What Harness Coding is
Harness Coding is the developer side of DeepSeek Harness. Open a project folder, describe what you want, such as "the login test fails, find out why and fix it", and the agent gets to work. It searches the codebase, reads the relevant files, makes edits, runs your test suite, reads the output and keeps going until the task is done or it needs your input.
It's an agent, not autocomplete. It handles multi-step, multi-file work you'd normally do yourself: tracking a bug through three modules, adding a feature end to end, or updating every call site after an API change. The V4.1-Flash-1017 update made this kind of project work noticeably faster and cheaper.
Your whole repo
Searches and opens the files it needs, rather than only what you paste in.
Tests - commands
Runs your build, linter and tests, with your approval, and reacts to the results.
Reviewable diffs
Every change is a diff you can accept, adjust or reject.
Harness Coding vs other DeepSeek coding options
| Chat app | Coder in your IDE | Harness Coding | |
|---|---|---|---|
| What it is | Ask questions, paste snippets | Inline autocomplete and fill-in-the-middle | An agent working in your repo |
| Sees your code | Only what you paste | The current file and nearby context | The whole project folder |
| Edits files | No | As you type | Yes, across many files |
| Runs tests | No | No | Yes, with approval |
| Best for | Learning, quick answers, explanations | Writing code faster line by line | Bugs, features, refactors and upgrades |
| Learn more | Coding in chat | DeepSeek Coder | This page |
Coding tasks you can hand over
| Task | What Harness does | Example request |
|---|---|---|
| Fix a bug | Reproduces it, finds the cause, fixes it, and adds a test so it stays fixed | "Orders over 100 items crash checkout. Fix it." |
| Build a feature | Plans the change, edits models, routes, UI and tests, then runs everything | "Add CSV export to the reports page." |
| Refactor | Restructures code without changing behaviour, using tests as a safety net | "Split utils.py into smaller modules." |
| Write tests | Adds unit and integration tests for untested code, including edge cases | "Get payments/ to 80% test coverage." |
| Explain a codebase | Maps the structure and explains how a feature flows through it | "How does a request reach the database here?" |
| Upgrade dependencies | Bumps versions, fixes breaking changes, and reruns the tests | "Upgrade to the latest major version of our web framework." |
| Code review | Reviews a branch or diff for bugs, security problems and readability | "Review my changes on this branch." |
| Docs - comments | Writes READMEs, docstrings, changelogs and setup guides | "Write a README that explains local setup." |
| Scripts - automation | Writes one-off or recurring scripts that process data or files | "Write a script that resizes every image in /assets." |
How a coding session works
The loop between edit and run tests is what makes an agent useful. When a test fails, Harness reads the error, works out why and tries again, often several times, before it hands the result back to you. You step in when it asks a question, needs approval to run a command, or has finished.
Worked examples
Example 1 - Fix a failing test
test_discount fails for orders over Rs 5,000. Find the cause, make the smallest fix, add tests for the boundary cases, and run the full suite.Harness runs the failing test first to see the real error, follows the code to the cause, changes only what's needed, and adds tests for exactly Rs 5,000 as well as amounts just below and above it. You get a two-line diff and a green test run to review.
Example 2 - Add a feature end to end
Add pagination to the /orders API: 20 per page by default, ?page= and ?per_page= parameters (max 100), and include total and next_page in the response. Update the frontend list and add tests. Show me the plan first.
- apiUpdate
routes/orders.pyto read and validate the page parameters, and query with offset and limit. - schemaAdd
totalandnext_pageto the response schema. - uiAdd Next and Previous buttons to
OrdersList, keeping the page number in the URL. - testsAPI tests for the default, a custom size, an over-limit size and the last page, plus a UI test for the buttons.
- verifyRun the full suite and the linter, then summarise every changed file.
Six files changed and nine tests added, all passing. You approve the plan, review each diff, and commit on a feature branch.
Example 3 - Understand an unfamiliar repo
I'm new to this codebase. Explain the folder structure, how a request flows from the router to the database, where configuration lives, and how to run it locally. Don't change any files.
A plain-English tour with file paths: entry point, middleware, service layer, data models and test setup, plus the exact commands to install, configure and run the project. Ask it to save this as ONBOARDING.md for the next new starter (see Harness for documents).
Set up your project for best results
- Open the project root in Harness: the folder with your package file and tests, not your whole home directory. New to Harness? See install - set up.
- Make sure the tests run. If
npm test,pytestor your equivalent already works, Harness can check its own changes. - Start from a clean Git state, so every change Harness makes is easy to see and undo.
- Add a short notes file (or keep your README current) explaining how to install, test and lint, plus any project rules such as "use our logger, not print".
