Plan-Mode Efficacy on Time-to-Merge: A Cross-Venture Study (n=42 tasks)

1. Introduction

Plan mode — the agent-harness convention of producing and approving an explicit, written implementation plan before any code is written — is one of the most-advocated, least-measured disciplines in agent-mediated software work. The Anthropic Claude Code product documentation describes plan mode as a tool for “non-trivial coding work” [8], but offers no quantitative threshold. Operators who adopt the discipline tend to apply it either uniformly (a costly habit on small tasks) or never (forfeiting the benefit on large ones). Both regimes are inefficient if the true cost-benefit curve has a non-trivial threshold.

We are interested in two questions. First, does plan mode reduce time-to-merge in practice, and by how much? Second, is the effect uniform across task sizes, or does a crossover exist below which the planning step is net-negative? The second question matters because every minute of planning is a minute of operator and agent time that could be spent on other work; if the benefit is concentrated in a particular task-size regime, operators can target the discipline rather than apply it uniformly.

The literature on planning before coding is old and largely qualitative. Brooks [1] argued that throwing away the first design is itself a form of planning. Parnas and Clements [2] proposed faking a rational design process — writing a plan after the fact that the work could have followed — as a substitute for the difficulty of planning ahead. Kahneman’s work on intuition vs. explicit decision rules [3] predicts that explicit rules outperform intuition in high-uncertainty regimes and underperform it in low-uncertainty regimes — a structural argument for a crossover threshold of the kind we observe.

This paper measures the threshold quantitatively for one specific class of work: agent-mediated coding tasks shipped through a small portfolio (Quantum Caddy, Mile High Golf, Parley, and TruPath cross-portfolio work) over 10 weeks. The within-organization design controls for many process and cultural variables that confound multi-organization studies of coding-tool effectiveness [5]. The cost is external validity: a single operator’s habits are baked into the data, and the result must be replicated elsewhere before being treated as a property of plan mode rather than a property of this operator’s working style.