[Failed to forward HTTP request to MATLAB with error: 400]

[Kubadrew]

Summary

Hello All,

When I start matlab-proxy I see following error
Failed to forward HTTP request to MATLAB with error: 400, message="Content-Length can't be present with Transfer-Encoding:\n\n b'Content-Length: 87636'\n ^", url='https://127.0.0.1:31518/msh/matlab/content/bundle.mwBundle.agent-spf-ui-matlab.js?mre=http://localhost:8888/'

Can you suggest what could be the reason ??

Browser(s) used

No response

Script output

============================
MATLAB

matlab - /usr/local/bin/matlab �[32mOK�[0m

============================
matlab-proxy-app

matlab-proxy-app - None �[31m X�[0m
Recommendation: matlab-proxy-app is not installed. Please install matlab-proxy-app.

============================
Python and pip executables

python - None �[31m X�[0m
Recommendation: python is not installed. Please install python.

pip - /usr/local/bin/pip �[32mOK�[0m

pip 26.0.1 from /usr/local/lib/python3.11/dist-packages/pip (python 3.11)

python3 - /usr/local/bin/python3 �[32mOK�[0m

Python 3.11.14

============================
Xvfb

Xvfb - /usr/bin/Xvfb �[32mOK�[0m

============================
OS information

Linux
6.6.87.2-microsoft-standard-WSL2
Linux-6.6.87.2-microsoft-standard-WSL2-x86_64-with-glibc2.35
uname_result(system='Linux', node='ab4031f4b0fd', release='6.6.87.2-microsoft-standard-WSL2', version='#1 SMP PREEMPT_DYNAMIC Thu Jun 5 18:30:46 UTC 2025', machine='x86_64')

============================
Conda information

conda - None

============================
Installed packages

/bin/sh: 1: python: not found

============================
Environment variables

MWI_BASE_URL=/matlab
MWI_MATLAB_STARTUP_SCRIPT=run('/usr/local/MATLAB/R2025b/matlab_web_interface_init.m')
MWI_ENABLE_TOKEN_AUTH=False
MWI_APP_PORT=8888
MWI_LOG_FILE=/tmp/matlab_proxy_logs.txt
MATLAB_IMAGE_VERSION=1.13.0-rc1
MATLAB_ENGINE_SESSION=MATLAB_GUI
MWI_LOG_LEVEL=INFO

