# Errors (https://www.cal-sdk.com/docs/reference/errors)

When a call fails, the error's class tells you what went wrong and its fields tell you where. All errors extend `CalsdkError`, which extends `Error`. Every class has a static `isInstance(error)` check that works across duplicated package copies. Errors from a provider keep the original failure in `cause`.

```ts
import { NotFoundError } from '@cal-sdk/core';

if (NotFoundError.isInstance(error)) console.log(error.resource, error.id);
```

[Errors and retries](/docs/concepts/errors) covers how to handle them.

## Input and setup [#input-and-setup]

| Class | Fields | Thrown when |
| --- | --- | --- |
| `InvalidArgumentError` | `argument` | An option is invalid. `argument` is its path, such as `'event.start.timeZone'` |
| `NoSuchProviderVersionError` | `provider`, `version` | The provider implements a contract version this calsdk does not know |
| `UnsupportedFunctionalityError` | `provider`, `functionality` | The provider cannot do what was asked |

## Provider responses [#provider-responses]

| Class | Fields | Thrown when |
| --- | --- | --- |
| `AuthenticationError` | `provider` | Credentials were rejected (401) |
| `PermissionDeniedError` | `provider` | The user may not do this (403) |
| `NotFoundError` | `provider`, `resource` (`'calendar'` or `'event'`), `id` | The calendar or event does not exist, was deleted or is hidden (404, 410) |
| `ConflictError` | `provider`, `eventId` | `ifMatch` no longer matches (412) |
| `RateLimitError` | `provider`, `retryAfterMs?`, `isRetryable` (always `true`) | The provider is throttling (429) |
| `SyncCursorExpiredError` | `provider`, `calendarId` | The provider no longer accepts the sync cursor |
| `InvalidResponseDataError` | `data` | The provider returned data in a shape calsdk does not expect. The message leaves the data out |
| `APICallError` | `url`, `statusCode?`, `responseHeaders?`, `responseBody?`, `isRetryable` | Any other failed request, including network errors |

`APICallError.isRetryable` is `true` for 408, 429, 5xx and network failures. Request headers are never stored on it, since they carry the token.

## Free time [#free-time]

| Class | Fields | Thrown when |
| --- | --- | --- |
| `FreeBusyUnavailableError` | `provider`, `calendarId` | `findFreeTime` could not read one of the calendars |

`NotFoundError`, `SyncCursorExpiredError` and `FreeBusyUnavailableError` leave calendar ids out of their messages, since they are often email addresses. Read them from the fields.