role:system messages inside `messages` are mapped to assistant -> upstream 400 invalid_message_role

[awcator]

Summary

A /v1/messages request whose messages array ends with a role: "system" entry is translated to an OpenAI payload that ends on an assistant turn, which upstream (Copilot's Gemini backend) rejects with:

400 {"error":{"message":"The last message in a Gemini request must not be from the assistant. Ensure the conversation ends with a user or tool message.","code":"invalid_message_role"}}

Root cause

In src/routes/messages/non-stream-translation.ts, translateAnthropicMessagesToOpenAI only special-cases role === "user" and routes everything else through handleAssistantMessage:

const otherMessages = anthropicMessages.flatMap((message) =>
  message.role === "user" ?
    handleUserMessage(message)
  : handleAssistantMessage(message),
)

So a role: "system" message — which Anthropic clients such as Claude Code place in messages[] rather than the top-level system field — becomes an OpenAI assistant message. When that message is the last one, the forwarded request ends on an assistant turn and upstream returns invalid_message_role.

Repro

Works (single user message):

{"model":"gemini-3.6-flash","max_tokens":100,"messages":[{"role":"user","content":"hello"}]}

Fails (trailing role: "system" — what Claude Code actually sends):

{"model":"gemini-3.6-flash","max_tokens":100,"messages":[{"role":"user","content":"hello"},{"role":"system","content":"some instructions"}]}

role: "system" is not part of the AnthropicUserMessage | AnthropicAssistantMessage union in anthropic-types.ts, which is why the translator never handles it.

Suggested fix (minimal)

Fold any role: "system" message's text into the OpenAI system prompt instead of routing it to the assistant branch, so the last OpenAI message is never an assistant turn:

• Treat role === "system" messages like the top-level system field (append text to the leading system message, or emit one if none exists).
• Ensure a system-only/assistant-only tail can never reach upstream.

Related: #261 covers the adjacent "trailing assistant prefill" case but not inline role:"system" messages.