diff --git a/src/app/api/quickbooks/invoice/invoice.service.ts b/src/app/api/quickbooks/invoice/invoice.service.ts
index fd69504f..91d71718 100644
--- a/src/app/api/quickbooks/invoice/invoice.service.ts
+++ b/src/app/api/quickbooks/invoice/invoice.service.ts
@@ -550,7 +550,7 @@ export class InvoiceService extends BaseService {
   /**
    * Pre-flights QBO for invoices whose DocNumber starts with the Assembly
    * invoice number and returns the lowest free slot (`<n>`, `<n>-1`, …).
-   * Used by webhookInvoiceCreated to dodge 6240 collisions when a customer
+   * Used by webhookInvoiceCreated to dodge 6140 collisions when a customer
    * has already created an invoice with the same DocNumber in QBO manually.
    */
   private async resolveAvailableDocNumber(
@@ -771,7 +771,7 @@ export class InvoiceService extends BaseService {
       customer?.Id || existingCustomer?.qbCustomerId
 
     // Resolve a DocNumber that won't collide in QBO. Pre-flight a prefix
-    // query, pick the lowest free slot (`<n>`, `<n>-1`, `<n>-2`, …). On 6240
+    // query, pick the lowest free slot (`<n>`, `<n>-1`, `<n>-2`, …). On 6140
     // race (customer manually created the slot we picked between our query
     // and our create), re-walk once and retry. After that, throw and let
     // resync handle it.
@@ -820,13 +820,13 @@ export class InvoiceService extends BaseService {
     } catch (err) {
       if (!isQBODuplicateDocNumberError(err)) throw err
       console.info(
-        `InvoiceService#webhookInvoiceCreated | 6240 on DocNumber=${docNumber}; re-walking once`,
+        `InvoiceService#webhookInvoiceCreated | 6140 on DocNumber=${docNumber}; re-walking once`,
       )
       docNumber = await this.resolveAvailableDocNumber(
         intuitApiService,
         assemblyInvoiceNumber,
       )
-      addSyncBreadcrumb('Retrying invoice creation in QBO after 6240', {
+      addSyncBreadcrumb('Retrying invoice creation in QBO after 6140', {
         invoiceNumber: assemblyInvoiceNumber,
         docNumber,
       })
diff --git a/src/app/api/quickbooks/invoice/invoice.utils.ts b/src/app/api/quickbooks/invoice/invoice.utils.ts
index 3fe4bf8d..8fd7c3e1 100644
--- a/src/app/api/quickbooks/invoice/invoice.utils.ts
+++ b/src/app/api/quickbooks/invoice/invoice.utils.ts
@@ -1,3 +1,5 @@
+import { QBOErrorCodes } from '@/constant/intuitErrorCode'
+
 export const formatAssemblyInvoicePrivateNote = (
   invoiceNumber: string,
 ): string => `Assembly invoice: ${invoiceNumber}`
@@ -39,8 +41,17 @@ export const findNextAvailableDocNumber = (
   )
 }
 
