DeepSeek Coder Guide: Code Completion, Generation, Debugging and How to Use It

What is DeepSeek Coder?

DeepSeek Coder is a family of language models trained predominantly on source code and technical text, rather than general conversation. That focus makes it fast and accurate at the things developers do all day: completing the line you're typing, writing a function from its docstring, translating code between languages, explaining what a block does and spotting why it breaks.

Coder comes in several sizes, from small models that run on a laptop for instant autocomplete to large ones that understand whole repositories. Its weights are published openly, so you can run it in your own IDE or on your own servers as well as through DeepSeek's API and Harness Coding. New to DeepSeek altogether? Start with how to use DeepSeek AI, then come back here.

What DeepSeek Coder can do

Languages: Python, JavaScript/TypeScript, Java, C/C++, C#, Go, Rust, PHP, Ruby, Kotlin, Swift, SQL, Bash, HTML/CSS and many more - strongest on the most widely used ones.

How fill-in-the-middle (FIM) works

Ordinary models only continue text from the end. FIM gives Coder both sides of the gap - the code before your cursor (prefix) and the code after it (suffix) - so the completion fits perfectly between them. It's what makes Coder feel natural inside an editor, where you're usually editing the middle of a file, not appending to the end.

Worked examples

Example 1 - Generate a function from a description

from datetime import date

def count_weekends(dates: list[str]) -> int:
    """Count dates that fall on Saturday or Sunday; skip invalid ones."""
    n = 0
    for d in dates:
        try:
            if date.fromisoformat(d).weekday() >= 5:
                n += 1
        except ValueError:
            continue
    return n

def test_counts_weekends():
    assert count_weekends(["2026-11-07", "2026-11-09"]) == 1  # Sat, Mon

def test_skips_invalid():
    assert count_weekends(["not-a-date", "2026-11-08"]) == 1

Example 2 - Find and fix a bug

Example 3 - Explain unfamiliar code

Coder vs other DeepSeek models for programming

Full model comparison: DeepSeek models explained · DeepSeek vs ChatGPT for coding.

How to access DeepSeek Coder

Harness isn't only for code. The same desktop agent can draft documents such as READMEs and release notes, build spreadsheets from test or benchmark results, run scheduled tasks like a nightly test run that summarises failures, and connect to your tools through plugins.

API quick start

The DeepSeek API is OpenAI-compatible, so existing SDKs work with a different base URL. Get a key from the API platform first - full walkthrough in the API guide.

# pip install openai
from openai import OpenAI
import os

client = OpenAI(api_key=os.environ["DEEPSEEK_API_KEY"],
                base_url="https://api.deepseek.com")

resp = client.chat.completions.create(
    model="deepseek-coder",   # confirm current name in the API docs
    messages=[
        {"role": "system", "content": "You are a senior Python engineer. Reply with code only."},
        {"role": "user", "content": "Write a function that slugifies a title."},
    ],
    temperature=0,
)
print(resp.choices[0].message.content)

# Fill-in-the-middle uses the completions endpoint (beta base URL)
from openai import OpenAI
import os

client = OpenAI(api_key=os.environ["DEEPSEEK_API_KEY"],
                base_url="https://api.deepseek.com/beta")

resp = client.completions.create(
    model="deepseek-coder",   # confirm current name in the API docs
    prompt='def is_valid_email(s):\n    """Return True if s looks like an email."""\n',
    suffix="    return ok\n",
    max_tokens=128,
)
print(resp.choices[0].text)   # just the missing middle

curl https://api.deepseek.com/chat/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $DEEPSEEK_API_KEY" \
  -d '{
    "model": "deepseek-coder",
    "temperature": 0,
    "messages": [
      {"role": "user", "content": "Write a Bash one-liner that finds files over 100 MB."}
    ]
  }'

Set temperature to 0 for deterministic code. Token costs are on the pricing page; for grounding answers in current docs, see search - fetch.

Prompting tips for DeepSeek Coder

Limits and cost

DeepSeek Coder - frequently asked questions