diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7e398a71..185775a2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+### v4.0.3 (2026-02-05)
+* * *
+
+### New Features:
+* Add custom field support for sub-resources in HostedPage, PricingPageSession, Quote, Subscription, and Gift operations.
+
 ### v4.0.2 (2026-01-30)
 * * *
 
diff --git a/VERSION b/VERSION
index 4d54dadd..c4e41f94 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-4.0.2
+4.0.3
diff --git a/build.gradle.kts b/build.gradle.kts
index 7ded6db8..85664f20 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -7,7 +7,7 @@ plugins {
 }
 
 group = "com.chargebee"
-version = "4.0.2"
+version = "4.0.3"
 description = "Java client library for ChargeBee"
 
 // Project metadata
diff --git a/dist/chargebee-java-4.0.1-javadoc.jar b/dist/chargebee-java-4.0.1-javadoc.jar
deleted file mode 100644
index 3ba04acc..00000000
Binary files a/dist/chargebee-java-4.0.1-javadoc.jar and /dev/null differ
diff --git a/dist/chargebee-java-4.0.1-sources.jar b/dist/chargebee-java-4.0.1-sources.jar
deleted file mode 100644
index 891f5688..00000000
Binary files a/dist/chargebee-java-4.0.1-sources.jar and /dev/null differ
diff --git a/dist/chargebee-java-4.0.1.jar b/dist/chargebee-java-4.0.1.jar
deleted file mode 100644
index bd218b61..00000000
Binary files a/dist/chargebee-java-4.0.1.jar and /dev/null differ
diff --git a/src/main/java/com/chargebee/v4/models/addon/params/AddonListParams.java b/src/main/java/com/chargebee/v4/models/addon/params/AddonListParams.java
index 1121c762..76923f83 100644
--- a/src/main/java/com/chargebee/v4/models/addon/params/AddonListParams.java
+++ b/src/main/java/com/chargebee/v4/models/addon/params/AddonListParams.java
@@ -11,7 +11,6 @@
 import com.chargebee.v4.filters.StringFilter;
 import com.chargebee.v4.filters.NumberFilter;
 import com.chargebee.v4.filters.TimestampFilter;
-
 import com.chargebee.v4.filters.CustomFieldSelector;
 
 import java.util.Collections;
diff --git a/src/main/java/com/chargebee/v4/models/coupon/params/CouponListParams.java b/src/main/java/com/chargebee/v4/models/coupon/params/CouponListParams.java
index 447d6151..ffc9dadd 100644
--- a/src/main/java/com/chargebee/v4/models/coupon/params/CouponListParams.java
+++ b/src/main/java/com/chargebee/v4/models/coupon/params/CouponListParams.java
@@ -10,7 +10,6 @@
 import com.chargebee.v4.internal.Recommended;
 import com.chargebee.v4.filters.StringFilter;
 import com.chargebee.v4.filters.TimestampFilter;
-
 import com.chargebee.v4.filters.CustomFieldSelector;
 
 import java.util.Collections;
diff --git a/src/main/java/com/chargebee/v4/models/customer/params/CustomerListParams.java b/src/main/java/com/chargebee/v4/models/customer/params/CustomerListParams.java
index a4d75587..adbdb360 100644
--- a/src/main/java/com/chargebee/v4/models/customer/params/CustomerListParams.java
+++ b/src/main/java/com/chargebee/v4/models/customer/params/CustomerListParams.java
@@ -10,7 +10,6 @@
 import com.chargebee.v4.internal.Recommended;
 import com.chargebee.v4.filters.StringFilter;
 import com.chargebee.v4.filters.TimestampFilter;
-
 import com.chargebee.v4.filters.CustomFieldSelector;
 
 import java.util.Collections;
diff --git a/src/main/java/com/chargebee/v4/models/feature/params/FeatureListParams.java b/src/main/java/com/chargebee/v4/models/feature/params/FeatureListParams.java
index 7e6db631..c723cf82 100644
--- a/src/main/java/com/chargebee/v4/models/feature/params/FeatureListParams.java
+++ b/src/main/java/com/chargebee/v4/models/feature/params/FeatureListParams.java
@@ -9,7 +9,6 @@
 
 import com.chargebee.v4.internal.Recommended;
 import com.chargebee.v4.filters.StringFilter;
-
 import com.chargebee.v4.filters.CustomFieldSelector;
 
 import java.util.Collections;
diff --git a/src/main/java/com/chargebee/v4/models/gift/params/GiftCreateParams.java b/src/main/java/com/chargebee/v4/models/gift/params/GiftCreateParams.java
index 427fc417..4cd3ce00 100644
--- a/src/main/java/com/chargebee/v4/models/gift/params/GiftCreateParams.java
+++ b/src/main/java/com/chargebee/v4/models/gift/params/GiftCreateParams.java
@@ -9,6 +9,7 @@
 import com.chargebee.v4.internal.Recommended;
 import com.chargebee.v4.internal.JsonUtil;
 
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.List;
@@ -1107,6 +1108,8 @@ public static final class SubscriptionParams {
 
     private final String planQuantityInDecimal;
 
