Harness#

In scion, a Harness is an adapter that allows a specific LLM tool to run within the Scion orchestration layer. It handles provisioning, configuration, and execution for that tool inside an OCI container.

Purpose#

The harness ensures that generic Scion commands (start, stop, attach, resume) work consistently regardless of the underlying agent software.

Interface#

Each harness implements (Go):

DiscoverAuth() — Locate credentials on the host
GetEnv() — Map credentials to container environment variables
GetCommand() — Build the correct CLI invocation
Provision() — Harness-specific setup during agent creation
PropagateFiles() — Copy config files into agent home
GetVolumes() — Define volume mounts

Supported Harnesses#

Harness	Target Tool	Notes
`gemini`	gemini-cli	Default harness. API key / OAuth / Vertex AI auth
`claude`	claude-code	Anthropic API key / Vertex AI auth. See claude-code for full capabilities.
`opencode`	opencode	Experimental. No hook support
`codex`	codex	Runs `--full-auto` by default
`generic`	Any CLI tool	Fallback adapter

Capability Matrix#

Capability	Gemini	Claude	OpenCode	Codex
Resume	✅	✅	✅	✅
Resume with Prompt	✅	✅	✅	❌
Hooks	✅	✅	❌	❌
OpenTelemetry	✅	✅	❌	✅
System Prompt Override	✅	✅	❌	❌

Extensibility#

New harnesses can be added via the plugin-system (hashicorp/go-plugin over gRPC) without modifying the core codebase.