Cursor Composer 2.5 For .NET

Cursor Composer 2.5 is worth looking at because it pushes AI coding further away from autocomplete and closer to a real development loop. For .NET engineers, thats where things start looking interesting.

Most AI coding examples are still too small. They ask the model to write a method, generate a DTO, or create a unit test. Thats cool, but it doesnt reflect how software is actually built. Real .NET work usually means moving through a solution, understanding projects, following naming conventions, respecting dependency boundaries, adding tests, reading build failures, and making another change without losing track of the original intent.

Its aimed at that longer loop. Cursor says it improves sustained work, complex instruction following, communication style, and effort calibration. The training write-up also talks about more difficult reinforcement learning environments, targeted textual feedback, and far more synthetic coding tasks than Composer 2.

That sounds great until you put it inside a .NET solution. A coding agent that can keep going through a feature slice, run tests, fix compile errors, and avoid ignoring half your instructions is a different tool from a chat window that gives you a decent first draft.

Why .NET is a good test for coding agents

.NET projects are a strong test for agentic coding because the codebase usually has structure. You might have an API project, an application layer, infrastructure, test projects, shared contracts, migration files, background workers, and CI rules. Even in a modular monolith, a small change can cross several files. A new endpoint might need a request model, validator, handler, persistence change, tests, OpenAPI metadata, and logging.

That makes .NET a good place to see whether it is actually useful. The model has to understand shape, not just syntax. It has to follow the existing architecture instead of inventing a new one. It has to avoid pushing infrastructure concerns into the application layer. It has to know when a DbContext should stay scoped, when a CancellationToken should flow through the call chain, and when a generated abstraction is just noise. Thats the standard I would use for judging Composer 2.5 in a serious .NET codebase.

The useful workflow is agent plus tests plus review

The best use is not asking it to produce perfect code in one go. The better pattern is to give it a bounded task, let it inspect the solution, make the smallest useful set of changes, run the relevant tests, and then explain what it changed.

This is where agentic coding starts to make sense. The agent is not replacing review. It is reducing the drag between intent and a working patch. For .NET, that means Composer should be pushed towards tasks that already have a clear engineering boundary. A vertical slice. A test project. A handler. A migration. A background worker. A failing build. A small refactor with a measurable end state.

Loose prompts create loose code. That is true with people and it is even more true with agents.

A realistic .NET task

A good Composer task should sound like a well-written ticket. It should name the module, describe the change, set boundaries, and define the verification step. It should also include your house style. For example, if your codebase uses vertical slices, minimal APIs, and FluentValidation, say that directly.

In the Payments module, add idempotency support to the CreatePayment endpoint.

Follow the existing vertical-slice structure.
Use the current Minimal API style.
Do not introduce a shared service unless the existing code already uses one.
Keep domain logic out of the endpoint.
Use FluentValidation if the feature already uses it.
Add tests for duplicate idempotency keys.
Run the Payments test project and fix any failures.
Before editing, inspect the existing CreatePayment implementation and summarise the files you plan to change.

It removes a lot of choice from the model. It tells Composer where to look, what shape to preserve, what not to invent, and how to prove the change works. You are not asking for a grand design. You are asking for a patch.

What the generated shape should look like

If its working well in a .NET codebase, it should end up with something close to the existing project style. For a minimal API vertical slice, that might mean an endpoint that stays thin, a command that carries the request, and a handler that owns the workflow.

public static class CreatePaymentEndpoint
{
    public static IEndpointRouteBuilder MapCreatePayment(this IEndpointRouteBuilder app)
    {
        app.MapPost("/payments", async (
                CreatePaymentRequest request,
                string? idempotencyKey,
                ISender sender,
                CancellationToken stopToken) =>
            {
                var command = new CreatePayment.Command(
                    request.AccountId,
                    request.Amount,
                    request.Currency,
                    idempotencyKey);

                var result = await sender.Send(command, stopToken);

                return Results.Created($"/payments/{result.PaymentId}", result);
            })
            .WithName("CreatePayment")
            .WithOpenApi();

        return app;
    }
}

The important part is not this exact code. The important part is the separation of concerns. The endpoint maps transport data into a command and returns the result. It does not own idempotency, persistence, retries, or business rules.

