Quarterly estimated-tax package (Form 1040-ES): derives your current-year total tax (federal income + self-employment + FICA + state) from income, applies the safe harbor (lesser of 90% current or 100%/110% prior-year tax), and returns the four equal 2026 installments net of withholding with their due dates.

Quarterly estimated-tax package (Form 1040-ES): derives your current-year total tax (federal income + self-employment + FICA + state) from income, applies the safe harbor (lesser of 90% current or 100%/110% prior-year tax), and returns the four equal 2026 installments net of withholding with their due dates.

Payment: x402 ($1.00 USDC on eip155:8453). Call unpaid to receive an HTTP 402 with requirements, then retry with the PAYMENT-SIGNATURE header. Method: current-year tax composed from the verified tax-estimate engine, fed into the 1040-ES safe-harbor engine; four equal installments = (required - withholding) / 4