+const DUP_DOC_NUMBER_CODE = QBOErrorCodes.DUPLICATE_DOC_NUMBER
+// QBOErrorCode is a numeric literal union, so the interpolation below has
+// no regex-special characters and needs no escaping. If that type ever
+// widens to a string, escape the interpolation before using it here.
+const DUP_DOC_NUMBER_PATTERN = new RegExp(
+  `${DUP_DOC_NUMBER_CODE}|Duplicate Document Number`,
+  'i', // case insensitive
+)
+
 /**
- * Recognizes QBO Error 6240 "Duplicate Document Number". Reads `.status`,
+ * Recognizes QBO's Duplicate Document Number error. Reads `.status`,
  * `.code`, `errors[].code`, and message text so it works whether the caller
  * surfaces the parsed APIError directly or wraps/normalises it.
  */
@@ -60,16 +71,19 @@ export const isQBODuplicateDocNumberError = (err: unknown): boolean => {
         Detail?: string
         Message?: string
       }
-      if (fault.code === 6240 || fault.code === '6240') return true
+      if (Number(fault.code) === DUP_DOC_NUMBER_CODE) return true
       if (
-        /6240|Duplicate Document Number/i.test(fault.Detail ?? '') ||
-        /6240|Duplicate Document Number/i.test(fault.Message ?? '')
+        DUP_DOC_NUMBER_PATTERN.test(fault.Detail ?? '') ||
+        DUP_DOC_NUMBER_PATTERN.test(fault.Message ?? '')
       ) {
         return true
       }
     }
   }
-  if (e.status === 6240 || e.status === '6240') return true
-  if (e.code === 6240 || e.code === '6240') return true
-  return /6240|Duplicate Document Number/i.test(e.message ?? '')
+  if (
+    Number(e.status) === DUP_DOC_NUMBER_CODE ||
+    Number(e.code) === DUP_DOC_NUMBER_CODE
+  )
+    return true
+  return DUP_DOC_NUMBER_PATTERN.test(e.message ?? '')
 }
diff --git a/src/constant/intuitErrorCode.ts b/src/constant/intuitErrorCode.ts
index f142f737..db1718a8 100644
--- a/src/constant/intuitErrorCode.ts
+++ b/src/constant/intuitErrorCode.ts
@@ -1,7 +1,16 @@
 // Doc: https://developer.intuit.com/app/developer/qbo/docs/develop/troubleshooting/error-codes
-export const AccountErrorCodes = [
-  6190, // account suspended
-  6000, // business validation error
+export const QBOErrorCodes = {
+  BUSINESS_VALIDATION: 6000,
+  DUPLICATE_DOC_NUMBER: 6140,
+  ACCOUNT_SUSPENDED: 6190,
+  DUPLICATE_NAME_EXISTS: 6240, // customer/vendor/employee name collision
+} as const
+
+export type QBOErrorCode = (typeof QBOErrorCodes)[keyof typeof QBOErrorCodes]
+
+export const AccountErrorCodes: readonly number[] = [
+  QBOErrorCodes.ACCOUNT_SUSPENDED,
+  QBOErrorCodes.BUSINESS_VALIDATION,
 ]
 
 export const OAuthErrorCodes = {
diff --git a/src/utils/intuitAPI.ts b/src/utils/intuitAPI.ts
index 492292db..c8d3f5c2 100644
--- a/src/utils/intuitAPI.ts
+++ b/src/utils/intuitAPI.ts
@@ -68,7 +68,7 @@ export type IntuitAPITokensType = Pick<
 
 export const IntuitAPIErrorMessage = '#IntuitAPIErrorMessage#'
 
-// APIError.status carries the QBO fault code (e.g. 6240) when available,
+// APIError.status carries the QBO fault code (e.g. 6140) when available,
 // else BAD_REQUEST. withErrorHandler clamps non-HTTP codes downstream.
 export function assertNotQBFault(raw: unknown, opName: string): void {
   const result = QBFaultSchema.safeParse(raw)
@@ -734,7 +734,7 @@ export default class IntuitAPI {
    * Returns all QBO invoices whose DocNumber starts with `prefix`. Used by
    * findNextAvailableDocNumber to detect collisions and pick the next free
    * suffix before createInvoice. Caps at maxresults=100; if a single prefix
-   * has more matches, the caller falls back to catch-6240 retry semantics.
+   * has more matches, the caller falls back to catch-6140 retry semantics.
    */
   async _findInvoicesByDocNumberPrefix(
     prefix: string,
diff --git a/test/integration/quickbooks/invoiceCreated/qboDocNumberCollision.test.ts b/test/integration/quickbooks/invoiceCreated/qboDocNumberCollision.test.ts
index 64af9013..608eee3b 100644
--- a/test/integration/quickbooks/invoiceCreated/qboDocNumberCollision.test.ts
+++ b/test/integration/quickbooks/invoiceCreated/qboDocNumberCollision.test.ts
@@ -2,6 +2,7 @@ import { describe, it, expect, vi } from 'vitest'
 
 import { db } from '@/db'
 import { QBInvoiceSync } from '@/db/schema/qbInvoiceSync'
+import { QBOErrorCodes } from '@/constant/intuitErrorCode'
 
