facet_agent

The agent serving surface for facet apps: enable() before app.run, and a connected agent can read the UI and act through it.

[dependencies]
facet_agent = "*"
facet_runtime = "*"
# plus the agent stack it serves (agent_core/mcp/inapp + the platform
# overlay) — `cpc pm add . facet_agent` writes the closure

import "facet_agent/agent" as agent;
import "facet_runtime/runtime" as runtime;

fn run() -> i32 {
    agent::enable();                 // register the serving hooks
    runtime::agent_mcp("myapp");     // serve under this NAME, before startup
    let app = runtime::App::new("myapp");
    app.window("main", Home::boxed);
    let _run = app.run("main");
    return 0 as i32;
}

An id, not an address: the platform derives where it listens from the id and this process's pid. macOS and Linux expose a 0600 Unix socket plus an HTTP port; Windows, iOS, and Android expose the loopback HTTP port. A launcher that spawned the app knows the pid, so it can work the address out without being told. The app also prints it and writes it to /tmp/mcp-<id>-<pid>.json.

Without an application policy, a connection receives operator() plus cap_edit_tree(): it can read and drive ordinary nodes and use the inspector extension, but still cannot cross Protected or Private tiers. Install a fn(auth::Request) -> auth::Grant with set_policy before enable() when the surface must be narrower. facet_agent/consent is a ready-made policy that asks the user once per client.

import "facet_agent/agent" resolves by platform override: AppKit on macOS, GTK on Linux, Win32 on Windows, UIKit on iOS, and the Android agent backend on Android. It installs into Facet's application seam (application::install_agent) — facet itself knows nothing about the agent stack.

This is facet's OPTIONAL tier as a package boundary (2026-08-17): an app that never imports facet_agent links none of the agent machinery, by construction rather than by promise.

Tests: cd vendor/facet_agent && cpc test. The suite compiles the serving surface on the active platform — the vocab::Agent → policy translation is live code no other build type-checks, and a wrong arm there is a card number readable by every agent that connects.