Undersign — an on-device e-signature app — designed and built in one short night with an AI agent (Claude Code) under human direction. This is the timeline behind it, reconstructed from the build log.
Six phases. Sixteen minutes.
Each phase committed and pushed on its own — six green check-ins in a quarter of an hour.
Real: the document engine (pdf.js render, pdf-lib flatten), signature capture, the audit trail, and SHA-256 tamper-evidence — all running with zero backend, nothing leaving the device. Simulated / out of scope: cross-device delivery (no server, so signers role-switch in one browser), and regulated eIDAS/ESIGN certification or identity verification. Knowing exactly where the moat is — workflow, compliance, integrations — is the point.