Multi-select compatibility probes

Slack's multi-select reference lists Section, Actions, and Input as supported parents. During the 12 October 2026 stress run, Slack rejected multi-select elements inside Actions in tested messages across all eight implementations with unsupported element:multiselect. Native Section accessories were accepted. This is a dated discrepancy between the documented contract and the tested Slack workspace, not evidence that Actions should be removed from slackblocks.

If your workspace rejects an Actions multi-select, construct the same native element as a Section accessory. For a form, put it in an Input block inside a Modal with a submit button. Each placement needs its own native parent constructor; do not rewrite serialized JSON or disable validation to work around an API rejection.

The executable probes cover static, external, users, conversations, and channels menus in every parent, using all ten maintained native APIs. They emit 15 payloads per language and include a structural check executed for all ten consumers by the documentation build. External options still require an authenticated options handler; constructing an external menu does not implement that handler.

For an authorized live reproduction:

1. Record the slackblocks version, runtime, test date, app scopes, surface, and exact sanitized payload. Use the same menu family and equivalent inputs for each parent. Keep test workspace identifiers private.
2. Send one Actions and one Section message through the same API transport. Open the Input Modal through a genuine fresh interaction trigger. Record API success or the complete sanitized error separately from whether the client visibly renders the control.
3. Select actual values and verify the callback and saved selection. An accepted payload alone does not prove that option loading, interaction routing, or application state works.
4. Preserve the rejected Actions request alongside the compatible native request. If current Slack behavior differs, report the new date and result instead of treating the historical rejection as a permanent validation rule.

Do not automatically retry an unchanged invalid_blocks request. The API rejected its content, so repair the native construction or investigate the documented/runtime mismatch first. This recipe changes the application layout; it does not change the shared Actions schema or promise identical behavior across every Slack client. Mobile verification remains a separate test.