Deserialize error data and populate exception.Data on client

Co-authored-by: halter73 <54385+halter73@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/McpSessionHandler.cs

@@ -453,7 +453,29 @@ public async Task<JsonRpcResponse> SendRequestAsync(JsonRpcRequest request, Canc
             if (response is JsonRpcError error)
             {
                 LogSendingRequestFailed(EndpointName, request.Method, error.Error.Message, error.Error.Code);
-                throw new McpProtocolException($"Request failed (remote): {error.Error.Message}", (McpErrorCode)error.Error.Code);
+                var exception = new McpProtocolException($"Request failed (remote): {error.Error.Message}", (McpErrorCode)error.Error.Code);
+                
+                // Populate exception.Data with the error data if present
+                if (error.Error.Data is not null)
+                {
+                    // The data could be a Dictionary<string, JsonElement> or a JsonElement containing an object
+                    if (error.Error.Data is Dictionary<string, JsonElement> dataDict)
+                    {
+                        foreach (var kvp in dataDict)
+                        {
+                            exception.Data[kvp.Key] = kvp.Value;
+                        }
+                    }
+                    else if (error.Error.Data is JsonElement jsonElement && jsonElement.ValueKind == JsonValueKind.Object)
+                    {
+                        foreach (var property in jsonElement.EnumerateObject())
+                        {
+                            exception.Data[property.Name] = property.Value;
+                        }
+                    }
+                }
+                
+                throw exception;
             }
 
             if (response is JsonRpcResponse success)

tests/ModelContextProtocol.Tests/McpProtocolExceptionDataTests.cs

@@ -3,6 +3,7 @@
 using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Server;
 using ModelContextProtocol.Tests.Utils;
+using System.Text.Json;
 
 namespace ModelContextProtocol.Tests;
 
@@ -72,6 +73,28 @@ public async Task Exception_With_Serializable_Data_Propagates_To_Client()
 
         Assert.Equal("Request failed (remote): Resource not found", exception.Message);
         Assert.Equal((McpErrorCode)(-32002), exception.ErrorCode);
+        
+        // Verify the data was propagated to the exception
+        // The Data collection should contain the expected keys
+        var hasUri = false;
+        var hasCode = false;
+        foreach (System.Collections.DictionaryEntry entry in exception.Data)
+        {
+            if (entry.Key is string key)
+            {
+                if (key == "uri") hasUri = true;
+                if (key == "code") hasCode = true;
+            }
+        }
+        Assert.True(hasUri, "Exception.Data should contain 'uri' key");
+        Assert.True(hasCode, "Exception.Data should contain 'code' key");
+        
+        // Verify the values (they should be JsonElements)
+        var uriValue = Assert.IsType<JsonElement>(exception.Data["uri"]);
+        Assert.Equal("file:///path/to/resource", uriValue.GetString());
+        
+        var codeValue = Assert.IsType<JsonElement>(exception.Data["code"]);
+        Assert.Equal(404, codeValue.GetInt32());
     }
 
     [Fact]
@@ -87,6 +110,23 @@ public async Task Exception_With_NonSerializable_Data_Still_Propagates_Error_To_
 
         Assert.Equal("Request failed (remote): Resource not found", exception.Message);
         Assert.Equal((McpErrorCode)(-32002), exception.ErrorCode);
+        
+        // Verify that only the serializable data was propagated (non-serializable was filtered out)
+        var hasUri = false;
+        var hasNonSerializable = false;
+        foreach (System.Collections.DictionaryEntry entry in exception.Data)
+        {
+            if (entry.Key is string key)
+            {
+                if (key == "uri") hasUri = true;
+                if (key == "nonSerializable") hasNonSerializable = true;
+            }
+        }
+        Assert.True(hasUri, "Exception.Data should contain 'uri' key");
+        Assert.False(hasNonSerializable, "Exception.Data should not contain 'nonSerializable' key");
+        
+        var uriValue = Assert.IsType<JsonElement>(exception.Data["uri"]);
+        Assert.Equal("file:///path/to/resource", uriValue.GetString());
     }
 
     [Fact]
@@ -100,6 +140,10 @@ public async Task Exception_With_Only_NonSerializable_Data_Still_Propagates_Erro
 
         Assert.Equal("Request failed (remote): Resource not found", exception.Message);
         Assert.Equal((McpErrorCode)(-32002), exception.ErrorCode);
+        
+        // When all data is non-serializable, the Data collection should be empty
+        // (the server's ConvertExceptionData returns null when no serializable data exists)
+        Assert.Empty(exception.Data);
     }
 
     /// <summary>