diff --git a/packages/mosaic/framework/tools/tmux/agent-send.sh b/packages/mosaic/framework/tools/tmux/agent-send.sh
index 675f4d63..68ce783e 100755
--- a/packages/mosaic/framework/tools/tmux/agent-send.sh
+++ b/packages/mosaic/framework/tools/tmux/agent-send.sh
@@ -155,15 +155,19 @@ B64=$(printf '%s' "$FULL" | base64 -w0)
 
 vflag=""; [ "$VERBOSE" = 1 ] && vflag="-v"
 
-# Socket default resolution (B1, 2026-08-29): explicit -L wins; else the
-# launcher-exported MOSAIC_TMUX_SOCKET; else the tmux default socket. When
-# the target session exists on MULTIPLE socket servers and no explicit -L
-# was given, refuse with a disambiguation error instead of silently picking
-# one (measured 2026-08-28/29: tasking sends landed in a stale default-
-# socket twin of the target seat; rc 0 reported honest delivery to the
-# wrong pane). Local targets only; remote targets pass -L through as-is.
+# Socket default resolution (B1, 2026-08-29). Precedence: explicit -L >
+# launcher-exported MOSAIC_TMUX_SOCKET > unique socket hit > refusal on
+# ambiguity > tmux default socket. The ambiguity refusal fires ONLY when
+# no explicit or env choice exists and the session name lives on multiple
+# servers (measured 2026-08-28/29: tasking sends landed in a stale
+# default-socket twin; rc 0 reported honest delivery to the wrong pane).
+# Socket discovery scans tmux's own socket dir, ${TMUX_TMPDIR:-/tmp}/tmux-UID
+# (codex PR #1466: TMPDIR is not where tmux keeps -L sockets).
+if [ -z "$SOCKET_NAME" ] && [ -n "${MOSAIC_TMUX_SOCKET:-}" ]; then
+  SOCKET_NAME="$MOSAIC_TMUX_SOCKET"
+fi
 if [ -z "$SOCKET_NAME" ] && [ -z "$SSH_TARGET" ]; then
-  socket_dir="${TMPDIR:-/tmp}/tmux-$(id -u)"
+  socket_dir="${TMUX_TMPDIR:-/tmp}/tmux-$(id -u)"
   hits=""
   for sf in "$socket_dir"/*; do
     [ -S "$sf" ] || continue
@@ -173,17 +177,12 @@ if [ -z "$SOCKET_NAME" ] && [ -z "$SSH_TARGET" ]; then
   hit_count=$(printf '%s' "$hits" | wc -w)
   if [ "$hit_count" -gt 1 ]; then
     echo "agent-send.sh: REFUSING - session '$DST_SESSION' exists on multiple sockets:$hits" >&2
-    echo "  Pass -L <socket> explicitly (or set MOSAIC_TMUX_SOCKET)." >&2
+    echo "  Pass -L <socket> explicitly (or export MOSAIC_TMUX_SOCKET to disambiguate)." >&2
     exit 4
   elif [ "$hit_count" -eq 1 ]; then
     SOCKET_NAME="${hits# }"
-  elif [ -n "${MOSAIC_TMUX_SOCKET:-}" ]; then
-    SOCKET_NAME="$MOSAIC_TMUX_SOCKET"
   fi
 fi
-if [ -z "$SOCKET_NAME" ] && [ -n "${MOSAIC_TMUX_SOCKET:-}" ] && [ -n "$SSH_TARGET" ]; then
-  SOCKET_NAME="$MOSAIC_TMUX_SOCKET"
-fi
 
 socket_args=()
 if [ -n "$SOCKET_NAME" ]; then
diff --git a/packages/mosaic/framework/tools/tmux/test-send-message-socket.sh b/packages/mosaic/framework/tools/tmux/test-send-message-socket.sh
index 5e72b555..79c3434a 100755
--- a/packages/mosaic/framework/tools/tmux/test-send-message-socket.sh
+++ b/packages/mosaic/framework/tools/tmux/test-send-message-socket.sh
@@ -88,17 +88,34 @@ done
 tmux -L "$SOCKET" new-session -d -s "$TWIN" -c "$TMPDIR" 'PS1="❯ " exec bash --noprofile --norc -i'
 tmux new-session -d -s "$TWIN" -c "$TMPDIR" 'PS1="❯ " exec bash --noprofile --norc -i'
 amb_rc=0
-"$AGENT_SEND" -s "$TWIN" -m "must refuse" >/tmp/amb.out 2>/tmp/amb.err || amb_rc=$?
+env -u MOSAIC_TMUX_SOCKET "$AGENT_SEND" -s "$TWIN" -m "must refuse" >/tmp/amb.out 2>/tmp/amb.err || amb_rc=$?
 [ "$amb_rc" -eq 4 ] || fail "ambiguity refusal: rc=$amb_rc want 4 (stderr: $(cat /tmp/amb.err))"
 grep -q "multiple sockets" /tmp/amb.err || fail "ambiguity refusal message missing socket list"
 grep -qF "$SOCKET" /tmp/amb.err || fail "ambiguity refusal message does not name the test socket"
 tmux kill-session -t "$TWIN" >/dev/null 2>&1 || true
 tmux -L "$SOCKET" kill-session -t "$TWIN" >/dev/null 2>&1 || true
 
+# Arm A2: with MOSAIC_TMUX_SOCKET exported, a twin session is NOT ambiguous:
+# the env var disambiguates by precedence (codex PR #1466 blocker).
+tmux -L "$SOCKET" new-session -d -s "$TWIN" -c "$TMPDIR" 'PS1="❯ " exec bash --noprofile --norc -i'
+tmux new-session -d -s "$TWIN" -c "$TMPDIR" 'PS1="❯ " exec bash --noprofile --norc -i'
+a2_rc=0
+MOSAIC_TMUX_SOCKET="$SOCKET" "$AGENT_SEND" -s "$TWIN" -m "env disambiguated" >/tmp/a2.out 2>/tmp/a2.err || a2_rc=$?
+[ "$a2_rc" -eq 0 ] || fail "env disambiguation: rc=$a2_rc (stderr: $(cat /tmp/a2.err))"
+sleep 0.2
+a2_pane="$(tmux -L "$SOCKET" capture-pane -t "=$TWIN:0.0" -p)" || fail "cannot capture twin (arm A2)"
+grep -qF "env disambiguated" <<<"$a2_pane" || fail "env disambiguation did not deliver on the named socket"
+a2_default="$(tmux capture-pane -t "=$TWIN:0.0" -p)" || true
+if grep -qF "env disambiguated" <<<"$a2_default"; then
+  fail "env disambiguation cross-delivered to the default-socket twin"
+fi
+tmux kill-session -t "$TWIN" >/dev/null 2>&1 || true
+tmux -L "$SOCKET" kill-session -t "$TWIN" >/dev/null 2>&1 || true
+
 # Arm B: session unique to ONE socket, no -L -> auto-resolve to that socket
 # and deliver there.
 uniq_rc=0
-"$AGENT_SEND" -s "$TARGET" -m "autoresolved hello" >/tmp/uniq.out 2>/tmp/uniq.err || uniq_rc=$?
+env -u MOSAIC_TMUX_SOCKET "$AGENT_SEND" -s "$TARGET" -m "autoresolved hello" >/tmp/uniq.out 2>/tmp/uniq.err || uniq_rc=$?
 [ "$uniq_rc" -eq 0 ] || fail "unique auto-resolution: rc=$uniq_rc (stderr: $(cat /tmp/uniq.err))"
 sleep 0.2
 auto_pane="$(capture_named)" || fail "could not capture named socket pane (arm B)"