diff --git a/src/plugins/api/api.ts b/src/plugins/api/api.ts
index 1f4e8a4bd40..af883cec162 100644
--- a/src/plugins/api/api.ts
+++ b/src/plugins/api/api.ts
@@ -8,6 +8,8 @@ import type { AccountInfoResponse } from "./models/AccountInfo";
 import type { AccountLoginRequest, AccountLoginResponse } from "./models/AccountLogin";
 import type { TitleStatsResponse } from "./models/TitleStats";
 import type { UpdateAllSavedataRequest } from "./models/UpdateAllSavedata";
+import type { UpdateSessionSavedataRequest } from "./models/UpdateSessionSavedata";
+import type { UpdateSystemSavedataRequest } from "./models/UpdateSystemSavedata";
 import type { VerifySavedataResponse } from "./models/VerifySavedata";
 
 type DataType = "json" | "form-urlencoded";
@@ -66,10 +68,7 @@ export class Api {
     try {
       const response = await this.doPost<AccountLoginRequest>(
         "/account/login",
-        {
-          username,
-          password,
-        },
+        { username, password },
         "form-urlencoded"
       );
 
@@ -186,17 +185,17 @@ export class Api {
 
   /**
    * Update a system savedata.
-   * @param clientSessionId The savedata session ID
+   * @param updateData The {@linkcode UpdateSystemSavedataRequest} to send
    * @param rawSystemData The raw {@linkcode SystemSaveData}
    * @returns an error message if something went wrong
    */
-  public async updateSystemSavedata(clientSessionId: string, rawSystemData: string) {
+  public async updateSystemSavedata(updateData: UpdateSystemSavedataRequest, rawSystemData: string) {
     try {
-      const params = new URLSearchParams();
-      params.append("clientSessionId", clientSessionId);
+      const updateArr = Object.entries(updateData).map(([key, value]) => [key, String(value)]);
+      const params = new URLSearchParams(updateArr);
       const response = await this.doPost<string>(`/savedata/system/update?${params}`, rawSystemData);
 
-      return (await response.json()) as string;
+      return await response.text();
     } catch (err) {
       console.warn("Could not update system savedata!", err);
     }
@@ -225,6 +224,27 @@ export class Api {
     }
   }
 
+  /**
+   * Update a session savedata.
+   * @param updateData The {@linkcode UpdateSessionSavedataRequest} to send
+   * @param rawSavedata The raw savedata (as `string`)
+   * @returns an error message if something went wrong
+   */
+  public async updateSessionSavedata(updateData: UpdateSessionSavedataRequest, rawSavedata: string) {
+    try {
+      const updateArr = Object.entries(updateData).map(([key, value]) => [key, String(value)]);
+      const params = new URLSearchParams(updateArr);
+
+      const response = await this.doPost<string>(`/savedata/session/update?${params}`, rawSavedata);
+
+      return await response.text();
+    } catch (err) {
+      console.warn("Could not update session savedata!", err);
+    }
+
+    return null;
+  }
+
   /**
    * Delete a session savedata slot.
    * @param slotId The slot ID to load
diff --git a/src/plugins/api/models/UpdateSessionSavedata.ts b/src/plugins/api/models/UpdateSessionSavedata.ts
new file mode 100644
index 00000000000..8e450e40f6c
--- /dev/null
+++ b/src/plugins/api/models/UpdateSessionSavedata.ts
@@ -0,0 +1,6 @@
+export class UpdateSessionSavedataRequest {
+  slot: number;
+  trainerId: number;
+  secretId: number;
+  clientSessionId: string;
+}
diff --git a/src/plugins/api/models/UpdateSystemSavedata.ts b/src/plugins/api/models/UpdateSystemSavedata.ts
new file mode 100644
index 00000000000..ebf74acfc7e
--- /dev/null
+++ b/src/plugins/api/models/UpdateSystemSavedata.ts
@@ -0,0 +1,5 @@
+export class UpdateSystemSavedataRequest {
+  clientSessionId: string;
+  trainerId?: number;
+  secretId?: number;
+}
diff --git a/src/system/game-data.ts b/src/system/game-data.ts
index e29a509d0c7..73984d18682 100644
--- a/src/system/game-data.ts
+++ b/src/system/game-data.ts
@@ -397,7 +397,7 @@ export class GameData {
       localStorage.setItem(`data_${loggedInUser?.username}`, encrypt(systemData, bypassLogin));
 
       if (!bypassLogin) {
-        api.updateSystemSavedata(clientSessionId, systemData)
+        api.updateSystemSavedata({clientSessionId}, systemData)
           .then(error => {
             this.scene.ui.savingIcon.hide();
             if (error) {
@@ -1454,14 +1454,14 @@ export class GameData {
                     if (!success[0]) {
                       return displayError(`Could not contact the server. Your ${dataName} data could not be imported.`);
                     }
-                    let url: string;
+                    const { trainerId, secretId } = this;
+                    let updatePromise: Promise<string | null>;
                     if (dataType === GameDataType.SESSION) {
-                      url = `savedata/session/update?slot=${slotId}&trainerId=${this.trainerId}&secretId=${this.secretId}&clientSessionId=${clientSessionId}`;
+                      updatePromise = api.updateSessionSavedata({slot: slotId, trainerId, secretId, clientSessionId}, dataStr);
                     } else {
-                      url = `savedata/system/update?trainerId=${this.trainerId}&secretId=${this.secretId}&clientSessionId=${clientSessionId}`;
+                      updatePromise = api.updateSystemSavedata({trainerId, secretId, clientSessionId}, dataStr);
                     }
-                    Utils.apiPost(url, dataStr, undefined, true)
-                      .then(response => response.text())
+                    updatePromise
                       .then(error => {
                         if (error) {
                           console.error(error);