diff --git a/src/ModelContextProtocol.Core/Client/McpClientImpl.cs b/src/ModelContextProtocol.Core/Client/McpClientImpl.cs
index 4205c28e1..5beebb6df 100644
--- a/src/ModelContextProtocol.Core/Client/McpClientImpl.cs
+++ b/src/ModelContextProtocol.Core/Client/McpClientImpl.cs
@@ -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);
diff --git a/tests/ModelContextProtocol.Tests/Client/McpClientTests.cs b/tests/ModelContextProtocol.Tests/Client/McpClientTests.cs
index 262efbd40..b97d4d3ab 100644
--- a/tests/ModelContextProtocol.Tests/Client/McpClientTests.cs
+++ b/tests/ModelContextProtocol.Tests/Client/McpClientTests.cs
@@ -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()
     {