depth mastery of the PayPal Subscriptions REST API, webhook signature validation, and plan revision/downgrade workflows.
  • How to combine LLM unstructured document parsing with deterministic business logic to prevent false positives in financial transactions.
  • Designing defensive microservices with atomic transactions and synchronous SQLite adapters for zero-latency UI responsiveness.

What's Next for Sentinel

  • Autonomous Auto-Pause Rules: Allowing users to set automated policies (e.g., "If no logins are recorded on this gym pass for 30 consecutive days, automatically pause billing").
  • Smart Plan Optimization: Automatically querying merchant catalogs to recommend cheaper student, duo, or promotional annual tiers.
  • Multi-Currency Global Support: Real-time currency conversions and localized subscription price benchmark alerts for international PayPal users.

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