Flow ExecutionContext with JsonRpcMessage (#616)

The primary goal of this change is to support IHttpContextAccessor in tool calls when the Streamable HTTP is in its default non-Stateless mode.

src/ModelContextProtocol.AspNetCore/HttpServerTransportOptions.cs

@@ -35,6 +35,20 @@ public class HttpServerTransportOptions
     /// </remarks>
     public bool Stateless { get; set; }
 
+    /// <summary>
+    /// Gets or sets whether the server should use a single execution context for the entire session.
+    /// If <see langword="false"/>, handlers like tools get called with the <see cref="ExecutionContext"/>
+    /// belonging to the corresponding HTTP request which can change throughout the MCP session.
+    /// If <see langword="true"/>, handlers will get called with the same <see cref="ExecutionContext"/>
+    /// used to call <see cref="ConfigureSessionOptions" /> and <see cref="RunSessionHandler"/>.
+    /// </summary>
+    /// <remarks>
+    /// Enabling a per-session <see cref="ExecutionContext"/> can be useful for setting <see cref="AsyncLocal{T}"/> variables
+    /// that persist for the entire session, but it prevents you from using IHttpContextAccessor in handlers.
+    /// Defaults to <see langword="false"/>.
+    /// </remarks>
+    public bool PerSessionExecutionContext { get; set; }
+
     /// <summary>
     /// Gets or sets the duration of time the server will wait between any active requests before timing out an MCP session.
     /// </summary>

src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

@@ -188,6 +188,7 @@ internal sealed class StreamableHttpHandler(
             transport = new()
             {
                 SessionId = sessionId,
+                FlowExecutionContextFromRequests = !HttpServerTransportOptions.PerSessionExecutionContext,
             };
             context.Response.Headers[McpSessionIdHeaderName] = sessionId;
         }