============================
matlab-proxy logs

[CRITICAL 2026-02-18 10:54:34,742 MATLABProxyApp] Access MATLAB at:
http://localhost:8888/matlab
[INFO 2026-02-18 10:54:34,742 MATLABProxyApp] MATLAB Root: /usr/local/MATLAB/R2025b
[INFO 2026-02-18 10:55:58,671 MATLABProxyApp] Starting MATLAB...
[INFO 2026-02-18 10:55:58,736 MATLABProxyApp] Started Xvfb with PID=8165 on DISPLAY=0
[INFO 2026-02-18 10:55:58,736 MATLABProxyApp] Starting window manager with DISPLAY=:0
[INFO 2026-02-18 10:55:58,737 MATLABProxyApp] The results of executing MWI_MATLAB_STARTUP_SCRIPT are stored at: /home/361jdrewniak/.matlab/MWI/hosts/ab4031f4b0fd/ports/8888/startup_code_output.txt
[ERROR 2026-02-18 10:56:31,040 MATLABProxyApp] Failed to forward HTTP request to MATLAB with error: 400, message="Content-Length can't be present with Transfer-Encoding:\n\n b'Content-Length: 48330'\n ^", url='https://127.0.0.1:31516/matlab/msh/matlab/content/bundle.mwBundle.agent-spf-ui-matlab.js?mre=http://localhost:8888/matlab/'
[INFO 2026-02-18 11:13:23,002 MATLABProxyApp] Waiting for MATLAB to exit...
[ERROR 2026-02-18 11:13:27,995 MATLABProxyApp] Failed to send HTTP request to Embedded connector
[INFO 2026-02-18 11:13:27,995 MATLABProxyApp] Failed to stop MATLAB gracefully. Attempting to terminate the process.
[INFO 2026-02-18 11:13:36,859 MATLABProxyApp] MATLAB has shutdown with exit code: 0
[INFO 2026-02-18 11:13:46,247 MATLABProxyApp] Shutting down MATLAB Desktop...
[INFO 2026-02-18 11:13:46,352 MATLABProxyApp] Finished shutting down. Thank you for using the MATLAB proxy.
[CRITICAL 2026-02-18 11:17:10,996 MATLABProxyApp] Access MATLAB at:
http://localhost:8888
[INFO 2026-02-18 11:17:10,996 MATLABProxyApp] MATLAB Root: /usr/local/MATLAB/R2025b
[INFO 2026-02-18 11:17:11,807 MATLABProxyApp] Starting MATLAB...
[INFO 2026-02-18 11:17:11,878 MATLABProxyApp] Started Xvfb with PID=15581 on DISPLAY=0
[INFO 2026-02-18 11:17:11,879 MATLABProxyApp] Starting window manager with DISPLAY=:0
[INFO 2026-02-18 11:17:11,880 MATLABProxyApp] The results of executing MWI_MATLAB_STARTUP_SCRIPT are stored at: /home/361jdrewniak/.matlab/MWI/hosts/ab4031f4b0fd/ports/8888/startup_code_output.txt
[ERROR 2026-02-18 11:17:30,744 MATLABProxyApp] Failed to forward HTTP request to MATLAB with error: 400, message="Content-Length can't be present with Transfer-Encoding:\n\n b'Content-Length: 87636'\n ^", url='https://127.0.0.1:31516/msh/matlab/content/bundle.mwBundle.agent-spf-ui-matlab.js?mre=http://localhost:8888/'
[ERROR 2026-02-18 12:04:34,224 MATLABProxyApp] Failed to forward HTTP request to MATLAB with error: 400, message="Content-Length can't be present with Transfer-Encoding:\n\n b'Content-Length: 87636'\n ^", url='https://127.0.0.1:31516/msh/matlab/content/bundle.mwBundle.agent-spf-ui-matlab.js?mre=http://localhost:8888/'
[INFO 2026-02-18 14:34:54,103 MATLABProxyApp] Waiting for MATLAB to exit...
[INFO 2026-02-18 14:39:10,986 MATLABProxyApp] Shutting down MATLAB Desktop...
[ERROR 2026-02-18 14:39:49,125 MATLABProxyApp]
[INFO 2026-02-18 14:39:49,125 MATLABProxyApp] Failed to stop MATLAB gracefully. Attempting to terminate the process.
[INFO 2026-02-18 16:01:51,986 MATLABProxyApp] MATLAB has shutdown with exit code: 0
[INFO 2026-02-18 16:01:52,008 MATLABProxyApp] Finished shutting down. Thank you for using the MATLAB proxy.
[CRITICAL 2026-02-18 16:13:11,169 MATLABProxyApp] Access MATLAB at:
http://localhost:8888
[INFO 2026-02-18 16:13:11,170 MATLABProxyApp] MATLAB Root: /usr/local/MATLAB/R2025b
[INFO 2026-02-18 16:13:11,944 MATLABProxyApp] Starting MATLAB...
[INFO 2026-02-18 16:13:12,614 MATLABProxyApp] Started Xvfb with PID=74956 on DISPLAY=0
