Is the choice of JsonIgnoreCondition.WhenWritingNull intentional?

[glen-84]

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Question Category

• Protocol Specification
• SDK Usage
• Server Implementation
• General Implementation
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Your Question

var result = await mcpClient.CallToolAsync(
    "ExampleTool",
    new Dictionary<string, object?>
    {
        {
            "arg",
            new { field1A = new { field1B = new[] { new { field1C = (string?)null } } } }
        }
    });

I was surprised that field1C was dropped because it has a null value – is the use of JsonIgnoreCondition.WhenWritingNull intentional?

(I know that I can specify the JsonSerializerOptions – I'm just wondering why this is the default.)

[eiriktsarpalis]

We've adopted it as a default in the core protocol we found it beneficial to not transmit null properties. As you said, you can override this behavior for your own tool models.

[glen-84]

There's a semantic difference between null and "not set" (undefined) though:

• "Set this value to null"
• "Don't set this value at all"

The latter can result in a default value being used, as is the case with GraphQL. So instead of the intended value of null being persisted, you end up with the default value, violating POLA.

[eiriktsarpalis]

Can you point to particular instances of the MCP core protocol where this is happening?

[glen-84]

I'm not sure what you mean.

[eiriktsarpalis]

You mentioned that there are cases where the absense of an explicit null property can result in a "default" value being used. Are you referring to particular cases in the MCP protocol where this is a problem, or are you expressing a broader concern?

[glen-84]

I'm not referring to something in the protocol itself, but in the SDK.

If you look at this test, removing the serializerOptions results in the following snapshot change:

        "objectWithNullDefault": {
          "field1A": {
            "field1B": [
              {
-               "field1C": null
+               "field1C": "12:00:00"
              }
            ]
          }
        },

The "user" passes an explicit null for field1C, but the tool handler gets nothing, which has the effect of using the default value for the GraphQL field. If the "user" wanted the default value, they could have omitted the field from the input, but how do they provide an explicit null as an argument? It's two different things.

If you use JsonIgnoreCondition.WhenWritingNull, then two inputs are equivalent:

1. "field1C": null
2. (nothing)

But again, these are semantically different, and should not be equivalent.