Harness design goals

Model quality depends on the agent harness: tool schemas, edit format, context handling, and control flow. The same weights can succeed or fail depending on those choices.

Primary mechanisms

1. Edit format. Formats that are hard to emit cause apply failures and retries. Hashline (and model-specific edit prompts) is the main write path in packages/coding-agent.
2. Control flow. Stop when verification passes; bound retries; budget context and subagent fan-out. Plan mode, goal mode, and tool-approval tiers encode parts of this in the engine.

Design consequences

• Hashline-aware edit / write with native verification (see The hashline edit engine).
• Explicit model slots and optional roles (modelRoles, catalog selectors, thinking levels).
• Engine-enforced modes: plan file + resolve path, goal continuation, approval tiers.
• Schema-based tool-call repair before argument validation (see Repair).

Related

• Mechanisms
• Repair
• The hashline edit engine