Harness Handbook – Making Agent Harnesses Understandable, Auditable and Editable

12 hours ago

Agent harness behavior is scattered across many implementation sites, making it hard to understand, audit, or adapt.
Harness Handbook organizes implementation into a behavior-level manual with three levels (L1, L2, L3) linked to code evidence.
Key problems include complex codebases (e.g., Codex has 2,267 files) and need for behavior localization to answer specific questions.
The Handbook supports three goals: understand system flow, audit behavior against expectations, and adapt for building custom agents.
Behavior-Guided Progressive Disclosure (BGPD) allows users to start from a behavior question and trace to code evidence layer by layer.
Experiments show Handbook-assisted coding agents achieve higher localization accuracy with lower token cost, especially for complex tasks.
Handbook Studio provides an interactive workbench for reading, verifying, and making changes on the behavior map.
Example change (temporary environment variables) demonstrates how Handbook reduces fan-out of edits across 10 files and 14 sites.
Takeaway: complex harnesses need a behavior-level map for understanding, auditing, and adapting, with code evidence at every step.

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