# DeepSeek V4.1 Flash API: get a key and make your first call

By SeedRouter · Published 2026-09-28 · Updated 2026-09-28

To call DeepSeek V4.1 Flash, you need an API key from a platform that serves it and a request with its model ID. On DeepSeek's own API the model is `deepseek-flash`, and the old names `deepseek-v4-flash` and `deepseek-v4-flash-vision-exp` are routed to it. On SeedRouter the model ID is `deepseek-v4.1-flash`, and one key calls it pay-as-you-go in the official request format: point the OpenAI SDK at `https://api.seedrouter.ai/v1` and keep your code.

This guide uses SeedRouter; the request bodies are the same as DeepSeek's own API.

## How do I get a DeepSeek V4.1 Flash API key?

1. Sign in to SeedRouter and open **API keys**.
2. Create a key and copy it; it is shown once.
3. Add credit when you need it. New accounts start with a small free balance, and there is no subscription.

Keep the key in an environment variable such as `SEEDROUTER_API_KEY`, and only use it from server-side code.

## How do I call DeepSeek V4.1 Flash from Python?

```python
import os
from openai import OpenAI

client = OpenAI(
    api_key=os.environ["SEEDROUTER_API_KEY"],
    base_url="https://api.seedrouter.ai/v1",
)

completion = client.chat.completions.create(
    model="deepseek-v4.1-flash",
    messages=[{"role": "user", "content": "Give me three names for a coffee shop."}],
)
print(completion.choices[0].message.content)
```

Thinking is on by default, so the message also carries the model's reasoning in `reasoning_content`, next to the answer in `content`.

## How do I call it from Node.js or cURL?

```javascript
import OpenAI from "openai";

const client = new OpenAI({
  apiKey: process.env.SEEDROUTER_API_KEY,
  baseURL: "https://api.seedrouter.ai/v1",
});

const completion = await client.chat.completions.create({
  model: "deepseek-v4.1-flash",
  messages: [{ role: "user", content: "Give me three names for a coffee shop." }],
});
console.log(completion.choices[0].message.content);
```

```bash
curl https://api.seedrouter.ai/v1/chat/completions \
  -H "Authorization: Bearer $SEEDROUTER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model": "deepseek-v4.1-flash", "messages": [{"role": "user", "content": "Give me three names for a coffee shop."}]}'
```

The same key also answers the Responses API (`/v1/responses`) and the Anthropic Messages format (`/v1/messages`) for `deepseek-v4.1-flash`.

## How do I turn thinking off or set its effort?

Thinking is on by default at `high` effort. Turn it off with `thinking`, or choose an effort with `reasoning_effort`. The OpenAI SDK passes `thinking` through `extra_body`. The first request turns thinking off for the fastest, cheapest answer; the second uses the hardest effort:

```python
completion = client.chat.completions.create(
    model="deepseek-v4.1-flash",
    messages=[{"role": "user", "content": "Classify: 'my card was charged twice'"}],
    extra_body={"thinking": {"type": "disabled"}},
)

completion = client.chat.completions.create(
    model="deepseek-v4.1-flash",
    messages=[{"role": "user", "content": "How many primes are there below 150?"}],
    reasoning_effort="max",
)
```

| `reasoning_effort` | Effect               |
| ------------------ | -------------------- |
| `none`             | Thinking off         |
| `low`              | Short reasoning      |
| `high` (default)   | Most tasks           |
| `max`              | The hardest problems |

DeepSeek also accepts `minimal` (runs as `low`), `medium` and `xhigh` (run as `high`). In our test on a prime-counting question, thinking off used 2 output tokens, `low` 258 and `max` 319. Reasoning is billed as output.

## How do I stream the answer?

Add `stream=True`. With thinking on, the reasoning arrives first in `delta.reasoning_content`, then the answer in `delta.content`, and the last chunk carries the token usage:

```python
stream = client.chat.completions.create(
    model="deepseek-v4.1-flash",
    messages=[{"role": "user", "content": "Write a haiku about latency."}],
    stream=True,
)
for chunk in stream:
    if chunk.choices and chunk.choices[0].delta.content:
        print(chunk.choices[0].delta.content, end="", flush=True)
```

## Can I send images?

Yes. DeepSeek V4.1 Flash reads images natively. Send a public URL or a base64 data URI in an `image_url` part:

```python
completion = client.chat.completions.create(
    model="deepseek-v4.1-flash",
    messages=[{
        "role": "user",
        "content": [
            {"type": "image_url", "image_url": {"url": "https://example.com/chart.png"}},
            {"type": "text", "text": "What does this chart show?"},
        ],
    }],
)
```

An image URL may be up to 8,192 characters long and point to a file of up to 32 MiB.

## What errors should I expect?

| Error                           | Cause                                        | Fix                                                                 |
| ------------------------------- | -------------------------------------------- | ------------------------------------------------------------------- |
| 400 on `response_format`        | `json_schema` is not supported               | Use `{"type": "json_object"}` and describe the shape in the prompt  |
| 400 on `temperature` or `top_p` | Above 2 or above 1                           | Keep them in range; with thinking on they have little effect anyway |
| 400 on an image URL             | The file could not be downloaded as an image | Check the URL is public and points to an image                      |
| 401                             | Missing or wrong key                         | Check the `Authorization` header                                    |

Errors return `{"error": {"code": ..., "message": "..."}}`, and a request that fails is not charged.

## Frequently asked questions

### Is the DeepSeek V4.1 Flash API OpenAI-compatible?

Yes. It takes the Chat Completions and Responses formats, so the OpenAI SDK works with only the base URL and model changed. It also takes the Anthropic Messages format.

### Why is the model ID different from DeepSeek's?

DeepSeek names the model `deepseek-flash` on its own API. SeedRouter uses `deepseek-v4.1-flash` so the version is part of the name. The request body is otherwise the same.

### How much does a DeepSeek V4.1 Flash request cost?

It is billed per token, at peak or off-peak rates by the hour. The [DeepSeek V4.1 Flash pricing guide](https://seedrouter.ai/blog/deepseek-v4-1-flash-api-pricing) has the live rates and worked examples.

### Where is the full parameter list?

The [DeepSeek V4.1 Flash API reference](https://seedrouter.ai/docs/deepseek-v4-1-flash) lists every field, with the [DeepSeek V4.1 Flash page](https://seedrouter.ai/models/deepseek-v4-1-flash) for a playground and live prices.