diff --git a/packages/mosaic/framework/tools/quality/test-enumeration-exclusions.txt b/packages/mosaic/framework/tools/quality/test-enumeration-exclusions.txt
index fa48e012..a4e88867 100644
--- a/packages/mosaic/framework/tools/quality/test-enumeration-exclusions.txt
+++ b/packages/mosaic/framework/tools/quality/test-enumeration-exclusions.txt
@@ -29,6 +29,10 @@ packages/mosaic/framework/tools/git/test-lane-brief-pr-linkage.sh | unmeasured i
 # --- tools/tmux: require a live tmux server ---
 packages/mosaic/framework/tools/tmux/test-send-message-socket.sh | requires a real tmux server on a throwaway socket; CI image ships no tmux; #1017 burndown (needs tmux in image or a signed permanent exclusion)
 packages/mosaic/framework/tools/tmux/test-send-message-verdict.sh | requires real tmux-pane fixtures on a throwaway socket; CI image ships no tmux; #1017 burndown (same condition as its sibling)
+# The entry below is NOT covered by the #1017 signature block above: it was
+# signed by jarvis-enhance (dragon-lin, 2026-08-24) at a later base, for the
+# test added alongside the send-message.sh exact-target fix.
+packages/mosaic/framework/tools/tmux/test-send-message-target.sh | requires a real tmux server on a throwaway socket, and specifically a MULTI-WINDOW session (the bug it guards is invisible on a single-window fixture); CI image ships no tmux; same burndown condition as its two siblings above
 
 # --- single-suite directories: unmeasured in CI ---
 
diff --git a/packages/mosaic/framework/tools/tmux/send-message.sh b/packages/mosaic/framework/tools/tmux/send-message.sh
index 56144d54..84e07bb9 100755
--- a/packages/mosaic/framework/tools/tmux/send-message.sh
+++ b/packages/mosaic/framework/tools/tmux/send-message.sh
@@ -64,13 +64,31 @@ if [ -n "$SOCKET_NAME" ]; then
   tmux_cmd+=(-L "$SOCKET_NAME")
 fi
 
-# tmux accepts `=session` for some commands, but pane-level commands such as
-# capture-pane require a pane-qualified target. Keep exact-session addressing
-# convenient while avoiding accidental prefix matches.
+# Normalise the target to an EXACT session plus a window part, because tmux
+# resolves the two halves with different and individually dangerous defaults:
+#
+#   * An unpinned name is a PREFIX match. With a session `foobar` alive and
+#     no session `foo`, `-t foo` resolves to `foobar` at rc=0, so a message is
+#     delivered, verified and reported OK against the wrong agent's pane.
+#   * A bare `=name` is not enough on its own: capture-pane REJECTS it
+#     ("can't find pane") while list-panes silently PREFIX-MATCHES it, so the
+#     validation below would pass on a session the capture cannot read.
+#   * A trailing `:` follows the session's ACTIVE window. Pinning `:0.0`
+#     instead addresses window 0 unconditionally, and since the paste, the
+#     Enter and the verifying capture all use EFFECTIVE_TARGET, a multi-window
+#     agent gets typed into window 0 and confirmed by reading window 0 --
+#     a false "delivered" rather than a loud failure.
+#
+# Explicit tmux ids (%pane, @window, $session) are passed through untouched;
+# prefixing `=` to them would break addressing that is already unambiguous.
 EFFECTIVE_TARGET=$TARGET
-if [[ "$TARGET" == =* && "$TARGET" != *:* ]]; then
-  EFFECTIVE_TARGET="${TARGET}:0.0"
-fi
+case "$TARGET" in
+  =*|%*|@*|\$*) ;;
+  *) EFFECTIVE_TARGET="=$TARGET" ;;
+esac
+case "$EFFECTIVE_TARGET" in
+  =*) [[ "$EFFECTIVE_TARGET" == *:* ]] || EFFECTIVE_TARGET="${EFFECTIVE_TARGET}:" ;;
+esac
 
