Keep Stateless as a non-obsolete convenience property

Retain the bool as shorthand for the Stateless and Stateful session modes,
while SessionMode remains available for selecting hybrid behavior. Remove the
MCP9008 diagnostic and related obsolete documentation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: saicharanpardhu

docs/concepts/stateless/stateless.md

@@ -43,7 +43,7 @@ The `SessionMode` property is the single most important setting for forward-proo
 
 <!-- mlc-disable-next-line -->
 > [!NOTE]
-> The older `bool` <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions.Stateless> property is obsolete ([`MCP9008`](xref:list-of-diagnostics#obsolete-apis)) because it cannot express hybrid mode. It remains a compatibility proxy over `SessionMode`: assigning `true` selects `Stateless` and assigning `false` selects `Stateful`, while reading it returns `true` only for `Stateless`. Both properties update the same underlying value, so the last assignment wins.
+> The `bool` <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions.Stateless> property remains a convenient shorthand for the two most common modes: assigning `true` selects `Stateless` and assigning `false` selects `Stateful`, while reading it returns `true` only for `Stateless`. Use `SessionMode` when you need `StatefulForInitializeClients`. Both properties update the same underlying value, so the last assignment wins.
 
 ### The 2026-07-28 protocol revision
 
@@ -466,7 +466,7 @@ builder.Services.AddMcpServer()
 | Property | Type | Default | Description |
 |----------|------|---------|-------------|
 | <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions.SessionMode> | <xref:ModelContextProtocol.AspNetCore.HttpServerSessionMode> | `Stateless` | Selects how the server tracks state between requests: `Stateless` (no sessions), `Stateful` (sessions for every client, `2026-07-28` refused), or `StatefulForInitializeClients` ([hybrid](#hybrid-mode-sessions-for-initialize-clients-only)). |
-| <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions.Stateless> | `bool` | `true` | _Obsolete (`MCP9008`)._ Compatibility proxy over `SessionMode`: `true` maps to `Stateless`, `false` maps to `Stateful`, and hybrid mode reads as `false`. Use `SessionMode` instead. |
+| <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions.Stateless> | `bool` | `true` | Convenience proxy over `SessionMode`: `true` maps to `Stateless`, `false` maps to `Stateful`, and hybrid mode reads as `false`. Use `SessionMode` to select hybrid mode. |
 | <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions.IdleTimeout> | `TimeSpan` | 2 hours | _Stateful only (`MCP9006`)._ Duration of inactivity before a session is closed. Checked every 5 seconds. |
 | <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions.MaxIdleSessionCount> | `int` | 10,000 | _Stateful only (`MCP9006`)._ Maximum idle sessions before the oldest are forcibly terminated. |
 | <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions.ConfigureSessionOptions> | `Func<HttpContext, McpServerOptions, CancellationToken, Task>?` | `null` | Per-session callback to customize `McpServerOptions` with access to `HttpContext`. In stateless mode (including all `2026-07-28` requests), this runs on every HTTP request. |

