diff --git a/src/app/(dashboard)/[portSlug]/invoices/new/page.tsx b/src/app/(dashboard)/[portSlug]/invoices/new/page.tsx
index 605d3b1..3a9d9c0 100644
--- a/src/app/(dashboard)/[portSlug]/invoices/new/page.tsx
+++ b/src/app/(dashboard)/[portSlug]/invoices/new/page.tsx
@@ -55,7 +55,13 @@ export default function NewInvoicePage() {
     },
   });
 
-  const { register, handleSubmit, watch, setValue, formState: { errors } } = methods;
+  const {
+    register,
+    handleSubmit,
+    watch,
+    setValue,
+    formState: { errors },
+  } = methods;
 
   const watchedValues = watch();
   const lineItems = watchedValues.lineItems ?? [];
@@ -87,7 +93,7 @@ export default function NewInvoicePage() {
   async function goNext() {
     if (step === 1) {
       const valid = await methods.trigger([
-        'clientName',
+        'billingEntity',
         'billingEmail',
         'billingAddress',
         'dueDate',
@@ -112,11 +118,7 @@ export default function NewInvoicePage() {
     <div className="max-w-2xl mx-auto space-y-6">
       {/* Header */}
       <div className="flex items-center gap-3">
-        <Button
-          variant="ghost"
-          size="sm"
-          onClick={() => router.push(`/${portSlug}/invoices`)}
-        >
+        <Button variant="ghost" size="sm" onClick={() => router.push(`/${portSlug}/invoices`)}>
           <ChevronLeft className="h-4 w-4" />
         </Button>
         <h1 className="text-xl font-semibold">New Invoice</h1>
@@ -131,22 +133,16 @@ export default function NewInvoicePage() {
                 step > s.id
                   ? 'bg-primary text-primary-foreground'
                   : step === s.id
-                  ? 'bg-primary text-primary-foreground'
-                  : 'bg-muted text-muted-foreground'
+                    ? 'bg-primary text-primary-foreground'
+                    : 'bg-muted text-muted-foreground'
               }`}
             >
               {step > s.id ? <Check className="h-3.5 w-3.5" /> : s.id}
             </div>
-            <span
-              className={`text-sm ${
-                step === s.id ? 'font-medium' : 'text-muted-foreground'
-              }`}
-            >
+            <span className={`text-sm ${step === s.id ? 'font-medium' : 'text-muted-foreground'}`}>
               {s.label}
             </span>
-            {idx < STEPS.length - 1 && (
-              <div className="w-8 h-px bg-border mx-1" />
-            )}
+            {idx < STEPS.length - 1 && <div className="w-8 h-px bg-border mx-1" />}
           </div>
         ))}
       </div>
@@ -161,17 +157,36 @@ export default function NewInvoicePage() {
               </CardHeader>
               <CardContent className="space-y-4">
                 <div className="space-y-1">
-                  <Label htmlFor="clientName">
-                    Client Name <span className="text-destructive">*</span>
+                  <Label htmlFor="billingEntityType">
+                    Billing Entity <span className="text-destructive">*</span>
                   </Label>
-                  <Input
-                    id="clientName"
-                    {...register('clientName')}
-                    placeholder="Client or company name"
-                  />
-                  {errors.clientName && (
-                    <p className="text-xs text-destructive">{errors.clientName.message}</p>
+                  <div className="grid grid-cols-2 gap-2">
+                    <Select
+                      defaultValue="client"
+                      onValueChange={(v) =>
+                        setValue('billingEntity.type', v as 'client' | 'company')
+                      }
+                    >
+                      <SelectTrigger id="billingEntityType">
+                        <SelectValue placeholder="Type" />
+                      </SelectTrigger>
+                      <SelectContent>
+                        <SelectItem value="client">Client</SelectItem>
+                        <SelectItem value="company">Company</SelectItem>
+                      </SelectContent>
+                    </Select>
+                    <Input {...register('billingEntity.id')} placeholder="Entity ID" />
+                  </div>
+                  {errors.billingEntity && (
+                    <p className="text-xs text-destructive">
+                      {errors.billingEntity.message ??
+                        errors.billingEntity.id?.message ??
+                        errors.billingEntity.type?.message}
+                    </p>
                   )}
+                  <p className="text-xs text-muted-foreground">
+                    Picker UI is coming in Task 10.2 — for now paste the client or company ID.
+                  </p>
                 </div>
 
