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Improve protocol version handling (#468)

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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.
This commit is contained in:
Stephen Toub 2025-06-02 23:03:57 -04:00 • committed by GitHub
commit e0b058f260
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2 changed files with 45 additions and 12 deletions
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12
src/ModelContextProtocol.Core/Client/McpClient.cs
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@ -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

13
src/ModelContextProtocol.Core/Client/McpClientOptions.cs
View file

@ -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.