docs/list-of-diagnostics.md

@@ -46,4 +46,3 @@ When APIs are marked as obsolete, a diagnostic is emitted to warn users that the
 | `MCP9005` | In place | The Roots, Sampling, and Logging features are deprecated as of specification version 2026-07-28 and may be removed in a future version. See [SEP-2577](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2577) for more information. |
 | `MCP9006` | In place | The stateful Streamable HTTP configuration knobs on <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions> — `EventStreamStore`, `SessionMigrationHandler`, `PerSessionExecutionContext`, `IdleTimeout`, and `MaxIdleSessionCount` — only apply when the request is served with a session. Starting with the `2026-07-28` protocol revision, Streamable HTTP no longer supports sessions, and the SDK now defaults `SessionMode` to `HttpServerSessionMode.Stateless`. These knobs remain available for back-compat with the legacy stateful Streamable HTTP transport but new code should target the stateless path. |
 | `MCP9007` | In place | `AuthorizationRedirectDelegate` and `ClientOAuthOptions.AuthorizationRedirectDelegate` are retained for source and binary compatibility but cannot provide the authorization-response state or RFC 9207 issuer. State and issuer validation are skipped when these APIs are used. Use `ClientOAuthOptions.AuthorizationCallbackHandler` for response-bound, issuer-aware authorization flows. |
-| `MCP9008` | In place | <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions.Stateless> is a two-value flag that cannot express <xref:ModelContextProtocol.AspNetCore.HttpServerSessionMode.StatefulForInitializeClients>. Use <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions.SessionMode> instead. `Stateless` remains a compatibility proxy over `SessionMode`: `true` maps to `Stateless` and `false` maps to `Stateful`. See [Stateless and stateful mode](xref:stateless#hybrid-mode-sessions-for-initialize-clients-only) for details. |

src/Common/Obsoletions.cs

@@ -50,8 +50,4 @@ internal static class Obsoletions
     public const string AuthorizationRedirectDelegate_DiagnosticId = "MCP9007";
     public const string AuthorizationRedirectDelegate_Message = "AuthorizationRedirectDelegate cannot provide the RFC 9207 issuer and is retained for compatibility only. Use AuthorizationCallbackHandler instead.";
     public const string AuthorizationRedirectDelegate_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#obsolete-apis";
-
-    public const string StatelessProperty_DiagnosticId = "MCP9008";
-    public const string StatelessProperty_Message = "HttpServerTransportOptions.Stateless cannot express the hybrid session mode. Use HttpServerTransportOptions.SessionMode instead.";
-    public const string StatelessProperty_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#obsolete-apis";
 }

src/ModelContextProtocol.AspNetCore/HttpServerTransportOptions.cs

@@ -101,14 +101,13 @@ public class HttpServerTransportOptions
     /// set to <see langword="false"/> only when you need to support legacy clients that rely on session affinity.
     /// </value>
     /// <remarks>
-    /// This property is a compatibility proxy over <see cref="SessionMode"/>. Reading it returns
+    /// This property is a convenience proxy over <see cref="SessionMode"/>. Reading it returns
     /// <see langword="true"/> only when <see cref="SessionMode"/> is <see cref="HttpServerSessionMode.Stateless"/>,
     /// so <see cref="HttpServerSessionMode.StatefulForInitializeClients"/> reads as <see langword="false"/>.
     /// Assigning <see langword="true"/> selects <see cref="HttpServerSessionMode.Stateless"/> and assigning
     /// <see langword="false"/> selects <see cref="HttpServerSessionMode.Stateful"/>. Because both properties
     /// update the same underlying value, the last assignment wins when both are configured.
     /// </remarks>
-    [Obsolete(Obsoletions.StatelessProperty_Message, DiagnosticId = Obsoletions.StatelessProperty_DiagnosticId, UrlFormat = Obsoletions.StatelessProperty_Url)]
     public bool Stateless
     {
         get => SessionMode is HttpServerSessionMode.Stateless;

tests/ModelContextProtocol.AspNetCore.Tests/HttpServerTransportOptionsTests.cs

@@ -8,7 +8,6 @@ public void SessionMode_DefaultsToStateless()
         Assert.Equal(HttpServerSessionMode.Stateless, new HttpServerTransportOptions().SessionMode);
     }
 
-#pragma warning disable MCP9008 // Stateless is obsolete; these tests verify the compatibility proxy.
     [Theory]
     [InlineData(true, HttpServerSessionMode.Stateless)]
     [InlineData(false, HttpServerSessionMode.Stateful)]
@@ -41,5 +40,4 @@ public void AssigningBothProperties_DoesNotThrow_AndLastAssignmentWins()
         options.Stateless = true;
         Assert.Equal(HttpServerSessionMode.Stateless, options.SessionMode);
     }
-#pragma warning restore MCP9008
 }