diff --git a/src/argentSessionService.ts b/src/argentSessionService.ts
index 6aa915d..a15a317 100644
--- a/src/argentSessionService.ts
+++ b/src/argentSessionService.ts
@@ -96,7 +96,7 @@ export class ArgentSessionService {
         calldata: compiledCalldata,
         nonce: txDetailsV3.nonce.toString(),
         version: num.toBigInt(txDetailsV3.version).toString(10),
-        chainId: num.toBigInt(txDetailsV3.chainId).toString(10),
+        chain_id: num.toBigInt(txDetailsV3.chainId).toString(10),
         resource_bounds: {
           l1_gas: {
             max_amount: txDetailsV3.resourceBounds.l1_gas.max_amount.toString(),
diff --git a/src/sessionTypes.ts b/src/sessionTypes.ts
index a67c52c..cc1fdd1 100644
--- a/src/sessionTypes.ts
+++ b/src/sessionTypes.ts
@@ -115,7 +115,7 @@ export type ArgentServiceSessionTxV3Body = {
   calldata: Calldata
   nonce: string
   version: string
-  chainId: string
+  chain_id: string
   resource_bounds: {
     l1_gas: {
       max_amount: string