The transcript admits only sendable messages

Every message committed to a session’s event log serialises to a request every supported provider accepts. The event log is the single source of truth for the next request; once a malformed or empty message is committed, it is re-serialised on every subsequent round-trip, so a single bad commit can wedge the whole run on a strict backend (Anthropic 400s, the run dies — misclassified as fatal). The protocol state machine (`event.rs`) enforces role alternation; this invariant enforces per-message admissibility at the one place messages enter the log: the apply commit boundary in agent.

Three commit-time obligations, all in Session::apply (the deep-review X-tags):

• Tool-call arguments are objects (X2). A ContentPart::ToolCall whose fn_arguments is not a JSON object is repaired to {} by admit_assistant before append_assistant. genai’s Anthropic adapter repairs only a null argument, so a bare string / number / array re-serialises verbatim and strict backends reject every later request carrying it.
• No empty assistant message (X7). An assistant turn with no substantive part — no tool call, no non-empty text, no binary — would serialise as "content": [] (or an empty text block) and is rejected once a later message (the empty-turn nudge’s user prompt) makes it non-final. admit_assistant substitutes a short stub; the turn still surfaces as Empty so the nudge recovers it.
• A MaxTokens turn with captured tool calls dispatches, not strands (X6). Returning Truncated there leaves the assistant message carrying tool_ids and the session in AwaitingToolResults, where the nudge’s next append_user fails “tool results pending”. apply instead dispatches the captured calls and continues the loop; only a MaxTokens turn with no tool call raises Truncated to nudge.

Two adjacent obligations keep the invariant whole:

• Auto-compaction runs where it can (X1). Compaction needs ReadyForUser (can_compact), which holds at the top of apply, never mid-loop in AwaitingAssistantAfterToolResults — the prior placement was dead code.
• A JSON-body 4xx is classified, not lost (X3). parse_4xx_status matches both the status: <code> token and a JSON "code": <code> body, so an OpenRouter-shaped {"code":400,…} becomes a structured Api error rather than an opaque Other.

The hard rule: a new shape of committed message must pass through the same admission step — never append an assistant message (or a tool result) the serialiser would render in a form a supported provider rejects. This is the exarch analogue of the wire-hop discipline (exhaustive maps): the seam admits only validated, complete values.

See also turn-ends-ready (the role-alternation half), agent (apply, admit_assistant, compact), provider (from_genai, parse_4xx_status).