Patterns

How do I accept one search key while rejecting both an empty query and a query with several keys?

Build one variant per key: require that key and forbid every other key with optional `never`.

The recipe

type ExactlyOne<T> = {
  [K in keyof T]:
    Required<Pick<T, K>> &
    Partial<Record<Exclude<keyof T, K>, never>>
}[keyof T]

type Search = ExactlyOne<{
  id: string
  email: string
  handle: string
}>

The build compiles this and checks each result below.

How it works

01
```
Required<Pick<T, K>>
```
The current variant requires its selected key.
02
```
Partial<Record<Exclude<keyof T, K>, never>>
```
Every unselected key is optional but forbidden when present.

What you get

{ id: string } extends Search ? true : false

→true

```
{} extends Search ? true : false
```
→false

{ id: string; email: string } extends Search ? true : false

→false

Where it goes wrong

Like `AtLeastOne`, this creates one union member per key. Keep the source object small or the resulting diagnostics and editor work will grow quickly.

Takeaway

Require the selected key, forbid the rest, then index the mapped table to form the union.