What's new in ASP.NET Core in .NET 11 Preview 6

[Copilot]

Adds Preview 6 content to the cumulative "What's new in ASP.NET Core in .NET 11" article, generated from the dotnet/core release notes (dotnet/core#10456) via the whats-new-from-release-notes agent.

Note on parameters: The issue body listed PREVIEW = preview5, but the referenced source PR and the issue title are both Preview 6. Processed Preview 6 accordingly.

New include files

• Minimal APIs — async-validation-minimal-apis-preview-6.md (IAsyncValidatableObject, AsyncValidationAttribute, Validator.ValidateObjectAsync), short-circuit-endpoints-attribute-preview-6.md ([ShortCircuit])
• OpenAPI — openapi-3-2-default-preview-6.md (documents now target 3.2 by default)
• SignalR — signalr-authentication-refresh-preview-6.md (token refresh without reconnect), signalr-cancel-hub-invocations-preview-6.md (client-side cancellation of non-streaming invocations)

Updated files

• csharp-unions-preview-6.md — expanded to reflect the grown source: unions now span Minimal APIs, MVC/Razor Pages, SignalR JSON hub protocol, and Blazor, plus OpenAPI anyOf schema behavior and non-body binding limits.
• aspnetcore-11.md — added [!INCLUDE] directives in the mapped sections; bumped ms.date to 07/10/2026; Breaking changes section left intact.

Excluded (per content rules)

• Blazor Virtualize scroll-to-item — owned by the Blazor docs team; flagged, not authored here.
• Bug fixes and the standalone contributors list — inline contributor thank-yous (@Youssef1313, @Porozhniakov) are preserved within their feature includes.

Conventions

Includes use H3 headings, no front matter, -preview-6 suffixes, relative Learn links, and present-tense phrasing. Verified <xref:> IDs are used where docs exist; brand-new Preview 6 APIs without published docs carry <!-- TODO: Update to <xref:> ... --> markers for later conversion.

---

Internal previews

📄 File	🔗 Preview link
aspnetcore/release-notes/aspnetcore-11.md	What's new in ASP.NET Core in .NET 11

[wadepickett]

@tdykstra and @guardrex,
I started a draft for this. There may be udpates of the release notes to sync with between now and the final version. This does not include any update related to Blazor yet @guardrex since you usually like to do you own, but you can add to this draft if you want. As a reminder I am not available all next week. Tdykstra will take this over from here for any sync.

[guardrex]

@wadepickett ... Thanks!👍 Illona has a PR up, and I have one with only the Blazor C# union bits on it. I'll check next week to see what else Blazor needs. I'll review this Monday morning (without commits from me) to assist @tdykstra with the approval he'll need.

[guardrex]

Approving with a handful of NIT suggestions.

[guardrex]

Suggested change

Asynchronous validation lets a validation rule do real work, such as a database lookup or a remote API call, without blocking a thread. A model implements `IAsyncValidatableObject` and returns validation results as an `IAsyncEnumerable<ValidationResult>`. Because `IAsyncValidatableObject` extends `IValidatableObject`, also implement the synchronous `Validate` method. When a type validates asynchronously only, throw from `Validate` so it isn't silently validated through the synchronous APIs:

Asynchronous validation lets a validation rule do real work, such as a database lookup or a remote API call, without blocking a thread. A model implements `IAsyncValidatableObject` and returns validation results as an `IAsyncEnumerable<ValidationResult`. Because `IAsyncValidatableObject` extends `IValidatableObject`, also implement the synchronous `Validate` method. When a type validates asynchronously only, throw from `Validate` so it isn't silently validated through the synchronous APIs:

[guardrex]

Optional; but AFAIK (and what I've been doing in the Blazor node and samples), framework coding calls for a line between the start of an if expression and a preceding code line.

Suggested change

	if (!await rooms.HasAvailabilityAsync(Date, cancellationToken))
	if (!await rooms.HasAvailabilityAsync(Date, cancellationToken))

[guardrex]

You could also place the C# language reference cross-link here as well, but this is What's New, so perhaps it's best to leave it as-is. I went through on the Blazor PR and updated all of the non-What's New references to use the reference doc, so we won't be saddled with pointing devs to a What's New language feature article in reference content when the language reference article exists. Here's the link in case you want to see or use it ...

https://learn.microsoft.com/dotnet/csharp/language-reference/builtin-types/union

[guardrex]

Suggested change

	A few limits apply. Only JSON request bodies and responses are supported — binding a union from the query string, route values, headers, or form fields isn't yet available. When multiple cases serialize to the same JSON shape, disambiguate them with a `[JsonUnion]` classifier. SignalR unions require the JSON hub protocol; the MessagePack and Newtonsoft.Json protocols don't support unions.
	A few limits apply. Only JSON request bodies and responses are supported—binding a union from the query string, route values, headers, or form fields isn't yet available. When multiple cases serialize to the same JSON shape, disambiguate them with a `[JsonUnion]` classifier. SignalR unions require the JSON hub protocol; the MessagePack and `Newtonsoft.Json` protocols don't support unions.

[guardrex]

Suggested change

	Generated OpenAPI documents now target OpenAPI 3.2 by default. Documents continue to generate as before; set the document version explicitly if you need to target an earlier version for tooling that hasn't adopted 3.2 yet.
	Generated OpenAPI documents now target OpenAPI 3.2 by default. Documents continue to generate as before. Set the document version explicitly if you need to target an earlier version for tooling that hasn't adopted 3.2 yet.

[guardrex]

I still need to understand if the repo's convention for code-fencing all filenames and paths remains in effect after the "freshness" pass.

Suggested change

	Short-circuiting is useful for endpoints that don't need authentication, CORS, or other middleware — for example a health check or a robots.txt response — and it avoids the cost of running that middleware. The endpoint still runs and produces its response. Pass an optional status code, such as `[ShortCircuit(404)]`, to set the response status code.
	Short-circuiting is useful for endpoints that don't need authentication, CORS, or other middleware—for example a health check or a `robots.txt` response—and it avoids the cost of running that middleware. The endpoint still runs and produces its response. Pass an optional status code, such as `[ShortCircuit(404)]`, to set the response status code.