swift scripts run in the background with swift launched via swiftly will make the current shell exit

[Frizlab]

Steps to Reproduce

• Create a file yolo.swift that contains the following content:

#!/usr/bin/env swift
print("hello")

• Make it executable
• Run it like so with swiftly properly installed and in the PATH:

./yolo.swift &

Expected Behavior

The script should run in the background, output hello and then exit, and the shell should stay alive.

Actual Behavior

The script launches in the background, but zsh errors out reading the TTY and exits:

prompt) ./toto.swift &
[1] 64386
prompt)  
zsh: error on TTY read: Input/output error
zsh: warning: 1 jobs SIGHUPed
[1]  + hangup     ./toto.swift
prompt) 
Saving session...
...copying shared history...
...saving history...truncating history files...
...completed.

[Process completed]

Environment

• macOS 26.2 (25C56)
• Terminal 2.15 (466)
• zsh 5.9 (arm64-apple-darwin25.0)
• swiftly 1.1.1
• swift from Xcode:

swift-driver version: 1.127.14.1 Apple Swift version 6.2 (swiftlang-6.2.3.3.20 clang-1700.6.3.2)
Target: arm64-apple-macosx26.0

My Analysis

I’m not 100% sure that’s the problem, but I’m pretty sure the issue actually comes from swift-subprocess. I actually had this exact same issue with my own “subprocess” implementation (I really should write a Readme…).

The fix consisted in not setting the current process group ID of the input file descriptor if the process was in the background (which I determined by checking whether the current process group ID of the file descriptor is my own process group ID).