[Essential for first interviews — Tier 1] Diagnose and correct inappropriate singleton, scoped, and transient dependency-injection lifetimes in an ASP.NET Core request path.

[Essential for first interviews — Tier 1] Assemble authentication, authorization, routing, exception handling, and custom middleware in a defensible execution order.

[Essential for first interviews — Tier 1] Implement an asynchronous API operation that propagates CancellationToken and contains no sync-over-async calls.

[Essential for first interviews — Tier 1] Implement one portfolio REST endpoint with appropriate HTTP semantics, validation, and Problem Details errors.

[Essential for first interviews — Tier 2] Build a typed market-price client with IHttpClientFactory and an explicit timeout.

[Essential for first interviews — Tier 2] Bind and validate strongly typed options with environment-specific configuration overrides.

[Essential for first interviews — Tier 3] Select an API-versioning and backward-compatibility strategy for a stated client-change scenario.

[Essential for first interviews — Tier 3] Use an AI assistant to propose a constrained code change, then independently verify its correctness, security, tests, and documentation.