Properly clarify content model visibility - content vs structuredContent vs _meta

[liady]

There were questions from the community about the proper field to use for passing data to the UI without the passing through the model.

The spec mentions (in a somewhat hard-to-find location) that content is passed to the model, and that both structuredContent and _meta are not added to the context. This needs further clarification.
In addition - it's not clear whether content needs to be shown in addition to the UI.

As a comparison - Apps SDK spec clearly specifies that content and structuredContent are being passed to the model, while _meta isn't.

We should probably make it clearer in the spec.

[ochafik]

We should stick to the MCP semantics (and upcoming clarification SEP 1624):

• As far as host is concerned / what it sends to the model:
  • Use content for model-bound information
  • Only pass structuredContent if content was empty
• As far as apps / tools are concerned:
  • Pass data the model doesn't need to see in _meta
  • Use content to give the model a high-level executive summary of what's going on, so it's not overwhelmed w/ data it doesn't need (and isn't tempted to talk about said content for pages and pages). Can be as simple as "A view was displayed w/ the content of item 123 (a talking car), which the user can now interact with" (Also note you can call ui/update-model-context / app.updateModelContext w/ updates of whatever changes in the UI w/ a similar model-readable text)
  • Consistently w/ MCP spec, if passing structuredContent, also pass a text version of it in content (note: text doesn't mean necessarily JSON.stringify, can be any textualization of it)

[aharvard]

TYSM for clarifying this for host implementation!