I assume because the discovery is branching. If the an agent using the CLI for for GitHub needs to make an issue, it can check the help message for the issue sub-command and go from there, doesn't need to know anything about pull requests, or pipelines, or account configuration, etc, so it doesn't query those subcommands.

Compare this to an MCP, where my understanding is that the entire API usage is injected into the context.

> How is progressive discovery not more expensive due to the increased number of steps?

Why not run the discovery (whether MCP or CLI) in a subagent that returns only the relevant tools. I mean, discovery can be done on a local model, right?

In short: JSON. Plan prose or markdown is way more token efficient than JSON. I think that responding in JSON was always a mistake in the spec; it should have been free-form text (which could then be JSON if required).

It depends on what your "currency" is: inference cost vs. models getting dumber/slower with a fuller context.