Full-Stack Developer to Complete an Internal Marketing Tool

We are looking for a senior full-stack developer to complete and improve an internal tool that connects product activity with marketing campaigns. The tool is partially built, and much of the infrastructure is already in place. We need someone to review the existing work, understand what we are trying to accomplish, and turn it into a reliable production system. The project: Our product team defines actions that users need to complete. The system evaluates activity stored in Supabase to determine whether each user has completed the required action. We need the tool to provide: - A central record connecting each user’s internal IDs, email addresses, phone numbers, HubSpot ID, and other identifiers - A reliable view of relevant user activity and completion status - Saved email templates - Filters for selecting campaign recipients - Email delivery through Mailgun - Tracking for deliveries, failures, bounces, complaints, unsubscribes, opens, and clicks - Follow-up emails based on whether a user completed a required product action For example, we may send an email asking users to complete an action. A second email should go only to people who received the first email but have not completed that action. The system needs to produce consistent results, prevent duplicate sends, check eligibility immediately before sending, process Mailgun events reliably, and make failures visible. Our current tools include: Supabase and PostgreSQL HubSpot Mailgun Railway Vercel Codex Claude Code HubSpot will remain part of our sales and marketing system. We need someone who can make sound decisions about what should remain in HubSpot, what should be handled by the custom tool, and how data should move between them. The existing system was built internally with help from Codex and Claude Code. Some of it may be reusable, and some of it may need to be changed. We want someone who will evaluate the existing work before recommending an approach. You will work directly with our Head of Growth and coordinate with our product and engineering team. Product will define the required user actions and completion criteria. You will be responsible for translating those requirements into a reliable and maintainable system. What we are looking for: - Strong full-stack development and PostgreSQL experience - Experience completing and improving existing software - Experience with Supabase or similar platforms - Experience with HubSpot APIs and data - Experience with Mailgun or another email delivery service - Experience with APIs, webhooks, user permissions, testing, and deployment - Regular use of Codex, Claude Code, Cursor, or similar development tools - The ability to review and correct AI-generated code - Clear communication with both technical and nontechnical team members - Good judgment about when to build something and when to use an existing tool This is a fixed-price project with a budget of $8,000. We will discuss the final scope and payment milestones with the selected developer after an initial review of the existing system. We are looking for an individual developer who will personally perform the work, not an agency. To apply, please answer: - What similar internal tool have you personally built or inherited? - What experience do you have with Supabase, PostgreSQL, HubSpot, and email delivery systems? - How do you use Codex, Claude Code, Cursor, or similar tools in your work? - How do you approach reviewing and completing a partially built application? - What would you need to know before recommending an approach? - Will you personally perform the work?