The package OTP application — the single shared supervision tree that the per-connection subtrees and the reattach extension depend on.

What it starts

The Connection/Session per-connection subtree is started PER CONNECTION (library mode via Agent.child_spec/1 / Client.child_spec/1, or standalone via ConnectionsSupervisor). But three things are package-level shared — a durable session outlives every connection (process identity ≠ session identity), and the attach funnel is one per node:

1. Raxol.AgentClientProtocol.SessionRegistry — the ONE package-level unique Registry keyed {conn_pid, session_id}. The Connection reads it for session/cancel direct dispatch, so its name is contract.
2. Raxol.AgentClientProtocol.Ext.Journal.WriterRegistry + Raxol.AgentClientProtocol.Ext.Journal.WriterSupervisor — the journal single-publisher Writers, started lazily on first append/subscribe. Ext.Journal.ensure_writer/3 starts them here.
3. Raxol.AgentClientProtocol.Ext.AttachPolicy.TaskSupervisor — the Task.Supervisor the fail-closed admission funnel isolates policy tasks under. Without it, a REAL attach denies :policy_infra on every request — the fail-closed leg, correct but useless for a legitimate attach.

It also starts Raxol.AgentClientProtocol.ConnectionsSupervisor, the standalone-mode DynamicSupervisor an embedder DynamicSupervisor.start_child/2s a ConnectionSupervisor under (Agent.child_spec/1 / Client.child_spec/1's start MFA, wrapped in a plain child spec) when they want this module's shared tree to also host the connection subtree itself, rather than splicing Agent.child_spec/1/Client.child_spec/1 into their own supervisor. There is no mix acp.serve task and no connect/3 function: standalone mode is this DynamicSupervisor plus the embedder's own start_child/2 call.

When it starts

The application module is declared in mix.exs (mod:), so in a project that depends on this package it starts before the project's own application, with all five children: two Registrys, two DynamicSupervisors and one Task.Supervisor. None of them opens a file, a port or a listener. Use them as they are: do not also start children/0 in your own supervisor while this application runs, or your supervisor fails with {:already_started, _}.

Only this package's own MIX_ENV=test build starts an empty tree, so its suite (which starts each named supervisor manually, per test, with the SAME names) is never disturbed by a colliding auto-started one. The env is captured at COMPILE time (@auto_start), and a dependency is compiled with MIX_ENV=prod unless its :env option says otherwise, so a consumer's tests get the full tree too. No Mix call happens at runtime or in a release.

Library mode (an embedder that keeps it from starting)

An embedder that wants these processes under its own supervisor lists the package in included_applications in its mix.exs application/0: the application is then loaded (and included in a release) but not started, and the embedder splices children/0 into its own tree. The child specs are the exact list this module starts, so the names the modules resolve against (Runner's default Task.Supervisor, Writer.registry/0, Session.registry/0) are satisfied with no further wiring. A project that depends on raxol_agent cannot do this: raxol_agent lists this package among the applications it starts.