diff --git a/backend/src/mcp/tools/accounting.ts b/backend/src/mcp/tools/accounting.ts
index 807f9e9..22a6b43 100644
--- a/backend/src/mcp/tools/accounting.ts
+++ b/backend/src/mcp/tools/accounting.ts
@@ -78,7 +78,6 @@ const buildOutgoingDocumentPayload = (
 
     const stringFields = [
         "state",
-        "documentNumber",
         "documentDate",
         "deliveryDate",
         "deliveryDateEnd",
@@ -118,8 +117,11 @@ const buildOutgoingDocumentPayload = (
     return payload
 }
 
-const shouldAssignDocumentNumber = (args: Record<string, unknown>, state?: unknown) =>
-    args.assignDocumentNumber === true || (state && state !== "Entwurf" && !stringArg(args, "documentNumber"))
+const assertNoManualDocumentNumber = (args: Record<string, unknown>) => {
+    if (args.documentNumber !== undefined || args.assignDocumentNumber !== undefined) {
+        throw new Error("Belegnummern werden nur beim Finalisieren vergeben")
+    }
+}
 
 export const accountingTools: McpTool[] = [
     {
@@ -193,21 +195,18 @@ export const accountingTools: McpTool[] = [
     {
         name: "accounting.outgoing_documents.create",
         title: "Ausgangsbeleg erstellen",
-        description: "Erstellt einen Ausgangsbeleg im aktiven Mandanten. Ohne assignDocumentNumber bleibt der Beleg als Entwurf ohne neue Nummer möglich.",
+        description: "Erstellt einen Ausgangsbeleg-Entwurf im aktiven Mandanten. Belegnummern werden erst beim Finalisieren vergeben.",
         requiredPermissions: ["accounting.outgoing_documents.write"],
         inputSchema: {
             type: "object",
             required: ["type"],
             properties: {
                 type: { type: "string" },
-                state: { type: "string", default: "Entwurf" },
-                assignDocumentNumber: { type: "boolean", default: false },
                 customer: { type: "number" },
                 contact: { type: "number" },
                 contract: { type: "number" },
                 project: { type: "number" },
                 plant: { type: "number" },
-                documentNumber: { type: "string" },
                 documentDate: { type: "string" },
                 deliveryDate: { type: "string" },
                 deliveryDateEnd: { type: "string" },
@@ -228,12 +227,9 @@ export const accountingTools: McpTool[] = [
             },
         },
         async handler(context, args) {
+            assertNoManualDocumentNumber(args)
             const payload = buildOutgoingDocumentPayload(args, context.userId, context.tenantId, true)
-
-            if (shouldAssignDocumentNumber(args, payload.state) && !payload.documentNumber) {
-                const result = await useNextNumberRangeNumber(context.server, context.tenantId, String(payload.type))
-                payload.documentNumber = result.usedNumber
-            }
+            payload.state = "Entwurf"
 
             const [created] = await context.server.db
                 .insert(createddocuments)
@@ -246,7 +242,7 @@ export const accountingTools: McpTool[] = [
     {
         name: "accounting.outgoing_documents.update",
         title: "Ausgangsbeleg aktualisieren",
-        description: "Aktualisiert einen Ausgangsbeleg im aktiven Mandanten.",
+        description: "Aktualisiert einen Ausgangsbeleg im aktiven Mandanten, solange er noch nicht finalisiert ist.",
         requiredPermissions: ["accounting.outgoing_documents.write"],
         inputSchema: {
             type: "object",
@@ -255,13 +251,11 @@ export const accountingTools: McpTool[] = [
                 id: { type: "number" },
                 type: { type: "string" },
                 state: { type: "string" },
-                assignDocumentNumber: { type: "boolean", default: false },
                 customer: { type: "number" },
                 contact: { type: "number" },
                 contract: { type: "number" },
                 project: { type: "number" },
                 plant: { type: "number" },
-                documentNumber: { type: "string" },
                 documentDate: { type: "string" },
                 deliveryDate: { type: "string" },
                 deliveryDateEnd: { type: "string" },
@@ -284,6 +278,7 @@ export const accountingTools: McpTool[] = [
         async handler(context, args) {
             const id = numberArg(args, "id")
             if (!id) throw new Error("id ist erforderlich")
+            assertNoManualDocumentNumber(args)
 
             const [existing] = await context.server.db
                 .select()
@@ -292,14 +287,60 @@ export const accountingTools: McpTool[] = [
                 .limit(1)
 
             if (!existing) throw new Error("Ausgangsbeleg nicht gefunden")
+            if (existing.state !== "Entwurf") throw new Error("Finalisierte Ausgangsbelege können nicht über update geändert werden")
 
             const payload = buildOutgoingDocumentPayload(args, context.userId, context.tenantId)
-            const nextType = String(payload.type || existing.type)
+            payload.state = "Entwurf"
 
-            if (shouldAssignDocumentNumber(args, payload.state || existing.state) && !payload.documentNumber && !existing.documentNumber) {
-                const result = await useNextNumberRangeNumber(context.server, context.tenantId, nextType)
-                payload.documentNumber = result.usedNumber
-            }
+            const [updated] = await context.server.db
+                .update(createddocuments)
+                .set(payload as any)
+                .where(and(eq(createddocuments.id, id), eq(createddocuments.tenant, context.tenantId)))
+                .returning()
+
+            return { document: updated }
+        },
+    },
+    {
+        name: "accounting.outgoing_documents.finalize",
+        title: "Ausgangsbeleg finalisieren",
+        description: "Finalisiert einen Ausgangsbeleg, setzt den Status auf Gebucht und vergibt dabei genau einmal eine Belegnummer.",
+        requiredPermissions: ["accounting.outgoing_documents.write"],
+        inputSchema: {
+            type: "object",
+            required: ["id"],
+            properties: {
+                id: { type: "number" },
+                documentDate: { type: "string" },
+                deliveryDate: { type: "string" },
+                deliveryDateEnd: { type: "string" },
+                paymentDays: { type: "number" },
+                payment_type: { type: "string" },
+                taxType: { type: "string" },
+                rows: { type: "array" },
+                report: { type: "object" },
+                availableInPortal: { type: "boolean" },
+            },
+        },
+        async handler(context, args) {
+            const id = numberArg(args, "id")
+            if (!id) throw new Error("id ist erforderlich")
+            assertNoManualDocumentNumber(args)
+
+            const [existing] = await context.server.db
+                .select()
+                .from(createddocuments)
+                .where(and(eq(createddocuments.id, id), eq(createddocuments.tenant, context.tenantId)))
+                .limit(1)
+
+            if (!existing) throw new Error("Ausgangsbeleg nicht gefunden")
+            if (existing.documentNumber) throw new Error("Ausgangsbeleg wurde bereits finalisiert")
+
+            const payload = buildOutgoingDocumentPayload(args, context.userId, context.tenantId)
+            const result = await useNextNumberRangeNumber(context.server, context.tenantId, existing.type)
+
+            payload.state = "Gebucht"
+            payload.documentNumber = result.usedNumber
 
             const [updated] = await context.server.db
                 .update(createddocuments)