This week in Cursor + .NET — 7 rules (week ending September 20, 2026)

A weekly digest from the Agentic Architect persistence kit: 7 senior C#/.NET rules for engineers keeping Cursor honest across sessions.

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.

7 daily senior rules

Rule 5: Minimal API Groups

Sun 20 Sep

Use MapGroup and EndPoints for .NET 9 services. It keeps the context window lean and focused, helping the AI provide more accurate code suggestions than it can with massive, bloated Controllers.

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Rule 4: Validator Enforcement

Sat 19 Sep

Every MediatR Request must have a FluentValidation rule. Force the AI to check the validator before writing handler logic. This prevents "context rot" where the AI forgets your business constraints.

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Rule 3: Structured Logging

Fri 18 Sep

Enforce ILogger with structured templates. Never use string interpolation in logs. This ensures that when you feed logs back into Cursor for debugging, the AI recognizes the patterns immediately.

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Rule 2: Persistence Boundaries

Thu 17 Sep

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.

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Rule 1: The Result Pattern

Wed 16 Sep

Stop using nulls for flow control. Use a Result object to force Cursor to handle success and failure cases explicitly. This prevents 90% of AI-generated logic errors in Web APIs.

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Rule 25: ActivitySource for OpenTelemetry

Tue 15 Sep

Logs alone won't debug a distributed system. Add a static readonly ActivitySource per project and wrap every external call (DB, HTTP, queue) in StartActivity. Cursor never adds OTEL spans on its own — give it a rule that recognises external-call patterns and proposes the trace.

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Rule 24: ValueTask Only When Justified

Mon 14 Sep

ValueTask is a perf optimisation for hot paths that often return synchronously. It is not a drop-in for Task. Cursor swaps them around without thinking. Flag ValueTask returns and ask whether the method is actually mostly synchronous. If not, revert to Task.

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Try one rule before you trust the whole kit

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 £9.00.

• Free sample: arch-core-lite.mdc on GitHub
• Full kit (£9.00, one-time): Agentic Architect Kit
• Daily rules feed: https://agentic-architect.dev/blog/

Canonical home for everything in this digest: https://agentic-architect.dev/blog/.