The harness needs one internal language for messages that the loop, session log, and all plugins speak. Options: mirror the DeepSeek/OpenAI chat-completions shape (zero mapping for the first provider, awkward for rich content), adopt Anthropic's Messages block structure verbatim (battle-tested, but our canonical types would mirror a third-party API we don't target first), or own a vocabulary.
Own it: messages are arrays of typed content blocks (`text`, `reasoning`, `tool-call`, `tool-result`, `image`), with the union derived from the merge-extensible `ContentBlockMap` so plugins add block types via declaration merging. The same merge-extensible-map pattern types every "stringly" field (`MessageSource`, `FinishReason`, `TurnTrigger`, `TurnEndReason`). Streaming is a raw chunk protocol; `BlockAssembler` is the single shared assembly implementation. Adapters translate to provider wire formats — mapping cost lives in adapters, where it belongs.
In-session context injection (`context/message`, `steering/message`) renders as tagged user-role envelopes (the system-reminder pattern) rather than a new role, so adapters carry zero burden. TODO(review): revisit once the DeepSeek V4 adapter exists.