 import invoiceCreatedPayload from '@test/fixtures/invoiceCreated.webhook'
 import {
@@ -68,7 +69,12 @@ describe('POST /api/quickbooks/webhook — invoice.created (an invoice with this
       const duplicateError = Object.assign(
         new Error('Duplicate Document Number'),
         {
-          errors: [{ code: '6240', Detail: 'Duplicate Document Number error' }],
+          errors: [
+            {
+              code: String(QBOErrorCodes.DUPLICATE_DOC_NUMBER),
+              Detail: 'Duplicate Document Number error',
+            },
+          ],
         },
       )
       const createInvoice = vi
diff --git a/test/unit/app/api/quickbooks/invoice/invoice.service.docNumber.test.ts b/test/unit/app/api/quickbooks/invoice/invoice.service.docNumber.test.ts
index 0bcafa06..a9b20aef 100644
--- a/test/unit/app/api/quickbooks/invoice/invoice.service.docNumber.test.ts
+++ b/test/unit/app/api/quickbooks/invoice/invoice.service.docNumber.test.ts
@@ -1,7 +1,7 @@
 /**
  * Unit coverage for InvoiceService#resolveAvailableDocNumber — the pre-flight
- * helper used by webhookInvoiceCreated to dodge 6240 collisions. The public
- * retry-after-6240 branch is covered end-to-end in
+ * helper used by webhookInvoiceCreated to dodge 6140 collisions. The public
+ * retry-after-6140 branch is covered end-to-end in
  * test/integration/quickbooks/invoiceCreated/qboDocNumberCollision.test.ts;
  * this file pins the focused pieces in isolation.
  *
diff --git a/test/unit/app/api/quickbooks/invoice/invoice.utils.test.ts b/test/unit/app/api/quickbooks/invoice/invoice.utils.test.ts
index 3c27128c..e01e3058 100644
--- a/test/unit/app/api/quickbooks/invoice/invoice.utils.test.ts
+++ b/test/unit/app/api/quickbooks/invoice/invoice.utils.test.ts
@@ -5,6 +5,10 @@ import {
   isQBODuplicateDocNumberError,
   MAX_SUFFIX_ATTEMPTS,
 } from '@/app/api/quickbooks/invoice/invoice.utils'
+import { QBOErrorCodes } from '@/constant/intuitErrorCode'
+
+const DUP_DOC_NUMBER = QBOErrorCodes.DUPLICATE_DOC_NUMBER
+const DUP_DOC_NUMBER_STR = String(DUP_DOC_NUMBER)
 
 describe('formatAssemblyInvoicePrivateNote', () => {
   it('formats an invoice number into the canonical PrivateNote string', () => {
@@ -80,14 +84,14 @@ describe('findNextAvailableDocNumber', () => {
 
 describe('isQBODuplicateDocNumberError', () => {
   it('matches the real APIError shape thrown by intuitAPI._createInvoice', () => {
-    // This is the actual shape: status=400, message=boilerplate, errors=array
-    // of QBO fault objects. The 6240 code lives in errors[i].code.
+    // Actual shape: status=400, message=boilerplate, errors=array of QBO
+    // fault objects. The duplicate code lives in errors[i].code.
     const realApiError = {
       status: 400,
       message: '#IntuitAPIErrorMessage#createInvoice',
       errors: [
         {
-          code: '6240',
+          code: DUP_DOC_NUMBER_STR,
           Message: 'Duplicate Document Number Error',
           Detail:
             'Duplicate Document Number Error : You must specify a different number. This number has already been used.',
@@ -102,7 +106,7 @@ describe('isQBODuplicateDocNumberError', () => {
     expect(
       isQBODuplicateDocNumberError({
         status: 400,
-        errors: [{ code: 6240 }],
+        errors: [{ code: DUP_DOC_NUMBER }],
       }),
     ).toBe(true)
   })
@@ -116,12 +120,14 @@ describe('isQBODuplicateDocNumberError', () => {
     ).toBe(true)
   })
 
-  it('matches top-level .status as 6240 (defense-in-depth)', () => {
-    expect(isQBODuplicateDocNumberError({ status: 6240 })).toBe(true)
+  it('matches the duplicate code at the top-level .status (defense-in-depth)', () => {
+    expect(isQBODuplicateDocNumberError({ status: DUP_DOC_NUMBER })).toBe(true)
   })
 
-  it('matches top-level .code as 6240 (defense-in-depth)', () => {
-    expect(isQBODuplicateDocNumberError({ code: '6240' })).toBe(true)
+  it('matches the duplicate code at the top-level .code (defense-in-depth)', () => {
+    expect(isQBODuplicateDocNumberError({ code: DUP_DOC_NUMBER_STR })).toBe(
+      true,
+    )
   })
 
   it('matches top-level .message text (defense-in-depth)', () => {