A city compares the same permitting instructions in Claude Code versus a Messages API app. What design insight matters?

Cities comparing the same permitting instructions in Claude Code versus a Messages API app must expect instruction interpretation differences across Claude interfaces and validate each surface. That design insight matters because product surfaces can differ in defaults, tool availability, system-prompt placement, memory, and interaction patterns even when the written permitting guidance looks identical. Validating each shipped interface prevents false confidence that one successful Claude Code trial proves a Messages API permitting assistant will behave the same for planners. Reusing Claude Code system prompts directly in Messages API system blocks fails by concept: interface parity is not guaranteed and must be empirically checked. Centralizing permitting instruction tuning in Claude Code configuration for shared API behavior fails by concept: Messages API apps are full products that need their own instruction engineering. Matching sampling parameters like temperature across interfaces to normalize permitting instruction effects fails by concept: Claude Code workflows used by city staff also require correct permitting instruction design, and matching temperature won't make interfaces interpret instructions identically. Exam caveat: validation should use representative permitting cases and refusal tests on each surface, and differences may require surface-specific prompt adjustments rather than reusing Claude Code system prompts directly in Messages API system blocks. Operational check: run the same suite of permitting instruction test cases through Claude Code and the Messages API app, document behavioral deltas, and gate release on surface-specific fixes where interpretation diverges.