diff --git a/backend/src/services/contractService.js b/backend/src/services/contractService.js
index 4eae5f6f..e443b4ef 100644
--- a/backend/src/services/contractService.js
+++ b/backend/src/services/contractService.js
@@ -765,7 +765,9 @@ async function createContract(payload, adminId) {
   const hasEventCols = await hasColumnCached('contracts', 'event_name');
 
   return await db.transaction(async (trx) => {
-    const contractNumber = await nextContractNumber();
+    // Pass trx so the sequence claim joins our outer transaction —
+    // SQLite deadlocks otherwise (1-connection default).
+    const contractNumber = await nextContractNumber(trx);
     const row = {
       contract_number: contractNumber,
       customer_account_id: payload.customerAccountId,
@@ -1594,7 +1596,9 @@ async function createFromQuote(quoteId, adminId) {
   const hasContractEventCols = await hasColumnCached('contracts', 'event_name');
 
   return await db.transaction(async (trx) => {
-    const contractNumber = await nextContractNumber();
+    // Pass trx so the sequence claim joins our outer transaction —
+    // SQLite deadlocks otherwise (1-connection default).
+    const contractNumber = await nextContractNumber(trx);
     const contractRow = {
       contract_number: contractNumber,
       customer_account_id: quote.customer_account_id,
diff --git a/backend/src/services/invoiceService.js b/backend/src/services/invoiceService.js
index c50fe62f..ab02c247 100644
--- a/backend/src/services/invoiceService.js
+++ b/backend/src/services/invoiceService.js
@@ -812,7 +812,9 @@ async function createInvoice(payload, adminId, trx = db) {
   // has been ruled out. Previously this was at the top of the function
   // which leaked one number per multi-installment save (the spawner
   // claims its own numbers and never used this one).
-  const invoiceNumber = await nextInvoiceNumber();
+  // Pass trx so the sequence claim joins our outer transaction —
+  // SQLite deadlocks otherwise (1-connection default).
+  const invoiceNumber = await nextInvoiceNumber(trx);
   const row = {
     invoice_number: invoiceNumber,
     customer_account_id: payload.customerAccountId,
@@ -976,7 +978,7 @@ async function spawnInstallmentInvoices({ trx, eventId, quoteId, customer, curre
     const rowStatus = isDeliveryTrigger ? 'pending_delivery' : 'scheduled';
     const rowScheduledSendAt = isDeliveryTrigger ? null : scheduledSendAt;
 
-    const invoiceNumber = await nextInvoiceNumber();
+    const invoiceNumber = await nextInvoiceNumber(trx);
     const dueDate = computeDueDate(scheduledSendAt, resolvedNetDays).toISOString().slice(0, 10);
 
     const row = {
@@ -2049,7 +2051,9 @@ async function createStorno(originalId, adminId, trx = db) {
   // Generate the Storno's sequence number from the same gap-free
   // series as regular invoices (single sequence — decision locked
   // with the maintainer; satisfies §14 (4) Nr. 4 UStG).
-  const stornoNumber = await nextInvoiceNumber();
+  // Pass trx so the sequence claim joins the caller's transaction —
+  // SQLite deadlocks otherwise (1-connection default).
+  const stornoNumber = await nextInvoiceNumber(trx);
   const now = new Date();
   const issueDate = now.toISOString().slice(0, 10);
 
diff --git a/backend/src/services/quoteService.js b/backend/src/services/quoteService.js
index b93c7528..0c993ca0 100644
--- a/backend/src/services/quoteService.js
+++ b/backend/src/services/quoteService.js
@@ -491,7 +491,11 @@ async function createQuote(payload, adminId) {
   const bank = await businessProfileService.resolveBankAccountForCurrency(currency, payload.businessBankAccountId);
 
   return await db.transaction(async (trx) => {
-    const quoteNumber = await nextQuoteNumber();
+    // SQLite's 1-connection default deadlocks when claimNextSequence
+    // opens its own micro-transaction inside this outer one — thread
+    // trx so both run on the same connection. Postgres tolerates
+    // either form but the consistency is worth it.
+    const quoteNumber = await nextQuoteNumber(trx);
     const row = {
       quote_number: quoteNumber,
       customer_account_id: payload.customerAccountId,