The handler is where the behaviour belongs.

public static class CreatePayment
{
    public sealed record Command(
        Guid AccountId,
        decimal Amount,
        string Currency,
        string? IdempotencyKey) : IRequest<Response>;

    public sealed record Response(Guid PaymentId, string Status);

    public sealed class Handler(
        PaymentsDbContext db,
        ISystemClock clock) : IRequestHandler<Command, Response>
    {
        public async Task<Response> Handle(Command command, CancellationToken stopToken)
        {
            if (!string.IsNullOrWhiteSpace(command.IdempotencyKey))
            {
                var existing = await db.PaymentRequests
                    .Where(x => x.IdempotencyKey == command.IdempotencyKey)
                    .Select(x => new Response(x.PaymentId, x.Status))
                    .SingleOrDefaultAsync(stopToken);

                if (existing is not null)
                {
                    return existing;
                }
            }

            var payment = new Payment
            {
                PaymentId = Guid.NewGuid(),
                AccountId = command.AccountId,
                Amount = command.Amount,
                Currency = command.Currency,
                Status = "Received",
                IdempotencyKey = command.IdempotencyKey,
                CreatedAtUtc = clock.UtcNow
            };

            db.Payments.Add(payment);

            await db.SaveChangesAsync(stopToken);

            return new Response(payment.PaymentId, payment.Status);
        }
    }
}

That example is deliberately incomplete for production because real idempotency needs a uniqueness constraint and safe duplicate handling under concurrency. That is exactly the sort of detail you should make Composer handle explicitly rather than hoping it guesses.

A stronger follow-up prompt would be:

Now harden this for concurrent duplicate requests.

Add a unique database constraint on IdempotencyKey where the key is not null.
Update the handler so two simultaneous requests with the same key return the same payment result.
Do not use an in-memory lock.
Add a test that sends two concurrent CreatePayment commands with the same idempotency key.

Thats how Id use it. Keep the first task narrow. Then ask it to harden a specific risk.

Where Composer 2.5 should help most in .NET

The obvious use case is feature work, but I think the better use cases are the awkward middle-sized jobs that developers postpone.

Moving a controller endpoint to a minimal API endpoint is a good example. The shape is repetitive, but the details still need care. It can inspect the existing controller, map route metadata, preserve response codes, keep auth attributes equivalent, and add tests.

Another useful case is test backfilling. A lot of .NET Engineers have decent production code and patchy tests. It can inspect a handler and generate focused tests around the existing behaviour. That is safer than asking it to invent new features because the expected behaviour is already in the code. It can also help with dependency clean-up. For example, finding where a service is injected but only used to access one method, replacing it with a narrower interface, and updating tests. Thats the kind of tedious refactor a human can do, but it is also the kind of task that eats time and attention.

The agent needs project rules

Cursor works better when the repo tells the agent how to behave. For .NET, I would keep a short project rule file that explains the architecture in plain language. Do not write an essay. The point is to stop the model making avoidable mistakes.

This solution uses vertical slices.

Endpoint files should only handle transport concerns.
Business behaviour belongs in the command handler.
Infrastructure implementations stay in Infrastructure.
Application abstractions stay with the feature or application layer.
Do not create shared helper classes unless at least two existing features need them.
Use decimal for money and keep database precision explicit.
Prefer small focused tests over broad integration tests unless persistence behaviour is involved.

This is more useful than telling the model to "write clean code". Clean code means nothing unless the agent knows what your team accepts. The same idea applies to prompts. The more specific the boundary, the better the result.

Refactor only the Payments/CreatePayment vertical slice.
Do not modify other payment endpoints.
Do not rename public contracts.
Do not change database schema unless required for the idempotency constraint.
Do not add a new package.
After editing, show the diff summary grouped by file.

That is the level of direction I would use.

The .NET build loop is where agents prove themselves

A coding agent becomes far more useful when it can run the normal feedback loop. For .NET, that loop is simple. Restore, build, test, inspect the failure, fix the cause, and repeat.

The last step is important. You want the agent to tell you what it changed and what risk remains. That helps you review with intent rather than scanning a large diff cold.

A good final response from the agent should look like this:

Changed Payments/CreatePayment/CreatePaymentEndpoint.cs to pass the idempotency key into the command.
Changed Payments/CreatePayment/CreatePayment.cs to return an existing result for duplicate keys.
Added a unique filtered index on IdempotencyKey.
Added a concurrency test for duplicate requests.
Ran dotnet test tests/Payments.Tests/Payments.Tests.csproj.

Remaining risk:
The handler catches duplicate key errors based on the SQL Server exception number. If this provider changes, the error mapping should move behind a persistence abstraction.

That is useful. It gives you enough context to review the patch properly.

Where I would be careful

I wouldnt hand Composer 2.5 a vague instruction like "improve our payment architecture". Thats how you get a confident mess. Architecture work needs context the model may not have. Production constraints, team skill, release risk, etc all affect the right decision. Composer can help explore options, produce diagrams, or draft a migration path, but it should not silently redesign the system. I would also be careful with authentication, authorisation, payment state machines, database migrations, background processing, and anything that touches production data. Let the agent prepare a patch, but keep the decision-making with the engineer.