+    private final Map<String, String> customFields;
+
     private SubscriptionParams(SubscriptionBuilder builder) {
 
       this.planId = builder.planId;
@@ -1114,6 +1117,11 @@ private SubscriptionParams(SubscriptionBuilder builder) {
       this.planQuantity = builder.planQuantity;
 
       this.planQuantityInDecimal = builder.planQuantityInDecimal;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getPlanId() {
@@ -1128,6 +1136,10 @@ public String getPlanQuantityInDecimal() {
       return planQuantityInDecimal;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -1147,6 +1159,8 @@ public Map<String, Object> toFormData() {
         formData.put("plan_quantity_in_decimal", this.planQuantityInDecimal);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -1164,6 +1178,8 @@ public static final class SubscriptionBuilder {
 
       private String planQuantityInDecimal;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private SubscriptionBuilder() {}
 
       public SubscriptionBuilder planId(String value) {
@@ -1181,6 +1197,42 @@ public SubscriptionBuilder planQuantityInDecimal(String value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public SubscriptionParams build() {
         return new SubscriptionParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutExistingForItemsParams.java b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutExistingForItemsParams.java
index b23db35c..541ac2a9 100644
--- a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutExistingForItemsParams.java
+++ b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutExistingForItemsParams.java
@@ -8,6 +8,7 @@
 
 import com.chargebee.v4.internal.Recommended;
 
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.List;
@@ -752,6 +753,8 @@ public static final class SubscriptionParams {
 
     private final Integer contractTermBillingCycleOnRenewal;
 
+    private final Map<String, String> customFields;
+
     private SubscriptionParams(SubscriptionBuilder builder) {
 
       this.id = builder.id;
@@ -771,6 +774,11 @@ private SubscriptionParams(SubscriptionBuilder builder) {
       this.invoiceNotes = builder.invoiceNotes;
 
       this.contractTermBillingCycleOnRenewal = builder.contractTermBillingCycleOnRenewal;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -809,6 +817,10 @@ public Integer getContractTermBillingCycleOnRenewal() {
       return contractTermBillingCycleOnRenewal;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -859,6 +871,8 @@ public Map<String, Object> toFormData() {
             "contract_term_billing_cycle_on_renewal", this.contractTermBillingCycleOnRenewal);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -888,6 +902,8 @@ public static final class SubscriptionBuilder {
 
       private Integer contractTermBillingCycleOnRenewal;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private SubscriptionBuilder() {}
 
       public SubscriptionBuilder id(String value) {
@@ -937,6 +953,42 @@ public SubscriptionBuilder contractTermBillingCycleOnRenewal(Integer value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public SubscriptionParams build() {
         return new SubscriptionParams(this);
       }
@@ -1035,6 +1087,8 @@ public static final class CustomerParams {
 
     private final String entityIdentifierStandard;
 
+    private final Map<String, String> customFields;
+
     private CustomerParams(CustomerBuilder builder) {
 
       this.vatNumber = builder.vatNumber;
@@ -1046,6 +1100,11 @@ private CustomerParams(CustomerBuilder builder) {
       this.entityIdentifierScheme = builder.entityIdentifierScheme;
 
       this.entityIdentifierStandard = builder.entityIdentifierStandard;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getVatNumber() {
@@ -1068,6 +1127,10 @@ public String getEntityIdentifierStandard() {
       return entityIdentifierStandard;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -1097,6 +1160,8 @@ public Map<String, Object> toFormData() {
         formData.put("entity_identifier_standard", this.entityIdentifierStandard);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -1118,6 +1183,8 @@ public static final class CustomerBuilder {
 
       private String entityIdentifierStandard;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private CustomerBuilder() {}
 
       public CustomerBuilder vatNumber(String value) {
@@ -1145,6 +1212,42 @@ public CustomerBuilder entityIdentifierStandard(String value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public CustomerBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public CustomerBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public CustomerParams build() {
         return new CustomerParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutExistingParams.java b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutExistingParams.java
index 09a7b434..3e00cf7b 100644
--- a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutExistingParams.java
+++ b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutExistingParams.java
@@ -8,6 +8,7 @@
 
 import com.chargebee.v4.internal.Recommended;
 
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.List;
@@ -600,6 +601,8 @@ public static final class SubscriptionParams {
 
     private final Integer contractTermBillingCycleOnRenewal;
 
+    private final Map<String, String> customFields;
+
     private SubscriptionParams(SubscriptionBuilder builder) {
 
       this.id = builder.id;
@@ -629,6 +632,11 @@ private SubscriptionParams(SubscriptionBuilder builder) {
       this.invoiceNotes = builder.invoiceNotes;
 
       this.contractTermBillingCycleOnRenewal = builder.contractTermBillingCycleOnRenewal;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -687,6 +695,10 @@ public Integer getContractTermBillingCycleOnRenewal() {
       return contractTermBillingCycleOnRenewal;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -762,6 +774,8 @@ public Map<String, Object> toFormData() {
             "contract_term_billing_cycle_on_renewal", this.contractTermBillingCycleOnRenewal);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -801,6 +815,8 @@ public static final class SubscriptionBuilder {
 
       private Integer contractTermBillingCycleOnRenewal;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private SubscriptionBuilder() {}
 
       public SubscriptionBuilder id(String value) {
@@ -874,6 +890,42 @@ public SubscriptionBuilder contractTermBillingCycleOnRenewal(Integer value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public SubscriptionParams build() {
         return new SubscriptionParams(this);
       }
@@ -966,11 +1018,18 @@ public static final class CustomerParams {
 
     private final String vatNumberPrefix;
 
