diff --git a/dist/platforms/mac/steps/activate.sh b/dist/platforms/mac/steps/activate.sh
index f3fc91d..65d96c5 100644
--- a/dist/platforms/mac/steps/activate.sh
+++ b/dist/platforms/mac/steps/activate.sh
@@ -13,7 +13,15 @@ ACTIVATE_MAX_ATTEMPTS="${UNITY_LICENSE_RETRY_MAX_ATTEMPTS:-4}"
 ACTIVATE_RETRY_DELAY_SECONDS=20
 ACTIVATE_TRANSIENT_LICENSE_ERROR_PATTERN='TimeoutPolicy did not complete|Access token is unavailable|entitlement groups and 0 free entitlements|License activation has failed|No valid Unity Editor license found|License is not active'
 
-if [[ -n "$UNITY_LICENSE" ]] || [[ -n "$UNITY_LICENSE_FILE" ]]; then
+# Serial mode is preferred over personal-license (below) whenever both are
+# configured: a manually-activated .ulf is bound to the machine fingerprint
+# of whatever machine originally requested it, which real CI evidence shows
+# genuinely doesn't match every runner - confirmed via unity-test-runner's
+# own CI, where the identical .ulf activates cleanly through this same
+# script's ubuntu counterpart but fails windows with "Machine bindings
+# don't match" every single time. Serial credentials have no such
+# constraint, so given a choice, prefer them.
+if [[ -z "$UNITY_SERIAL" || -z "$UNITY_EMAIL" || -z "$UNITY_PASSWORD" ]] && { [[ -n "$UNITY_LICENSE" ]] || [[ -n "$UNITY_LICENSE_FILE" ]]; }; then
   #
   # PERSONAL LICENSE MODE
   #
diff --git a/dist/platforms/ubuntu/steps/activate.sh b/dist/platforms/ubuntu/steps/activate.sh
index c451755..5b91ea0 100644
--- a/dist/platforms/ubuntu/steps/activate.sh
+++ b/dist/platforms/ubuntu/steps/activate.sh
@@ -12,7 +12,12 @@ UNITY_ACTIVATE_MAX_ATTEMPTS="${UNITY_LICENSE_RETRY_MAX_ATTEMPTS:-4}"
 UNITY_ACTIVATE_RETRY_DELAY_SECONDS=20
 UNITY_ACTIVATE_TRANSIENT_PATTERN='TimeoutPolicy did not complete|Access token is unavailable|entitlement groups and 0 free entitlements|License activation has failed|No valid Unity Editor license found|License is not active'
 
-if [[ -n "$UNITY_LICENSE" ]] || [[ -n "$UNITY_LICENSE_FILE" ]]; then
+# Serial mode is preferred over personal-license (below) whenever both are
+# configured - see mac/steps/activate.sh's matching comment: a manually-
+# activated .ulf is bound to the machine fingerprint of whatever machine
+# originally requested it, which doesn't necessarily match every runner.
+# Serial credentials have no such constraint, so given a choice, prefer them.
+if [[ -z "$UNITY_SERIAL" || -z "$UNITY_EMAIL" || -z "$UNITY_PASSWORD" ]] && { [[ -n "$UNITY_LICENSE" ]] || [[ -n "$UNITY_LICENSE_FILE" ]]; }; then
   #
   # PERSONAL LICENSE MODE
   #
diff --git a/dist/platforms/windows/activate.ps1 b/dist/platforms/windows/activate.ps1
index 89d6242..59acb9f 100644
--- a/dist/platforms/windows/activate.ps1
+++ b/dist/platforms/windows/activate.ps1
@@ -19,7 +19,16 @@ $MaxAttempts = if ($Env:UNITY_LICENSE_RETRY_MAX_ATTEMPTS) { [int]$Env:UNITY_LICE
 $RetryDelaySeconds = 20
 $TransientPattern = 'TimeoutPolicy did not complete|Access token is unavailable|entitlement groups and 0 free entitlements|License activation has failed|No valid Unity Editor license found|License is not active'
 
-if ($Env:UNITY_LICENSE -or $Env:UNITY_LICENSE_FILE) {
+# Serial mode is preferred over personal-license (below) whenever both are
+# configured - see mac/steps/activate.sh's matching comment: a manually-
+# activated .ulf is bound to the machine fingerprint of whatever machine
+# originally requested it, which real CI evidence shows genuinely doesn't
+# match every runner (confirmed "Machine bindings don't match" on windows
+# specifically, with the identical .ulf activating cleanly elsewhere).
+# Serial credentials have no such constraint, so given a choice, prefer them.
+$HasSerialCredentials = $Env:UNITY_SERIAL -and $Env:UNITY_EMAIL -and $Env:UNITY_PASSWORD
+
+if ((-not $HasSerialCredentials) -and ($Env:UNITY_LICENSE -or $Env:UNITY_LICENSE_FILE)) {
   #
   # PERSONAL LICENSE MODE
   #