diff --git a/packages/mosaic/framework/tools/tmux/agent-send.sh b/packages/mosaic/framework/tools/tmux/agent-send.sh
index 1d0b9c09..3c16555b 100755
--- a/packages/mosaic/framework/tools/tmux/agent-send.sh
+++ b/packages/mosaic/framework/tools/tmux/agent-send.sh
@@ -168,7 +168,13 @@ vflag=""; [ "$VERBOSE" = 1 ] && vflag="-v"
 # (probing '==name' for an already-exact input was a false-negative hit).
 DST_TARGET="$DST_SESSION"
 case "$DST_SESSION" in
-  =*|*:*) ;;
+  =*) ;;
+  *:*)
+    # Compound target (session:win.pane): pin the SESSION component exact
+    # (=session:win.pane); unpinned, the session part still prefix-matches
+    # (codex PR #1466: 'agent:0.0' can resolve into 'agent-old').
+    DST_TARGET="=${DST_SESSION%%:*}:${DST_SESSION#*:}"
+    ;;
   *) DST_TARGET="=$DST_SESSION" ;;
 esac
 
diff --git a/packages/mosaic/framework/tools/tmux/test-send-message-socket.sh b/packages/mosaic/framework/tools/tmux/test-send-message-socket.sh
index 073818b8..0d2ef2fe 100755
--- a/packages/mosaic/framework/tools/tmux/test-send-message-socket.sh
+++ b/packages/mosaic/framework/tools/tmux/test-send-message-socket.sh
@@ -139,6 +139,20 @@ fi
 tmux kill-session -t "$PSEUDO" >/dev/null 2>&1 || true
 rm -f "$A3_ERR"
 
+# Arm A4: compound targets pin the SESSION component exact (codex PR
+# #1466): 'TWIN:0.0' must not resolve into the prefix-named session.
+PSEUDO2="${TWIN}-old"
+tmux new-session -d -s "$PSEUDO2" -c "$TMPDIR" 'PS1="❯ " exec bash --noprofile --norc -i'
+A4_ERR=$(mktemp)
+a4_rc=0
+env -u MOSAIC_TMUX_SOCKET "$AGENT_SEND" -s "$TWIN:0.0" -m "compound trap" >/dev/null 2>"$A4_ERR" || a4_rc=$?
+[ "$a4_rc" -eq 1 ] || fail "compound prefix false-hit: rc=$a4_rc want 1 (stderr: $(cat "$A4_ERR"))"
+if tmux capture-pane -t "=$PSEUDO2:0.0" -p 2>/dev/null | grep -qF "compound trap"; then
+  fail "compound delivery landed in the prefix-named session"
+fi
+tmux kill-session -t "$PSEUDO2" >/dev/null 2>&1 || true
+rm -f "$A4_ERR"
+
 uniq_rc=0
 env -u MOSAIC_TMUX_SOCKET "$AGENT_SEND" -s "$TARGET" -m "autoresolved hello" >$UNIQ_OUT 2>$UNIQ_ERR || uniq_rc=$?
 [ "$uniq_rc" -eq 0 ] || fail "unique auto-resolution: rc=$uniq_rc (stderr: $(cat $UNIQ_ERR))"