I've been exploring various use-cases for a binary wire format for MCP.
My exploration went beyond the media/base64 case: I also experimented with replacing JSON with MessagePack entirely.

I wrote up the full exploration here: Binary vs JSON for MCP: I Went Looking

Today, when a tool returns media inline (an image, audio, or a blob), the MCP content schema gives you exactly one way to carry the bytes: a base64-encoded JSON string. That's a fixed ~33% size tax on the wire, plus the CPU to encode on the way out and decode on the way in.

To be clear about scope: I'm not proposing a binary transport. I'm suggesting a content-schema change limited to the three inline-binary fields (ImageContent.data, AudioContent.data, BlobResourceContents.blob), allowing a native binary representation alongside today's base64 string.