The three methods answer different questions.
| Method | Main constraint | Primary question | Durable output |
|---|---|---|---|
| IDD | Product meaning is uncertain | What behavior should be adopted? | Human decision and approved contract candidate |
| TDD / contract-first | Behavior is known enough to specify | Does the implementation satisfy the agreed behavior? | Repeatable tests, contracts, and regression protection |
| TWDD | Meaningful real validation is scarce | How can one safe window yield maximum replayable evidence? | Evidence bundle, replay assets, and batched repair plan |
IDD probe
→ human selects behavior
→ tests and contracts fix that behavior
→ TWDD prepares a scarce real-world validation window
→ live findings become offline regression tests
→ independent delivery gates remain in force
TWDD can also support a conventional project without IDD. It does not replace TDD when behavior is already clear, and it does not make uncertain product behavior correct merely because a live run completed.