+    private final Map<String, String> customFields;
+
     private CustomerParams(CustomerBuilder builder) {
 
       this.vatNumber = builder.vatNumber;
 
       this.vatNumberPrefix = builder.vatNumberPrefix;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getVatNumber() {
@@ -981,6 +1040,10 @@ public String getVatNumberPrefix() {
       return vatNumberPrefix;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -995,6 +1058,8 @@ public Map<String, Object> toFormData() {
         formData.put("vat_number_prefix", this.vatNumberPrefix);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -1010,6 +1075,8 @@ public static final class CustomerBuilder {
 
       private String vatNumberPrefix;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private CustomerBuilder() {}
 
       public CustomerBuilder vatNumber(String value) {
@@ -1022,6 +1089,42 @@ public CustomerBuilder vatNumberPrefix(String value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public CustomerBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public CustomerBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public CustomerParams build() {
         return new CustomerParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutGiftParams.java b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutGiftParams.java
index 3554a55a..a9c78b97 100644
--- a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutGiftParams.java
+++ b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutGiftParams.java
@@ -8,6 +8,7 @@
 
 import com.chargebee.v4.internal.Recommended;
 
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.List;
@@ -217,6 +218,8 @@ public static final class SubscriptionParams {
 
     private final String coupon;
 
+    private final Map<String, String> customFields;
+
     private SubscriptionParams(SubscriptionBuilder builder) {
 
       this.planId = builder.planId;
@@ -226,6 +229,11 @@ private SubscriptionParams(SubscriptionBuilder builder) {
       this.planQuantityInDecimal = builder.planQuantityInDecimal;
 
       this.coupon = builder.coupon;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getPlanId() {
@@ -244,6 +252,10 @@ public String getCoupon() {
       return coupon;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -268,6 +280,8 @@ public Map<String, Object> toFormData() {
         formData.put("coupon", this.coupon);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -287,6 +301,8 @@ public static final class SubscriptionBuilder {
 
       private String coupon;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private SubscriptionBuilder() {}
 
       public SubscriptionBuilder planId(String value) {
@@ -310,6 +326,42 @@ public SubscriptionBuilder coupon(String value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public SubscriptionParams build() {
         return new SubscriptionParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutNewForItemsParams.java b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutNewForItemsParams.java
index 9250eee1..7d2a19c9 100644
--- a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutNewForItemsParams.java
+++ b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutNewForItemsParams.java
@@ -8,6 +8,7 @@
 
 import com.chargebee.v4.internal.Recommended;
 
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.List;
@@ -612,6 +613,8 @@ public static final class SubscriptionParams {
 
     private final Integer contractTermBillingCycleOnRenewal;
 
+    private final Map<String, String> customFields;
+
     private SubscriptionParams(SubscriptionBuilder builder) {
 
       this.id = builder.id;
@@ -633,6 +636,11 @@ private SubscriptionParams(SubscriptionBuilder builder) {
       this.poNumber = builder.poNumber;
 
       this.contractTermBillingCycleOnRenewal = builder.contractTermBillingCycleOnRenewal;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -675,6 +683,10 @@ public Integer getContractTermBillingCycleOnRenewal() {
       return contractTermBillingCycleOnRenewal;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -730,6 +742,8 @@ public Map<String, Object> toFormData() {
             "contract_term_billing_cycle_on_renewal", this.contractTermBillingCycleOnRenewal);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -761,6 +775,8 @@ public static final class SubscriptionBuilder {
 
       private Integer contractTermBillingCycleOnRenewal;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private SubscriptionBuilder() {}
 
       public SubscriptionBuilder id(String value) {
@@ -814,6 +830,42 @@ public SubscriptionBuilder contractTermBillingCycleOnRenewal(Integer value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public SubscriptionParams build() {
         return new SubscriptionParams(this);
       }
@@ -930,6 +982,8 @@ public static final class CustomerParams {
 
     private final EinvoicingMethod einvoicingMethod;
 
