diff --git a/.changeset/forty-ties-think.md b/.changeset/forty-ties-think.md
new file mode 100644
index 000000000..6f552c988
--- /dev/null
+++ b/.changeset/forty-ties-think.md
@@ -0,0 +1,5 @@
+---
+"@rebilly/client-php": patch
+---
+
+Make Plan.id readOnly Rebilly/api-definitions#1651
diff --git a/src/Model/FlexiblePlan.php b/src/Model/FlexiblePlan.php
index adc03be7c..1ac5826a1 100644
--- a/src/Model/FlexiblePlan.php
+++ b/src/Model/FlexiblePlan.php
@@ -287,14 +287,14 @@ public function setLinks(null|array $links): static
         return $this;
     }
 
-    public function getRecurringInterval(): ?SubscriptionOrderPlanRecurringInterval
+    public function getRecurringInterval(): SubscriptionOrderPlanRecurringInterval
     {
-        return $this->fields['recurringInterval'] ?? null;
+        return $this->fields['recurringInterval'];
     }
 
-    public function setRecurringInterval(null|SubscriptionOrderPlanRecurringInterval|array $recurringInterval): static
+    public function setRecurringInterval(SubscriptionOrderPlanRecurringInterval|array $recurringInterval): static
     {
-        if ($recurringInterval !== null && !($recurringInterval instanceof SubscriptionOrderPlanRecurringInterval)) {
+        if (!($recurringInterval instanceof SubscriptionOrderPlanRecurringInterval)) {
             $recurringInterval = SubscriptionOrderPlanRecurringInterval::from($recurringInterval);
         }
 
@@ -303,15 +303,15 @@ public function setRecurringInterval(null|SubscriptionOrderPlanRecurringInterval
         return $this;
     }
 
-    public function getTrial(): ?PlanTrial
+    public function getTrial(): TrialOnlyPlanTrial
     {
-        return $this->fields['trial'] ?? null;
+        return $this->fields['trial'];
     }
 
-    public function setTrial(null|PlanTrial|array $trial): static
+    public function setTrial(TrialOnlyPlanTrial|array $trial): static
     {
-        if ($trial !== null && !($trial instanceof PlanTrial)) {
-            $trial = PlanTrial::from($trial);
+        if (!($trial instanceof TrialOnlyPlanTrial)) {
+            $trial = TrialOnlyPlanTrial::from($trial);
         }
 
         $this->fields['trial'] = $trial;
diff --git a/src/Model/OrderItemPlan.php b/src/Model/OrderItemPlan.php
index 37938474d..4f0ad065c 100644
--- a/src/Model/OrderItemPlan.php
+++ b/src/Model/OrderItemPlan.php
@@ -287,14 +287,14 @@ public function setLinks(null|array $links): static
         return $this;
     }
 
-    public function getRecurringInterval(): ?SubscriptionOrderPlanRecurringInterval
+    public function getRecurringInterval(): SubscriptionOrderPlanRecurringInterval
     {
-        return $this->fields['recurringInterval'] ?? null;
+        return $this->fields['recurringInterval'];
     }
 
-    public function setRecurringInterval(null|SubscriptionOrderPlanRecurringInterval|array $recurringInterval): static
+    public function setRecurringInterval(SubscriptionOrderPlanRecurringInterval|array $recurringInterval): static
     {
-        if ($recurringInterval !== null && !($recurringInterval instanceof SubscriptionOrderPlanRecurringInterval)) {
+        if (!($recurringInterval instanceof SubscriptionOrderPlanRecurringInterval)) {
             $recurringInterval = SubscriptionOrderPlanRecurringInterval::from($recurringInterval);
         }
 
@@ -303,15 +303,15 @@ public function setRecurringInterval(null|SubscriptionOrderPlanRecurringInterval
         return $this;
     }
 
-    public function getTrial(): ?PlanTrial
+    public function getTrial(): TrialOnlyPlanTrial
     {
-        return $this->fields['trial'] ?? null;
+        return $this->fields['trial'];
     }
 
-    public function setTrial(null|PlanTrial|array $trial): static
+    public function setTrial(TrialOnlyPlanTrial|array $trial): static
     {
-        if ($trial !== null && !($trial instanceof PlanTrial)) {
-            $trial = PlanTrial::from($trial);
+        if (!($trial instanceof TrialOnlyPlanTrial)) {
+            $trial = TrialOnlyPlanTrial::from($trial);
         }
 
         $this->fields['trial'] = $trial;
diff --git a/src/Model/Plan.php b/src/Model/Plan.php
index 0af62ad42..f99b6c690 100644
--- a/src/Model/Plan.php
+++ b/src/Model/Plan.php
@@ -98,13 +98,6 @@ public function getId(): ?string
         return $this->fields['id'] ?? null;
     }
 
-    public function setId(null|string $id): static
-    {
-        $this->fields['id'] = $id;
-
-        return $this;
-    }
-
     public function getName(): string
     {
         return $this->fields['name'];
@@ -287,14 +280,14 @@ public function setLinks(null|array $links): static
         return $this;
     }
 
-    public function getRecurringInterval(): ?SubscriptionOrderPlanRecurringInterval
+    public function getRecurringInterval(): SubscriptionOrderPlanRecurringInterval
     {
-        return $this->fields['recurringInterval'] ?? null;
+        return $this->fields['recurringInterval'];
     }
 
-    public function setRecurringInterval(null|SubscriptionOrderPlanRecurringInterval|array $recurringInterval): static
+    public function setRecurringInterval(SubscriptionOrderPlanRecurringInterval|array $recurringInterval): static
     {
-        if ($recurringInterval !== null && !($recurringInterval instanceof SubscriptionOrderPlanRecurringInterval)) {
+        if (!($recurringInterval instanceof SubscriptionOrderPlanRecurringInterval)) {
             $recurringInterval = SubscriptionOrderPlanRecurringInterval::from($recurringInterval);
         }
 
@@ -303,15 +296,15 @@ public function setRecurringInterval(null|SubscriptionOrderPlanRecurringInterval
         return $this;
     }
 
-    public function getTrial(): ?PlanTrial
+    public function getTrial(): TrialOnlyPlanTrial
     {
-        return $this->fields['trial'] ?? null;
+        return $this->fields['trial'];
     }
 
-    public function setTrial(null|PlanTrial|array $trial): static
+    public function setTrial(TrialOnlyPlanTrial|array $trial): static
     {
-        if ($trial !== null && !($trial instanceof PlanTrial)) {
-            $trial = PlanTrial::from($trial);
+        if (!($trial instanceof TrialOnlyPlanTrial)) {
+            $trial = TrialOnlyPlanTrial::from($trial);
         }
 
         $this->fields['trial'] = $trial;
@@ -421,6 +414,13 @@ public function jsonSerialize(): array
         return $data;
     }
 
+    private function setId(null|string $id): static
+    {
+        $this->fields['id'] = $id;
+
+        return $this;
+    }
+
     private function setCurrencySign(null|string $currencySign): static
     {
         $this->fields['currencySign'] = $currencySign;
diff --git a/src/Model/PlanTrial.php b/src/Model/TrialOnlyPlanTrial.php
similarity index 97%
rename from src/Model/PlanTrial.php
rename to src/Model/TrialOnlyPlanTrial.php
index 32b40b298..c2afbe7ba 100644
--- a/src/Model/PlanTrial.php
+++ b/src/Model/TrialOnlyPlanTrial.php
@@ -15,7 +15,7 @@
 
 use JsonSerializable;
 
-class PlanTrial implements JsonSerializable
+class TrialOnlyPlanTrial implements JsonSerializable
 {
     private array $fields = [];