---
title: Error API
description: Reference for TaggedError, exhaustive and partial matching, UnhandledException, codec errors, and Panic.
---

## `TaggedError(tag)`

Factory for a generic Error subclass:

```ts
class NotFound extends TaggedError("NotFound")<{
  id: string;
  message: string;
}> {}
```

Instances expose `_tag`, declared properties, native Error fields, `toJSON()`, exhaustive `.match(handlers)`, and generator iterator behavior. Concrete classes expose `.is(value)`.

## Guards

| API                       | Narrows to            |
| ------------------------- | --------------------- |
| `TaggedError.is(value)`   | `AnyTaggedError`      |
| `isTaggedError(value)`    | `AnyTaggedError`      |
| `ConcreteError.is(value)` | the concrete subclass |
| `Panic.is(value)`         | `Panic`               |
| `isPanic(value)`          | `Panic`               |

## Matching

| API                                            | Behavior                                             |
| ---------------------------------------------- | ---------------------------------------------------- |
| `error.match(handlers)`                        | Exhaustive instance match over every `_tag`          |
| `matchError(error, handlers)`                  | Exhaustive standalone match over every `_tag`        |
| `matchError(handlers)(error)`                  | Data-last exhaustive match                           |
| `matchErrorPartial(error, handlers)`           | Handle selected tags; return others unchanged        |
| `matchErrorPartial(error, handlers, fallback)` | Handle selected tags; transform others with fallback |
| `matchErrorPartial(handlers, fallback)(error)` | Data-last partial match                              |

Handler return types are inferred as a union when they differ. If an exhaustive `.match()` or `matchError` handler throws, the operation throws `Panic` with the original exception as `cause`. `matchErrorPartial` retains its fallback and handler exception behavior.

The `match` property name is reserved on `TaggedError` payloads; TypeScript also rejects incompatible subclass members with that name.

## Built-in error classes

### `UnhandledException`

Returned by `Result.try` and `Result.tryPromise` when no custom catch translator is supplied.

| Property  | Type                   |
| --------- | ---------------------- |
| `_tag`    | `"UnhandledException"` |
| `cause`   | `unknown`              |
| `message` | `string`               |

### `ResultSerializationError`

Returned when a selected serialization schema reports validation issues. `serializeUnsafe` throws `Panic` with this error as its cause instead of returning it.

| Property | Type                                     |
| -------- | ---------------------------------------- |
| `_tag`   | `"ResultSerializationError"`             |
| `value`  | `unknown`                                |
| `issues` | optional readonly Standard Schema issues |

### `ResultDeserializationError`

Returned for an invalid serialized envelope or when a selected payload schema reports issues. `deserializeUnsafe` throws `Panic` with this error as its cause while preserving valid decoded domain Err values.

| Property | Type                                                                     |
| -------- | ------------------------------------------------------------------------ |
| `_tag`   | `"ResultDeserializationError"`                                           |
| `value`  | `unknown`                                                                |
| `issues` | optional readonly Standard Schema issues; absent for an invalid envelope |

### `Panic`

Thrown for defects. Exposes `_tag: "Panic"`, `message`, optional `cause`, `toJSON()`, static `.is`, and generator iterator behavior.

## `panic(message, cause?)`

Throws a new `Panic` and returns `never` at the type level.