+    private final Map<String, String> customFields;
+
     private CustomerParams(CustomerBuilder builder) {
 
       this.id = builder.id;
@@ -959,6 +1013,11 @@ private CustomerParams(CustomerBuilder builder) {
       this.entityIdentifierStandard = builder.entityIdentifierStandard;
 
       this.einvoicingMethod = builder.einvoicingMethod;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -1017,6 +1076,10 @@ public EinvoicingMethod getEinvoicingMethod() {
       return einvoicingMethod;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -1091,6 +1154,8 @@ public Map<String, Object> toFormData() {
         formData.put("einvoicing_method", this.einvoicingMethod);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -1130,6 +1195,8 @@ public static final class CustomerBuilder {
 
       private EinvoicingMethod einvoicingMethod;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private CustomerBuilder() {}
 
       public CustomerBuilder id(String value) {
@@ -1202,6 +1269,42 @@ public CustomerBuilder einvoicingMethod(EinvoicingMethod value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public CustomerBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public CustomerBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public CustomerParams build() {
         return new CustomerParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutNewParams.java b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutNewParams.java
index 7ed14a0b..1ad3e831 100644
--- a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutNewParams.java
+++ b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutNewParams.java
@@ -8,6 +8,7 @@
 
 import com.chargebee.v4.internal.Recommended;
 
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.List;
@@ -532,6 +533,8 @@ public static final class SubscriptionParams {
 
     private final Integer contractTermBillingCycleOnRenewal;
 
+    private final Map<String, String> customFields;
+
     private SubscriptionParams(SubscriptionBuilder builder) {
 
       this.id = builder.id;
@@ -563,6 +566,11 @@ private SubscriptionParams(SubscriptionBuilder builder) {
       this.affiliateToken = builder.affiliateToken;
 
       this.contractTermBillingCycleOnRenewal = builder.contractTermBillingCycleOnRenewal;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -625,6 +633,10 @@ public Integer getContractTermBillingCycleOnRenewal() {
       return contractTermBillingCycleOnRenewal;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -705,6 +717,8 @@ public Map<String, Object> toFormData() {
             "contract_term_billing_cycle_on_renewal", this.contractTermBillingCycleOnRenewal);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -746,6 +760,8 @@ public static final class SubscriptionBuilder {
 
       private Integer contractTermBillingCycleOnRenewal;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private SubscriptionBuilder() {}
 
       public SubscriptionBuilder id(String value) {
@@ -824,6 +840,42 @@ public SubscriptionBuilder contractTermBillingCycleOnRenewal(Integer value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public SubscriptionParams build() {
         return new SubscriptionParams(this);
       }
@@ -934,6 +986,8 @@ public static final class CustomerParams {
 
     private final Boolean consolidatedInvoicing;
 
+    private final Map<String, String> customFields;
+
     private CustomerParams(CustomerBuilder builder) {
 
       this.id = builder.id;
@@ -957,6 +1011,11 @@ private CustomerParams(CustomerBuilder builder) {
       this.vatNumberPrefix = builder.vatNumberPrefix;
 
       this.consolidatedInvoicing = builder.consolidatedInvoicing;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -1003,6 +1062,10 @@ public Boolean getConsolidatedInvoicing() {
       return consolidatedInvoicing;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -1062,6 +1125,8 @@ public Map<String, Object> toFormData() {
         formData.put("consolidated_invoicing", this.consolidatedInvoicing);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -1095,6 +1160,8 @@ public static final class CustomerBuilder {
 
       private Boolean consolidatedInvoicing;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private CustomerBuilder() {}
 
       public CustomerBuilder id(String value) {
@@ -1152,6 +1219,42 @@ public CustomerBuilder consolidatedInvoicing(Boolean value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public CustomerBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public CustomerBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public CustomerParams build() {
         return new CustomerParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutOneTimeForItemsParams.java b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutOneTimeForItemsParams.java
index fe08f184..2031155b 100644
--- a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutOneTimeForItemsParams.java
+++ b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutOneTimeForItemsParams.java
@@ -8,6 +8,7 @@
 
 import com.chargebee.v4.internal.Recommended;
 
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.List;
@@ -558,6 +559,8 @@ public static final class CustomerParams {
 
     private final Boolean consolidatedInvoicing;
 
+    private final Map<String, String> customFields;
+
     private CustomerParams(CustomerBuilder builder) {
 
       this.id = builder.id;
@@ -589,6 +592,11 @@ private CustomerParams(CustomerBuilder builder) {
       this.entityIdentifierStandard = builder.entityIdentifierStandard;
 
       this.consolidatedInvoicing = builder.consolidatedInvoicing;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -651,6 +659,10 @@ public Boolean getConsolidatedInvoicing() {
       return consolidatedInvoicing;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -730,6 +742,8 @@ public Map<String, Object> toFormData() {
         formData.put("consolidated_invoicing", this.consolidatedInvoicing);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -771,6 +785,8 @@ public static final class CustomerBuilder {
 
       private Boolean consolidatedInvoicing;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private CustomerBuilder() {}
 
       public CustomerBuilder id(String value) {
@@ -848,6 +864,42 @@ public CustomerBuilder consolidatedInvoicing(Boolean value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public CustomerBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public CustomerBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public CustomerParams build() {
         return new CustomerParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutOneTimeParams.java b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutOneTimeParams.java
index a8493b2e..761ba08e 100644
--- a/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutOneTimeParams.java
+++ b/src/main/java/com/chargebee/v4/models/hostedPage/params/HostedPageCheckoutOneTimeParams.java
@@ -8,6 +8,7 @@
 
 import com.chargebee.v4.internal.Recommended;
 