That doesnt make it less valuable. It means the value is in acceleration, not delegation without oversight.

A better prompt for architecture-sensitive work

For larger .NET work, I would split the task into analysis first, then implementation.

Analyse the current payment submission flow.

Do not edit files yet.

Find the endpoint, command handler, persistence model, tests, and any background workers involved.
Summarise the current flow.
Identify where idempotency should be enforced.
Identify any database constraints needed.
Identify risks around concurrent requests.
Suggest the smallest safe implementation plan.

Wait for approval before making changes.

That prompt gives you control. Composer can do the repo-reading and planning work, while you keep the authority to approve the design. Once the plan is right, the implementation prompt can be much narrower.

Implement option 1 from the approved plan.

Keep the change inside the Payments module.
Add the filtered unique index.
Add the duplicate request tests.
Run the Payments test project.
Do not alter public API contracts.

This is the core shift with agentic coding. You get better results by treating the model like a fast contributor who needs clear tickets, guardrails, and review.

What about cost?

Cursor lists Composer 2.5 standard at $0.50 per million input tokens and $2.50 per million output tokens. The fast variant has the same stated intelligence but costs $3.00 per million input tokens and $15.00 per million output tokens, with fast as the default.

For a single developer, the difference may not feel huge. For a team, default behaviour becomes spend. Id use the fast variant when latency affects the flow. Interactive debugging, pair programming style edits, and short feedback loops are good candidates. I would use standard for slower background work, test generation, documentation passes, and analysis tasks where waiting a little longer is acceptable. The cost conversation becomes more important once agents start reading larger repositories. A task that touches a .NET solution can pull in endpoint files, handlers, validators, entity mappings, migrations, and tests. That context is useful, but it is not free. The practical answer is to keep tasks scoped. Smaller tasks are easier to review, cheaper to run, and less likely to drift.

Composer 2.5 and senior engineering judgement

Better agents make senior judgement more valuable, not less. A junior developer might trust a large generated patch because it compiles. A senior engineer asks different questions. Did this preserve the module boundary? Did it change the public contract? Does the test prove the right behaviour? Is the database constraint safe? What happens under concurrency? What happens during deployment? Is the migration reversible? Does this create a support problem six months from now?

Composer can help you move faster through the mechanical parts of the work. It can inspect files, write tests, propose edits, and respond to failures. It cannot fully understand the production consequences unless you bring that context into the task. Thats the line I would hold. Use the agent to reduce friction. Dont use it to avoid thinking.

What I would actually use it for this week

If I had Composer 2.5 inside a .NET repo, I would start with tasks like this.

Find all endpoints in the Claims module that return Results.BadRequest with plain strings.
Replace them with the existing ProblemDetails pattern used elsewhere in the module.
Add or update tests for the changed responses.
Do not change route names or response status codes.
Run the Claims API test project.

Id also use it for targeted test work.

Add tests for CreateUserPermission.

Use the same test style as CreateRole.
Cover successful creation, duplicate permission name, invalid role id, and cancellation token flow.
Do not change production code unless a test reveals an obvious bug.

And Id use it for safe refactoring.

In the Users module, inspect the CreateRole and CreateUserPermission vertical slices.

Suggest a small refactor that removes duplication without introducing a shared generic abstraction.
Do not edit yet.
Show the proposed before and after shape.

Those are the jobs where it should earn its keep. They are real enough to be useful, small enough to review, and structured enough for the agent to succeed.

The anti-pattern is still the same

The worst way to use it is to throw a vague goal at it and accept the patch because it looks professional. AI-generated code often looks cleaner than it is. It can use the right names, the right syntax, and the right architecture vocabulary while quietly missing an important behaviour. Thats especially risky in .NET backend systems where the hard part is not writing C#, its preserving the contract and runtime behaviour. A generated migration can compile and still be dangerous. A generated retry policy can look sensible and still duplicate payments. Thats why the workflow around Composer matters as much as the model itself.

The real value for .NET teams

Composer 2.5 looks useful because it is moving towards the way developers actually work. It is trained for longer coding sessions, harder tasks, and better behaviour inside an agent loop. That lines up well with .NET development, where a lot of work involves moving through a structured solution and making changes across several files without breaking the shape of the system. For .NET teams, the opportunity is not to replace engineers. The opportunity is to reduce the drag around small and medium-sized engineering tasks. Turn vague work into bounded tickets. Give the agent rules. Make it inspect before editing. Make it run tests. Make it explain the diff. Review the output properly. Thats the practical version of agentic coding.

Cursor Composer 2.5 announcementncement

Cursor Composer 2.5 changeloger 2.5 changelog

Cursor Composer 2 technical reportal report