                 <div className="space-y-1">
@@ -202,11 +217,7 @@ export default function NewInvoicePage() {
                     <Label htmlFor="dueDate">
                       Due Date <span className="text-destructive">*</span>
                     </Label>
-                    <Input
-                      id="dueDate"
-                      type="date"
-                      {...register('dueDate')}
-                    />
+                    <Input id="dueDate" type="date" {...register('dueDate')} />
                     {errors.dueDate && (
                       <p className="text-xs text-destructive">{errors.dueDate.message}</p>
                     )}
@@ -216,7 +227,9 @@ export default function NewInvoicePage() {
                     <Label>Payment Terms</Label>
                     <Select
                       defaultValue="net30"
-                      onValueChange={(v) => setValue('paymentTerms', v as CreateInvoiceInput['paymentTerms'])}
+                      onValueChange={(v) =>
+                        setValue('paymentTerms', v as CreateInvoiceInput['paymentTerms'])
+                      }
                     >
                       <SelectTrigger>
                         <SelectValue placeholder="Select terms" />
@@ -284,8 +297,10 @@ export default function NewInvoicePage() {
               <CardContent className="space-y-4">
                 <div className="grid grid-cols-2 gap-4 text-sm">
                   <div>
-                    <span className="text-muted-foreground">Client</span>
-                    <p className="font-medium mt-0.5">{watchedValues.clientName}</p>
+                    <span className="text-muted-foreground">Billing Entity</span>
+                    <p className="font-medium mt-0.5">
+                      {watchedValues.billingEntity?.type}: {watchedValues.billingEntity?.id}
+                    </p>
                   </div>
                   <div>
                     <span className="text-muted-foreground">Due Date</span>
@@ -293,9 +308,7 @@ export default function NewInvoicePage() {
                   </div>
                   <div>
                     <span className="text-muted-foreground">Payment Terms</span>
-                    <p className="font-medium mt-0.5 capitalize">
-                      {watchedValues.paymentTerms}
-                    </p>
+                    <p className="font-medium mt-0.5 capitalize">{watchedValues.paymentTerms}</p>
                   </div>
                   <div>
                     <span className="text-muted-foreground">Currency</span>
@@ -354,12 +367,7 @@ export default function NewInvoicePage() {
 
           {/* Navigation */}
           <div className="flex items-center justify-between">
-            <Button
-              type="button"
-              variant="outline"
-              onClick={goBack}
-              disabled={step === 1}
-            >
+            <Button type="button" variant="outline" onClick={goBack} disabled={step === 1}>
               <ChevronLeft className="mr-1.5 h-4 w-4" />
               Back
             </Button>
diff --git a/src/lib/services/invoices.ts b/src/lib/services/invoices.ts
index 766967b..7bc2a2e 100644
--- a/src/lib/services/invoices.ts
+++ b/src/lib/services/invoices.ts
@@ -2,20 +2,17 @@ import { eq, and, desc, like, lt, sql, gte, lte, inArray, ne } from 'drizzle-orm
 import type { PgColumn } from 'drizzle-orm/pg-core';
 
