diff --git a/src/f.sh b/src/f.sh
index dd67ee5..c600702 100644
--- a/src/f.sh
+++ b/src/f.sh
@@ -25,7 +25,18 @@ f() {
         printf '\n[LEARN] "%s" failed. Enter a fix (blank to give up): ' "$*" >&2
         read -r n
         [ -n "$n" ] || return 1
-        echo "$*=$n" >> p
+        # printf, not echo: POSIX leaves it implementation-defined whether
+        # echo interprets backslash escapes in its argument, and dash/
+        # busybox ash both do by default (bash's echo doesn't, unless
+        # -e). A taught fix containing a literal "\n" -- e.g. any fix
+        # that itself uses printf '%s\n' internally, which is common and
+        # idiomatic -- got silently corrupted into a real embedded
+        # newline on write, breaking p's one-fix-per-line format the
+        # moment that fix was replayed. Confirmed live: identical fix
+        # text taught under bash was fine; the same text taught under
+        # `sh` (dash) truncated mid-line on replay. printf's %s never
+        # interprets escapes in the substituted value, under any shell.
+        printf '%s=%s\n' "$*" "$n" >> p
         FIX="$n"
     done
 }