The approved-action replay appends the sealed tool result with no assistant
prose for that round, which produced an assistant message with content "".
Anthropic's Messages API rejects a non-final assistant message with empty
content, so a resumed turn after a tool approval failed before the model saw
the result. A turn carrying neither prose nor reasoning has nothing to say to
any provider, so it is no longer appended. A round with prose, and a
reasoning-only round that DeepSeek thinking mode needs, both still append.
