# Explain a slot (/docs/api-reference/scheduling-availability/explain-scheduling-slot)

Why one start can or can't be booked right now, host by host, decided the same way a booking is. For support and debugging: it says when a host is busy, so don't show it to attendees.

`GET https://api.coss.com/v1/scheduling/event_types/{event_type}/slots/explain`

Authenticate with your API key as a bearer token (`Authorization: Bearer coss_test_…`). Responses are JSON with snake_case fields; errors are `{ error: { type, code, message, param?, issues?, request_id } }` (https://coss.com/docs/api-reference/errors.md).

## Path parameters

- `event_type` (string, required): The event type's id or uid.

## Query parameters

- `start` (string, required): An ISO 8601 instant with an offset.
- `host` (string): Only this host (id or uid).
- `duration_minutes` (integer, 5-1440)
- `attributes` (string): A JSON object narrowing a pool, such as `{"licensed_in":"CA"}`.

## Example request

TypeScript (the `coss` package):

```ts
import { Coss } from "coss";

const coss = new Coss({
  apiKey: process.env.COSS_API_KEY,
  baseUrl: "https://api.coss.com",
});

const { data, error } = await coss.scheduling.slots.explain({
  value: "evtype_01kd2m9a0b1c2d3e4f5g6h7j8m",
}, {
  start: "2027-01-04T15:00:00.000Z",
});
```

HTTP:

```sh
curl -X GET "https://api.coss.com/v1/scheduling/event_types/evtype_01kd2m9a0b1c2d3e4f5g6h7j8m/slots/explain?start=2027-01-04T15%3A00%3A00.000Z" \
  -H "Authorization: Bearer $COSS_API_KEY"
```

## Response 200

Whether it can be booked, and why.

- `object` ("scheduling.slot_explanation", required)
- `start` (string, required): The start asked about.
- `end` (string, required): When it would end.
- `available` (boolean, required): Whether it can be booked right now.
- `reason` (string | null, required): Why it can't: `not_on_grid`, `too_soon`, `outside_booking_window`, `calendar_unverified`, `outside_availability`, `host_busy`, `already_claimed`, `limit_reached` or `session_full`, or `event_type_inactive` or `no_hosts`. `null` when it can. With several hosts refused, the first one's reason: see `hosts`.
- `hosts` (object[], required): Everyone who could take this event type, narrowed by the query.
  - `host` (string, required): The host's id.
  - `role` ("required" | "member", required): `required` hosts must all be free. Only one `member` needs to be.
  - `available` (boolean, required): Whether this host could take it.
  - `reason` (string | null, required): Why not: `not_on_grid`, `too_soon`, `outside_booking_window`, `calendar_unverified`, `outside_availability`, `host_busy`, `already_claimed`, `limit_reached` or `session_full`. `null` when they could.
  - `calendar_reason` (string | null, required): With `calendar_unverified`: `calendar_needs_reauth`, `calendar_stale` or `calendar_not_covered`.

Example:

```json
{
  "object": "scheduling.slot_explanation",
  "start": "2027-01-04T15:00:00.000Z",
  "end": "2027-01-04T15:30:00.000Z",
  "available": false,
  "reason": "host_busy",
  "hosts": [
    {
      "host": "host_01kd2m9a0b1c2d3e4f5g6h7j8m",
      "role": "member",
      "available": false,
      "reason": "host_busy",
      "calendar_reason": null
    }
  ]
}
```

## Errors

- `400`: The `Coss-Version` header names an unknown API version (`invalid_api_version`).
- `401`: The API key is missing, invalid, revoked or expired (`authentication_error`).
- `402`: In live environments: the plan doesn't include this (`plan_feature_unavailable`), or it's past the plan's limit (`plan_limit_reached`).
- `403`: The API key lacks the permission this endpoint needs (`permission_error`), or its organization is suspended (`organization_suspended`).
- `404`: Nothing with this id or uid exists in the environment (`not_found`).
- `422`: The request failed validation (`validation_error`, see `issues`), or `start`, `end` or `duration_minutes` is invalid: `end` isn't after `start`, the window is longer than 62 days, or the duration isn't one of the event type's (`invalid_time`).
- `429`: Too many requests with this API key (`rate_limited`), or too many requests with invalid keys from this address (`too_many_failed_authentications`). Retry after `Retry-After` seconds. Successful responses carry `RateLimit-Limit`, `RateLimit-Remaining` and `RateLimit-Reset` (seconds until the limit is fully restored).
- `500`: Unexpected server error (`api_error` / `internal_error`).
- `503`: The API key could not be verified right now; retry (`service_unavailable`).