As someone who's worked multiple gigs and contract positions, I know how nerve-wracking tax season can be when you're not sure if you've withheld enough. The standard W-4 calculator assumes one employer, and most payroll systems don't show you the bigger picture across all your income sources. Add 1099 work, where nothing is withheld at all, and it's easy to end the year with a surprise bill.

I built Tax Gap Tracker to solve this exact problem. You log paychecks and client payments as they come in, and the app projects your full-year tax, compares it with what you've paid through withholding and estimated payments, and shows the gap while there's still time to close it.

Key Features:

W-2 and 1099 Income in One Place - Track every job and client with its own pay schedule, W-4 settings, and year-to-date totals. Each source is projected to December 31 on its own schedule, and income from a job that ended still counts toward the year.

A Full-Year Tax Estimate - Uses 2026 federal brackets and standard deductions, self-employment tax, the qualified business income deduction with its phase-out, 1099 business expenses, and the child and other-dependent credits. The dashboard breaks the estimate down line by line.

Multi-State Support - Your home state taxes all your income while other states tax only the work done there, with a credit for tax paid to the other state. Covers the seven supported states with an income tax plus Texas, Florida, and Washington.

Quarterly Estimated Payments - Record payments to the IRS and your state and see each installment on the 1040-ES due dates, including California's 30/40/0/30 split. Enter last year's tax and the schedule targets the safe harbor that avoids an underpayment penalty, using each state's own rules.

AI Tax Assistant - A chat assistant powered by Claude Sonnet 5.5 that sees the same numbers as your dashboard, so you can ask "How much should I pay for Q4?" or "Should I raise my withholding?" and get answers based on your data. It explains what its numbers rest on, says when it isn't sure, and points you to a tax professional for decisions that need one.

Google Sign-In - Google OAuth with account linking through NextAuth v5. Users can sign up with email and password and link their Google account later, or the other way around.

Technical Highlights:

Built with Next.js 16 (App Router) and deployed to Vercel with automatic CI/CD. Used Prisma 8 with PostgreSQL for type-safe database access, adding each schema change as a reviewed, additive update. All of the tax math lives in one shared module that the dashboard, the API, and the AI assistant all call, so every screen reports the same numbers. The assistant runs on the Vercel AI SDK with the user's full tax picture passed into each request, and it's instructed to be upfront about the limits of a simplified estimate.

This project gave me hands-on experience with the latest Next.js features, production OAuth, modeling real tax rules in code, and integrating AI into an application responsibly. It's the kind of tool I actually use myself, and exactly the type of practical, user-focused software I want to build for clients.