diff --git a/packages/mosaic/framework/tools/tmux/agent-send.sh b/packages/mosaic/framework/tools/tmux/agent-send.sh
index 9da54052..d44e46ca 100755
--- a/packages/mosaic/framework/tools/tmux/agent-send.sh
+++ b/packages/mosaic/framework/tools/tmux/agent-send.sh
@@ -126,7 +126,16 @@ if [ -z "$SRC_LABEL" ]; then
   src_host=$(hostname -s 2>/dev/null || echo "?")
   src_sess=${MOSAIC_AGENT_NAME:-}
   if [ -z "$src_sess" ]; then
-    src_sess=$(tmux display-message -p '#S' 2>/dev/null || echo "?")
+    if [ -n "${TMUX:-}" ]; then
+      # Inside tmux: display-message resolves against this client's own session.
+      src_sess=$(tmux display-message -p '#S' 2>/dev/null || echo "?")
+    else
+      # Outside tmux with no name: display-message reports the LAST-ACTIVE
+      # session — someone else's identity (measured 2026-08-20: a nameless
+      # non-tmux sender was stamped "peggy", a live seat, forged silently).
+      # Stamp an explicit unverified label instead; deliberate senders use -S.
+      src_sess="unverified"
+    fi
   fi
   SRC_LABEL="${src_host}:${src_sess}"
 fi
diff --git a/packages/mosaic/framework/tools/tmux/agent-send.test.sh b/packages/mosaic/framework/tools/tmux/agent-send.test.sh
index 87a1ed90..69fbc757 100755
--- a/packages/mosaic/framework/tools/tmux/agent-send.test.sh
+++ b/packages/mosaic/framework/tools/tmux/agent-send.test.sh
@@ -61,8 +61,15 @@ no()   { FAIL=$((FAIL+1)); printf 'FAIL %s\n   %s\n' "$1" "$2"; }
 
 # Run the tool with the stub injected; echoes captured payload on stdout.
 run() { AGENT_SEND_SENDER="$STUB" bash "$TOOL" -S a:src -n dsthost "$@"; }
-run_auto() {
-  env -u MOSAIC_AGENT_NAME \
+# Hermetic auto-label runs: TMUX is controlled explicitly so results never
+# depend on whether the caller running this suite sits inside tmux.
+run_auto() {          # models a sender OUTSIDE tmux (no client context)
+  env -u MOSAIC_AGENT_NAME -u TMUX \
+    AGENT_SEND_SENDER="$STUB" PATH="$FAKE_BIN:$PATH" \
+    bash "$TOOL" -n dsthost "$@"
+}
+run_auto_in_tmux() {  # models a sender INSIDE tmux (client context exists)
+  env -u MOSAIC_AGENT_NAME TMUX=/fake/socket \
     AGENT_SEND_SENDER="$STUB" PATH="$FAKE_BIN:$PATH" \
     bash "$TOOL" -n dsthost "$@"
 }
@@ -145,7 +152,9 @@ want="[$src_host:authoritative-agent -> dsthost:mos] env identity"
   || no "MOSAIC_AGENT_NAME is authoritative across sockets" "got=[$got] want=[$want]"
 
 # 9. Without the env identity, self-lookup uses local tmux, not destination -L.
-got=$(FAKE_TMUX_MODE=sessions run_auto -L destination-socket -s mos -m "local fallback")
+#    Sender is INSIDE tmux: the only context where display-message self-lookup
+#    is safe (it resolves against this client's own session).
+got=$(FAKE_TMUX_MODE=sessions run_auto_in_tmux -L destination-socket -s mos -m "local fallback")
 want="[$src_host:local-agent -> dsthost:mos] local fallback"
 [ "$got" = "$want" ] && ok "cross-socket fallback uses local sender session" \
   || no "cross-socket fallback uses local sender session" "got=[$got] want=[$want]"
@@ -153,8 +162,17 @@ want="[$src_host:local-agent -> dsthost:mos] local fallback"
   && ok "cross-socket fallback rejects destination holder identity" \
   || no "cross-socket fallback rejects destination holder identity" "got=[$got]"
 
+# 9b. NO tmux context: display-message answers with the LAST-ACTIVE session —
+#     someone else's identity (forgery vector). The label must be `unverified`,
+#     never a borrowed name, even though a tmux server exists here and the fake
+#     would confidently answer `local-agent`.
+got=$(FAKE_TMUX_MODE=sessions run_auto -s mos -m "no tmux context")
+want="[$src_host:unverified -> dsthost:mos] no tmux context"
+[ "$got" = "$want" ] && ok "no-tmux sender labeled unverified, never borrowed" \
+  || no "no-tmux sender labeled unverified, never borrowed" "got=[$got] want=[$want]"
+
 # 10. If neither env nor local tmux identifies the sender, preserve '?'.
-got=$(FAKE_TMUX_MODE=unavailable run_auto -L destination-socket -s mos -m "unknown fallback")
+got=$(FAKE_TMUX_MODE=unavailable run_auto_in_tmux -L destination-socket -s mos -m "unknown fallback")
 want="[$src_host:? -> dsthost:mos] unknown fallback"
 [ "$got" = "$want" ] && ok "unknown sender falls back to ?" \
   || no "unknown sender falls back to ?" "got=[$got] want=[$want]"