diff --git a/packages/mosaic/framework/tools/fleet/test-start-agent-session.sh b/packages/mosaic/framework/tools/fleet/test-start-agent-session.sh
index 182b1ee6..3343e60a 100755
--- a/packages/mosaic/framework/tools/fleet/test-start-agent-session.sh
+++ b/packages/mosaic/framework/tools/fleet/test-start-agent-session.sh
@@ -422,8 +422,12 @@ ln -sf "$FAKE_BIN/npm" "$NODE_LAUNCHER_BIN/npm"
 node_pane_environment=$(tr '\0' '\n' < "$HOME_NODE/fleet/pane-environment")
 # Colon-pad and match a whole element. A regex with `(^|:)` after `.*` looks like it
 # does this and does not: an anchor cannot match mid-pattern, so it silently requires
-# a leading colon and rejects the directory in FIRST position — which is exactly where
-# the fix puts it. That produced a failure reading "pane ran but PANE_PATH does not
+# a leading colon and rejects the directory in FIRST position — which is where THIS
+# FIXTURE puts it: it runs under `env -i` with no MOSAIC_RUNTIME_BIN, so the bootstrap
+# directory leads. That is a property of the fixture, not of the fix — in general the
+# directory sits second, after MOSAIC_RUNTIME_BIN. The colon padding makes the
+# assertion position-independent either way, which is why it is written this way and
+# not with an anchor. That produced a failure reading "pane ran but PANE_PATH does not
 # carry <dir>" against a PATH whose first element was that dir.
 node_pane_path=":$(printf '%s\n' "$node_pane_environment" | sed -n 's/^PATH=//p' | head -1):"
 case "$node_pane_path" in