framework tools/tmux: B1 — already-exact targets no longer bypass discovery; rc 4 in EXIT CODES section

DST_TARGET computed BEFORE socket discovery and used for the probes
(probing '==name' for already-exact input was a false negative that
fell back to the default socket; codex blocker). Exit 4 moved into the
canonical EXIT CODES header (codex should-fix).

packages/mosaic/framework/tools/tmux/agent-send.sh

@@ -66,8 +66,10 @@
 #     group 1 = src label   group 2 = dst host:session
 #     group 3 = class (absent => actionable)   group 4 = message body
 #
-# EXIT CODES  (passed through from send-message.sh)
+# EXIT CODES  (passed through from send-message.sh, except 4)
 #   0 delivered/queued · 1 target not found · 2 still draft · 3 usage error
+#   4 agent-send refusal: local target session exists on multiple socket
+#     servers and no -L / MOSAIC_TMUX_SOCKET disambiguated it (B1)
 set -uo pipefail
 
 SELF_DIR=$(cd -- "$(dirname -- "$0")" && pwd)
@@ -158,6 +160,18 @@ B64=$(printf '%s' "$FULL" | base64 -w0)
 
 vflag=""; [ "$VERBOSE" = 1 ] && vflag="-v"
 
+# Exact session matching for the sender target (codex PR #1466): without
+# '=', tmux target syntax accepts an unambiguous PREFIX, so a delivery
+# aimed at session X can land in X-old. Compound targets (session:win.pane)
+# and already-exact ('=...') forms pass through untouched. Computed BEFORE
+# socket discovery so the discovery probes use the same target semantics
+# (probing '==name' for an already-exact input was a false-negative hit).
+DST_TARGET="$DST_SESSION"
+case "$DST_SESSION" in
+  =*|*:*) ;;
+  *) DST_TARGET="=$DST_SESSION" ;;
+esac
+
 # Socket default resolution (B1, 2026-08-29). Precedence: explicit -L >
 # launcher-exported MOSAIC_TMUX_SOCKET > unique socket hit > refusal on
 # ambiguity > tmux default socket. The ambiguity refusal fires ONLY when
@@ -181,7 +195,7 @@ if [ -z "$SOCKET_NAME" ] && [ -z "$SSH_TARGET" ]; then
     # '=' forces exact session-name matching: tmux target syntax otherwise
     # accepts an unambiguous PREFIX, so a session named X-old on a socket
     # would count as a false hit for target X (codex PR #1466).
-    tmux -L "$sname" has-session -t "=$DST_SESSION" 2>/dev/null && hits="$hits$sname"$'\n'
+    tmux -L "$sname" has-session -t "$DST_TARGET" 2>/dev/null && hits="$hits$sname"$'\n'
   done
   hit_count=$(printf '%s' "$hits" | grep -c . || true)
   if [ "$hit_count" -gt 1 ]; then
@@ -199,16 +213,6 @@ if [ -n "$SOCKET_NAME" ]; then
   socket_args=(-L "$SOCKET_NAME")
 fi
 
-# Exact session matching for the sender target too (codex PR #1466): without
-# '=', tmux target syntax accepts an unambiguous PREFIX, so a delivery aimed
-# at session X can land in X-old. Compound targets (session:win.pane) and
-# already-exact ('=...') forms pass through untouched.
-DST_TARGET="$DST_SESSION"
-case "$DST_SESSION" in
-  =*|*:*) ;;
-  *) DST_TARGET="=$DST_SESSION" ;;
-esac
-
 if [ -z "$SSH_TARGET" ]; then
   # Local pane: call the canonical sender directly.
   exec "$SENDER" "${socket_args[@]}" -t "$DST_TARGET" -b "$B64" -r "$RETRIES" $vflag