sed: unknown option to `s'

The errors

sed: -e expression #1, char 11: unknown option to `s'
sed: -e expression #1, char 10: unterminated `s' command

Why they happen

s/old/new/ uses / to separate its parts. When old or new is a path, its slashes end the parts early, and sed reads what's left as options, like the g in /g:

sed -i 's/src/app/old.ts/src/app/new.ts/g' index.ts    # unknown option to `s'

A backslash right before the delimiter escapes it, so the command never ends: unterminated. A Windows path that ends in a backslash does it:

sed 's/old/C:\/' paths.txt    # unterminated `s' command

The fix

Use a delimiter that isn't in the text. Any character works after s:

sed -i 's#src/app/old.ts#src/app/new.ts#g' index.ts

For a coding agent, an edit tool is safer still: it replaces exact text, with no regex and no quoting to get wrong.