Codex server compaction never triggers when Claude Code appends a system-role message after the user turn (hooks / output styles) #204

Versions: claude-code-proxy 0.1.47 (macOS, behind a local shim), Claude Code 2.1.319, codex.serverCompaction: true.

What happens: a manual /compact in Claude Code 2.1.319 (and, by the same request shape, automatic compaction) reaches the proxy as a normal /v1/messages request with the x-claude-code-session-id header, but server_compaction_triggered is never logged and no compaction_trigger request goes to Codex. Claude Code still gets its portable text summary, so nothing looks broken.

Why (from a CCP_TRAFFIC_LOG=1 capture of the actual request):

1. The request keeps Claude Code's normal system prompt (cache-safe). It does not contain You are a helpful AI assistant tasked with summarizing conversations, so is_compact_request(system) is false.
2. The compaction instruction (CRITICAL: Respond with TEXT ONLY. Do NOT call any tools. … Your task is to create a detailed summary of the conversation so far …) is appended to the text of the last user message; both markers are present there.
3. But messages.last() is not that user message. When a UserPromptSubmit hook or an output style emits text, Claude Code appends a system-role message after the user turn (ours begins Concise output style is active. … UserPromptSubmit hook success: …). is_compact_messages_request only inspects request.messages.last() and requires role == "user" (src/providers/codex/translate/request.rs), so detection returns false. Roles of the captured request, in order: assistant, user, system, assistant, user, system.

Repro: Claude Code 2.1.319 with any UserPromptSubmit hook that prints a line (or an active output style), a Codex model through the proxy, two short turns, then /compact → no server_compaction_* event in the proxy log (three runs here: 0 events).
Control: the same kind of body sent with curl to the proxy with the session header and the compaction instruction as the final user message → server_compaction_triggered and server_compaction_completed within 5 s.

Fix: look at the last user message, skipping trailing system-role messages (PR follows). A trailing assistant message should still mean "not a boundary".

Side observation for the docs: in 2.1.319 a /compact on a short context (one summary plus two turns) produced a new summary without any upstream request at all, so a boundary is not always one summary request.