This week in Cursor + .NET — 3 rules + 4 essays (week ending May 22, 2026)

This article summarizes a week of daily rules for senior C#/.NET engineers using the Cursor AI coding assistant, covering topics such as preventing AI from corrupting error models, catching silent DI lifetime bugs, and enforcing async CancellationToken usage. It also promotes a free Cursor rule file called "arch-core-lite.mdc" designed to stop the AI from forgetting architectural constraints, with a full kit available for £19.99. The canonical versions of these posts are hosted on the Agentic Architect blog.

Every weekday a single, opinionated rule for senior C /.NET engineers using Cursor. Here's the full week in one read — canonical posts live on the Agentic Architect blog. Thu 21 May · C patterns Stop the AI from undoing your Result/ErrorOr error model with throw and null on every new prompt. Wed 20 May · Cursor rules The correction spiral — and the circuit-breaker rule that ends it before you type the stop phrase. Tue 19 May · .NET DI The .NET lifetime bug that ships to production silently — and the one Cursor rule that catches it before merge. Mon 18 May · Architecture The hidden cost senior engineers pay every morning — and a four-rule architecture for eliminating it. Fri 22 May The single most expensive .NET runtime bug: a Singleton holding a Scoped service. Cursor cheerfully writes this without warning. Audit constructor parameters of any class registered as Singleton — if any are typically Scoped DbContext, repositories, MediatR sender , flag it before merge. Thu 21 May Every async method in your codebase should accept and forward a CancellationToken. Make it a rule that any new async signature without one is flagged. Stops the AI from quietly losing cancellation half-way down a call chain. Wed 20 May Never let the AI write database logic in your Controllers. Enforce a strict boundary where all IQueryable access stays in the Infrastructure layer. This keeps your business logic "pure" and readable for the LLM. The free arch-core-lite.mdc is one drop-in Cursor rule that ends the morning re-explanation ritual. Install in 60 seconds, see whether Cursor actually remembers your DI lifetimes, and decide for yourself whether the full kit is worth £19.99. arch-core-lite.mdc on GitHub Canonical home for everything in this digest: https://agenticstandardcontact-byte.github.io/agentic-architect/blog/.