diff --git a/scripts/redeploy-fleetd.sh b/scripts/redeploy-fleetd.sh
index 36a423d..e3b0fb6 100755
--- a/scripts/redeploy-fleetd.sh
+++ b/scripts/redeploy-fleetd.sh
@@ -178,6 +178,44 @@ swap_staged_jar() {
         may recover this once you find out why the move failed."
 }
 
+# fleetd #521 — the swap decision, and the step that acts on it.
+#
+# The defect: the swap step used to be guarded inline by `if [ "$DO_BUILD" = 1 ]` in the main flow.
+# Changing that to `if false` left the suite green and the swap never ran, so a redeploy reported
+# every step succeeding while the daemon started on no jar at all (stage_built_jar has already moved
+# the freshly built one to $JAR_STAGED by then) or on a stale one.
+# test_swap_ordered_after_wait_and_before_start could not catch it: it reads this script's own text
+# and compares line positions, and a same-line edit moves no line.
+#
+# Why these are TWO functions, and why the second one exists at all. Extracting only the predicate
+# — `should_swap`, which is what #521 asked for — is not enough, and this was measured, not guessed:
+# with the main flow calling `if should_swap "$DO_BUILD"; then`, changing THAT to `if false; then`
+# still left the whole suite at exit 0 with no failures. Tests that call a predicate directly prove
+# the predicate is right; nothing makes the code that does the work consult it. Extraction had moved
+# the untested decision one level up rather than removing it.
+#
+# So the decision and the action live together in swap_if_built, and the main flow has no guard of
+# its own to get wrong — it calls one function unconditionally. A test then calls swap_if_built with
+# both values of do_build and checks whether the swap actually happened, which fails if the guard is
+# removed, inverted, or stops being consulted. should_swap stays a separate predicate because it is
+# the decision itself and is worth naming and testing on its own.
+#
+# What this still does not pin: deleting the swap_if_built call from the main flow altogether. That
+# is the ordering test's job — its needle is that call site — and no test in this file can do better,
+# because sourcing stops before the main flow ever runs (see the SOURCED guard below).
+should_swap() {
+  local do_build="$1"
+  [ "$do_build" = 1 ]
+}
+
+swap_if_built() {
+  local do_build="$1"
+  should_swap "$do_build" || return 0
+  say "swap"
+  swap_staged_jar "$JAR_STAGED" "$JAR"
+  ok "jar in place: $(jar_id)"
+}
+
 # `launchctl list <label>` exits 0 iff the label is loaded (registered with launchd) — true whether
 # or not it is currently running, which is exactly "supervision is active" for our purposes. Read-
 # only: neither helper below changes anything, so both are also safe under --check.
@@ -720,11 +758,7 @@ fi
 # NOW is it safe to put the freshly built jar at the path the NEXT `java -jar` (direct, or via
 # launchd/systemd's ExecStart) will read from — this mv is the one and only write to $JAR anywhere
 # in this script's mutating flow. If it fails, do not start: die() below exits before "start" runs.
-if [ "$DO_BUILD" = 1 ]; then
-  say "swap"
-  swap_staged_jar "$JAR_STAGED" "$JAR"
-  ok "jar in place: $(jar_id)"
-fi
+swap_if_built "$DO_BUILD"
 
 # ------------------------------------------------------------------ start
 # Unsupervised: login shell (zsh -l) is what puts the secrets on the daemon's environment, and cwd
diff --git a/scripts/test-redeploy-fleetd.sh b/scripts/test-redeploy-fleetd.sh
index 9647a3e..dad2c2c 100755
--- a/scripts/test-redeploy-fleetd.sh
+++ b/scripts/test-redeploy-fleetd.sh
@@ -365,23 +365,92 @@ test_wait_for_daemon_exit_times_out_if_pid_never_clears() {
   source "$ROOT/scripts/redeploy-fleetd.sh"   # restore the real running_pid/sleep for later tests
 }
 
