Why

Remote compaction v2 was still using context_compaction as both the request trigger and the compacted output shape. The Responses API now has the landed contract for this flow: Codex sends a dedicated { "type": "compaction_trigger" } input item, and the backend returns the standard compaction output item with encrypted content.

This aligns the v2 path with that wire contract while preserving the existing local compacted-history post-processing behavior.

What changed

• Add ResponseItem::CompactionTrigger and regenerate the app-server protocol schema fixtures.
• Send compaction_trigger from remote_compaction_v2 instead of a payload-less context_compaction.
• Collect exactly one backend compaction output item, then reuse the existing compacted-history rebuilding path.
• Treat the trigger item as a transient request marker rather than model output or persisted rollout/memory content.

Verification

• cargo test -p codex-protocol compaction_trigger
• cargo test -p codex-core remote_compact_v2
• cargo test -p codex-core compact_remote_v2
• cargo test -p codex-core responses_websocket_sends_response_processed_after_remote_compaction_v2
• just write-app-server-schema
• cargo test -p codex-app-server-protocol schema_fixtures