diff --git a/modules/sdk-api/src/bitgoAPI.ts b/modules/sdk-api/src/bitgoAPI.ts
index 504868e629..11cfb313fe 100644
--- a/modules/sdk-api/src/bitgoAPI.ts
+++ b/modules/sdk-api/src/bitgoAPI.ts
@@ -234,6 +234,38 @@ function deriveTokenIssuanceEcdhSecret(ecdhXprv: string, derivationPath: string,
   ).toString('hex');
 }
 
+type PasswordUpdatePayload = {
+  keychains: Record<string, string>;
+  v2_keychains: Record<string, string>;
+  version: number;
+  oldPassword: string;
+  password: string;
+};
+
+function serializedMapLength(keychains: Record<string, string>): number {
+  return Object.entries(keychains).reduce(
+    (length, [id, value], index) =>
+      length + (index > 0 ? 1 : 0) + JSON.stringify(id).length + 1 + JSON.stringify(value).length,
+    2
+  );
+}
+
+function passwordUpdatePayloadSizeKB(updatePasswordParams: PasswordUpdatePayload): number {
+  const emptyPayloadLength = JSON.stringify({
+    ...updatePasswordParams,
+    keychains: {},
+    v2_keychains: {},
+  }).length;
+  // Replace the two "{}" map placeholders with their serialized entries.
+  const payloadLength =
+    emptyPayloadLength -
+    4 +
+    serializedMapLength(updatePasswordParams.keychains) +
+    serializedMapLength(updatePasswordParams.v2_keychains);
+
+  return Math.ceil(payloadLength / 1024);
+}
+
 export class BitGoAPI implements BitGoBase {
   // v1 types
   protected _keychains: any;
@@ -2156,9 +2188,8 @@ export class BitGoAPI implements BitGoBase {
         password: hmacNewPassword,
       };
 
-      // Calculate payload size in KB
-      const payloadSizeBytes = JSON.stringify(updatePasswordParams).length;
-      const payloadSizeKB = Math.ceil(payloadSizeBytes / 1024);
+      // Calculate payload size in KB without serializing both keychain maps together.
+      const payloadSizeKB = passwordUpdatePayloadSizeKB(updatePasswordParams);
 
       // Check if batching flow is enabled
       let useBatchingFlow = false;
diff --git a/modules/sdk-api/test/unit/bitgoAPI.ts b/modules/sdk-api/test/unit/bitgoAPI.ts
index b6da41d55b..06d0b49548 100644
--- a/modules/sdk-api/test/unit/bitgoAPI.ts
+++ b/modules/sdk-api/test/unit/bitgoAPI.ts
@@ -1104,6 +1104,47 @@ describe('Constructor', function () {
       changePassScope.isDone().should.be.true();
     });
 
+    it('checks batching without serializing the complete keychain payload', async function () {
+      const largeKeychains = Object.fromEntries(
+        Array.from({ length: 200 }, (_, index) => [`keychain-${index}`, 'x'.repeat(50)])
+      );
+      const updatePasswordParams = {
+        keychains: largeKeychains,
+        v2_keychains: { v2k1: 'v2enc' },
+        version: 25,
+        oldPassword: 'hashed-pw',
+        password: 'hashed-pw',
+      };
+      const expectedPayloadSizeKB = Math.ceil(JSON.stringify(updatePasswordParams).length / 1024);
+      v1UpdatePasswordStub.resolves({ keychains: largeKeychains, version: 25 });
+
+      nock(ROOT)
+        .get('/api/v2/user/checkBatchingPasswordFlow')
+        .query({ payloadSize: expectedPayloadSizeKB.toString() })
+        .reply(200, { isBatchingFlowEnabled: true, maxBatchSizeKB: 1 });
+      const batchPutScope = nock(ROOT).persist().put('/api/v2/user/keychains').reply(200, {});
+      const changePassScope = nock(ROOT)
+        .post('/api/v1/user/changepassword', (body: any) => !body.keychains && !body.v2_keychains)
+        .reply(200, {});
+
+      const originalStringify = JSON.stringify;
+      const stringifyStub = sandbox.stub(JSON, 'stringify').callsFake((value: any) => {
+        if (value?.keychains && Object.keys(value.keychains).length > 100) {
+          throw new Error('complete keychain payload was serialized');
+        }
+        return originalStringify(value);
+      });
+
+      try {
+        await bitgo.changePassword({ oldPassword: 'oldpw', newPassword: 'newpw' });
+      } finally {
+        stringifyStub.restore();
+      }
+
+      batchPutScope.isDone().should.be.true();
+      changePassScope.isDone().should.be.true();
+    });
+
     it('stages an empty batch and changes password when no keychains are re-encryptable', async function () {
       v1UpdatePasswordStub.resolves({ keychains: {}, version: 25 });
       v2UpdatePasswordStub.resolves({});