Symptom: HarnessLaunch starts, but the agent does not come up. The browser opens and never loads anything, because the local server cannot start.
Cause: DeepSeek Harness runs a small local server, and that server needs port 3080 to be free. If another program already holds it — often a previous run that did not shut down cleanly — the new one cannot start.
This is a Windows problem, not a DeepSeek Harness problem, and it is usually fixed in under two minutes.
Step 1: Find out what is holding port 3080
Open Command Prompt or PowerShell and run:
netstat -ano | findstr :3080You will get one or more lines ending in a number. That last number is the process ID (PID).
To find out which program it is:
tasklist /FI "PID eq 12345"Replace 12345 with the PID you found. In PowerShell you can also ask directly:
Get-Process -Id 12345Step 2: Decide what to do about it
There are only two real cases.
Case A — it is a leftover DeepSeek Harness process. This is the common one. A previous run was closed badly and its server is still alive in the background, holding the port.
Fix: open Task Manager, find the leftover process, end it. Then start HarnessLaunch again. If you are not sure which process it is, restarting the machine clears every orphaned listener and is a perfectly reasonable first move.
Case B — it is a different program. Something else on your machine has claimed 3080. Stop that program before you start the agent, or run the agent at a time when that program is not running.
Step 3: Restart cleanly
If you ended a leftover process, start HarnessLaunch and press Run DeepSeek Harness again. It should now start the server and open the agent in your browser.
Step 4: If it still will not start
Check the troubleshooting page for the other two problems — the installer being blocked by SmartScreen, and file access inside the workspace. Those look similar from the outside: nothing happens, and nothing tells you why.
If it still fails, email [email protected] with:
- the output of
netstat -ano | findstr :3080 - what you expected and what happened
- the exact error text, copied rather than retyped
That output tells support immediately whether the port is actually the problem or whether something else is going on.
Why this happens more often than you would expect
A local server that runs in the background is easy to orphan. You close the window, the server keeps its port, and the next start fails with a symptom that looks like a broken installation. Nothing is broken — the port is simply still occupied.
That is also why a launcher does the waiting part for you. It starts the server, checks that it is actually answering, and only then opens the browser. When the port is blocked, the agent never answers — and knowing that this is the reason saves you from reinstalling something that was never broken.
HarnessLaunch runs DeepSeek Harness on Windows with one click. $4.57, paid once, no subscription. No account, no activation code. If it does not work on your machine, you get every cent back within 14 days.