Malformed initialize request with missing required params returns different JSON-RPC error codes per transport

[cclabadmin]

Describe the bug

When an initialize request has a valid JSON-RPC envelope and a recognized method name, but is missing required fields such as params.protocolVersion, the Kotlin SDK appears to return different JSON-RPC error codes depending on the transport:

• stdio returns -32603 Internal error, with the error message exposing the underlying Kotlin serialization exception.
• Streamable HTTP returns -32600 Invalid Request.

For the same JSON-RPC method and payload, I would expect the transports to classify the error consistently.

• Environment
  • Reproduced with stable release 0.13.0 (b7eeeb12058099f980fa98d1468506489a6b5f32)
  • Transports: stdio and Streamable HTTP

To reproduce

1. Start a Kotlin SDK MCP server over stdio or Streamable HTTP.
2. Send this malformed initialize request:

{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"capabilities":{},"clientInfo":{"name":"repro","version":"0.1.0"}}}

The request intentionally omits params.protocolVersion.

Expected behavior

Because the method exists but the method-specific params are incomplete, I would expect a consistent JSON-RPC error. The exact message can vary, but the error category should be stable across transports for the same input.

Logs

stdio returns -32603 Internal error. The error message appears to expose the underlying kotlinx.serialization.MissingFieldException message:

{"id":1,"error":{"code":-32603,"message":"Fields [protocolVersion, capabilities, clientInfo] are required for type with serial name 'io.modelcontextprotocol.kotlin.sdk.types.InitializeRequestParams', but they were missing"},"jsonrpc":"2.0"}

Streamable HTTP returns -32600 Invalid Request for the same request.

In both cases, the server remained usable.