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.List;
@@ -430,6 +431,8 @@ public static final class CustomerParams {
 
     private final Boolean consolidatedInvoicing;
 
+    private final Map<String, String> customFields;
+
     private CustomerParams(CustomerBuilder builder) {
 
       this.id = builder.id;
@@ -453,6 +456,11 @@ private CustomerParams(CustomerBuilder builder) {
       this.vatNumberPrefix = builder.vatNumberPrefix;
 
       this.consolidatedInvoicing = builder.consolidatedInvoicing;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -499,6 +507,10 @@ public Boolean getConsolidatedInvoicing() {
       return consolidatedInvoicing;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -558,6 +570,8 @@ public Map<String, Object> toFormData() {
         formData.put("consolidated_invoicing", this.consolidatedInvoicing);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -591,6 +605,8 @@ public static final class CustomerBuilder {
 
       private Boolean consolidatedInvoicing;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private CustomerBuilder() {}
 
       public CustomerBuilder id(String value) {
@@ -648,6 +664,42 @@ public CustomerBuilder consolidatedInvoicing(Boolean value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public CustomerBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public CustomerBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public CustomerParams build() {
         return new CustomerParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/itemFamily/params/ItemFamilyListParams.java b/src/main/java/com/chargebee/v4/models/itemFamily/params/ItemFamilyListParams.java
index b6bf7124..d33c6dd4 100644
--- a/src/main/java/com/chargebee/v4/models/itemFamily/params/ItemFamilyListParams.java
+++ b/src/main/java/com/chargebee/v4/models/itemFamily/params/ItemFamilyListParams.java
@@ -10,7 +10,6 @@
 import com.chargebee.v4.internal.Recommended;
 import com.chargebee.v4.filters.StringFilter;
 import com.chargebee.v4.filters.TimestampFilter;
-
 import com.chargebee.v4.filters.CustomFieldSelector;
 
 import java.util.Collections;
diff --git a/src/main/java/com/chargebee/v4/models/plan/params/PlanListParams.java b/src/main/java/com/chargebee/v4/models/plan/params/PlanListParams.java
index f626fba8..1e7611f4 100644
--- a/src/main/java/com/chargebee/v4/models/plan/params/PlanListParams.java
+++ b/src/main/java/com/chargebee/v4/models/plan/params/PlanListParams.java
@@ -11,7 +11,6 @@
 import com.chargebee.v4.filters.StringFilter;
 import com.chargebee.v4.filters.NumberFilter;
 import com.chargebee.v4.filters.TimestampFilter;
-
 import com.chargebee.v4.filters.CustomFieldSelector;
 
 import java.util.Collections;
diff --git a/src/main/java/com/chargebee/v4/models/pricingPageSession/params/PricingPageSessionCreateForExistingSubscriptionParams.java b/src/main/java/com/chargebee/v4/models/pricingPageSession/params/PricingPageSessionCreateForExistingSubscriptionParams.java
index bc1577dd..ec754b0c 100644
--- a/src/main/java/com/chargebee/v4/models/pricingPageSession/params/PricingPageSessionCreateForExistingSubscriptionParams.java
+++ b/src/main/java/com/chargebee/v4/models/pricingPageSession/params/PricingPageSessionCreateForExistingSubscriptionParams.java
@@ -9,6 +9,7 @@
 import com.chargebee.v4.internal.Recommended;
 import com.chargebee.v4.internal.JsonUtil;
 
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.List;
@@ -217,15 +218,26 @@ public static final class SubscriptionParams {
 
     private final String id;
 
+    private final Map<String, String> customFields;
+
     private SubscriptionParams(SubscriptionBuilder builder) {
 
       this.id = builder.id;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
       return id;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -235,6 +247,8 @@ public Map<String, Object> toFormData() {
         formData.put("id", this.id);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -248,6 +262,8 @@ public static final class SubscriptionBuilder {
 
       private String id;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private SubscriptionBuilder() {}
 
       public SubscriptionBuilder id(String value) {
@@ -255,6 +271,42 @@ public SubscriptionBuilder id(String value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public SubscriptionParams build() {
         return new SubscriptionParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/pricingPageSession/params/PricingPageSessionCreateForNewSubscriptionParams.java b/src/main/java/com/chargebee/v4/models/pricingPageSession/params/PricingPageSessionCreateForNewSubscriptionParams.java
index 53dbdef2..6dd3849b 100644
--- a/src/main/java/com/chargebee/v4/models/pricingPageSession/params/PricingPageSessionCreateForNewSubscriptionParams.java
+++ b/src/main/java/com/chargebee/v4/models/pricingPageSession/params/PricingPageSessionCreateForNewSubscriptionParams.java
@@ -9,6 +9,7 @@
 import com.chargebee.v4.internal.Recommended;
 import com.chargebee.v4.internal.JsonUtil;
 
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.List;
@@ -334,15 +335,26 @@ public static final class SubscriptionParams {
 
     private final String id;
 
