> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sailresearch.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Service tiers

> Trade off latency for lower token prices

Service tiers allow you to balance processing speed, latency, and cost for inference requests.

Sail serves low-latency inference by default for [core models](https://docs.sailresearch.com/models.md), at lower cost than traditional inference providers. You can opt in to the Balanced or Flex service tiers to cut token costs drastically for latency-tolerant work.

## Service tier comparison

| Tier | Queueing time | Target TPS (Experienced TPS varies by model and workload.) | Price vs. competitor model APIs |
| - | - | - | - |
| Default (**ASAP**) | None | 50-100 TPS | 30-70% less |
| **Balanced** | Up to 5 min for first turn, much faster for subsequent turns (seconds) | 30-50 TPS | 45-80% less |
| **Flex** | Up to 1 hour for first turn, much faster for subsequent turns (seconds) | 10-20 TPS | 55-85% less |

Check [Pricing](https://docs.sailresearch.com/pricing.md) for current per-model availability.

## Service tier details

### Default (ASAP)

Default, aka ASAP, is the standard low-latency tier for Sail’s core models. It is characterized by:

1. **No queueing delay** - TTFT is dependent on prompt size and model.
2. **High TPS** - targeting 50-100 TPS. Your experienced TPS will vary by workload and model.
3. **Foreground-only** - this tier is not compatible with background mode or batch requests.

Developers use ASAP for:

* Chat interfaces
* Realtime interactivity

Jump to:

* [Default (ASAP) notes and troubleshooting](#asap-requests)

### Flex

Flex is characterized by:

1. **A flexible start** - the first turn in a run is guaranteed to start within 1 hour.
2. **Much faster subsequent turns** - follow-up turns of an in-flight run see minimal (seconds) queueing delay.
3. **Low TPS** - targeting 10-20 TPS. Your experienced TPS will vary by workload and model.
4. Support for **extreme scale by design** - Flex offers the most generous capacity and highest rate limits.
5. **Background-only** - Flex is not compatible with streaming or foreground APIs.

> [!NOTE]
>
> Sail uses longest-prefix matching to automatically recognize subsequent turns.

Developers use Flex for:

* Proactive long-horizon agents
* Background agents and tasks
* Rollouts
* Automations and routines
* Large-scale evals
* High parallelism work
* Offline processing

Jump to:

* [How to use Flex](#how-to-use-flex)
* [Flex notes and troubleshooting](#flex-requests)

### Balanced

Balanced is the middle ground between Flex and Default, characterized by:

1. **A flexible start with a tighter bound than Flex** - the first turn in a run should start within 5 minutes.
2. **Much faster subsequent turns (Sail uses longest-prefix matching to automatically recognize subsequent turns.)** - like Flex, follow-up turns of an in-flight run see minimal (seconds) queueing delay.
3. **Medium TPS** - targeting 30-50 TPS. Your experienced TPS will vary by workload and model.
4. **Streaming, foreground, and background-compatible.**

Developers use Balanced for:

* Subagents
* Frequent automations and routines
* RL rollouts
* Evals
* Coding agents

Jump to:

* [How to use Balanced](#how-to-use-service-tiers)
* [Balanced notes and troubleshooting](#balanced-requests)

> [card] Multi-turn agents and rollouts with Balanced and Flex
>
> Balanced and Flex are designed to support multi-turn agents and rollouts. For
> the first turn in a run, Balanced and Flex have a “warm-up” queueing delay of
> up to 5 minutes (for Balanced) and 1 hour (for Flex), respectively.
> Afterwards, Sail automatically uses longest-prefix matching to recognize
> subsequent turns of an in-flight run. These follow-up turns are prioritized
> and incur minimal to no queueing time.

## How to use service tiers

### Via request body `metadata`

Set `metadata.completion_window` to `asap`, `balanced`, or `flex`:

```bash Responses theme={null}
curl https://api.sailresearch.com/v1/responses \
  -H "Authorization: Bearer $SAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "zai-org/GLM-5.3",
    "input": "Explain the key ideas behind transformers.",
    "metadata": {
      "completion_window": "balanced"
    }
  }'
```

```bash Chat theme={null}
curl https://api.sailresearch.com/v1/chat/completions \
  -H "Authorization: Bearer $SAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "zai-org/GLM-5.3",
    "messages": [
      {"role": "user", "content": "Explain the key ideas behind transformers."}
    ],
    "metadata": {
      "completion_window": "balanced"
    }
  }'
```

```bash Messages theme={null}
curl https://api.sailresearch.com/v1/messages \
  -H "Authorization: Bearer $SAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "zai-org/GLM-5.3",
    "max_tokens": 1024,
    "messages": [
      {"role": "user", "content": "Explain the key ideas behind transformers."}
    ],
    "metadata": {
      "completion_window": "balanced"
    }
  }'
```

To use Flex, you must use the [Responses API](https://docs.sailresearch.com/support.md#responses-api)
with `background=True` or submit requests through the [Batch API](https://docs.sailresearch.com/requests_at_scale.md#batch-api).

