Server unusable - JSONRPCResponse fields must not serialize to null

[fluidsonic]

Describe the bug
null-value properties of JSONRPCResponse (like _meta of RequestResult) incorrectly serialize to null in the response JSON, instead of being omitted.

According to the Spec, null is not an allowed value. It allows either omitting the property or an object value.

The MCP server is unusable.
⚠️ Claude & Claude Code do strict schema checking, which is valid. They reject the MCP response and assume the MCP connection fails.

To Reproduce
Steps to reproduce the behavior:

1. Connect to a kotlin-sdk MCP server with Claude Code.
2. Connection fails.

Expected behavior
Connection succeeds.

Logs
From Claude Code's debug log:

[DEBUG] MCP server "<name>": Connection failed after 36ms: [
  {
    "code": "invalid_union",
    "errors": [
      [
        {
          "expected": "string",
          "code": "invalid_type",
          "path": [
            "method"
          ],
          "message": "Invalid input: expected string, received undefined"
        },
        {
          "code": "unrecognized_keys",
          "keys": [
            "result"
          ],
          "path": [],
          "message": "Unrecognized key: \"result\""
        }
      ],
      [
        {
          "expected": "string",
          "code": "invalid_type",
          "path": [
            "method"
          ],
          "message": "Invalid input: expected string, received undefined"
        },
        {
          "code": "unrecognized_keys",
          "keys": [
            "id",
            "result"
          ],
          "path": [],
          "message": "Unrecognized keys: \"id\", \"result\""
        }
      ],
      [
        {
          "expected": "object",
          "code": "invalid_type",
          "path": [
            "result",
            "_meta"
          ],
          "message": "Invalid input: expected object, received null"
        }
      ],
      [
        {
          "expected": "object",
          "code": "invalid_type",
          "path": [
            "error"
          ],
          "message": "Invalid input: expected object, received undefined"
        },
        {
          "code": "unrecognized_keys",
          "keys": [
            "result"
          ],
          "path": [],
          "message": "Unrecognized key: \"result\""
        }
      ]
    ],
    "path": [],
    "message": "Invalid input"
  }
]

[kpavlov]

@fluidsonic, could you share a bit more about what’s going on? Also, what version of the MCP SDK and KotlinX Serialisation are you using? Is McpJson config used for serialization? I added a serialization test #583, but I can't reproduce the issue

[fluidsonic]

@kpavlov we use mcpStatelessStreamableHttp { … }.

Maybe we got our setup wrong, but I don't think McpJson is ever used.

kotlin-sdk/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/StreamableHttpServerTransport.kt

Line 200 in cc8cc76

activeStream.call.respond(payload)

This puts the JSONRPCResponse straight into the pipeline, to be serialized by whatever is installed in the pipeline.
We actually had to install the ContentNegotiation plugin just for that purpose, or otherwise we'd get an error that the response can't be serialized. We set up ContentNegotiation to use plain json().

Knowing about McpJson now, I moved the ContentNegotiation (we added globally) into the /mcp route and made it use McpJson:

install(ContentNegotiation) { // For MCP server.
	json(McpJson)
}

That works as a workaround!

[kpavlov]

Yes, it looks like McpJson was missing. Once #562 is completed, we should be good.
cc: @devcrocod