+# fleetd #521 — the swap step's guard, at two levels.
+#
+# The first two tests call the predicate should_swap() directly. They pin its logic, and that is all
+# they pin. On their own they did NOT close #521, and this was measured rather than argued: with the
+# main flow reading `if should_swap "$DO_BUILD"; then`, changing that line to `if false; then` left
+# this whole suite at exit 0 with zero FAIL lines, because nothing here made the code that performs
+# the swap consult the predicate at all. Extracting the decision had moved the untested decision up
+# a level, not removed it.
+#
+# So the last two tests call swap_if_built() — the function the main flow actually calls, holding the
+# guard and the swap together — with a recording stub in place of the real `mv`. Those fail if the
+# guard is removed, inverted, or stops being consulted.
+#
+# What none of these four can catch: deleting the `swap_if_built "$DO_BUILD"` line from the main flow
+# altogether. That is test_swap_ordered_after_wait_and_before_start's job below, because sourcing
+# stops before the main flow runs, so no test in this file can invoke it.
+test_should_swap_true_when_build_ran() {
+  should_swap 1 || fail "should_swap 1 (a build ran and staged a jar) must return true"
+}
+
+test_should_swap_false_when_build_skipped() {
+  if should_swap 0; then
+    fail "should_swap 0 (--no-build; nothing was staged this run) must return false"
+  fi
+}
+
+# Both of these re-source redeploy-fleetd.sh at the START, because a bash function definition is
+# global for the rest of the process and an earlier test may have left swap_staged_jar or jar_id
+# overridden (see the longer note on this at test_detect_supervisor_systemd_probe_error_is_unclear),
+# and again at the END, so their own stubs do not leak into every test that runs after them.
+test_swap_if_built_performs_the_swap_when_build_ran() {
+  source "$ROOT/scripts/redeploy-fleetd.sh"
+  local marker="$TMP/swap-if-built-ran"
+  rm -f "$marker"
+  swap_staged_jar() { printf '%s -> %s\n' "$1" "$2" > "$marker"; }
+  jar_id() { printf 'stubbed\n'; }
+  swap_if_built 1 > /dev/null
+  [ -f "$marker" ] \
+    || fail "swap_if_built 1 (a build ran and staged a jar) must perform the swap, and did not"
+  source "$ROOT/scripts/redeploy-fleetd.sh"
+}
+
+test_swap_if_built_skips_the_swap_when_build_skipped() {
+  source "$ROOT/scripts/redeploy-fleetd.sh"
+  local marker="$TMP/swap-if-built-skipped"
+  rm -f "$marker"
+  swap_staged_jar() { printf 'swapped\n' > "$marker"; }
+  jar_id() { printf 'stubbed\n'; }
+  swap_if_built 0 > /dev/null
+  if [ -f "$marker" ]; then
+    fail "swap_if_built 0 (--no-build; nothing was staged this run) must not swap, but it did"
+  fi
+  source "$ROOT/scripts/redeploy-fleetd.sh"
+}
+
 # fleetd #493 item 2: "put the swap after that wait, before the start." Sourcing stops before the
 # main flow ever runs (see the SOURCED guard in redeploy-fleetd.sh), so the ordering guarantee
 # itself — as opposed to the pure functions it's built from — can only be checked by reading the
 # script's own call sites, the same way test_recovery_patterns_match_source below checks Java
 # source shape instead of behavior it cannot invoke directly.
+#
+# Two details about the three greps below, both of which have already gone wrong here.
+#
+# The needle for the swap is the MAIN FLOW's call site, `swap_if_built "$DO_BUILD"` — not
+# `swap_staged_jar "$JAR_STAGED" "$JAR"`. Since fleetd #521 that second string lives inside
+# swap_if_built's body, which is defined near the top of the script, far ABOVE the stop step. Using
+# it made this test report "swap_staged_jar (line 215) is not after wait_for_daemon_exit (line 730)"
+# — a true statement about a function definition, and nothing at all about the order of the steps.
+#
+# Each grep ends in `|| true`. This file runs under `set -euo pipefail`, and `pipefail` makes the
+# pipeline's status grep's status, so a needle that is simply ABSENT failed the assignment and `set
+# -e` killed the whole suite on the spot — before reaching the `[ -n ... ] || fail` line written to
+# report exactly that. Measured: the suite exited 1 having printed zero bytes, no FAIL line and no
+# name of the missing call site. `|| true` lets the assignment succeed empty so the guard can speak.
 test_swap_ordered_after_wait_and_before_start() {
   local src="$ROOT/scripts/redeploy-fleetd.sh" wait_line swap_line start_line
-  wait_line="$(grep -Fn 'wait_for_daemon_exit "$STOP_WAIT"' "$src" | head -1 | cut -d: -f1)"
-  swap_line="$(grep -Fn 'swap_staged_jar "$JAR_STAGED" "$JAR"' "$src" | head -1 | cut -d: -f1)"
-  start_line="$(grep -Fn 'say "start"' "$src" | head -1 | cut -d: -f1)"
+  wait_line="$(grep -Fn 'wait_for_daemon_exit "$STOP_WAIT"' "$src" | head -1 | cut -d: -f1 || true)"
+  swap_line="$(grep -Fn 'swap_if_built "$DO_BUILD"' "$src" | head -1 | cut -d: -f1 || true)"
+  start_line="$(grep -Fn 'say "start"' "$src" | head -1 | cut -d: -f1 || true)"
   [ -n "$wait_line" ] || fail "could not find the wait-for-exit call site in redeploy-fleetd.sh"
   [ -n "$swap_line" ] || fail "could not find the swap call site in redeploy-fleetd.sh"
   [ -n "$start_line" ] || fail "could not find the start section in redeploy-fleetd.sh"
   [ "$swap_line" -gt "$wait_line" ] \
-    || fail "swap_staged_jar (line $swap_line) is not after wait_for_daemon_exit (line $wait_line)"
+    || fail "swap_if_built (line $swap_line) is not after wait_for_daemon_exit (line $wait_line)"
   [ "$swap_line" -lt "$start_line" ] \
-    || fail "swap_staged_jar (line $swap_line) is not before the start section (line $start_line)"
+    || fail "swap_if_built (line $swap_line) is not before the start section (line $start_line)"
 }
 
 # fleetd #511: the drain-gate abort message (fired when a build has staged a jar but the operator
@@ -670,6 +739,10 @@ test_stage_built_jar_dies_when_build_produced_nothing
 test_swap_staged_jar_moves_staged_onto_live
 test_swap_staged_jar_dies_without_staged_file
 test_swap_staged_jar_dies_when_mv_fails
+test_should_swap_true_when_build_ran
+test_should_swap_false_when_build_skipped
+test_swap_if_built_performs_the_swap_when_build_ran
+test_swap_if_built_skips_the_swap_when_build_skipped
 test_require_no_build_jar_dies_when_absent
 test_require_no_build_jar_accepts_present_jar
 test_wait_for_daemon_exit_returns_true_once_pid_clears