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Ultrafast mode is the fastest service tier in the OpenAI API. It is broadly available for GPT-6 Astra, with [preview access](https://openai.com/index/previewing-ultrafast/) for GPT-5.6 Sol. Use it when speed justifies the higher cost.
We strongly recommend [WebSockets](https://developers.openai.com/api/docs/guides/websocket-mode), especially for agentic applications that make many tool calls in quick succession. Without a persistent connection, network overhead can reduce the latency gains.
Ultrafast mode for GPT-6 Astra is currently available to all API users at [low
rate limits](https://developers.openai.com/api/docs/guides/ultrafast-mode#availability). If your organization works with an OpenAI account
team, contact them to request higher rate limits or preview access for GPT-5.6
Sol.
## Configure your request
Set `model` to `gpt-6-astra` and `service_tier` to `ultrafast` in each `response.create` event.
Use Ultrafast across turns on one WebSocket
Python
```
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51// Install: npm install openai ws
// Set OPENAI_API_KEY in your environment.
import OpenAI from "openai";
import { ResponsesWS } from "openai/resources/responses/ws";
const client = new OpenAI();
const ws = new ResponsesWS(client);
let previousResponseId = null;
try {
for (const input of [
"Explain why the sky is blue in one sentence.",
"Now explain why sunsets look red.",
]) {
let completed = false;
const events = ws.stream();
ws.send({
type: "response.create",
model: "gpt-6-astra",
service_tier: "ultrafast",
previous_response_id: previousResponseId,
input,
});
for await (const event of events) {
if (event.type === "error") throw event.error;
if (event.type !== "message") continue;
const message = event.message;
if (message.type === "response.output_text.delta") {
process.stdout.write(message.delta);
} else if (message.type === "response.completed") {
previousResponseId = message.response.id;
completed = true;
console.log();
break;
} else if (
message.type === "response.failed" ||
message.type === "response.incomplete"
) {
throw new Error(JSON.stringify(message));
}
}
if (!completed) {
throw new Error("Connection closed before the response finished.");
}
}
} finally {
ws.close();
}
```
```
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31# Install: pip install --upgrade "openai[realtime]"
# Set OPENAI_API_KEY in your environment.
from openai import OpenAI
client = OpenAI()
previous_response_id: str | None = None
prompts = [
"Explain why the sky is blue in one sentence.",
"Now explain why sunsets look red.",
]
with client.responses.connect() as connection:
for prompt in prompts:
connection.response.create(
model="gpt-6-astra",
service_tier="ultrafast",
previous_response_id=previous_response_id,
input=prompt,
)
for event in connection:
if event.type == "response.output_text.delta":
print(event.delta, end="", flush=True)
elif event.type == "response.completed":
previous_response_id = event.response.id
print()
break
elif event.type in {"response.failed", "response.incomplete", "error"}:
raise RuntimeError(event.to_json())
else:
raise RuntimeError("Connection closed before the response finished.")
```
The example streams two responses over the same connection. The second request sends the new prompt and passes the first response’s ID as `previous_response_id`. Reuse the connection for later turns and tool results. See [Continue with incremental inputs](https://developers.openai.com/api/docs/guides/websocket-mode#continue-with-incremental-inputs).
## HTTP alternative
Ultrafast also supports HTTP requests through the SDK. For agentic applications with frequent tool calls, use a persistent WebSocket connection to reduce overhead between requests.
Create an Ultrafast response over HTTP
Python
```
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10import OpenAI from "openai";
const client = new OpenAI();
const response = await client.responses.create({
model: "gpt-6-astra",
service_tier: "ultrafast",
input: "Explain why the sky is blue in one sentence.",
});
console.log(response.output_text);
```
```
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11from openai import OpenAI
client = OpenAI()
response = client.responses.create(
model="gpt-6-astra",
input="Explain why the sky is blue in one sentence.",
service_tier="ultrafast",
)
print(response.output_text)
```
```
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10client := openai.NewClient()
response, err := client.Responses.New(context.Background(), responses.ResponseNewParams{
Model: "gpt-6-astra",
ServiceTier: responses.ResponseNewParamsServiceTierUltrafast,
Input: responses.ResponseNewParamsInputUnion{OfString: openai.String("Explain why the sky is blue in one sentence.")},
})
if err != nil {
panic(err)
}
fmt.Println(response.OutputText())
```
```
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16import com.openai.client.OpenAIClient;
import com.openai.client.okhttp.OpenAIOkHttpClient;
import com.openai.models.responses.ResponseCreateParams;
ResponseCreateParams params =
ResponseCreateParams.builder()
.model("gpt-6-astra")
.input("Explain why the sky is blue in one sentence.")
.serviceTier(ResponseCreateParams.ServiceTier.ULTRAFAST)
.build();
client.responses().create(params).output().stream()
.flatMap(item -> item.message().stream())
.flatMap(message -> message.content().stream())
.flatMap(content -> content.outputText().stream())
.forEach(text -> System.out.println(text.text()));
```
```
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11require "openai"
client = OpenAI::Client.new
response = client.responses.create(
model: "gpt-6-astra",
service_tier: :ultrafast,
input: "Explain why the sky is blue in one sentence."
)
puts(response.output_text)
```
```
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8curl https://api.openai.com/v1/responses \
-H "Authorization: Bearer $OPENAI_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"model": "gpt-6-astra",
"input": "Explain why the sky is blue in one sentence.",
"service_tier": "ultrafast"
}'
```
This example waits for the complete response. To display output as it arrives, enable [streaming](https://developers.openai.com/api/docs/guides/streaming-responses?api-mode=responses).
## Availability
GPT-6 Astra has the following default Ultrafast token rate limits:
| API usage tier | Tokens per minute (TPM) |
| --- | --- |
| Tiers 1–3 | 500,000 |
| Tier 4 | 1,000,000 |
| Tier 5 | 5,000,000 |
See the [Ultrafast pricing table](https://developers.openai.com/api/docs/pricing?latest-pricing=ultrafast) for input, cached input, cache write, and output prices.
Ultrafast supports US data residency and global processing only. It does not support EU or other non-US regional processing endpoints.