diff --git a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/AccountCredentials.kt b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/AccountCredentials.kt
index 2ead74dc..df6e11c1 100644
--- a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/AccountCredentials.kt
+++ b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/AccountCredentials.kt
@@ -3,7 +3,7 @@ package net.codinux.banking.client.model
 import net.codinux.banking.client.model.config.NoArgConstructor
 
 @NoArgConstructor
-class AccountCredentials(
+open class AccountCredentials(
     var bankCode: String,
     var loginName: String,
     var password: String
diff --git a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/AccountTransaction.kt b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/AccountTransaction.kt
index d45f5282..e6045afc 100644
--- a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/AccountTransaction.kt
+++ b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/AccountTransaction.kt
@@ -4,7 +4,7 @@ import kotlinx.datetime.LocalDate
 import net.codinux.banking.client.model.config.NoArgConstructor
 
 @NoArgConstructor
-class AccountTransaction(
+open class AccountTransaction(
     val amount: Amount = Amount.Zero,
     val currency: String,
     val reference: String, // Alternative: purpose (or Remittance information)
diff --git a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/BankAccount.kt b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/BankAccount.kt
index 9136d72f..d77dd6ab 100644
--- a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/BankAccount.kt
+++ b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/BankAccount.kt
@@ -4,7 +4,7 @@ import kotlinx.datetime.LocalDate
 import net.codinux.banking.client.model.config.NoArgConstructor
 
 @NoArgConstructor
-class BankAccount(
+open class BankAccount(
     val identifier: String,
     var accountHolderName: String,
     val type: BankAccountType = BankAccountType.CheckingAccount,
diff --git a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/CustomerAccount.kt b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/CustomerAccount.kt
index 9b1fae92..a8cc62b6 100644
--- a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/CustomerAccount.kt
+++ b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/CustomerAccount.kt
@@ -3,7 +3,7 @@ package net.codinux.banking.client.model
 import net.codinux.banking.client.model.config.NoArgConstructor
 
 @NoArgConstructor
-class CustomerAccount(
+open class CustomerAccount(
     val bankCode: String,
     var loginName: String,
     /**
diff --git a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/UnbookedAccountTransaction.kt b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/UnbookedAccountTransaction.kt
index 66ef4882..7e8905f4 100644
--- a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/UnbookedAccountTransaction.kt
+++ b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/UnbookedAccountTransaction.kt
@@ -3,5 +3,5 @@ package net.codinux.banking.client.model
 import net.codinux.banking.client.model.config.NoArgConstructor
 
 @NoArgConstructor
-class UnbookedAccountTransaction {
+open class UnbookedAccountTransaction {
 }
\ No newline at end of file
diff --git a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/options/GetAccountDataOptions.kt b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/options/GetAccountDataOptions.kt
index cb1b5da6..02fdabcf 100644
--- a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/options/GetAccountDataOptions.kt
+++ b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/options/GetAccountDataOptions.kt
@@ -2,7 +2,7 @@ package net.codinux.banking.client.model.options
 
 import kotlinx.datetime.LocalDate
 
-data class GetAccountDataOptions(
+open class GetAccountDataOptions(
     val retrieveBalance: Boolean = true,
     val retrieveTransactions: RetrieveTransactions = RetrieveTransactions.OfLast90Days,
     val retrieveTransactionsFrom: LocalDate? = null,
diff --git a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/Error.kt b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/Error.kt
index efb9964c..70d885a2 100644
--- a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/Error.kt
+++ b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/Error.kt
@@ -3,7 +3,7 @@ package net.codinux.banking.client.model.response
 import net.codinux.banking.client.model.config.NoArgConstructor
 
 @NoArgConstructor
-class Error(
+open class Error(
     val type: ErrorType,
 
     /**
diff --git a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/GetAccountDataResponse.kt b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/GetAccountDataResponse.kt
index b91308cf..bcd084aa 100644
--- a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/GetAccountDataResponse.kt
+++ b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/GetAccountDataResponse.kt
@@ -6,7 +6,7 @@ import net.codinux.banking.client.model.config.JsonIgnore
 import net.codinux.banking.client.model.config.NoArgConstructor
 
 @NoArgConstructor
-class GetAccountDataResponse(
+open class GetAccountDataResponse(
     val customer: CustomerAccount
 ) {
 
diff --git a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/Response.kt b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/Response.kt
index 88f7335c..7f079c15 100644
--- a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/Response.kt
+++ b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/Response.kt
@@ -3,7 +3,7 @@ package net.codinux.banking.client.model.response
 import net.codinux.banking.client.model.config.NoArgConstructor
 
 @NoArgConstructor
-class Response<T> (
+open class Response<T> (
     val type: ResponseType,
     val data: T? = null,
     val error: Error? = null,
diff --git a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/TanRequired.kt b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/TanRequired.kt
index 9c13f24a..6abf4555 100644
--- a/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/TanRequired.kt
+++ b/BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/response/TanRequired.kt
@@ -3,7 +3,7 @@ package net.codinux.banking.client.model.response
 import net.codinux.banking.client.model.config.NoArgConstructor
 
 @NoArgConstructor
-class TanRequired (
+open class TanRequired (
     val tanRequestId: String,
     // TODO: add TAN model
 //    val tanChallenge: TanChallenge
diff --git a/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kBankingClient.kt b/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kBankingClient.kt
index b80fc8d6..27f9619f 100644
--- a/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kBankingClient.kt
+++ b/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kBankingClient.kt
@@ -8,7 +8,7 @@ import net.codinux.banking.client.model.response.Response
 import net.dankito.banking.fints.FinTsClient
 import net.dankito.banking.fints.callback.SimpleFinTsClientCallback
 
-class FinTs4kBankingClient : BankingClient {
+open class FinTs4kBankingClient : BankingClient {
 
     private val mapper = FinTs4kMapper()
 
diff --git a/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kBankingClientForCustomer.kt b/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kBankingClientForCustomer.kt
index 73c928f7..f877844c 100644
--- a/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kBankingClientForCustomer.kt
+++ b/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kBankingClientForCustomer.kt
@@ -3,7 +3,7 @@ package net.codinux.banking.client.fints4k
 import net.codinux.banking.client.BankingClientForCustomerBase
 import net.codinux.banking.client.model.AccountCredentials
 
-class FinTs4kBankingClientForCustomer(credentials: AccountCredentials)
+open class FinTs4kBankingClientForCustomer(credentials: AccountCredentials)
     : BankingClientForCustomerBase(credentials, FinTs4kBankingClient()) {
 
     constructor(bankCode: String, loginName: String, password: String)
diff --git a/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kMapper.kt b/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kMapper.kt
index 694c89b6..a6f75352 100644
--- a/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kMapper.kt
+++ b/FinTs4jBankingClient/src/commonMain/kotlin/net/codinux/banking/client/fints4k/FinTs4kMapper.kt
@@ -8,7 +8,7 @@ import net.dankito.banking.client.model.parameter.RetrieveTransactions
 import net.dankito.banking.client.model.response.ErrorCode
 import net.dankito.banking.fints.model.Money
 
-class FinTs4kMapper {
+open class FinTs4kMapper {
 
     fun mapToGetAccountDataParameter(credentials: AccountCredentials, options: GetAccountDataOptions) = GetAccountDataParameter(
         credentials.bankCode, credentials.loginName, credentials.password,