Clarify assembly requirements for AddValidation() in .NET 10 Minimal APIs

[Copilot]

AddValidation() uses a compile-time source generator that only discovers validatable types in the assembly where it's called. When endpoints live in a referenced assembly but AddValidation() is registered in the host, validation silently doesn't run—invalid payloads return 200 OK instead of 400 Bad Request. The docs didn't call this out.

Changes

• aspnetcore/fundamentals/minimal-apis.md
  • Added a [!NOTE] on the source-generator/assembly constraint.
  • Added a new Register validation in multi-assembly apps section covering the failing symptom, the extension-method workaround, the multi-assembly case, and troubleshooting.
  • Bumped ms.date.
• aspnetcore/release-notes/aspnetcore-10/includes/ValidationSupportMinAPI.md — Added a short [!NOTE] cross-referencing the new section.

Workaround documented

Register AddValidation() from within the endpoints' assembly, then call it from the host:

// In the assembly that defines the endpoints
public static class ServiceCollectionExtensions
{
    public static IServiceCollection AddApiValidation(this IServiceCollection services)
        => services.AddValidation();
}

// Host Program.cs
builder.Services.AddApiValidation();

Guidance mirrors the existing Blazor "Use validation models from a different assembly" precedent.

• Fixes Clarify source-generator/assembly requirements for AddValidation() in .NET 10 Minimal APIs #36899

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Internal previews

File	Preview link
aspnetcore/fundamentals/minimal-apis.md	aspnetcore/fundamentals/minimal-apis
aspnetcore/release-notes/aspnetcore-10/includes/ValidationSupportMinAPI.md	aspnetcore/release-notes/aspnetcore-10/includes/ValidationSupportMinAPI

[wadepickett]

Approved (after changes were made).

2nd Reviewer approval still required. @gumbarros does this look OK to you? Publish?

[gumbarros]

Approved (after changes were made).

2nd Reviewer approval still required. @gumbarros does this look OK to you? Publish?

Yes, perfect.

[wadepickett]

Sorry @gumbarros, I accidently sent a review request after you provided one. Please ignore. My I didn't notice my page had not refreshed.

Thanks very much for writing up the issue, your suggestions and providing reviews!!!

[tdykstra]

LGTM - Just a couple nits.