 import { db } from '@/lib/db';
-import {
-  invoices,
-  invoiceLineItems,
-  invoiceExpenses,
-  expenses,
-} from '@/lib/db/schema/financial';
+import { invoices, invoiceLineItems, invoiceExpenses, expenses } from '@/lib/db/schema/financial';
 import { files } from '@/lib/db/schema/documents';
 import { ports } from '@/lib/db/schema/ports';
 import { systemSettings } from '@/lib/db/schema/system';
+import { clients, clientAddresses } from '@/lib/db/schema/clients';
+import { companies, companyAddresses } from '@/lib/db/schema/companies';
 import { buildListQuery } from '@/lib/db/query-builder';
 import { createAuditLog } from '@/lib/audit';
 import { diffEntity } from '@/lib/entity-diff';
 import { withTransaction } from '@/lib/db/utils';
-import { NotFoundError, ConflictError } from '@/lib/errors';
+import { NotFoundError, ConflictError, ValidationError } from '@/lib/errors';
 import { emitToRoom } from '@/lib/socket/server';
 import { logger } from '@/lib/logger';
 import { generatePdf } from '@/lib/pdf/generate';
@@ -50,9 +47,7 @@ async function generateInvoiceNumber(portId: string, tx: typeof db): Promise<str
   const [existing] = await tx
     .select({ invoiceNumber: invoices.invoiceNumber })
     .from(invoices)
-    .where(
-      and(eq(invoices.portId, portId), like(invoices.invoiceNumber, `${prefix}-%`)),
-    )
+    .where(and(eq(invoices.portId, portId), like(invoices.invoiceNumber, `${prefix}-%`)))
     .orderBy(desc(invoices.invoiceNumber))
     .limit(1);
 
@@ -64,6 +59,88 @@ async function generateInvoiceNumber(portId: string, tx: typeof db): Promise<str
   return `${prefix}-${String(seq).padStart(3, '0')}`;
 }
 
+// ─── Resolve billing entity (polymorphic client | company) ────────────────
+
+/**
+ * Look up the billing entity referenced on invoice create and derive the
+ * display name + fallback billing email/address. Scoped to the tenant (portId);
+ * throws ValidationError on missing / cross-tenant lookups.
+ *
+ * Runs inside the caller's transaction so the lookup is consistent with the
+ * rest of the create operation.
+ */
+async function resolveBillingEntity(
+  tx: typeof db,
+  portId: string,
+  entity: { type: 'client' | 'company'; id: string },
+): Promise<{
+  clientName: string;
+  billingEmail: string | null;
+  billingAddress: string | null;
+}> {
+  if (entity.type === 'client') {
+    const client = await tx.query.clients.findFirst({
+      where: and(eq(clients.id, entity.id), eq(clients.portId, portId)),
+      with: {
+        contacts: true,
+      },
+    });
+    if (!client) throw new ValidationError('billing entity (client) not found');
+
+    // Prefer primary email contact, fall back to any email contact
+    const emailContact =
+      client.contacts?.find((c) => c.channel === 'email' && c.isPrimary) ??
+      client.contacts?.find((c) => c.channel === 'email');
+
+    const addressRow = await tx.query.clientAddresses.findFirst({
+      where: and(eq(clientAddresses.clientId, client.id), eq(clientAddresses.isPrimary, true)),
+    });
+    const billingAddress = addressRow
+      ? [
+          addressRow.streetAddress,
+          addressRow.city,
+          addressRow.stateProvince,
+          addressRow.postalCode,
+          addressRow.country,
+        ]
+          .filter(Boolean)
+          .join(', ')
+      : null;
+
+    return {
+      clientName: client.fullName,
+      billingEmail: emailContact?.value ?? null,
+      billingAddress: billingAddress || null,
+    };
+  }
+
+  const company = await tx.query.companies.findFirst({
+    where: and(eq(companies.id, entity.id), eq(companies.portId, portId)),
+  });
+  if (!company) throw new ValidationError('billing entity (company) not found');
+
+  const addressRow = await tx.query.companyAddresses.findFirst({
+    where: and(eq(companyAddresses.companyId, company.id), eq(companyAddresses.isPrimary, true)),
+  });
+  const billingAddress = addressRow
+    ? [
+        addressRow.streetAddress,
+        addressRow.city,
+        addressRow.stateProvince,
+        addressRow.postalCode,
+        addressRow.country,
+      ]
+        .filter(Boolean)
+        .join(', ')
+    : null;
+
+  return {
+    clientName: company.name,
+    billingEmail: company.billingEmail ?? null,
+    billingAddress: billingAddress || null,
+  };
+}
+
 // ─── List ─────────────────────────────────────────────────────────────────
 
