[🐛 Bug]: System.ArgumentException: 'The JSON value of length 492188000 is too large and not supported.'

[neominky]

Description

I want to run Firefox with a specific user profile.
But after a very long initialization time, I get a Json error.
The profile folder is 930 MB in size. This Profile has only visited two websites.
If I don't specify a Profile, it runs fine.

This is a capture of the call stack.

In the HttpRequestInfo method, an error occurs while executing ParametersAsJsonString.

Roughly tracing the process, I saw that it involved creating a compressed file, converting it to Base64, and then converting it back to JSON. It feels very inefficient, but how can I fix this error?

Thanks in advance for your help.

Reproducible Code

private async Task LaunchFirefox(FirefoxContext browser)
        {
            var options = new FirefoxOptions();
            options.EnableDevToolsProtocol = false;
            var profile = new FirefoxProfile(browser.ProfileLocation);
            options.Profile = profile;

            //options.AddArguments("--headless");
            FirefoxDriverService service = FirefoxDriverService.CreateDefaultService();
            var driver = new FirefoxDriver(service, options, TimeSpan.FromSeconds(60.0)); //<==error here

Debugging Logs

2025-08-19 17:36:20.810 TRACE DriverService: 1755592580803	geckodriver	INFO	Listening on 127.0.0.1:4507
2025-08-19 17:36:23.054 DEBUG HttpCommandExecutor: Executing command: []: newSession

[selenium-ci]

@neominky, thank you for creating this issue. We will troubleshoot it as soon as we can.

Selenium Triage Team: remember to follow the Triage Guide

[cgoldberg]

This looks like a limitation of System.Text.Json: dotnet/runtime#61089

The max size it can handle is 125MB. I don't think there is anything that can be done about this unless we use a different JSON serialization library or drastically change how we handle profiles. I'm not a .NET guy, so I can't speak to either of those.

[nvborisenko]

I am afraid we cannot fix it. For me it is strange to send 400MB data over wire. What if:

options.AddArguments("-profile", "/path/to/profile");

More details: https://developer.mozilla.org/en-US/docs/Web/WebDriver/Reference/Capabilities/firefoxOptions

To have geckodriver pick up an existing profile on the filesystem, please set the args field to {"args": ["-profile", "/path/to/your/profile"]}. Note that if you use a remote client targeting a server on a different system, the profile must already exist on the target system.