diff --git a/src/request-management/entities/schema/partyRole.enum.ts b/src/request-management/entities/schema/partyRole.enum.ts
index 2915653..5217e74 100644
--- a/src/request-management/entities/schema/partyRole.enum.ts
+++ b/src/request-management/entities/schema/partyRole.enum.ts
@@ -38,8 +38,8 @@ export class Person {
   @Prop({ type: Boolean })
   userNoCertificate?: boolean;
 
-  @Prop({ type: Number })
-  insurerBirthday?: number;
+  @Prop({ type: String })
+  insurerBirthday?: string;
 
   @Prop({ type: String })
   driverBirthday?: string | null;
@@ -90,7 +90,7 @@ export class Vehicle {
   @Prop() name?: string;
   @Prop() model?: string;
   @Prop() type?: string;
-  @Prop() isNew?: boolean;
+  @Prop({ type: Boolean }) isNewCar?: boolean;
   @Prop({ enum: ["CURRENT", "RECENTLY_TRANSFERRED"] })
   registrationState?: "CURRENT" | "RECENTLY_TRANSFERRED";
   @Prop() previousPlateId?: string;
diff --git a/src/request-management/inquiry-participant-persistence.spec.ts b/src/request-management/inquiry-participant-persistence.spec.ts
new file mode 100644
index 0000000..eaf64d9
--- /dev/null
+++ b/src/request-management/inquiry-participant-persistence.spec.ts
@@ -0,0 +1,118 @@
+import { model, Types } from "mongoose";
+import { BlameRequestType } from "src/Types&Enums/blame-request-management/blameRequestType.enum";
+import {
+  BlameRequest,
+  BlameRequestSchema,
+} from "./entities/schema/blame-cases.schema";
+import { PartyRole } from "./entities/schema/partyRole.enum";
+import { normalizeInquirySubmission } from "./inquiry-participant-resolver";
+
+describe("inquiry participant persistence", () => {
+  it("accepts slash-formatted Jalali birthdays from the structured payload", () => {
+    const submission = normalizeInquirySubmission(
+      BlameRequestType.THIRD_PARTY,
+      {
+        driver: {
+          nationalCode: "0022324224",
+          birthday: "1378/04/26",
+          hasDrivingLicense: true,
+          licenseNumber: "1001011111",
+          licenseType: "10",
+        },
+        vehicleOwner: {
+          nationalCode: "5560568341",
+          birthday: "1378/06/27",
+        },
+        thirdPartyPolicyholder: { sameAs: "VEHICLE_OWNER" },
+        vehicle: {
+          registrationState: "CURRENT",
+          currentPlate: {
+            leftDigits: "59",
+            centerAlphabet: "ی",
+            centerDigits: "419",
+            ir: "78",
+          },
+          isNewCar: false,
+        },
+      },
+    );
+    const BlameRequestModel = model<BlameRequest>(
+      "InquiryParticipantBirthdayPersistenceTest",
+      BlameRequestSchema,
+    );
+    const request = new BlameRequestModel({
+      requestNo: "BL-PARTICIPANT-BIRTHDAY",
+      publicId: "PARTICIPANT-BIRTHDAY",
+      type: BlameRequestType.THIRD_PARTY,
+      parties: [
+        {
+          role: PartyRole.FIRST,
+          person: {
+            insurerBirthday: submission.dto.insurerBirthday,
+            driverBirthday: submission.dto.driverBirthday,
+          },
+          participants: submission.participants.participants,
+          participantRoles: submission.participants.roles,
+        },
+      ],
+    });
+
+    expect(request.validateSync()).toBeUndefined();
+    expect(request.parties[0].person.insurerBirthday).toBe("1378/06/27");
+    expect(request.parties[0].person.driverBirthday).toBe("1378/04/26");
+  });
+
+  it("recasts populated party subdocuments during an inquiry-audit update", () => {
+    const BlameRequestModel = model<BlameRequest>(
+      "InquiryParticipantAuditPersistenceTest",
+      BlameRequestSchema,
+    );
+    const request = new BlameRequestModel({
+      requestNo: "BL-PARTICIPANT-AUDIT",
+      publicId: "PARTICIPANT-AUDIT",
+      type: BlameRequestType.THIRD_PARTY,
+      parties: [
+        {
+          role: PartyRole.FIRST,
+          person: {
+            userId: new Types.ObjectId(),
+            phoneNumber: "09120723528",
+          },
+          statement: {
+            acceptsExpertOpinion: false,
+            claimsDamage: false,
+            admitsGuilt: true,
+          },
+          participants: [
+            {
+              participantId: "DRIVER",
+              nationalCode: "0022324224",
+              birthday: "1378/04/26",
+              hasDrivingLicense: true,
+              licenseNumber: "1001011111",
+              licenseType: "10",
+            },
+          ],
+          participantRoles: {
+            driver: "DRIVER",
+            vehicleOwner: "DRIVER",
+            thirdPartyPolicyholder: "DRIVER",
+          },
+          vehicle: {
+            registrationState: "CURRENT",
+            plateId: "78-59-ی-419",
+          },
+        },
+      ],
+      inquiries: {},
+    });
+    const query = BlameRequestModel.findByIdAndUpdate(request._id, {
+      $set: {
+        inquiries: request.inquiries,
+        parties: request.parties,
+      },
+    });
+
+    expect(() => (query as any)._castUpdate(query.getUpdate())).not.toThrow();
+  });
+});
diff --git a/src/request-management/request-management.service.ts b/src/request-management/request-management.service.ts
index 7921b1a..a3f6c9c 100644
--- a/src/request-management/request-management.service.ts
+++ b/src/request-management/request-management.service.ts
@@ -1950,7 +1950,7 @@ export class RequestManagementService {
         (body.driverIsInsurer ? String(body.insurerBirthday) : null);
 
