A prompt optimizer that edits typed units instead of rewriting the whole string produces prompts 5x shorter for less optimization spend

arXiv 2608.28067·medium signalXBlueskyLinkedInCopy link

SEPO (arXiv 2608.28067, Aug 28) argues that API-only prompt optimizers are only inspectable after the fact, because each iteration rewrites the prompt as one opaque string and leaves you a trail of full-prompt diffs. Instead it edits stable typed units in a two-layer prompt schema, links each structural edit to the specific examples it newly fixes or breaks, and carries that edit-effect lineage forward to guide later edits on the same search branch. Across a 14-task held-out suite it beats GEPA by 3.1 points on Llama-3.1-8B-Instruct and 2.2 on Qwen3-8B, reaching 61.9% and 73.3% macro accuracy, while spending 2.9M optimization tokens against GEPA's 4.1M and emitting prompts over 5x shorter. Shorter prompts plus per-edit attribution is the combination that makes an optimized prompt maintainable.