Improve protocol version handling (#468)

Right now by default, the client and server hardcode the initial spec version, and if there's a mismatch, the request fails.

With this change, by default both client and server are allowed to float to any supported version.

src/ModelContextProtocol.Core/Client/McpClient.cs

@@ -129,11 +129,12 @@ internal sealed partial class McpClient : McpEndpoint, IMcpClient
             try
             {
                 // Send initialize request
+                string requestProtocol = _options.ProtocolVersion ?? McpSession.LatestProtocolVersion;
                 var initializeResponse = await this.SendRequestAsync(
                     RequestMethods.Initialize,
                     new InitializeRequestParams
                     {
-                        ProtocolVersion = _options.ProtocolVersion,
+                        ProtocolVersion = requestProtocol,
                         Capabilities = _options.Capabilities ?? new ClientCapabilities(),
                         ClientInfo = _options.ClientInfo ?? DefaultImplementation,
                     },
@@ -154,10 +155,13 @@ internal sealed partial class McpClient : McpEndpoint, IMcpClient
                 _serverInstructions = initializeResponse.Instructions;
 
                 // Validate protocol version
-                if (initializeResponse.ProtocolVersion != _options.ProtocolVersion)
+                bool isResponseProtocolValid =
+                    _options.ProtocolVersion is { } optionsProtocol ? optionsProtocol == initializeResponse.ProtocolVersion :
+                    McpSession.SupportedProtocolVersions.Contains(initializeResponse.ProtocolVersion);
+                if (!isResponseProtocolValid)
                 {
-                    LogServerProtocolVersionMismatch(EndpointName, _options.ProtocolVersion, initializeResponse.ProtocolVersion);
-                    throw new McpException($"Server protocol version mismatch. Expected {_options.ProtocolVersion}, got {initializeResponse.ProtocolVersion}");
+                    LogServerProtocolVersionMismatch(EndpointName, requestProtocol, initializeResponse.ProtocolVersion);
+                    throw new McpException($"Server protocol version mismatch. Expected {requestProtocol}, got {initializeResponse.ProtocolVersion}");
                 }
 
                 // Send initialized notification

src/ModelContextProtocol.Core/Client/McpClientOptions.cs

@@ -34,11 +34,18 @@ public class McpClientOptions
     /// Gets or sets the protocol version to request from the server, using a date-based versioning scheme.
     /// </summary>
     /// <remarks>
+    /// <para>
     /// The protocol version is a key part of the initialization handshake. The client and server must 
-    /// agree on a compatible protocol version to communicate successfully. If the server doesn't support
-    /// the requested version, it will respond with a version mismatch error.
+    /// agree on a compatible protocol version to communicate successfully.
+    /// </para>
+    /// <para>
+    /// If non-<see langword="null"/>, this version will be sent to the server, and the handshake
+    /// will fail if the version in the server's response does not match this version.
+    /// If <see langword="null"/>, the client will request the latest version supported by the server
+    /// but will allow any supported version that the server advertizes in its response.
+    /// </para>
     /// </remarks>
-    public string ProtocolVersion { get; set; } = "2024-11-05";
+    public string? ProtocolVersion { get; set; }
 
     /// <summary>
     /// Gets or sets a timeout for the client-server initialization handshake sequence.

src/ModelContextProtocol.Core/McpSession.cs

@@ -28,6 +28,16 @@ internal sealed partial class McpSession : IDisposable
     private static readonly Histogram<double> s_serverOperationDuration = Diagnostics.CreateDurationHistogram(
         "mcp.server.operation.duration", "Measures the duration of inbound message processing.", longBuckets: false);
 
+    /// <summary>The latest version of the protocol supported by this implementation.</summary>
+    internal const string LatestProtocolVersion = "2025-03-26";
+
+    /// <summary>All protocol versions supported by this implementation.</summary>
+    internal static readonly string[] SupportedProtocolVersions =
+    [
+        "2024-11-05",
+        LatestProtocolVersion,
+    ];
+
     private readonly bool _isServer;
     private readonly string _transportKind;
     private readonly ITransport _transport;

src/ModelContextProtocol.Core/Protocol/PaginatedRequest.cs

@@ -6,7 +6,7 @@ namespace ModelContextProtocol.Protocol;
 /// Provides a base class for paginated requests.
 /// </summary>
 /// <remarks>
-/// <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/2024-11-05/schema.json">See the schema for details</see>
+/// <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/">See the schema for details</see>
 /// </remarks>
 public class PaginatedRequestParams : RequestParams
 {

src/ModelContextProtocol.Core/Server/McpServer.cs

@@ -161,9 +161,20 @@ internal sealed class McpServer : McpEndpoint, IMcpServer
                 UpdateEndpointNameWithClientInfo();
                 GetSessionOrThrow().EndpointName = EndpointName;
 
+                // Negotiate a protocol version. If the server options provide one, use that.
+                // Otherwise, try to use whatever the client requested as long as it's supported.
+                // If it's not supported, fall back to the latest supported version.
+                string? protocolVersion = options.ProtocolVersion;
+                if (protocolVersion is null)
+                {
+                    protocolVersion = request?.ProtocolVersion is string clientProtocolVersion && McpSession.SupportedProtocolVersions.Contains(clientProtocolVersion) ?
+                        clientProtocolVersion :
+                        McpSession.LatestProtocolVersion;
+                }
+
                 return new InitializeResult
                 {
-                    ProtocolVersion = options.ProtocolVersion,
+                    ProtocolVersion = protocolVersion,
                     Instructions = options.ServerInstructions,
                     ServerInfo = options.ServerInfo ?? DefaultImplementation,
                     Capabilities = ServerCapabilities ?? new(),

src/ModelContextProtocol.Core/Server/McpServerOptions.cs

@@ -32,8 +32,11 @@ public class McpServerOptions
     /// <remarks>
     /// The protocol version defines which features and message formats this server supports.
     /// This uses a date-based versioning scheme in the format "YYYY-MM-DD".
+    /// If <see langword="null"/>, the server will advertize to the client the version requested
+    /// by the client if that version is known to be supported, and otherwise will advertize the latest
+    /// version supported by the server.
     /// </remarks>
-    public string ProtocolVersion { get; set; } = "2024-11-05";
+    public string? ProtocolVersion { get; set; }
 
     /// <summary>
     /// Gets or sets a timeout used for the client-server initialization handshake sequence.

tests/ModelContextProtocol.TestServer/Program.cs

@@ -46,7 +46,6 @@ internal static class Program
                 Logging = ConfigureLogging(),
                 Completions = ConfigureCompletions(),
             },
-            ProtocolVersion = "2024-11-05",
             ServerInstructions = "This is a test server with only stub functionality",
         };
 

tests/ModelContextProtocol.TestSseServer/Program.cs

@@ -34,7 +34,6 @@ public class Program
             Resources = new(),
             Prompts = new(),
         };
-        options.ProtocolVersion = "2024-11-05";
         options.ServerInstructions = "This is a test server with only stub functionality";
 
         Console.WriteLine("Registering handlers.");