Too Many Open Files

The number of concurrent network requests (MAX_CONCURRENCY) is set to 128 by default.

Every network request maps to an open socket, which is represented as a file on UNIX systems. If you see error messages like error trying to connect: tcp open error: Too many open files (os error 24) then you ran out of file handles.

You can either reduce the number of concurrent requests with the --max-concurrency option or increase the number of allowed open files on your system. For example, to reduce the number of concurrent requests to 50:

lychee --max-concurrency 50 https://example.com

Caution

Comes with a performance penalty

UNIX

To increase the number of allowed open files, you can use the ulimit command on UNIX systems. For example, to increase the limit to 4096:

ulimit -n 4096

Note

The ulimit command only affects the current shell session.

For permanent increases (e.g., across reboots), use launchctl to set system limits:

sudo launchctl limit maxfiles 4096 unlimited

Then apply per-session with ulimit -n 4096. This sets a soft limit of 4096 and removes the hard cap.

Note

You might need to restart your terminal or system for the changes to take effect.

For system-wide boot changes, create /Library/LaunchDaemons/limit.maxfiles.plist with XML keys for <key>LimitLoadToSessionType</key><string>System</string> and limits—see Apple docs.

Windows

On Windows, you can increase the limit by modifying the registry.

1. Open the Registry Editor (regedit).
2. Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip \Parameters.
3. Create a new DWORD value named MaxUserPort and set it to 65534.
4. Create another DWORD value named TcpTimedWaitDelay and set it to 30.
5. Restart your computer for the changes to take effect.

Caution

This addresses port exhaustion (common for socket errors); for raw file handles, check app-specific limits. Modifying the registry can have unintended consequences. Always back up your registry before making changes.