Aria2 Download Backend

ComfyUI Model Resolver can use aria2c as an optional download engine for model files. The default Python downloader still works without any extra setup, but aria2 is useful when you download large files and want better control over active transfers.

What aria2 adds

• Pause and resume support for downloads started through Model Resolver.
• Better handling of large files and interrupted transfers.
• Parallel/segmented downloading when the remote server supports it.
• A separate download process, so the UI can stay responsive while large files are being downloaded.

Recommended setup

1. Open Model Resolver.
2. Go to Options.
3. Find Download Engine.
4. Change the backend to aria2.
5. Click Install aria2.
6. Wait until the status message says that aria2 was installed and detected.
7. Click Save Options if you changed any other settings.

The installer downloads the official aria2 release for your platform when a matching prebuilt binary is available. It saves the executable inside this custom node directory under:

tools/aria2/

After installation, Model Resolver fills aria2c path automatically and uses that executable for future aria2 downloads.

Manual setup

If automatic installation is not available for your system, install aria2 manually and point Model Resolver to the executable.

Windows

Download aria2 from the official release page:

https://github.com/aria2/aria2/releases

Extract the archive and select the full path to aria2c.exe, for example:

D:\Tools\aria2\aria2c.exe

Paste that path into Options > Download Engine > aria2c path and click Check aria2.

Linux

Install aria2 with your package manager, for example:

sudo apt install aria2

or:

sudo pacman -S aria2

If aria2c is available in your system PATH, you can leave aria2c path empty. Otherwise, enter the full executable path, such as:

/usr/bin/aria2c

macOS

If you use Homebrew:

brew install aria2

Then leave aria2c path empty if aria2c is available in your PATH, or enter the full path reported by:

which aria2c

Checking the configuration

Use Check aria2 in Options. A valid setup should show that aria2 is available. If the daemon is not running yet, that is normal. Model Resolver starts the aria2 RPC process automatically when the first aria2 download begins.

How downloads behave

When the aria2 backend is active:

• New model downloads are sent to aria2.
• Pause and resume buttons work for aria2 downloads.
• Cancel removes the active aria2 transfer.
• Existing Python-download behavior remains available by switching the backend back to Python.

Troubleshooting

aria2c executable was not found

Model Resolver could not find aria2c.

Try one of these:

• Click Install aria2 again.
• Paste the full path to aria2c.exe or aria2c.
• Add aria2 to your system PATH.
• Switch back to the Python downloader.

Automatic install is not supported on my platform

The installer only uses official prebuilt aria2 binaries. If the official release does not provide a suitable binary for your OS/architecture, install aria2 manually and configure aria2c path.

CivitAI downloads fail with authorization or access errors

Some CivitAI downloads require a valid token or a resolved download URL. Make sure your CivitAI token is configured in Model Resolver options. If the same link works in the browser but not in aria2, switch temporarily to the Python backend and try again.

Downloads do not pause immediately

Pause/resume commands are sent to the aria2 RPC process. Usually this is immediate, but very busy downloads or remote server delays can take a moment to report the new state. The UI should update once aria2 confirms the transfer state.

Removing aria2 installed by Model Resolver

Close ComfyUI, then remove the downloaded aria2 folder from:

tools/aria2/

After that, either install aria2 again from Options or switch the download backend back to Python.