Explanation

Plugs wrap each other

Plugs are listed under setup and nest in that order: the first plug wraps all the others. Each plug has up to three handlers, and each handler receives a next function that runs everything inside it.

• onSession wraps one run of the session. Code before next() attaches the plug's resources, and code after it pauses them, in reverse order.
• onTeardown wraps the destruction of the environment, once the session is over.
• onMessage wraps every message of the Agent Client Protocol, in both directions.

A plug that does not define a handler is transparent for that event.

The pages

• How sessions find a runner: runners, instances, labels and requirements, queued work, and where a paused session resumes.
• Why plugs are stateless first: plugs derive what they need from the session id, and keep state only for what cannot be derived.
• Why time belongs to Blaise: idle detection, pausing and timeouts are agent settings that Blaise enforces.
• The security model: what plugs can do, what the agent sees, and which protections come from the host.
• Design alternatives: what was considered and why it lost.

For the whole picture, read How plugs wrap a session (the chain, the message proxy and hosts) and The life of a session (runs, pause and resume, teardown and what happens when something fails).

Hosts and where code runs covers what a host is, what runs where, why the runner token never reaches a host, and what each kind of host protects.