Fixed issue when accept:*/* and operation is a subscription.

src/HotChocolate/AspNetCore/src/AspNetCore/Serialization/DefaultHttpResponseFormatter.cs

@@ -551,7 +551,7 @@ protected virtual void OnWriteResponseHeaders(
                 return _multiPartFormat;
             }
 
-            if (mediaType.Kind is EventStream)
+            if (mediaType.Kind is EventStream or All)
             {
                 return _eventStreamFormat;
             }

src/HotChocolate/AspNetCore/test/AspNetCore.Tests.Utilities/ServerTestBase.cs

@@ -41,6 +41,7 @@ protected virtual TestServer CreateStarWarsServer(
                     .AddStarWarsTypes()
                     .AddTypeExtension<QueryExtension>()
                     .AddTypeExtension<SubscriptionsExtensions>()
+                    .AddType<Foo>()
                     .AddStarWarsRepositories()
                     .AddInMemorySubscriptions()
                     .UseInstrumentation()
@@ -165,4 +166,10 @@ protected virtual TestServer CreateServer(
                 .UseRouting()
                 .UseEndpoints(endpoints => configureConventions?.Invoke(endpoints)));
     }
+
+    [DirectiveType(DirectiveLocation.Subscription)]
+    public class Foo
+    {
+        public required int Bar { get; set; }
+    }
 }

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/GraphQLOverHttpSpecTests.cs

@@ -457,7 +457,102 @@ public async Task EventStream_Sends_KeepAlive()
         Snapshot
             .Create()
             .Add(response)
-            .MatchInline("""
+            .MatchInline(
+                """
+                Headers:
+                Cache-Control: no-cache
+                Content-Type: text/event-stream; charset=utf-8
+                -------------------------->
+                Status Code: OK
+                -------------------------->
+                event: next
+                data: {"data":{"delay":"next"}}
+
+                :
+
+                event: next
+                data: {"data":{"delay":"next"}}
+
+                :
+
+                event: complete
+
+
+                """);
+    }
+
+    [Fact]
+    public async Task EventStream_When_Accept_Is_All()
+    {
+        // arrange
+        var server = CreateStarWarsServer();
+        var client = server.CreateClient();
+        client.Timeout = TimeSpan.FromSeconds(30);
+
+        // act
+        using var request = new HttpRequestMessage(HttpMethod.Post, _url);
+        request.Content = JsonContent.Create(
+            new ClientQueryRequest
+            {
+                Query = "subscription {delay(count: 2, delay:15000)}",
+            });
+        request.Headers.Add("Accept", "*/*");
+
+        using var response = await client.SendAsync(request, ResponseHeadersRead);
+
+        // assert
+        Snapshot
+            .Create()
+            .Add(response)
+            .MatchInline(
+                """
+                Headers:
+                Cache-Control: no-cache
+                Content-Type: text/event-stream; charset=utf-8
+                -------------------------->
+                Status Code: OK
+                -------------------------->
+                event: next
+                data: {"data":{"delay":"next"}}
+
+                :
+
+                event: next
+                data: {"data":{"delay":"next"}}
+
+                :
+
+                event: complete
+
+
+                """);
+    }
+
+    [Fact]
+    public async Task EventStream_When_Accept_Is_All_And_Subscription_Directive()
+    {
+        // arrange
+        var server = CreateStarWarsServer();
+        var client = server.CreateClient();
+        client.Timeout = TimeSpan.FromSeconds(30);
+
+        // act
+        using var request = new HttpRequestMessage(HttpMethod.Post, _url);
+        request.Content = JsonContent.Create(
+            new ClientQueryRequest
+            {
+                Query = "subscription foo @foo(bar: 1) {delay(count: 2, delay:15000)}",
+            });
+        request.Headers.Add("Accept", "*/*");
+
+        using var response = await client.SendAsync(request, ResponseHeadersRead);
+
+        // assert
+        Snapshot
+            .Create()
+            .Add(response)
+            .MatchInline(
+                """
                 Headers:
                 Cache-Control: no-cache
                 Content-Type: text/event-stream; charset=utf-8

src/HotChocolate/Core/test/Types.Tests/Types/SubscriptionTypeTests.cs

@@ -6,6 +6,7 @@
 #pragma warning disable CS0618 // Type or member is obsolete
 #nullable enable
 
+using System.Runtime.CompilerServices;
 using CookieCrumble;
 using HotChocolate.Execution;
 using HotChocolate.Subscriptions;
@@ -647,6 +648,44 @@ public async Task Arguments_Can_Be_Declared_On_The_Stream()
             """);
     }
 
+    [Fact]
+    public async Task Subscription_Directives_Are_Allowed()
+    {
+        // arrange
+        // act
+        var executor = await new ServiceCollection()
+            .AddGraphQLServer()
+            .AddDocumentFromString(
+                """
+                type Subscription {
+                  bookAdded: String!
+                }
+
+                directive @bug(test: Int!) on SUBSCRIPTION
+                """)
+            .BindRuntimeType<SubscriptionWithDirective>("Subscription")
+            .ModifyOptions(o => o.StrictValidation = false)
+            .BuildRequestExecutorAsync();
+
+        var result = await executor.ExecuteAsync(
+            """
+            subscription test @bug(test: 123) {
+              bookAdded
+            }
+            """);
+
+        // assert
+        result.MatchInlineSnapshot(
+            """
+            {
+              "data": {
+                "bookAdded": "foo"
+              }
+            }
+
+            """);
+    }
+
     public class TestObservable : IObservable<string>, IDisposable
     {
         public bool DisposeRaised { get; private set; }
@@ -1047,4 +1086,18 @@ public string OnExplicit(
             [EventMessage] string message) =>
             message;
     }
+
+
+    public class SubscriptionWithDirective
+    {
+        [Subscribe(With = nameof(GetStream))]
+        public string BookAdded([EventMessage] string foo) => foo;
+
+        private async IAsyncEnumerable<string> GetStream(
+            [EnumeratorCancellation] CancellationToken ct = default)
+        {
+            await Task.Delay(200, ct);
+            yield return "foo";
+        }
+    }
 }