+    private final Map<String, String> customFields;
+
     private SubscriptionParams(SubscriptionBuilder builder) {
 
       this.id = builder.id;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
       return id;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -352,6 +364,8 @@ public Map<String, Object> toFormData() {
         formData.put("id", this.id);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -365,6 +379,8 @@ public static final class SubscriptionBuilder {
 
       private String id;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private SubscriptionBuilder() {}
 
       public SubscriptionBuilder id(String value) {
@@ -372,6 +388,42 @@ public SubscriptionBuilder id(String value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public SubscriptionParams build() {
         return new SubscriptionParams(this);
       }
@@ -394,6 +446,8 @@ public static final class CustomerParams {
 
     private final String locale;
 
+    private final Map<String, String> customFields;
+
     private CustomerParams(CustomerBuilder builder) {
 
       this.id = builder.id;
@@ -409,6 +463,11 @@ private CustomerParams(CustomerBuilder builder) {
       this.phone = builder.phone;
 
       this.locale = builder.locale;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -439,6 +498,10 @@ public String getLocale() {
       return locale;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -478,6 +541,8 @@ public Map<String, Object> toFormData() {
         formData.put("locale", this.locale);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -503,6 +568,8 @@ public static final class CustomerBuilder {
 
       private String locale;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private CustomerBuilder() {}
 
       public CustomerBuilder id(String value) {
@@ -540,6 +607,42 @@ public CustomerBuilder locale(String value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public CustomerBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public CustomerBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public CustomerParams build() {
         return new CustomerParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/quote/params/CreateSubscriptionItemsForCustomerQuoteParams.java b/src/main/java/com/chargebee/v4/models/quote/params/CreateSubscriptionItemsForCustomerQuoteParams.java
index 1a0aaf41..660c3739 100644
--- a/src/main/java/com/chargebee/v4/models/quote/params/CreateSubscriptionItemsForCustomerQuoteParams.java
+++ b/src/main/java/com/chargebee/v4/models/quote/params/CreateSubscriptionItemsForCustomerQuoteParams.java
@@ -595,6 +595,8 @@ public static final class SubscriptionParams {
 
     private final Integer contractTermBillingCycleOnRenewal;
 
+    private final Map<String, String> customFields;
+
     private SubscriptionParams(SubscriptionBuilder builder) {
 
       this.id = builder.id;
@@ -610,6 +612,11 @@ private SubscriptionParams(SubscriptionBuilder builder) {
       this.offlinePaymentMethod = builder.offlinePaymentMethod;
 
       this.contractTermBillingCycleOnRenewal = builder.contractTermBillingCycleOnRenewal;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -640,6 +647,10 @@ public Integer getContractTermBillingCycleOnRenewal() {
       return contractTermBillingCycleOnRenewal;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -680,6 +691,8 @@ public Map<String, Object> toFormData() {
             "contract_term_billing_cycle_on_renewal", this.contractTermBillingCycleOnRenewal);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -705,6 +718,8 @@ public static final class SubscriptionBuilder {
 
       private Integer contractTermBillingCycleOnRenewal;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private SubscriptionBuilder() {}
 
       public SubscriptionBuilder id(String value) {
@@ -743,6 +758,42 @@ public SubscriptionBuilder contractTermBillingCycleOnRenewal(Integer value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public SubscriptionParams build() {
         return new SubscriptionParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/quote/params/EditCreateSubscriptionCustomerQuoteForItemsParams.java b/src/main/java/com/chargebee/v4/models/quote/params/EditCreateSubscriptionCustomerQuoteForItemsParams.java
index 6b7dacdb..e119b0fa 100644
--- a/src/main/java/com/chargebee/v4/models/quote/params/EditCreateSubscriptionCustomerQuoteForItemsParams.java
+++ b/src/main/java/com/chargebee/v4/models/quote/params/EditCreateSubscriptionCustomerQuoteForItemsParams.java
@@ -579,6 +579,8 @@ public static final class SubscriptionParams {
 
     private final Integer contractTermBillingCycleOnRenewal;
 
+    private final Map<String, String> customFields;
+
     private SubscriptionParams(SubscriptionBuilder builder) {
 
       this.id = builder.id;
@@ -594,6 +596,11 @@ private SubscriptionParams(SubscriptionBuilder builder) {
       this.offlinePaymentMethod = builder.offlinePaymentMethod;
 
       this.contractTermBillingCycleOnRenewal = builder.contractTermBillingCycleOnRenewal;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -624,6 +631,10 @@ public Integer getContractTermBillingCycleOnRenewal() {
       return contractTermBillingCycleOnRenewal;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -664,6 +675,8 @@ public Map<String, Object> toFormData() {
             "contract_term_billing_cycle_on_renewal", this.contractTermBillingCycleOnRenewal);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -689,6 +702,8 @@ public static final class SubscriptionBuilder {
 
       private Integer contractTermBillingCycleOnRenewal;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private SubscriptionBuilder() {}
 
       public SubscriptionBuilder id(String value) {
@@ -727,6 +742,42 @@ public SubscriptionBuilder contractTermBillingCycleOnRenewal(Integer value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public SubscriptionParams build() {
         return new SubscriptionParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/quote/params/EditUpdateSubscriptionQuoteForItemsParams.java b/src/main/java/com/chargebee/v4/models/quote/params/EditUpdateSubscriptionQuoteForItemsParams.java
index aba9fee2..b88f4f4a 100644
--- a/src/main/java/com/chargebee/v4/models/quote/params/EditUpdateSubscriptionQuoteForItemsParams.java
+++ b/src/main/java/com/chargebee/v4/models/quote/params/EditUpdateSubscriptionQuoteForItemsParams.java
@@ -715,6 +715,8 @@ public static final class SubscriptionParams {
 
     private final Integer contractTermBillingCycleOnRenewal;
 
