Problem

For a single user turn, Claude Code can issue two API calls. One is a side call to generate a session title; the other is the real conversation. We observed that the real-conversation thread had no recorded prompt.

The cause is the mid-conversation-system-2026-04-07 beta. When enabled, the client appends a trailing role: "system" message after the user's text (e.g. an injected skills list):

"messages": [
  { "role": "user",   "content": [ "...system-reminder...", "hey how are ya" ] },
  { "role": "system", "content": "The following skills are available..." }
]

lastUserPrompt only inspected the last element of messages and required role == user. Since the last message is now system, it returned (nil, false) and the prompt was never recorded via RecordPromptUsage.

Fix

When the last message is role: system, step back exactly one message to find the user's prompt. Stepping back only past a single trailing system message means requests that contain no new user prompt (two trailing system messages, or a system message after an assistant turn) still record nothing, avoiding re-recording a stale prompt from an earlier turn.

Tests

Added lastUserPrompt cases covering:

• trailing system message stepping back to string and typed user content
• trailing system message after a non-user message (records nothing)
• a lone system message (no out-of-bounds)
• two trailing system messages (only steps back once)

All TestRequestPayloadLastUserPrompt subtests pass under -race.