A self-evolving trading agent rewrites its own prompts, but Steward locks risk rules beyond the type system and lost just 0.3% in a 24.6% sector drawdown
Autonomous investing agents must learn from outcomes without rewriting their own risk rules. Steward separates editable prompts from an invariant core: nine role-specialised agents decide jointly, code grades every trade nightly, and a quarterly reflector proposes only typed prompt edits that a deterministic gate applies reversibly. Over 102 trading days and 1,632 model calls in a paper account, the portfolio lost 0.3% while the semiconductor sector fell 24.6%, and no injected instruction was followed.