+    private final Map<String, String> customFields;
+
     private SubscriptionParams(SubscriptionBuilder builder) {
 
       this.setupFee = builder.setupFee;
@@ -730,6 +732,11 @@ private SubscriptionParams(SubscriptionBuilder builder) {
       this.offlinePaymentMethod = builder.offlinePaymentMethod;
 
       this.contractTermBillingCycleOnRenewal = builder.contractTermBillingCycleOnRenewal;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public Long getSetupFee() {
@@ -760,6 +767,10 @@ public Integer getContractTermBillingCycleOnRenewal() {
       return contractTermBillingCycleOnRenewal;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -800,6 +811,8 @@ public Map<String, Object> toFormData() {
             "contract_term_billing_cycle_on_renewal", this.contractTermBillingCycleOnRenewal);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -825,6 +838,8 @@ public static final class SubscriptionBuilder {
 
       private Integer contractTermBillingCycleOnRenewal;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private SubscriptionBuilder() {}
 
       @Deprecated
@@ -864,6 +879,42 @@ public SubscriptionBuilder contractTermBillingCycleOnRenewal(Integer value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public SubscriptionParams build() {
         return new SubscriptionParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/quote/params/QuoteListParams.java b/src/main/java/com/chargebee/v4/models/quote/params/QuoteListParams.java
index 7b1de744..f000901b 100644
--- a/src/main/java/com/chargebee/v4/models/quote/params/QuoteListParams.java
+++ b/src/main/java/com/chargebee/v4/models/quote/params/QuoteListParams.java
@@ -10,7 +10,6 @@
 import com.chargebee.v4.internal.Recommended;
 import com.chargebee.v4.filters.StringFilter;
 import com.chargebee.v4.filters.TimestampFilter;
-
 import com.chargebee.v4.filters.CustomFieldSelector;
 
 import java.util.Collections;
diff --git a/src/main/java/com/chargebee/v4/models/quote/params/UpdateSubscriptionQuoteForItemsParams.java b/src/main/java/com/chargebee/v4/models/quote/params/UpdateSubscriptionQuoteForItemsParams.java
index 735c80d0..f71a538a 100644
--- a/src/main/java/com/chargebee/v4/models/quote/params/UpdateSubscriptionQuoteForItemsParams.java
+++ b/src/main/java/com/chargebee/v4/models/quote/params/UpdateSubscriptionQuoteForItemsParams.java
@@ -734,6 +734,8 @@ public static final class SubscriptionParams {
 
     private final Integer contractTermBillingCycleOnRenewal;
 
+    private final Map<String, String> customFields;
+
     private SubscriptionParams(SubscriptionBuilder builder) {
 
       this.id = builder.id;
@@ -751,6 +753,11 @@ private SubscriptionParams(SubscriptionBuilder builder) {
       this.offlinePaymentMethod = builder.offlinePaymentMethod;
 
       this.contractTermBillingCycleOnRenewal = builder.contractTermBillingCycleOnRenewal;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -785,6 +792,10 @@ public Integer getContractTermBillingCycleOnRenewal() {
       return contractTermBillingCycleOnRenewal;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -830,6 +841,8 @@ public Map<String, Object> toFormData() {
             "contract_term_billing_cycle_on_renewal", this.contractTermBillingCycleOnRenewal);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -857,6 +870,8 @@ public static final class SubscriptionBuilder {
 
       private Integer contractTermBillingCycleOnRenewal;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private SubscriptionBuilder() {}
 
       public SubscriptionBuilder id(String value) {
@@ -901,6 +916,42 @@ public SubscriptionBuilder contractTermBillingCycleOnRenewal(Integer value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public SubscriptionBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public SubscriptionParams build() {
         return new SubscriptionParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/subscription/params/ImportSubscriptionParams.java b/src/main/java/com/chargebee/v4/models/subscription/params/ImportSubscriptionParams.java
index 2f8fb71f..3a975b4c 100644
--- a/src/main/java/com/chargebee/v4/models/subscription/params/ImportSubscriptionParams.java
+++ b/src/main/java/com/chargebee/v4/models/subscription/params/ImportSubscriptionParams.java
@@ -1053,6 +1053,8 @@ public static final class CustomerParams {
 
     private final String vatNumberPrefix;
 