 export async function listInvoices(portId: string, query: ListInvoicesInput) {
@@ -81,6 +158,12 @@ export async function listInvoices(portId: string, query: ListInvoicesInput) {
   if (query.dateTo) {
     filters.push(lte(invoices.dueDate, query.dateTo));
   }
+  if (query.billingEntityType) {
+    filters.push(eq(invoices.billingEntityType, query.billingEntityType));
+  }
+  if (query.billingEntityId) {
+    filters.push(eq(invoices.billingEntityId, query.billingEntityId));
+  }
 
   return buildListQuery({
     table: invoices,
@@ -139,14 +222,20 @@ export async function createInvoice(
   meta: ServiceAuditMeta,
 ) {
   const invoice = await withTransaction(async (tx) => {
+    // Resolve the polymorphic billing entity (client | company). Throws
+    // ValidationError if the entity is missing or belongs to another tenant.
+    const entitySnapshot = await resolveBillingEntity(tx, portId, data.billingEntity);
+    // clientName is always entity-derived on create (it's a snapshot).
+    // Caller-supplied billingEmail/billingAddress win over entity-derived values.
+    const effectiveClientName = entitySnapshot.clientName;
+    const effectiveBillingEmail = data.billingEmail ?? entitySnapshot.billingEmail;
+    const effectiveBillingAddress = data.billingAddress ?? entitySnapshot.billingAddress;
+
     const invoiceNumber = await generateInvoiceNumber(portId, tx);
 
     // Calculate subtotal from line items
     const lineItemsData = data.lineItems ?? [];
-    const subtotal = lineItemsData.reduce(
-      (sum, li) => sum + li.quantity * li.unitPrice,
-      0,
-    );
+    const subtotal = lineItemsData.reduce((sum, li) => sum + li.quantity * li.unitPrice, 0);
 
     // BR-042: net10 discount — read from systemSettings
     let discountPct = 0;
@@ -155,10 +244,7 @@ export async function createInvoice(
         .select({ value: systemSettings.value })
         .from(systemSettings)
         .where(
-          and(
-            eq(systemSettings.key, 'invoice_net10_discount'),
-            eq(systemSettings.portId, portId),
-          ),
+          and(eq(systemSettings.key, 'invoice_net10_discount'), eq(systemSettings.portId, portId)),
         )
         .limit(1);
 
@@ -182,17 +268,12 @@ export async function createInvoice(
         .from(invoiceExpenses)
         .innerJoin(invoices, eq(invoices.id, invoiceExpenses.invoiceId))
         .where(
-          and(
-            inArray(invoiceExpenses.expenseId, expenseIds),
-            sql`${invoices.status} != 'draft'`,
-          ),
+          and(inArray(invoiceExpenses.expenseId, expenseIds), sql`${invoices.status} != 'draft'`),
         )
         .limit(1);
 
       if (alreadyLinked.length > 0) {
-        throw new ConflictError(
-          'One or more expenses are already linked to a non-draft invoice',
-        );
+        throw new ConflictError('One or more expenses are already linked to a non-draft invoice');
       }
     }
 
@@ -201,9 +282,11 @@ export async function createInvoice(
       .values({
         portId,
         invoiceNumber,
-        clientName: data.clientName,
-        billingEmail: data.billingEmail ?? null,
-        billingAddress: data.billingAddress ?? null,
+        billingEntityType: data.billingEntity.type,
+        billingEntityId: data.billingEntity.id,
+        clientName: effectiveClientName,
+        billingEmail: effectiveBillingEmail,
+        billingAddress: effectiveBillingAddress,
         dueDate: data.dueDate,
         paymentTerms: data.paymentTerms ?? 'net30',
         currency: data.currency ?? 'USD',
@@ -297,10 +380,7 @@ export async function updateInvoice(
     // Recalculate totals if line items changed
     if (data.lineItems !== undefined) {
       const lineItemsData = data.lineItems;
-      const subtotal = lineItemsData.reduce(
-        (sum, li) => sum + li.quantity * li.unitPrice,
-        0,
-      );
+      const subtotal = lineItemsData.reduce((sum, li) => sum + li.quantity * li.unitPrice, 0);
 
