In live tests' CommandTestsBase.CallToolAsync(), when the mcpClient.CallToolAsync call throws an McpException containing the string An error occurred, CommandTestsBase returns null rather than bubbling that tool call exception.

This obfuscates server errors during live tests making troubleshooting difficult. If a tool call causes an exception, that exception should be the reason for the test failure. If a test is asserting that a specific call should throw an exception, XUnit has appropriate testing patterns:

 // Act & Assert var exception = await Assert.ThrowsAsync<McpException>(async () => await CallToolAsync(command, parameters)); // Optionally, verify the exception message Assert.Contains("An error occurred", exception.Message);