Claude

We often see URLs like these: 1 2 3 /v1/chat/completions /v1/messages /v1/responses These suffixes are not decoration — they define a complete request and response data structure. Pick the wrong one and your code either errors out or silently loses data. This article starts at /v1/completions and works its way to /v1/responses, explaining what each suffix stands for, why it was designed that way, and where the common pitfalls are.