       const paymentTerms = data.paymentTerms ?? existing.paymentTerms;
       let discountPct = 0;
@@ -364,17 +444,15 @@ export async function updateInvoice(
           .limit(1);
 
         if (alreadyLinked.length > 0) {
-          throw new ConflictError(
-            'One or more expenses are already linked to a non-draft invoice',
-          );
+          throw new ConflictError('One or more expenses are already linked to a non-draft invoice');
         }
       }
 
       await tx.delete(invoiceExpenses).where(eq(invoiceExpenses.invoiceId, id));
       if (data.expenseIds.length > 0) {
-        await tx.insert(invoiceExpenses).values(
-          data.expenseIds.map((expenseId) => ({ invoiceId: id, expenseId })),
-        );
+        await tx
+          .insert(invoiceExpenses)
+          .values(data.expenseIds.map((expenseId) => ({ invoiceId: id, expenseId })));
       }
     }
 
@@ -416,11 +494,7 @@ export async function updateInvoice(
 
 // ─── Delete (draft only) ──────────────────────────────────────────────────
 
-export async function deleteInvoice(
-  id: string,
-  portId: string,
-  meta: ServiceAuditMeta,
-) {
+export async function deleteInvoice(id: string, portId: string, meta: ServiceAuditMeta) {
   const existing = await getInvoiceById(id, portId);
   if (existing.status !== 'draft') {
     throw new ConflictError('Only draft invoices can be deleted');
@@ -429,9 +503,7 @@ export async function deleteInvoice(
   await withTransaction(async (tx) => {
     await tx.delete(invoiceExpenses).where(eq(invoiceExpenses.invoiceId, id));
     await tx.delete(invoiceLineItems).where(eq(invoiceLineItems.invoiceId, id));
-    await tx
-      .delete(invoices)
-      .where(and(eq(invoices.id, id), eq(invoices.portId, portId)));
+    await tx.delete(invoices).where(and(eq(invoices.id, id), eq(invoices.portId, portId)));
   });
 
   void createAuditLog({
@@ -453,11 +525,7 @@ export async function deleteInvoice(
 
 // ─── Generate PDF ─────────────────────────────────────────────────────────
 
-export async function generateInvoicePdf(
-  id: string,
-  portId: string,
-  meta: ServiceAuditMeta,
-) {
+export async function generateInvoicePdf(id: string, portId: string, meta: ServiceAuditMeta) {
   const invoice = await getInvoiceById(id, portId);
 
   const [port] = await db
@@ -519,11 +587,7 @@ export async function generateInvoicePdf(
 
 // ─── Send invoice ─────────────────────────────────────────────────────────
 
-export async function sendInvoice(
-  id: string,
-  portId: string,
-  meta: ServiceAuditMeta,
-) {
+export async function sendInvoice(id: string, portId: string, meta: ServiceAuditMeta) {
   const invoice = await getInvoiceById(id, portId);
 
   // Generate PDF if not exists
@@ -621,11 +685,7 @@ export async function detectOverdue(portId: string) {
     .select({ id: invoices.id, invoiceNumber: invoices.invoiceNumber, dueDate: invoices.dueDate })
     .from(invoices)
     .where(
-      and(
-        eq(invoices.portId, portId),
-        eq(invoices.status, 'sent'),
-        lt(invoices.dueDate, today),
-      ),
+      and(eq(invoices.portId, portId), eq(invoices.status, 'sent'), lt(invoices.dueDate, today)),
     );
 
   if (overdueInvoices.length === 0) return;
@@ -636,9 +696,10 @@ export async function detectOverdue(portId: string) {
       .set({ status: 'overdue', updatedAt: new Date() })
       .where(eq(invoices.id, inv.id));
 
