diff --git a/Resources/bin/start-cmux-profiling b/Resources/bin/start-cmux-profiling
index 5c404182464a..a08f1ba54105 100755
--- a/Resources/bin/start-cmux-profiling
+++ b/Resources/bin/start-cmux-profiling
@@ -154,6 +154,15 @@ if ! [[ "$duration" =~ ^[0-9]+$ ]] || [ "$duration" -lt 1 ]; then
   exit 2
 fi
 
+# The timeout helpers do arithmetic on these; a non-integer would abort the
+# surrounding command instead of timing it out.
+for timeout_var in CMUX_PROFILE_SYSTEM_PROFILER_TIMEOUT_SECONDS CMUX_PROFILE_TOOL_VERSION_TIMEOUT_SECONDS CMUX_PROFILE_TOC_TIMEOUT_SECONDS; do
+  if [ -n "${!timeout_var:-}" ] && ! [[ "${!timeout_var}" =~ ^[0-9]+$ ]]; then
+    echo "start-cmux-profiling: $timeout_var must be a whole number of seconds" >&2
+    exit 2
+  fi
+done
+
 case "$target_channel" in
   ""|stable|nightly|staging|dev)
     ;;
@@ -248,14 +257,17 @@ run_output_with_timeout() {
   local timeout_seconds="$1"
   shift
   local output_file="${TMPDIR:-/tmp}/cmux-profile-probe.$$.$RANDOM"
-  local child_pid elapsed
+  local child_pid ticks
+  # Poll in tenths of a second: a whole-second poll cost every probe at least
+  # 1 s, even an instant one like `xcodebuild -version`.
+  local limit=$((10#$timeout_seconds * 10))
 
   "$@" > "$output_file" 2>/dev/null &
   child_pid="$!"
-  elapsed=0
+  ticks=0
 
   while kill -0 "$child_pid" >/dev/null 2>&1; do
-    if [ "$elapsed" -ge "$timeout_seconds" ]; then
+    if [ "$ticks" -ge "$limit" ]; then
       kill "$child_pid" >/dev/null 2>&1 || true
       sleep 1
       kill -9 "$child_pid" >/dev/null 2>&1 || true
@@ -263,8 +275,8 @@ run_output_with_timeout() {
       rm -f "$output_file"
       return 124
     fi
-    sleep 1
-    elapsed=$((elapsed + 1))
+    sleep 0.1
+    ticks=$((ticks + 1))
   done
 
   if wait "$child_pid"; then
@@ -659,10 +671,12 @@ wait_with_timeout() {
   local child_pid="$1"
   local timeout_seconds="$2"
   local log_file="$3"
-  local elapsed=0
+  local ticks=0
+  # Tenths of a second, like run_output_with_timeout.
+  local limit=$((10#$timeout_seconds * 10))
 
   while kill -0 "$child_pid" >/dev/null 2>&1; do
-    if [ "$elapsed" -ge "$timeout_seconds" ]; then
+    if [ "$ticks" -ge "$limit" ]; then
       echo "Timed out after ${timeout_seconds}s; terminating pid ${child_pid}" >> "$log_file"
       kill "$child_pid" >/dev/null 2>&1 || true
       sleep 1
@@ -670,8 +684,8 @@ wait_with_timeout() {
       wait "$child_pid" >/dev/null 2>&1 || true
       return 124
     fi
-    sleep 1
-    elapsed=$((elapsed + 1))
+    sleep 0.1
+    ticks=$((ticks + 1))
   done
 
   wait "$child_pid"
diff --git a/tests/test_start_cmux_profiling.sh b/tests/test_start_cmux_profiling.sh
index 7a72e02d49c8..404123be59f2 100755
--- a/tests/test_start_cmux_profiling.sh
+++ b/tests/test_start_cmux_profiling.sh
@@ -182,7 +182,8 @@ if [ "${1:-}" = "xctrace" ] && [ "${2:-}" = "record" ]; then
 fi
 
 if [ "${1:-}" = "xctrace" ] && [ "${2:-}" = "export" ]; then
-  sleep 5
+  # Only the TOC-timeout case asks for a slow export.
+  sleep "${FAKE_XCTRACE_EXPORT_SECONDS:-0}"
   exit 0
 fi
 
@@ -191,7 +192,7 @@ EOF
 chmod +x "$fake_bin/xcrun"
 
 timeout_out="$TMP_DIR/timeout-out"
-HOME="$TMP_DIR" PATH="$fake_bin:$PATH" CMUX_PROFILE_TOC_TIMEOUT_SECONDS=1 "$SCRIPT" \
+HOME="$TMP_DIR" PATH="$fake_bin:$PATH" CMUX_PROFILE_TOC_TIMEOUT_SECONDS=1 FAKE_XCTRACE_EXPORT_SECONDS=5 "$SCRIPT" \
   --test-ps-file "$ps_file" \
   --channel dev \
   --tag dog \
@@ -238,6 +239,19 @@ if grep -Fq "$TMP_DIR/cmux DEV dog.app" "$timeout_out/summary.md" ||
   exit 1
 fi
 
+# A timeout that is not a whole number of seconds is refused up front; inside the
+# timeout helpers' arithmetic it would abort the command it was meant to time.
+for bad_timeout in abc 1.5 5s; do
+  for timeout_var in CMUX_PROFILE_SYSTEM_PROFILER_TIMEOUT_SECONDS CMUX_PROFILE_TOOL_VERSION_TIMEOUT_SECONDS CMUX_PROFILE_TOC_TIMEOUT_SECONDS; do
+    rc=0
+    bad_err="$(env "$timeout_var=$bad_timeout" "$SCRIPT" --list-targets --test-ps-file "$ps_file" 2>&1 >/dev/null)" || rc=$?
+    if [ "$rc" -ne 2 ] || [[ "$bad_err" != *"$timeout_var must be a whole number of seconds"* ]]; then
+      echo "FAIL: $timeout_var=$bad_timeout must be rejected with exit 2, got $rc: $bad_err" >&2
+      exit 1
+    fi
+  done
+done
+
 failing_system_profiler="$TMP_DIR/failing-system-profiler"
 cat > "$failing_system_profiler" <<'EOF'
 #!/usr/bin/env bash
@@ -245,7 +259,8 @@ exit 9
 EOF
 chmod +x "$failing_system_profiler"
 display_failed_out="$TMP_DIR/display-failed-out"
-PATH="$fake_bin:$PATH" CMUX_PROFILE_SYSTEM_PROFILER="$failing_system_profiler" "$SCRIPT" \
+# The 1 s export outlives several 0.1 s polls, then succeeds inside the timeout.
+PATH="$fake_bin:$PATH" CMUX_PROFILE_SYSTEM_PROFILER="$failing_system_profiler" FAKE_XCTRACE_EXPORT_SECONDS=1 "$SCRIPT" \
   --test-ps-file "$ps_file" \
   --channel dev \
   --tag dog \