> [!WARNING]
>
> Flex is **not** compatible with Chat Completions, Messages, or **foreground**
> (`background=False`) Responses API requests.

```bash Responses highlight={7} theme={null}
curl https://api.sailresearch.com/v1/responses \
  -H "Authorization: Bearer $SAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "zai-org/GLM-5.3",
    "input": "Explain the key ideas behind transformers.",
    "background": true,
    "metadata": {
      "completion_window": "flex"
    }
  }'
```

```bash Batch theme={null}
# Batch always uses flex, so no service tier is specified.
curl https://api.sailresearch.com/v1/batches \
  -H "Authorization: Bearer $SAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "endpoint": "/v1/responses",
    "requests": [
      {
        "custom_id": "request-1",
        "params": {
          "model": "zai-org/GLM-5.3",
          "input": "Explain the key ideas behind transformers."
        }
      }
    ]
  }'
```

### Via request header

Alternatively, if your client cannot add metadata to a request body, you can
set the service tier using the `X-Sail-Completion-Window` request header.

```bash Responses highlight={4} theme={null}
curl https://api.sailresearch.com/v1/responses \
  -H "Authorization: Bearer $SAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -H "X-Sail-Completion-Window: balanced" \
  -d '{
    "model": "zai-org/GLM-5.3",
    "input": "Explain the key ideas behind transformers."
  }'
```

```bash Chat highlight={4} theme={null}
curl https://api.sailresearch.com/v1/chat/completions \
  -H "Authorization: Bearer $SAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -H "X-Sail-Completion-Window: balanced" \
  -d '{
    "model": "zai-org/GLM-5.3",
    "messages": [
      {"role": "user", "content": "Explain the key ideas behind transformers."}
    ]
  }'
```

```bash Messages highlight={4} theme={null}
curl https://api.sailresearch.com/v1/messages \
  -H "Authorization: Bearer $SAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -H "X-Sail-Completion-Window: balanced" \
  -d '{
    "model": "zai-org/GLM-5.3",
    "max_tokens": 1024,
    "messages": [
      {"role": "user", "content": "Explain the key ideas behind transformers."}
    ]
  }'
```

### Default behavior

For inference requests for [core models](https://docs.sailresearch.com/models.md), the default behavior (when no `completion_window` is specified) is low-latency inference (`asap`).
The exceptions to this default behavior are:

* [Batch inference requests](https://docs.sailresearch.com/requests_at_scale.md#batch-api), which default to Flex and do not support other service tiers.
* [LoRA requests](https://docs.sailresearch.com/loras.md), which are Flex-only.
* Requests that use custom tools, which default to Balanced for core models because ASAP does not support them.
* Responses API requests with `background=True` use Balanced or Flex by default, since they do not support ASAP.

For [Flex-only models](https://docs.sailresearch.com/models.md#flex-only-models), the default (and only) service tier is Flex.

## Troubleshooting

### ASAP requests

* ASAP does not support custom tools (`tools` with `type="custom"`) on the
  Responses or Chat Completions APIs. Use Balanced, or omit the service
  tier to default to Balanced for core models.
* ASAP does not support Responses requests with `background=True` or the
  Batch API. Use Balanced or Flex for Responses requests with `background=True`.
* Setting `metadata.completion_window="asap"` with custom tools or
  `background=True` returns HTTP `400` with error code `unsupported_asap_request`.

### Balanced requests

* You can send Balanced requests synchronously or as [Responses requests with `background=True`](https://docs.sailresearch.com/support#responses-api).
* A synchronous streaming Balanced request opens its stream right away and keeps the
  connection alive while it waits. Output starts when the model begins generating.

### Flex requests

* Chat Completions and Messages reject Flex with HTTP `400`, with or without
  streaming. Use ASAP or Balanced on those endpoints.
* Responses requests using Flex reject an omitted `background` value, `background=None`, or
  `background=False` with HTTP `400`. Set `background=True` explicitly.

### Request settings

* If you set both `X-Sail-Completion-Window` and `metadata.completion_window`,
  the value in the request body takes precedence.
* The model must support the tier you choose. Check [Pricing](https://docs.sailresearch.com/pricing.md) for
  availability.

### Batch and Background

* Responses requests with `background=True` cannot use `stream=True`.
  Retrieve results by polling or using [webhooks](https://docs.sailresearch.com/webhooks.md).
* A polling timeout does not cancel a Responses request with `background=True`.
  Keep polling the same response ID. See [Retrying requests](https://docs.sailresearch.com/retries.md) if you
  lose the connection before receiving an ID.
* The Batch API does not use `X-Sail-Completion-Window`.
* Batch always uses Flex and runs in the background. Setting a subrequest's
  service tier to `asap` or `balanced` returns HTTP `400`.
* Batch rejects models that do not support Flex.

### Capacity errors

* For ASAP requests, Sail may wait briefly for capacity. If none becomes
  available, it returns `503` with code `model_capacity_unavailable`
  without queueing the request.

* If the service is overloaded, Sail can return `503` with code
  `service_overloaded` before accepting a request in any service tier.

* If the response includes `Retry-After`, wait at least that many seconds.
  See [Retrying requests](https://docs.sailresearch.com/retries.md) before resubmitting after a timeout or another
  server error, since the request may already have started.