Coach: Sending a Message Diagram

Three doors before the model

No session or unfinished setup: 401. The AI gate says no (paused, spend line, your own daily allowance, maintenance): 503 with a plain sentence
The per-user message slot fails closed: if Redis is down the message is refused, never sent unmetered
Past a spend line the Coach drops from the Pro model to the Flash model instead of stopping

Prompt and tools

Each thread kind (chat, lesson, review, mock) registers its own prompt and tools; the client sends only the new message, history comes from Postgres
A shared scope rule keeps the Coach on interview prep; anything a person wrote is fenced as untrusted text the model must not obey
Read tools run freely; action tools only PROPOSE and wait for a tap

Streaming, saving, mock scoring

The answer is stored even if the reader leaves; only an explicit Stop aborts the model
A provider error shows 'Coach couldn't answer'; the failed spend is still logged when usage is known
Ending a mock re-checks the gate, claims a short lock, scores the transcript once and saves the rubric, score and feedback; nobody answered means 'abandoned', no score