API tracing and schema-change tests reveal both runtime failures and compatibility regressions. Static endpoints do not prove integration resilience. The architecture implied by the combination of Options A and D is the one that survives real workloads: separate responsibilities, explicit contracts, and measurable runtime behavior. Together, A states "Implement comprehensive API call tracing with latency measurement, success rates per endpoint, and correlation analysis between tool failures and task completion."; D states "Design integration tests simulating API version changes, schema modifications, and backward compatibility scenarios to ensure reliable tool connections across updates.", so the answer covers both sides of the requirement instead of solving only the model or only the infrastructure layer. The practical pattern is schema- bound tool invocation, typed parameters, timeout envelopes, retry policy, and traceable function execution. In NVIDIA terms, the Agent Toolkit model is to expose tools as reusable workflow components; that is what makes multi-tool agents testable under schema changes. The distractors fail because embedding tools inside the agent loop makes security review, timeout handling, and version control unnecessarily difficult. This is exactly where NVIDIA's stack is strongest: separating acceleration, orchestration, policy, and observability.