diff --git a/scripts/test-zb.sh b/scripts/test-zb.sh
index 1ec2266bf..4dbcc51bf 100755
--- a/scripts/test-zb.sh
+++ b/scripts/test-zb.sh
@@ -12,20 +12,33 @@ while [ ! -e "$T/go" ]; do sleep 0.1; done
 exit \$(cat "$T/status" 2>/dev/null || echo 0)
 FAKE
 chmod +x "$T/zig"
+unset FIZZY_BUILD_SKIP FIZZY_BUILD_WAIT
 export FIZZY_BUILD_ZIG=$T/zig FIZZY_BUILD_SLOTS=2 FIZZY_BUILD_MAX_LOAD=100000 \
     FIZZY_BUILD_SLOTS_DIR=$T/slots FIZZY_BUILD_POLL=0.2
 fail() { echo "FAIL: $1"; exit 1; }
 alive() { kill -0 "$1" 2>/dev/null; }
+# poll <what> <file> <pattern>: poll for the pattern in the file (a slow runner is not a failure),
+# failing after 20s with what the file held.
+poll() {
+    i=0
+    while ! grep -q "$3" "$2" 2>/dev/null; do
+        i=$((i + 1))
+        [ $i -le 200 ] || fail "$1 never showed '$3' in $2 after 20s: $(cat "$2" 2>/dev/null) ran: $(cat "$T/ran" 2>/dev/null) slots: $(cat "$T"/slots/*/pid 2>/dev/null) env: $(env | grep FIZZY_BUILD | tr '\n' ' ')"
+        sleep 0.1
+    done
+}
 
 # Two builds hold the two slots; the third waits, then runs when they finish.
 ./zb build a >"$T/o1" 2>&1 & echo $! >>"$T/pids"
 ./zb build b >"$T/o2" 2>&1 & echo $! >>"$T/pids"
-sleep 1
+poll "builds a and b" "$T/ran" '^build b'
+poll "builds a and b" "$T/ran" '^build a'
+poll "slot 0 held" "$T/slots/slot.0/pid" '^[0-9]'
+poll "slot 1 held" "$T/slots/slot.1/pid" '^[0-9]'
 ./zb build c >"$T/o3" 2>&1 & p3=$!; echo $p3 >>"$T/pids"
-sleep 1
-alive $p3 || fail "third build did not wait"
-grep -q 'zb: waiting for a build slot' "$T/o3" || fail "no waiting line: $(cat "$T/o3")"
-grep -q '^build c' "$T/ran" && fail "third build ran while both slots were held"
+poll "third build" "$T/o3" 'zb: waiting for a build slot'
+alive $p3 || fail "third build did not wait: $(cat "$T/o3") ran: $(cat "$T/ran")"
+grep -q '^build c' "$T/ran" && fail "third build ran while both slots were held: $(cat "$T/ran")"
 echo 3 >"$T/status"
 touch "$T/go"
 wait $p3; [ $? = 3 ] || fail "zig's exit status not passed on"
@@ -60,9 +73,8 @@ grep -q '^build i' "$T/ran" || fail "build i did not run"
 
 # Over the load ceiling, a build waits (and says so) until it is lifted.
 FIZZY_BUILD_MAX_LOAD=-1 ./zb build e >"$T/o5" 2>&1 & p5=$!; echo $p5 >>"$T/pids"
-sleep 1
+poll "build above the load ceiling" "$T/o5" 'waiting for load'
 alive $p5 || fail "build ran above the load ceiling"
-grep -q 'waiting for load' "$T/o5" || fail "no load line: $(cat "$T/o5")"
 kill $p5
 
 # Light commands pass straight through, and FIZZY_BUILD_SKIP bypasses the wait.
diff --git a/scripts/zb b/scripts/zb
index 8135277a5..5758b374b 100755
--- a/scripts/zb
+++ b/scripts/zb
@@ -56,29 +56,51 @@ alive() { kill -0 "$1" 2>/dev/null; }
 
 slot_pid() { cat "$1/pid" 2>/dev/null; }
 
-# Take a free slot, reclaiming one whose holder died. Sets $mine. A slot with no pid file yet is
-# its taker between `mkdir` and the write, so it counts as held until it is a minute old.
+# Take a free slot, reclaiming one whose holder died. Sets $mine. A slot appears with its pid
+# already inside (a rename of a directory made in private), so a live holder is never seen without
+# one and is never reclaimed by age; a slot with no pid file is a taker that died mid-way, held
+# until it is a minute old, then claimed by an exclusive create of its pid. A rename onto a slot
+# taken meanwhile nests inside it instead of failing, so the take counts only when the slot's pid
+# is ours.
 mine=
 try_slot() {
     i=0
     while [ "$i" -lt "$slots" ]; do
         s=$dir/slot.$i
         i=$((i + 1))
-        if mkdir "$s" 2>/dev/null; then
-            echo "$$" >"$s/pid"
-            mine=$s
-            return 0
+        if [ ! -e "$s" ]; then
+            rm -rf "$dir/take.$$"
+            mkdir "$dir/take.$$" && echo "$$" >"$dir/take.$$/pid" || return 1
+            mv "$dir/take.$$" "$s" 2>/dev/null
+            if [ "$(slot_pid "$s")" = "$$" ]; then
+                mine=$s
+                return 0
+            fi
+            rm -rf "$dir/take.$$" "$s/take.$$"
+            continue
         fi
         p=$(slot_pid "$s")
         if [ -z "$p" ]; then
             [ -n "$(find "$s" -maxdepth 0 -mmin +1 2>/dev/null)" ] || continue
+            # Exclusive create: it fails if a taker has put its pid there since we looked.
+            if (set -C && echo "$$" >"$s/pid") 2>/dev/null; then
+                mine=$s
+                return 0
+            fi
+            continue
         elif alive "$p"; then
             continue
         fi
-        # Rename is atomic, so only one waiter wins a dead slot; it retries the take next pass.
-        if mv "$s" "$dir/dead.$$" 2>/dev/null; then
-            rm -rf "$dir/dead.$$"
-            i=$((i - 1))
+        # Moving the pid file away leaves a young pid-less slot that every other waiter holds, and
+        # fails if another reclaimer got there first. What was moved is checked: if the slot was
+        # taken after we read the pid, it holds a live pid, which goes back.
+        if mv "$s/pid" "$s/pid.$$" 2>/dev/null; then
+            if [ "$(cat "$s/pid.$$")" = "$p" ] && ! alive "$p"; then
+                rm -rf "$s"
+                i=$((i - 1))
+            else
+                mv "$s/pid.$$" "$s/pid"
+            fi
         fi
     done
     return 1