[Bug] `Browser.new_context: Browser closed` when using WebSurfer with docker browser

[sunnykim1206]

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 install into 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

---

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:/... and Connected to browser.

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.

[matheusmaldaner]

Hi @sunnykim1206, while I haven't replicated your issue, I found these threads describing similar problems pointing to Docker shared memory limits:

• https://issues.chromium.org/issues/40517415
• Chrome crashing due to /dev/shm filesystem lacking execution permissions for Docker Desktop and the WSL2 setup. SeleniumHQ/docker-selenium#1309

Docker defaults to 64MB for /dev/shm, which can cause Chromium to crash when creating browser contexts.

Can you try adding shm_size="2g", after auto_remove=True, in both:

• src/magentic_ui/tools/playwright/browser/headless_docker_playwright_browser.py (line ~131)
• src/magentic_ui/tools/playwright/browser/vnc_docker_playwright_browser.py (line ~169)

Let me know if this helps and I can make a PR with this fix!

[moontato]

Hi @sunnykim1206, while I haven't replicated your issue, I found these threads describing similar problems pointing to Docker shared memory limits:

* https://issues.chromium.org/issues/40517415

* [Chrome crashing due to /dev/shm filesystem lacking execution permissions for Docker Desktop and the WSL2 setup. SeleniumHQ/docker-selenium#1309](https://github.com/SeleniumHQ/docker-selenium/issues/1309)

Docker defaults to 64MB for /dev/shm, which can cause Chromium to crash when creating browser contexts.

Can you try adding shm_size="2g", after auto_remove=True, in both:

* [src/magentic_ui/tools/playwright/browser/headless_docker_playwright_browser.py](https://github.com/microsoft/magentic-ui/blob/main/src/magentic_ui/tools/playwright/browser/headless_docker_playwright_browser.py) (line ~131)

* [src/magentic_ui/tools/playwright/browser/vnc_docker_playwright_browser.py](https://github.com/microsoft/magentic-ui/blob/main/src/magentic_ui/tools/playwright/browser/vnc_docker_playwright_browser.py) (line ~169)

Let me know if this helps and I can make a PR with this fix!

Hello, I am having this issue as well. Adding shm_size="2g" in both files unfortunately does not fix it.