# TSR1501: Type '…' cannot be checked at runtime: ….

> parse<T> was given a type with no runtime check, such as a function type, an abstract class or an unregistered brand. Validate a data type instead.

## What it means

`parse<T>` and the other `@ts-r.dev/guard` helpers expand into a runtime validator built from the type's declarations. Some types have no runtime check: function and constructor types, `Promise`, `this` types, conditional and mapped types that don't resolve on declarations, `typeof` queries, unique symbols, `any` and `never`. `{1}` gives the reason.

Your own classes are checked with `instanceof`, so the validator must be able to import the class. Abstract classes, generic classes, classes imported with `import type`, `declare class` outside a `.d.ts` file and unexported classes are TSR1501.

Branded types such as `string & { readonly __brand: "UserId" }` look like a plain `string` at run time. They need a predicate registered in `[sound.guard] refinements`; without one they're TSR1501, and the reason names the configuration key.

## Example

```ts
// A function type has no runtime shape to check.
import { parse } from "@ts-r.dev/guard";

const raw: unknown = null;
const f = parse<(x: number) => string>(raw); // @expect TSR1501
export const out: string = f(1);
```

## How to fix

- Validate a plain data type: primitives, literals, object types, arrays, tuples, unions, `Record`, template literal types and the supported utility types.
- For values that hold functions, validate the data part and construct the rest in code.
- For an abstract class, validate a concrete subclass. Export the class, and import it with a value import, not `import type`.
- For a branded type, register a predicate: `[sound.guard] refinements = { UserId = "./src/ids#isUserId" }`, where `isUserId` is an exported `(v: unknown) => v is UserId`. The validator checks the base type, then calls it.

Sound mode is defined in [design record 0007](https://github.com/Webb-Ventures/typescript-rust/blob/main/docs/design/0007-sound-mode.md) (accepted 2026-10-15).

Source: https://ts-r.dev/errors/TSR1501/