BUG: Claude Code 2.1.215 embedded system messages fail validation

[SimianW]

Kiro Gateway Version

Current main at a5292ca (v2.4 development branch)

What happened?

Claude Code v2.1.215 can place runtime system reminders in messages[] using role: "system". Kiro Gateway rejects the request during Pydantic validation because AnthropicMessage.role only accepts user and assistant.

Minimal sanitized request shape:

{
  "model": "<model-id>",
  "messages": [
    {"role": "user", "content": "<redacted>"},
    {
      "role": "system",
      "content": [
        {
          "type": "text",
          "text": "<runtime instructions redacted>",
          "cache_control": {"type": "ephemeral"}
        }
      ]
    }
  ],
  "system": [{"type": "text", "text": "<base system prompt redacted>"}]
}

The request never reaches the converter or Kiro runtime. A standard request through the same endpoint succeeds.

Expected behavior: before validating conversation roles, the Anthropic compatibility layer should preserve embedded system content by merging it into the top-level system field and remove those entries from messages[]. The same normalization should apply to /v1/messages/count_tokens.

Debug Logs

Sanitized validation output:

Validation error (422): [{
  "type": "literal_error",
  "loc": ["body", "messages", 1, "role"],
  "msg": "Input should be 'user' or 'assistant'",
  "input": "system"
}]

No prompts, tool definitions, credentials, account identifiers, local paths, or raw debug files are included.