[fix] add type field to client session config BaseModels

[bfineran]

Adds a pre-set type field to SseServerParameters, StreamableHttpParameters, and StdioServerParameters so serialized parameters will easily be loaded into the correct class.

Motivation and Context

All fields of SseServerParameters overlap with StreamableHttpParameters, so a third party application that would want to connect to multiple session with a config such as:

class ClientConfig(BaseModel):
    mcp_configs: List[Union[StreamableHttpParameters, SseServerParameters]]

would not have a way to be instantiated from a serialized config (like a yaml file) since the following is ambiguous.

mcp_configs:
  - url: ""

Additionally, if someone were to serialize either the sse or http config classes they would not be able to reload the second class listed in the union.

Adding the preset type field should fix this in a backwards compatible way.

How Has This Been Tested?

Existing tests cover backwards compatibility, confirmed this PR addresses the described use case

Breaking Changes

No, added fields include the only valid Literal default.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[Kludex]

I don't think this is necessary. You can handle the serialization logic in your application.

[felixweinberger]

Hi @bfineran thank you for your contribution! And apologies for the time it took to get back tot his.

I don't think this is necessary. You can handle the serialization logic in your application.

Agree with this, this seems like a concern that should be handled at the application layer and not in the SDK - within the SDK we can leverage isinstance to compare the Pydantic Types of already instantiated objects.

If you're storing these configurations somehow and need to discriminate between different types, you could have a wrapper class that does this for you without having to modify the SDK types for it.

Something like this:

  class ServerConfig(BaseModel):
      type: Literal["stdio", "sse", "streamable_http"]
      config: dict[str, Any]

      def to_server_params(self) -> Union[StdioServerParameters, ...]:
          if self.type == "stdio":
              # do stuff
          elif self.type == "sse":
              # do stuff
          elif self.type == "streamable_http":
              # do stuff