How do I map fields between expression, invocation, and output steps?

The mapper in RPC Flow Methods (and the same mapping UI on Screens Flows) only offers targets that are already connected on the canvas. If expression inputs are empty, connect the expression output (or complete the step graph) first — that is expected Hub behavior, not a missing field.

Practical rules

• Map after the expression (or subflow) is wired to a downstream step.
• Expression input/output structures must include the fields you want. Changing a field’s type or nullability without refreshing the mapper leaves stale links.
• Empty arrays fail functions that assume at least one item — guard with a condition or a default.
• If the last row of the mapper table is clipped, that is a UI viewport issue: scroll or resize. The mapping may still exist.

Test the expression in isolation (Expression test) before blaming the invocation step.