Tool interface shape beats prompt engineering by a mile.

arXiv 2608.11386 ran 11,700 repository-issue-fixing trajectories across six tool architectures with capabilities held roughly equal. Structured interfaces improved run-to-run consistency up to 4.7x, natural-language search tools raised relevant-file discovery over 11%, and Python CodeAct-style interfaces cut steps 41.6% and tokens 56.3%. Cognitive-scaffolding tools like reasoning logs showed almost no effect. The highest-leverage knob on your coding agent is the tool schema, not the system prompt.