The two customer tools (list vs get-by-id) are clearly separated, and calculate_invoice vs compare_implementations have distinct purposes spelled out in their descriptions. There is minor conceptual overlap since compare_implementations implicitly runs both calculators, so an agent might pick calculate_invoice when it really wanted verification, but the descriptions mitigate this.

All four tools follow a clean snake_case verb_noun pattern (list_customers, get_customer, calculate_invoice, compare_implementations). The verbs are conventional and predictable for their actions.

Four tools is thin for a server labeled 'Billing'; the surface is essentially read-customers plus invoice math plus a migration check. The count is defensible for a narrow strangler-facade migration service, but feels borderline minimal for the stated domain.

Customers are read-only (no create/update) and there is no invoice persistence or retrieval — only calculation — plus no order or payment operations. The calculation and migration-verification path is well covered, but the broader billing lifecycle has notable gaps.