       if (!party.vehicle) party.vehicle = {} as any;
-      party.vehicle.isNew = body.isNewCar;
+      party.vehicle.isNewCar = body.isNewCar;
       party.vehicle.plateId =
         typeof body.plateId === "string"
           ? body.plateId
@@ -2382,7 +2382,7 @@ export class RequestManagementService {
         (body.driverIsInsurer ? String(body.insurerBirthday) : null);
 
       if (!party.vehicle) party.vehicle = {} as any;
-      party.vehicle.isNew = body.isNewCar;
+      party.vehicle.isNewCar = body.isNewCar;
       // Derive plateId from the plate parts returned by the VIN inquiry.
       // Plk2 may be a numeric letter code (e.g. 5 → "د") or already a Persian
       // letter string — resolvePlk2Letter handles both forms.
@@ -6633,7 +6633,7 @@ export class RequestManagementService {
           name: party.vehicle.name,
           model: party.vehicle.model,
           type: party.vehicle.type,
-          isNew: party.vehicle.isNew,
+          isNew: party.vehicle.isNewCar ?? (party.vehicle as any)?._doc?.isNew,
         }
       : undefined;
     const participants = Array.isArray(party.participants)
@@ -7134,7 +7134,7 @@ export class RequestManagementService {
         name: sandHubReport?.MapTypNam || sandHubReport?.CarName,
         model: sandHubReport?.MapTypNam,
         type: `${sandHubReport?.UsageField || ""} / ${sandHubReport?.MapUsageName || "-"}`,
-        isNew: firstPartyPlate.isNewCar,
+        isNewCar: firstPartyPlate.isNewCar,
         registrationState: inquirySubmission.vehicle?.registrationState,
         previousPlateId: inquirySubmission.vehicle?.previousPlate
           ? this.plateToPlateIdString(inquirySubmission.vehicle.previousPlate)
@@ -7429,7 +7429,7 @@ export class RequestManagementService {
           name: sandHubReport?.MapTypNam || sandHubReport?.CarName,
           model: sandHubReport?.MapTypNam,
           type: `${sandHubReport?.UsageField || ""} / ${sandHubReport?.MapUsageName || "-"}`,
-          isNew: plateDto.isNewCar,
+          isNewCar: plateDto.isNewCar,
           registrationState: inquirySubmission.vehicle?.registrationState,
           previousPlateId: inquirySubmission.vehicle?.previousPlate
             ? this.plateToPlateIdString(inquirySubmission.vehicle.previousPlate)