diff --git a/src/helper/error.ts b/src/helper/error.ts
new file mode 100644
index 0000000..4d564fa
--- /dev/null
+++ b/src/helper/error.ts
@@ -0,0 +1,6 @@
+export const ERROR = {
+  ACCOUNT_NOT_SOURCE:
+    "Transfer contains authorization entry for a different account",
+  AUTH_SUB_INVOCATIONS:
+    "Transfer authorizes sub-invocations to another contract",
+};
diff --git a/src/helper/horizon-rpc.ts b/src/helper/horizon-rpc.ts
index 91b7a4f..d5522db 100644
--- a/src/helper/horizon-rpc.ts
+++ b/src/helper/horizon-rpc.ts
@@ -248,7 +248,7 @@ export const submitTransaction = async (
       error: null,
     };
   } catch (e: any) {
-    if (e.response.status === 504) {
+    if (e.response?.status === 504) {
       // in case of 504, keep retrying this tx until submission succeeds or we get a different error
       // https://developers.stellar.org/api/errors/http-status-codes/horizon-specific/timeout
       // https://developers.stellar.org/docs/encyclopedia/error-handling
diff --git a/src/route/index.ts b/src/route/index.ts
index 410f4f4..19d470c 100644
--- a/src/route/index.ts
+++ b/src/route/index.ts
@@ -25,8 +25,10 @@ import {
   Transaction,
   TransactionBuilder,
   XdrLargeInt,
+  xdr,
 } from "stellar-sdk";
 import { buildTransfer, simulateTx } from "../helper/soroban-rpc";
+import { ERROR } from "../helper/error";
 
 const API_VERSION = "v1";
 
@@ -545,17 +547,41 @@ export async function initApiServer(
             const simulationResponse = (await server.simulateTransaction(
               tx
             )) as SorobanRpc.Api.SimulateTransactionSuccessResponse;
+
             const preparedTransaction = SorobanRpc.assembleTransaction(
               tx,
               simulationResponse
             );
+
+            const built = preparedTransaction.build();
+            switch (built.operations[0].type) {
+              case "invokeHostFunction": {
+                const sorobanOp = built
+                  .operations[0] as Operation.InvokeHostFunction;
+                const auths = sorobanOp.auth || [];
+
+                for (const auth of auths) {
+                  if (
+                    auth.credentials().switch() !==
+                    xdr.SorobanCredentialsType.sorobanCredentialsSourceAccount()
+                  ) {
+                    throw new Error(ERROR.ACCOUNT_NOT_SOURCE);
+                  }
+
+                  if (auth.rootInvocation().subInvocations().length) {
+                    throw new Error(ERROR.AUTH_SUB_INVOCATIONS);
+                  }
+                }
+              }
+            }
+
             const data = {
               simulationResponse,
-              preparedTransaction: preparedTransaction.build().toXDR(),
+              preparedTransaction: built.toXDR(),
             };
             reply.code(200).send(data);
           } catch (error) {
-            reply.code(400).send(JSON.stringify(error));
+            reply.code(400).send(error);
           }
         },
       });
diff --git a/src/service/mercury/index.ts b/src/service/mercury/index.ts
index 2bc4273..6bfd40b 100644
--- a/src/service/mercury/index.ts
+++ b/src/service/mercury/index.ts
@@ -34,7 +34,7 @@ enum NETWORK_URLS {
 }
 
 const ERROR_MESSAGES = {
-  JWT_EXPIRED: "jwt expired",
+  JWT_EXPIRED: "1_kJdMBB7ytvgRIqF1clh2iz2iI",
 };
 
 function getGraphQlError(error?: CombinedError) {
@@ -179,7 +179,7 @@ export class MercuryClient {
     } catch (error: unknown) {
       // renew and retry 0n 401, otherwise throw the error back up to the caller
       if (error instanceof Error) {
-        if (error.message === ERROR_MESSAGES.JWT_EXPIRED) {
+        if (error.message.includes(ERROR_MESSAGES.JWT_EXPIRED)) {
           await this.renewMercuryToken();
           this.logger.info("renewed expired jwt");
           return await method();
@@ -518,9 +518,10 @@ export class MercuryClient {
   ) => {
     const balances = [];
     const balanceMap = {} as Record<string, any>;
-    try {
-      const server = await getServer(network, customSorobanRpcUrl);
-      for (const id of contractIds) {
+
+    const server = await getServer(network, customSorobanRpcUrl);
+    for (const id of contractIds) {
+      try {
         const builder = await getTxBuilder(pubKey, network, server);
         const params = [new Address(pubKey).toScVal()];
         const balance = await getTokenBalance(id, params, server, builder);
@@ -530,11 +531,12 @@ export class MercuryClient {
           balance,
           ...tokenDetails,
         });
+      } catch (error) {
+        this.logger.error(error);
+        continue;
       }
-    } catch (error) {
-      this.logger.error(error);
-      return balanceMap;
     }
+
     for (const balance of balances) {
       balanceMap[`${balance.symbol}:${balance.id}`] = {
         token: {