- Keep secrets out. Use environment variables and a
.gitignored.envfile, never keys in source code.
Git workflow and safety
| Practice | Why it matters |
|---|---|
| Work on a branch | Ask Harness to create a branch such as fix/discount-boundary, so main stays untouched until you merge. |
| Review every diff | Read changes the way you'd review a colleague's pull request. Reject or adjust anything you don't understand. |
| Approve commands | Be careful with commands that delete files, reset branches, publish packages or touch production. Read them before approving. |
| Small, frequent commits | Commit after each working step, so any change can be rolled back on its own. |
| Protect secrets | Keep .env files and keys out of the working folder, or make sure they're ignored. File contents are sent to the model. |
| Trust, but verify | Passing tests don't prove the code is correct. Check the logic, especially in security, money and data-deletion code. |
| Beware untrusted repos | Files in a cloned project can contain instructions aimed at the AI. Review the plan before approving. See prompt injection. |
Which model to use for coding
| Task | Recommended model | Why |
|---|---|---|
| Most sessions: features, fixes, tests | V4.1 Flash | Fast and cheap for the many small steps an agent takes. Much improved for project work in the 1017 update. |
| Subtle bugs, race conditions, algorithms | R1 | Reasons step by step about state and edge cases. |
| Architecture, large refactors | V4 Pro | Keeps the most consistent view across a large codebase. |
| Focused code generation | Coder | Tight, code-only output. |
Prompting tips for coding sessions
| Do this | Why it helps | Example |
|---|---|---|
| Describe the symptom and the expected behaviour | Harness fixes the real problem instead of guessing. | "Expected 4,500, got 4,950 for a 5,000 order." |
| Say how to check the work | Gives the agent a clear "done" signal. | "Done when npm test and npm run lint pass." |
| Ask for the plan first | Catches the wrong approach before any code changes. | "Show the plan and which files you'll touch." |
| Set the scope | Prevents unrelated "improvements". | "Only change pricing.py. Don't reformat other files." |
| Name the conventions | Output matches your codebase. | "Follow the pattern in routes/users.py." |
| Prefer small steps | Smaller diffs are easier to review and safer to merge. | "Do the API part first, then stop." |
| Ask for a summary | Makes reviewing quick. | "List every changed file and why." |
Automate and extend
Nightly checks
Run the full test suite every night and get a short report on failures and likely causes. Set it up in scheduled tasks.
Dependency watch
Every week, check for outdated packages and known security advisories, and draft the upgrade on a branch.
Connect your tools
Use plugins to connect Git hosting, an issue tracker, CI or a database, or write your own.
Troubleshooting
| Problem | Likely cause | Fix |
|---|---|---|
| The agent loops on a failing test | A hard bug, or the model is stuck on one theory. | Stop it, share what you know, or switch to R1. |
| Tests won't run | Missing dependencies, environment variables or a database. | Make sure the tests run in a normal terminal first, and document the setup steps. |
| Changes touch too many files | The scope wasn't stated. | Reject the changes, restate what's in and out of scope, and ask for the plan again. |
| Code doesn't match your style | Conventions weren't given. | Point to an example file and your linter config. |
| Uses an outdated library API | The model's training data predates the version you use. | State the version, or let it read the current docs. |
| Large repo is slow | Too much code to read. | Point it to the relevant folders, and open a subfolder for focused tasks. |
| Can't sign in or "server busy" | An account issue or peak demand. | See the login guide and updates. |
Harness Coding - frequently asked questions
Which programming languages does it support?
Any language that lives in text files, including Python, JavaScript/TypeScript, Java, Go, Rust, C#, PHP and Ruby. It's strongest on the most widely used ones.
Does it replace my IDE?
No. Keep your editor for day-to-day work, and use Harness for multi-step tasks. For inline autocomplete, add DeepSeek Coder to your IDE.
Can it run commands on my computer?
Yes, such as tests, builds and linters, inside your project and with your approval. Read each command before you approve it.
Will it push to my main branch?
Only if you tell it to. Ask it to work on a branch, and merge after you've reviewed the changes.
Is my code sent anywhere?
The files it reads are sent to the DeepSeek model to be processed. Keep secrets out of the folder, and consider self-hosted models for sensitive code.
Is it free?
Harness is free during the public preview. With an API key, normal token rates apply. Coding sessions can use a lot of tokens, so Flash is the economical choice.
How does it compare with other AI coding tools?
It's open source, runs on low-cost DeepSeek models and can be extended with plugins. For a broader DeepSeek comparison, see DeepSeek vs ChatGPT. Get the mobile app for quick questions on the go.
Is this an official DeepSeek page?
No. DeepSeekR1.com is an independent guide. See about us, our disclaimer, privacy policy and terms.