 # Target must resolve to a live pane.
 if ! "${tmux_cmd[@]}" list-panes -t "$EFFECTIVE_TARGET" >/dev/null 2>&1; then
diff --git a/packages/mosaic/framework/tools/tmux/test-send-message-target.sh b/packages/mosaic/framework/tools/tmux/test-send-message-target.sh
new file mode 100755
index 00000000..2cbb12e2
--- /dev/null
+++ b/packages/mosaic/framework/tools/tmux/test-send-message-target.sh
@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+# Target normalisation: send-message.sh must address an EXACT session and the
+# session's ACTIVE window. Both halves have caused silent wrong-pane delivery:
+#   * an unpinned name prefix-matches, so a message for an absent session is
+#     delivered to a different agent and reported OK;
+#   * a `:0.0` pin addresses window 0 regardless of where the agent is, and
+#     because the paste, the Enter and the verifying capture share one target,
+#     the wrong window is also the window that confirms the send.
+# Both rows below FAIL against the pre-fix script, which is the point of them.
+set -euo pipefail
+
+SCRIPT_DIR=$(cd -- "$(dirname -- "$0")" && pwd)
+SEND_MESSAGE="$SCRIPT_DIR/send-message.sh"
+SOCKET="mosaic-test-target-$RANDOM-$$"
+TMPDIR=$(mktemp -d)
+trap 'tmux -L "$SOCKET" kill-server >/dev/null 2>&1 || true; rm -rf "$TMPDIR"' EXIT
+
+fail() { echo "FAIL: $*" >&2; exit 1; }
+command -v tmux >/dev/null 2>&1 || fail "tmux is required"
+
+tmux_() { tmux -L "$SOCKET" "$@"; }
+newsess() { tmux_ new-session -d -s "$1" -c "$TMPDIR" 'PS1="❯ " exec bash --noprofile --norc -i'; }
+hits() { tmux_ capture-pane -p -t "$1" 2>/dev/null | grep -cF "$2" || true; }
+
+# ── 1. an absent session must not prefix-match a live one ───────────────────
+newsess sibling-long
+nonce="absent-target-$RANDOM"
+rc=0; "$SEND_MESSAGE" -L "$SOCKET" -t sibling -m "$nonce" >/dev/null 2>&1 || rc=$?
+[ "$rc" -ne 0 ] || fail "send to absent session 'sibling' returned rc=0 (prefix-matched)"
+[ "$(hits sibling-long "$nonce")" -eq 0 ] || fail "message for absent 'sibling' was delivered to 'sibling-long'"
+
+# positive control: the detector above can see a real delivery
+nonce_ok="control-$RANDOM"
+"$SEND_MESSAGE" -L "$SOCKET" -t sibling-long -m "$nonce_ok" >/dev/null 2>&1 \
+  || fail "send to a live session failed"
+[ "$(hits sibling-long "$nonce_ok")" -gt 0 ] || fail "control: live delivery not observed — detector is blind"
+
+# ── 2. delivery follows the ACTIVE window, not window 0 ─────────────────────
+# A single-window fixture cannot tell `=s:` from `=s:0.0`; the active window
+# must be non-zero or this test proves nothing.
+newsess multi
+tmux_ new-window -t multi -c "$TMPDIR" 'PS1="❯ " exec bash --noprofile --norc -i'
+tmux_ select-window -t multi:1
+active=$(tmux_ display-message -p -t multi '#{window_index}')
+[ "$active" = "1" ] || fail "fixture setup: expected active window 1, got $active"
+
+for target in multi "=multi"; do
+  nonce="active-win-$RANDOM"
+  "$SEND_MESSAGE" -L "$SOCKET" -t "$target" -m "$nonce" >/dev/null 2>&1 \
+    || fail "send to '$target' failed"
+  [ "$(hits multi:1 "$nonce")" -gt 0 ] || fail "'$target' did not deliver to the active window"
+  [ "$(hits multi:0 "$nonce")" -eq 0 ] || fail "'$target' delivered to window 0 instead of the active window"
+done
+
+# ── 3. an explicit window part is preserved ─────────────────────────────────
+nonce="explicit-win-$RANDOM"
+"$SEND_MESSAGE" -L "$SOCKET" -t multi:0 -m "$nonce" >/dev/null 2>&1 || fail "send to 'multi:0' failed"
+[ "$(hits multi:0 "$nonce")" -gt 0 ] || fail "explicit 'multi:0' did not deliver to window 0"
+
+# ── 4. a unique prefix of a live session is still refused ───────────────────
+nonce="prefix-$RANDOM"
+rc=0; "$SEND_MESSAGE" -L "$SOCKET" -t mult -m "$nonce" >/dev/null 2>&1 || rc=$?
+[ "$rc" -ne 0 ] || fail "send to prefix 'mult' returned rc=0"
+[ "$(hits multi:1 "$nonce")" -eq 0 ] || fail "prefix 'mult' was delivered to 'multi'"
+
+echo "PASS: send-message.sh target normalisation"