Proposal: an optional input kind hint on PromptArgument (text | select | search) #3222

The gap. PromptArgument today carries name/description/required. A client rendering a prompt-input form cannot distinguish "free text" from "pick one of a known set" from "search a large corpus" - so even a client that faithfully implements completion/complete has to guess whether a bare textbox, a dropdown, or a type-to-filter combobox is the right control.

Concrete case. Our server (a Salesforce support-case tool) exposes prompts whose one argument is a case number. The ideal UX is a picker over the user's open cases: the server already implements completion/complete for it, including empty-input completion that returns the full list so a dropdown could populate with zero typing. But nothing in the declaration tells a client this argument IS pickable - the completions capability says the server can answer, not what the answer's shape means for the control.

Working server-side implementation (empty input = full option set, typed input narrows): https://gist.github.com/layibabalola/77616813ce7bfdd28b569c29b2e508d3

Proposal. An optional input hint on PromptArgument:

{ "name": "caseNumber", "required": true,
  "input": { "kind": "search" } }

• "select": the client SHOULD populate a dropdown from completion/complete with an empty value (the server returns the full option set).
• "search": the client SHOULD render a type-to-filter combobox backed by debounced completion/complete calls.
• "text" (or absent): today's behavior.

Why a hint and not UI schema. The client still owns the controls; the server states only the SHAPE of the answer. Clients that ignore input keep today's behavior, so it is fully backwards compatible - the same graceful-degradation posture as every other optional capability.

What we shipped meanwhile (evidence the gap has real cost): our prompts now declare zero arguments and instruct the model to detect the target from the chat or list options in-chat and wait for the user's pick. It works, but it spends a model round-trip and an extra user turn to do what one dropdown could.