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Claude Code sends the system prompt as the top-level `system` field and, separately, sends skill/plugin listings as `role: "system"` entries inside `messages` (#1459 made the transformer accept those). `transform()` unconditionally prepends the top-level field, so once both are present the OpenAI-compat payload ends up with two `system` messages that are not adjacent. `coalesceMessages` only merges consecutive same-role messages and explicitly skips `system`, so it cannot fix this. Strict OpenAI-compatible backends (LiteLLM among them) reject that shape with: 400 A 'system' message can only appear at index 0 of the messages array. Add `hoistSystemMessages`, run before `coalesceMessages`, which extracts every `system` message in encounter order and reinserts a single merged one at index 0. Content-preserving, no behavior change when at most one system message is present.