+    private final Map<String, String> customFields;
+
     private CustomerParams(CustomerBuilder builder) {
 
       this.id = builder.id;
@@ -1088,6 +1090,11 @@ private CustomerParams(CustomerBuilder builder) {
       this.vatNumber = builder.vatNumber;
 
       this.vatNumberPrefix = builder.vatNumberPrefix;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -1158,6 +1165,10 @@ public String getVatNumberPrefix() {
       return vatNumberPrefix;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -1247,6 +1258,8 @@ public Map<String, Object> toFormData() {
         formData.put("vat_number_prefix", this.vatNumberPrefix);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -1292,6 +1305,8 @@ public static final class CustomerBuilder {
 
       private String vatNumberPrefix;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private CustomerBuilder() {}
 
       public CustomerBuilder id(String value) {
@@ -1379,6 +1394,42 @@ public CustomerBuilder vatNumberPrefix(String value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public CustomerBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public CustomerBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public CustomerParams build() {
         return new CustomerParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/subscription/params/SubscriptionCreateParams.java b/src/main/java/com/chargebee/v4/models/subscription/params/SubscriptionCreateParams.java
index 84ca8fbb..a04a0c52 100644
--- a/src/main/java/com/chargebee/v4/models/subscription/params/SubscriptionCreateParams.java
+++ b/src/main/java/com/chargebee/v4/models/subscription/params/SubscriptionCreateParams.java
@@ -1348,6 +1348,8 @@ public static final class CustomerParams {
 
     private final CustomerType customerType;
 
+    private final Map<String, String> customFields;
+
     private CustomerParams(CustomerBuilder builder) {
 
       this.id = builder.id;
@@ -1401,6 +1403,11 @@ private CustomerParams(CustomerBuilder builder) {
       this.exemptionDetails = builder.exemptionDetails;
 
       this.customerType = builder.customerType;
+
+      this.customFields =
+          builder.customFields.isEmpty()
+              ? Collections.emptyMap()
+              : Collections.unmodifiableMap(new LinkedHashMap<>(builder.customFields));
     }
 
     public String getId() {
@@ -1507,6 +1514,10 @@ public CustomerType getCustomerType() {
       return customerType;
     }
 
+    public Map<String, String> customFields() {
+      return customFields;
+    }
+
     /** Get the form data for this request. */
     public Map<String, Object> toFormData() {
       Map<String, Object> formData = new LinkedHashMap<>();
@@ -1641,6 +1652,8 @@ public Map<String, Object> toFormData() {
         formData.put("customer_type", this.customerType);
       }
 
+      formData.putAll(customFields);
+
       return formData;
     }
 
@@ -1704,6 +1717,8 @@ public static final class CustomerBuilder {
 
       private CustomerType customerType;
 
+      private Map<String, String> customFields = new LinkedHashMap<>();
+
       private CustomerBuilder() {}
 
       public CustomerBuilder id(String value) {
@@ -1836,6 +1851,42 @@ public CustomerBuilder customerType(CustomerType value) {
         return this;
       }
 
+      /**
+       * Add a custom field to the request. Custom fields must start with "cf_".
+       *
+       * @param fieldName the name of the custom field (e.g., "cf_custom_field_name")
+       * @param value the value of the custom field
+       * @return this builder
+       * @throws IllegalArgumentException if fieldName doesn't start with "cf_"
+       */
+      public CustomerBuilder customField(String fieldName, String value) {
+        if (fieldName == null || !fieldName.startsWith("cf_")) {
+          throw new IllegalArgumentException("Custom field name must start with 'cf_'");
+        }
+        this.customFields.put(fieldName, value);
+        return this;
+      }
+
+      /**
+       * Add multiple custom fields to the request. All field names must start with "cf_".
+       *
+       * @param customFields map of custom field names to values
+       * @return this builder
+       * @throws IllegalArgumentException if any field name doesn't start with "cf_"
+       */
+      public CustomerBuilder customFields(Map<String, String> customFields) {
+        if (customFields != null) {
+          for (Map.Entry<String, String> entry : customFields.entrySet()) {
+            if (entry.getKey() == null || !entry.getKey().startsWith("cf_")) {
+              throw new IllegalArgumentException(
+                  "Custom field name must start with 'cf_': " + entry.getKey());
+            }
+            this.customFields.put(entry.getKey(), entry.getValue());
+          }
+        }
+        return this;
+      }
+
       public CustomerParams build() {
         return new CustomerParams(this);
       }
diff --git a/src/main/java/com/chargebee/v4/models/subscription/params/SubscriptionListParams.java b/src/main/java/com/chargebee/v4/models/subscription/params/SubscriptionListParams.java
index 06511e02..6c54a55a 100644
--- a/src/main/java/com/chargebee/v4/models/subscription/params/SubscriptionListParams.java
+++ b/src/main/java/com/chargebee/v4/models/subscription/params/SubscriptionListParams.java
@@ -11,7 +11,6 @@
 import com.chargebee.v4.filters.StringFilter;
 import com.chargebee.v4.filters.NumberFilter;
 import com.chargebee.v4.filters.TimestampFilter;
-
 import com.chargebee.v4.filters.CustomFieldSelector;
 
 import java.util.Collections;