Ponytail

Ponytail is a constraint you put on a coding agent's output: it pushes toward the smallest thing that works, preferring the standard library to new code, the platform to a dependency, and one line to fifty. Its claimed effect is a large reduction in generated code with no loss of safety.

The bias it corrects is not a model flaw but a training artefact. Models are rewarded for producing something that clearly works, and a visible, explicit implementation reads as more clearly correct than a terse one that leans on the standard library — so a fifty-line class can appear where a one-line comprehension belonged. The cost arrives later, as code someone has to read, review and maintain, and it compounds across a codebase. The caveat is that this is a heuristic applied to a judgement call: the terse version is sometimes genuinely worse, less clear to the next reader, and the reduction figure says nothing about which of those cases it produced. Treat it as a default that should be overridable, not a rule.