[Vercel AI Gateway] Anthropic system-message placement breaks Codex continuation and compaction

## Observed symptom AI Gateway: Anthropic system-message placement breaks Codex continuation and compaction ### Description Codex sessions using Claude Opus through AI Gateway fail when a refreshed developer instruction follows an assistant's final answer. The gateway converts the update to an inline Anthropic `system` message at a position Anthropic rejects. This blocks both normal continuation and Codex's inline summarization/compaction. We have hit this in multiple real sessions. The failure does not require a long conversation: four synthetic messages reproduce it. A virtual model with automatic caching also reproduces it, but a virtual model is not required. This was reproduced against the hosted gateway on September 30, 2026: | Endpoint/model | Input ordering | Result | | --- | --- | --- ## Literal error text ``` Anthropic system-message placement breaks Codex ``` ## Environment GitHub issue vercel/ai #21885 ## Trigger / context ### Description Codex sessions using Claude Opus through AI Gateway fail when a refreshed developer instruction follows an assistant's final answer. The gateway converts the update to an inline Anthropic `system` message at a position Anthropic rejects. This blocks both normal continuation and Codex's inline summarization/compaction. We have hit this in multiple real sessions. The failure does not require a long conversation: four synthetic messages reproduce it. A virtual model with automatic caching also reproduces it, but a virtual model is not required. This was reproduced against the hosted gateway on September 30, 2026: | Endpoint/model | Input ordering | Result | | --- | --- | --- | | `/codex/v1/responses`, `anthropic/claude-opus-5.5-fast` | user → assistant → developer → user | HTTP 400 | | `/codex/v1/responses`, same public model | user → assistant → user → developer | HTTP 200, `status: "completed"`, output `OK` | | `/codex/v1/responses`, auto-caching Opus virtual model | user → assistant → developer → user | HTTP 400 | | `/v1/responses`, same virtual model | user → assistant → developer → user | HTTP 400 | The second row is a diagnostic control, not a deployed fix. ## Source evidence - https://github.com/vercel/ai/issues/21885 (2026-10-01T16:32:55Z) — #21885 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:46:06.809152Z - scout_id: Cdc4736cd71 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome.