microsoft
[Bug] `Browser.new_context: Browser closed` when using WebSurfer with docker browser
Describe the bug
When I try to use the WebSurfer (Fara) agent in Magentic UI, the Playwright browser starts and Magentic UI logs Connected to browser, but then immediately fails with:
TargetClosedError: Browser.new_context: Browser closed
From the UI perspective, the web browsing run fails and the agent returns an error.
This does not look like a firewall issue: the WebSocket connection to ws://127.0.0.1:<port>/... is successfully established, and the browser Docker container stays in Up state. The failure happens when creating a new browser context.
Environment
- Magentic UI version:
0.1.6 - Install method: cloned from GitHub +
pip installinto a virtualenv - Playwright Python version:
1.51.0 - Python version:
3.12(virtualenv) - OS:
Ubuntu 24.04 / WSL2 on Windows 11> - Docker:
- Docker version:
Docker version 29.1.2, build 890dcca - Browser image:
ghcr.io/microsoft/magentic-ui-browser:0.0.2
- Docker version:
How I installed
I followed your instruction on Readmd.md for FARA-7B. But my VRAM size does not meet the spec, so I used llama_cpp_python instead of vLLM.
How to reproduce
Activate the virtualenv:
cd ~/Magentic_Fara
source .venv/bin/activate
Start llama_cpp
python -m llama_cpp.server --model models/microsoft_Fara-7B-Q4_K_M.gguf --host 0.0.0.0 --port 5000 --n_ctx 32000 --n_gpu_layers 999
Start Magentic UI (default/docker mode):
magentic-ui --fara --port 8081 --config fara_config.yaml
Open the web UI in a browser ( 127.0.0.1:8081) and start a new session with WebSurfer/Fara.
Select one of the sampel questions.
Observe that:
The browser Docker container starts.
Magentic UI logs show Browser started at ws://127.0.0.1:
Shortly after, the run fails with TargetClosedError: Browser.new_context: Browser closed.
Expected behavior
WebSurfer should successfully create a browser context and be able to navigate to the requested pages using the dockerized Playwright browser.
The run should complete normally and return browsing results.
Actual behavior
Host-side logs (Magentic UI):
2025-12-12 16:42:38.409 | INFO | magentic_ui.backend.web.routes.ws:run_websocket:44 - WebSocket connection established for run 4
2025-12-12 16:42:38.506 | INFO | magentic_ui.backend.web.routes.ws:run_websocket:53 - Received start request for run 4
2025-12-12 16:42:38.515 | INFO | magentic_ui.backend.teammanager.teammanager:prepare_run_paths:101 - Creating run dirs: /home/<USER>/.magentic_ui/files/user/<EMAIL>/4/4 and /home/<USER>/.magentic_ui/files/user/<EMAIL>/4/4
INFO: 127.0.0.1:57830 - "PUT /api/sessions/4?user_id=<EMAIL> HTTP/1.1" 200 OK
INFO: 127.0.0.1:57830 - "GET /api/sessions/?user_id=<EMAIL> HTTP/1.1" 200 OK
2025-12-12 16:42:38.951 | INFO | magentic_ui.tools.playwright.browser.base_playwright_browser:_start:186 - Browser started at ws://127.0.0.1:56271/27c5f429a370f283812ceb9f210aecf8
2025-12-12 16:42:39.174 | INFO | magentic_ui.tools.playwright.browser.base_playwright_browser:_start:189 - Connecting to browser at ws://127.0.0.1:56271/27c5f429a370f283812ceb9f210aecf8
2025-12-12 16:42:39.187 | INFO | magentic_ui.tools.playwright.browser.base_playwright_browser:connect_browser_with_retry:32 - Trying to establish connection to browser at ws://127.0.0.1:56271/27c5f429a370f283812ceb9f210aecf8...
2025-12-12 16:42:44.193 | WARNING | magentic_ui.tools.playwright.browser.base_playwright_browser:connect_browser_with_retry:35 - Retrying connection in 5 seconds: : WebSocket error: read ECONNRESET
...
2025-12-12 16:42:59.337 | INFO | magentic_ui.tools.playwright.browser.base_playwright_browser:_start:193 - Connected to browser
ERROR:autogen_core:Error processing publish message for web_surfer_...
...
File ".../magentic_ui/tools/playwright/browser/base_playwright_browser.py", line 194, in _start
self._context = await self._browser.new_context()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File ".../playwright/async_api/_generated.py", line 13930, in new_context
await self._impl_obj.new_context(
File ".../playwright/_impl/_browser.py", line 131, in new_context
channel = await self._channel.send("newContext", params)
File ".../playwright/_impl/_connection.py", line 528, in wrap_api_call
raise rewrite_error(error, f"{parsed_st['apiName']}: {error}") from None
playwright._impl._errors.TargetClosedError: Browser.new_context: Browser closed
ERROR:magentic_ui.backend.web.managers.connection:Stream error for run 4: TargetClosedError: Browser.new_context: Browser closed
...
RuntimeError: TargetClosedError: Browser.new_context: Browser closed
Docker containers:
$ docker ps --format 'table {{.ID}}\t{{.Image}}\t{{.Status}}\t{{.Names}}'
CONTAINER ID IMAGE STATUS NAMES
96fe37649a68 ghcr.io/microsoft/magentic-ui-browser:0.0.2 Up 6 seconds magentic-ui-vnc-browser_d65aa72219eb0f62f8772b312be8aaa5_56645
3b2a8d531f82 ghcr.io/microsoft/magentic-ui-browser:0.0.2 Up 38 minutes magentic-ui-vnc-browser_850e6f011abb7efd93ac922cf2b498d8_52823
Docker logs for the active browser container (ID 96fe37649a68):
Starting services...
DISPLAY=:99
2025-12-12 08:15:52,783 CRIT Supervisor is running as root. ...
2025-12-12 08:15:52,796 INFO supervisord started with pid 1
2025-12-12 08:15:53,804 INFO spawned: 'x11setup' with pid 11
2025-12-12 08:15:53,809 INFO spawned: 'x11vnc' with pid 13
2025-12-12 08:15:53,815 INFO spawned: 'playwright-server' with pid 15
2025-12-12 08:15:53,820 INFO spawned: 'novnc' with pid 17
2025-12-12 08:15:53,824 INFO spawned: 'openbox' with pid 20
2025-12-12 08:15:53,829 INFO spawned: 'xvfb' with pid 23
...
Starting Playwright server...
2025-12-12 08:15:56,872 INFO success: x11vnc entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)
Playwright server running: ws://localhost:59023/d65aa72219eb0f62f8772b312be8aaa5
Note: the browser container keeps running; it does not immediately exit/crash at the Docker level.
Additional context
When I test without docker ( magentic-ui --fara --port 8081 --config fara_config.yaml --run-without-docker ) it runs well.
