Scion adapter

Scion runs core-agent directly — no adapter binary. Two features in the main core-agent binary cover the Scion contract:

pkg/hooks — config-driven shell-command dispatch on tool/model/turn boundaries. Fires sciontool hook --dialect=core-agent to update Scion’s transient-activity display ($HOME/agent-info.json).
sciontool_status built-in tool — auto-registered when sciontool is on PATH. The model calls it to signal sticky lifecycle states (ask_user, blocked, task_completed, limits_exceeded) to Scion’s hub.

The bundle staged at extras/scion/ holds the Scion-side files (harness config.yaml, event dialect.yaml, README) that Scion needs to launch core-agent. Ownership will shift to Scion’s own harnesses/core-agent/ once upstream adopts the bundle — until then, extras/scion/ is the source of truth.

How Scion drives core-agent

Scion launches core-agent inside a tmux session in a container. tmux gives it a real PTY, so core-agent’s TUI auto-launches — scion attach shows the live TUI.
scion message <agent> "..." types into the TUI input field via tmux send-keys. No side-channel required.
As tool calls and model turns stream through the agent’s event iterator, pkg/hooks shells out to sciontool hook with a JSON envelope on stdin, which updates agent-info.json.
When the model wants to signal a sticky state, it calls the sciontool_status tool, which shells out to sciontool status <type> <message>.

Outside a Scion container (no sciontool on PATH) the sticky-state tool isn’t registered and the hooks config either goes unset or shells out to /bin/true-style no-ops — the same binary works for local development.

Configuration

.agents/config.json in the agent’s workspace:

{
  "model": {"provider": "gemini", "name": "gemini-3.6-flash"},
  "hooks": {
    "tool-start":   [{"command": "sciontool hook --dialect=core-agent"}],
    "tool-end":     [{"command": "sciontool hook --dialect=core-agent"}],
    "model-start":  [{"command": "sciontool hook --dialect=core-agent"}],
    "agent-end":    [{"command": "sciontool hook --dialect=core-agent"}]
  }
}

Each handler’s command is passed to /bin/sh -c with the envelope JSON on stdin. Handlers run sequentially with a per-command timeout (default 10s; override via timeout_seconds). See Hooks for the full mechanism.

Instructions to the model (typically in AGENTS.md) should mention the sticky-state tool: “Call sciontool_status(task_completed, "...") when the task is done; call sciontool_status(ask_user, "...") before waiting on user input.”

Container image

Any image with core-agent, tmux, and sciontool on PATH works. The scion-base image already ships tmux and sciontool; layering core-agent on top is a one-line Dockerfile:

FROM scion-base:latest
COPY core-agent /usr/local/bin/core-agent

Or build core-agent from source inside the image if you prefer.

Staging into a Scion checkout

Until Scion adopts the harness bundle upstream, copy it into a local Scion checkout:

cp -r extras/scion/. ../scion/harnesses/core-agent/
# then edit ../scion/harnesses/embed.go to include all:core-agent/*
cd ../scion && go build -o sciontool ./cmd/sciontool

Full instructions in extras/scion/README.md.

Env-var contract

Var	Set by	Purpose
`GOOGLE_API_KEY`	user / Scion	Gemini API key.
`GEMINI_API_KEY`	Scion’s Gemini harness	Read directly by core-agent’s gemini provider.
`ANTHROPIC_API_KEY`	user	Anthropic API key (when `model.provider: anthropic`).
`HOME`	container	Where `sciontool` writes `agent-info.json`.

Other env vars follow core-agent’s Providers documentation.

Sticky vs transient

Kind	Examples	Mechanism	Frequency
Transient	`thinking`, `executing`, `working`	`sciontool hook` invoked by `pkg/hooks` on each tool/model boundary	per agent / tool boundary
Sticky	`ask_user`, `blocked`, `task_completed`, `limits_exceeded`	`sciontool status <type> <message>` invoked by the `sciontool_status` ADK tool	invoked intentionally by the model

Scion’s sciontool binary owns $HOME/agent-info.json — core-agent doesn’t touch that file directly. Sticky states are also POSTed to Scion’s Hub when running in hosted mode.