-    const daysPastDue = Math.max(1, Math.ceil(
-      (Date.now() - new Date(inv.dueDate).getTime()) / (1000 * 60 * 60 * 24),
-    ));
+    const daysPastDue = Math.max(
+      1,
+      Math.ceil((Date.now() - new Date(inv.dueDate).getTime()) / (1000 * 60 * 60 * 24)),
+    );
     emitToRoom(`port:${portId}`, 'invoice:overdue', {
       invoiceId: inv.id,
       invoiceNumber: inv.invoiceNumber,
diff --git a/src/lib/validators/invoices.ts b/src/lib/validators/invoices.ts
index e0ea3b4..d26eb44 100644
--- a/src/lib/validators/invoices.ts
+++ b/src/lib/validators/invoices.ts
@@ -3,7 +3,10 @@ import { baseListQuerySchema } from '@/lib/api/route-helpers';
 
 export const createInvoiceSchema = z
   .object({
-    clientName: z.string().min(1).max(200),
+    billingEntity: z.object({
+      type: z.enum(['client', 'company']),
+      id: z.string().min(1),
+    }),
     billingEmail: z.string().email().optional(),
     billingAddress: z.string().max(500).optional(),
     dueDate: z.string().min(1),
@@ -35,9 +38,7 @@ export const updateInvoiceSchema = z.object({
   billingEmail: z.string().email().optional(),
   billingAddress: z.string().max(500).optional(),
   dueDate: z.string().min(1).optional(),
-  paymentTerms: z
-    .enum(['immediate', 'net10', 'net15', 'net30', 'net45', 'net60'])
-    .optional(),
+  paymentTerms: z.enum(['immediate', 'net10', 'net15', 'net30', 'net45', 'net60']).optional(),
   currency: z.string().length(3).optional(),
   notes: z.string().max(2000).optional(),
   lineItems: z
@@ -63,6 +64,8 @@ export const listInvoicesSchema = baseListQuerySchema.extend({
   clientName: z.string().optional(),
   dateFrom: z.string().optional(),
   dateTo: z.string().optional(),
+  billingEntityType: z.enum(['client', 'company']).optional(),
+  billingEntityId: z.string().optional(),
 });
 
