fix: accept inline system messages and merge them into system prompt

[aceaura]

What

Accept role: "system" / role: "developer" messages inside the messages array and merge their text into the system prompt instead of rejecting the request.

Fixes #190, #219, #255.

Why

Claude Code and Claude Desktop place runtime context (<system-reminder> blobs, memory files, session hooks) in a message with role: "system" inside messages. AnthropicMessage.role is Literal["user", "assistant"], so Pydantic rejects the request during FastAPI validation — before any downstream normalization runs — and the client cannot complete a single turn:

422 {"loc": ["body", "messages", 1, "role"],
     "msg": "Input should be 'user' or 'assistant'",
     "input": "system"}

How

Two parts, because widening the Literal alone is not enough:

1. models_anthropic.py — AnthropicMessage.role accepts system and developer, so the request passes validation. Unknown roles are still rejected, keeping the field constrained.

2. converters_anthropic.py — new split_inline_system_messages() peels those messages off and appends their text to the system prompt; anthropic_to_kiro() calls it before convert_anthropic_messages().

Widening the Literal on its own would let normalize_message_roles() fold the message into a user turn, which puts client context into the conversation as if the user had typed it and triggers a synthetic assistant turn to keep roles alternating. Merging into the system prompt instead mirrors what convert_openai_messages() already does for OpenAI role: "system", so both APIs now treat inline system messages the same way. Top-level system stays first; inline parts are appended in the order the client sent them.

This matches the approach @W0n9 proposed in #190.

Tests

tests/unit/test_models_anthropic.py — TestAnthropicMessageRoles (5 tests): each accepted role validates (parametrized), unknown roles still raise, and a full request carrying a system message validates.

tests/unit/test_converters_anthropic.py — TestSplitInlineSystemMessages (7 tests): extraction of system and developer roles, multiple system messages joined in order, text extracted from content blocks (the cache_control shape Claude Code sends), empty content skipped, normal conversations unchanged, empty list. TestAnthropicToKiroInlineSystem (4 tests): inline context reaches the Kiro payload, top-level system precedes inline text, the system message does not become a history turn, and a request of only system messages raises ValueError (surfaced as a client error by routes_anthropic).

Full suite: 1708 passed.

🤖 Generated with Claude Code

[cla-bot]

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[aceaura]

I have read the CLA and I accept its terms cla-bot[bot] ***@***.***>于2026年7月27日 周一20:03写道：

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