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Fix client to accept any supported server protocol version during negotiation #1537

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mikekistler wants to merge 2 commits into from
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Fix client to accept any supported server protocol version during negotiation #1537

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214df79
Add test for protocol version downgrade negotiation (#1536)
mikekistler Apr 28, 2026
888e978
Fix client to accept any supported server protocol version (#1536)
mikekistler Apr 28, 2026
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4 changes: 2 additions & 2 deletions src/ModelContextProtocol.Core/Client/McpClientImpl.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -569,8 +569,8 @@ public async Task ConnectAsync(CancellationToken cancellationToken = default)

// Validate protocol version
bool isResponseProtocolValid =
_options.ProtocolVersion is { } optionsProtocol ? optionsProtocol == initializeResponse.ProtocolVersion :
McpSessionHandler.SupportedProtocolVersions.Contains(initializeResponse.ProtocolVersion);
McpSessionHandler.SupportedProtocolVersions.Contains(initializeResponse.ProtocolVersion) ||
(_options.ProtocolVersion is { } optionsProtocol && optionsProtocol == initializeResponse.ProtocolVersion);
if (!isResponseProtocolValid)
{
LogServerProtocolVersionMismatch(_endpointName, requestProtocol, initializeResponse.ProtocolVersion);
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9 changes: 9 additions & 0 deletions tests/ModelContextProtocol.Tests/Client/McpClientTests.cs
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Expand Up @@ -587,6 +587,15 @@ public async Task ReturnsNegotiatedProtocolVersion(string? protocolVersion)
Assert.Equal(protocolVersion ?? "2025-11-25", client.NegotiatedProtocolVersion);
}

[Theory]
[InlineData("2025-11-25", "2025-06-18")]
public async Task AcceptsLowerServerProtocolVersion(string clientVersion, string serverVersion)
{
Server.ServerOptions.ProtocolVersion = serverVersion;
await using McpClient client = await CreateMcpClientForServer(new() { ProtocolVersion = clientVersion });
Assert.Equal(serverVersion, client.NegotiatedProtocolVersion);
}

[Fact]
public async Task EndToEnd_SamplingWithTools_ServerUsesIChatClientWithFunctionInvocation_ClientHandlesSamplingWithIChatClient()
{
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