It's more like this:

Right now when a model wants to call 1 or more tools, there is a fixed json schema to describe the tool calls.

Codemode is like: Why don't we just let the model write a program to call the tools and compose them however it wants? The key is that the programming language exposed to do this (usually javascript) will have APIs available to do some things internal to the harness (like call tools/mcp).