Stderr/Stdout Interleaving Broken in Claude Code's Bash Tool Execution

Bug Description
Claude Code's bash tool does not preserve interleaving of stdout and stderr. If you print to unbuffered stdout and stderr from Go with out1/err1/out2/err2 pattern, you get out1/out2/err1/err2, and this can cause confusing behavior when a tool uses both out and err channels and does correct line buffering / flushing.

Environment Info

• Platform: darwin
• Terminal: vscode
• Version: 1.0.33
• Feedback ID: 5df98393-266f-46d9-ac30-90fc2794d4c7

Test Program

interleave.go:

package main

import (
	"fmt"
	"os"
)

func main() {
	fmt.Fprintln(os.Stdout, "stdout line 1")
	fmt.Fprintln(os.Stderr, "stderr line 1")
	fmt.Fprintln(os.Stdout, "stdout line 2")
	fmt.Fprintln(os.Stderr, "stderr line 2")
}

Run in claude: !go run ./interleave.go

Outputs:

stdout line 1
stdout line 2
stderr line 1
stderr line 2

But it should output:

stdout line 1
stderr line 1
stdout line 2
stderr line 2

Workaround:

!go run ./interleave.go 2>&1