sed: can't read : No such file or directory

The error

$ f=$(rg -l "retries" src | head -1)
$ sed -i 's/retries = 3/retries = 5/' "$f"
sed: can't read : No such file or directory

Nothing between can't read and the colon: the file name was empty.

Why it happens

An empty variable. The search before it found nothing, so $f is empty and sed is asked to edit a file called "". Coding agents do this when they pick the file with rg -l … | head -1 and the pattern doesn't match anywhere.

macOS's sed -i '' on GNU sed, the sed on Linux and in Git Bash. GNU sed reads the '' as the script, and the real script as a file name:

$ sed -i '' 's/red/blue/' config.txt
sed: can't read s/red/blue/: No such file or directory

The fix

Check the name before sed uses it:

f=$(rg -l "retries" src | head -1)
if [ -n "$f" ]; then sed -i 's/retries = 3/retries = 5/' "$f"; else echo "no file matched"; fi

For a script that runs on both kinds of sed, give -i a suffix, then remove the backup it leaves:

sed -i.bak 's/red/blue/' config.txt && rm config.txt.bak

A coding agent is safer with an edit tool, which changes exact text in a file it names. Slashes in the expression are the other common sed failure: unknown option to `s'.