[INFO 2026-02-18 16:13:12,614 MATLABProxyApp] Starting window manager with DISPLAY=:0
[INFO 2026-02-18 16:13:12,615 MATLABProxyApp] The results of executing MWI_MATLAB_STARTUP_SCRIPT are stored at: /home/361jdrewniak/.matlab/MWI/hosts/ab4031f4b0fd/ports/8888/startup_code_output.txt
[ERROR 2026-02-18 16:13:36,603 MATLABProxyApp] Failed to forward HTTP request to MATLAB with error: 400, message="Content-Length can't be present with Transfer-Encoding:\n\n b'Content-Length: 87636'\n ^", url='https://127.0.0.1:31516/msh/matlab/content/bundle.mwBundle.agent-spf-ui-matlab.js?mre=http://localhost:8888/'
[INFO 2026-02-18 17:57:41,611 MATLABProxyApp] Waiting for MATLAB to exit...
[INFO 2026-02-18 17:57:55,116 MATLABProxyApp] MATLAB has shutdown with exit code: 0
[INFO 2026-02-18 17:57:58,023 MATLABProxyApp] Shutting down MATLAB Desktop...
[INFO 2026-02-18 17:57:58,128 MATLABProxyApp] Finished shutting down. Thank you for using the MATLAB proxy.
[CRITICAL 2026-02-23 14:13:12,081 MATLABProxyApp] Access MATLAB at:
http://localhost:8888
[INFO 2026-02-23 14:13:12,081 MATLABProxyApp] MATLAB Root: /usr/local/MATLAB/R2025b
[INFO 2026-02-23 14:13:55,569 MATLABProxyApp] Starting MATLAB...
[INFO 2026-02-23 14:13:55,632 MATLABProxyApp] Started Xvfb with PID=14107 on DISPLAY=0
[INFO 2026-02-23 14:13:55,632 MATLABProxyApp] Starting window manager with DISPLAY=:0
[INFO 2026-02-23 14:13:55,634 MATLABProxyApp] The results of executing MWI_MATLAB_STARTUP_SCRIPT are stored at: /home/361jdrewniak/.matlab/MWI/hosts/ab4031f4b0fd/ports/8888/startup_code_output.txt
[INFO 2026-02-23 14:18:33,031 MATLABProxyApp] Waiting for MATLAB to exit...
[INFO 2026-02-23 14:19:29,956 MATLABProxyApp] Shutting down MATLAB Desktop...
[INFO 2026-02-23 14:19:42,715 MATLABProxyApp] Finished shutting down. Thank you for using the MATLAB proxy.
[ERROR 2026-02-23 14:19:49,441 MATLABProxyApp] The port 8888 is not available. Please set another value for the environment variable MWI_APP_PORT
[ERROR 2026-02-23 14:19:57,464 MATLABProxyApp] The port 8888 is not available. Please set another value for the environment variable MWI_APP_PORT
[CRITICAL 2026-02-23 14:20:49,006 MATLABProxyApp] Access MATLAB at:
http://localhost:8888
[INFO 2026-02-23 14:20:49,006 MATLABProxyApp] MATLAB Root: /usr/local/MATLAB/R2025b
[INFO 2026-02-23 14:20:49,705 MATLABProxyApp] Starting MATLAB...
[INFO 2026-02-23 14:20:49,749 MATLABProxyApp] Started Xvfb with PID=17675 on DISPLAY=0
[INFO 2026-02-23 14:20:49,749 MATLABProxyApp] Starting window manager with DISPLAY=:0
[INFO 2026-02-23 14:20:49,750 MATLABProxyApp] The results of executing MWI_MATLAB_STARTUP_SCRIPT are stored at: /home/361jdrewniak/.matlab/MWI/hosts/ab4031f4b0fd/ports/8888/startup_code_output.txt
[ERROR 2026-02-23 14:21:06,241 MATLABProxyApp] Failed to forward HTTP request to MATLAB with error: 400, message="Content-Length can't be present with Transfer-Encoding:\n\n b'Content-Length: 87636'\n ^", url='https://127.0.0.1:31518/msh/matlab/content/bundle.mwBundle.agent-spf-ui-matlab.js?mre=http://localhost:8888/'

[rashedmyt]

Hi @Kubadrew,

Thank you for reaching out. The mentioned HTTP error printed in the terminal after starting matlab-proxy-app is expected and should not have any effect on accessing MATLAB from the browser.

Are you observing any unexpected behavior when using this package? If yes, please elaborate on the issue you are facing and possible reproduction steps to help us narrow the issue.

[Kubadrew]

Thanks for clarification. I don't have specific issue.

[rashedmyt]

Hi @Kubadrew

Thanks for the confirmation. Feel free to create a new issue if you discover any unexpected behavior.