 export type CreateInvoiceInput = z.infer<typeof createInvoiceSchema>;
diff --git a/tests/integration/invoices-billing-entity.test.ts b/tests/integration/invoices-billing-entity.test.ts
new file mode 100644
index 0000000..e0a45f8
--- /dev/null
+++ b/tests/integration/invoices-billing-entity.test.ts
@@ -0,0 +1,224 @@
+/**
+ * invoices.service billing-entity integration tests.
+ *
+ * Covers:
+ *  - createInvoice with billingEntity=client snapshots clientName from client.fullName
+ *  - createInvoice with billingEntity=company snapshots clientName from company.name
+ *  - createInvoice throws ValidationError when client does not exist
+ *  - createInvoice throws ValidationError when company is in another tenant (cross-tenant)
+ *  - createInvoice uses entity primary email + address when none provided
+ *  - createInvoice allows caller to override billingEmail and billingAddress
+ *
+ * Uses dynamic imports (PR 8 pattern) so env is loaded before service modules
+ * touch `db`.
+ */
+import { describe, it, expect, beforeAll } from 'vitest';
+
+describe('invoices.service — billing entity', () => {
+  let createInvoice: typeof import('@/lib/services/invoices').createInvoice;
+
+  let makePort: typeof import('../helpers/factories').makePort;
+  let makeClient: typeof import('../helpers/factories').makeClient;
+  let makeCompany: typeof import('../helpers/factories').makeCompany;
+  let makeAuditMeta: typeof import('../helpers/factories').makeAuditMeta;
+
+  let db: typeof import('@/lib/db').db;
+  let clientContacts: typeof import('@/lib/db/schema/clients').clientContacts;
+  let clientAddresses: typeof import('@/lib/db/schema/clients').clientAddresses;
+  let companyAddresses: typeof import('@/lib/db/schema/companies').companyAddresses;
+
+  let ValidationError: typeof import('@/lib/errors').ValidationError;
+
+  beforeAll(async () => {
+    const svc = await import('@/lib/services/invoices');
+    createInvoice = svc.createInvoice;
+
+    const factories = await import('../helpers/factories');
+    makePort = factories.makePort;
+    makeClient = factories.makeClient;
+    makeCompany = factories.makeCompany;
+    makeAuditMeta = factories.makeAuditMeta;
+
+    const dbMod = await import('@/lib/db');
+    db = dbMod.db;
+
+    const clientsSchema = await import('@/lib/db/schema/clients');
+    clientContacts = clientsSchema.clientContacts;
+    clientAddresses = clientsSchema.clientAddresses;
+
+    const companiesSchema = await import('@/lib/db/schema/companies');
+    companyAddresses = companiesSchema.companyAddresses;
+
+    const errors = await import('@/lib/errors');
+    ValidationError = errors.ValidationError;
+  });
+
+  it('creates an invoice with billingEntity=client and snapshots clientName from client.fullName', async () => {
+    const port = await makePort();
+    const client = await makeClient({
+      portId: port.id,
+      overrides: { fullName: 'Alice Admiral' },
+    });
+
+    const invoice = await createInvoice(
+      port.id,
+      {
+        billingEntity: { type: 'client', id: client.id },
+        dueDate: '2026-12-31',
+        paymentTerms: 'net30',
+        currency: 'USD',
+        lineItems: [{ description: 'Dockage', quantity: 1, unitPrice: 500 }],
+      },
+      makeAuditMeta({ portId: port.id }),
+    );
+
+    expect(invoice.id).toBeTruthy();
+    expect(invoice.billingEntityType).toBe('client');
+    expect(invoice.billingEntityId).toBe(client.id);
+    expect(invoice.clientName).toBe('Alice Admiral');
+    expect(invoice.portId).toBe(port.id);
+  });
+
+  it('creates an invoice with billingEntity=company and snapshots clientName from company.name', async () => {
+    const port = await makePort();
+    const company = await makeCompany({
+      portId: port.id,
+      overrides: { name: 'Poseidon Holdings Ltd' },
+    });
+
+    const invoice = await createInvoice(
+      port.id,
+      {
+        billingEntity: { type: 'company', id: company.id },
+        dueDate: '2026-12-31',
+        paymentTerms: 'net30',
+        currency: 'USD',
+        lineItems: [{ description: 'Marina services', quantity: 2, unitPrice: 1250 }],
+      },
+      makeAuditMeta({ portId: port.id }),
+    );
+
+    expect(invoice.billingEntityType).toBe('company');
+    expect(invoice.billingEntityId).toBe(company.id);
+    expect(invoice.clientName).toBe('Poseidon Holdings Ltd');
+  });
+
+  it('throws ValidationError when billing entity (client) does not exist', async () => {
+    const port = await makePort();
+
+    await expect(
+      createInvoice(
+        port.id,
+        {
+          billingEntity: { type: 'client', id: 'nonexistent-client-id' },
+          dueDate: '2026-12-31',
+          paymentTerms: 'net30',
+          currency: 'USD',
+          lineItems: [{ description: 'Dockage', quantity: 1, unitPrice: 100 }],
+        },
+        makeAuditMeta({ portId: port.id }),
+      ),
+    ).rejects.toBeInstanceOf(ValidationError);
+  });
+
+  it('throws ValidationError when billing entity (company) is in a different port', async () => {
+    const portA = await makePort();
+    const portB = await makePort();
+    const companyInB = await makeCompany({ portId: portB.id });
+
+    await expect(
+      createInvoice(
+        portA.id,
+        {
+          billingEntity: { type: 'company', id: companyInB.id },
+          dueDate: '2026-12-31',
+          paymentTerms: 'net30',
+          currency: 'USD',
+          lineItems: [{ description: 'Dockage', quantity: 1, unitPrice: 100 }],
+        },
+        makeAuditMeta({ portId: portA.id }),
+      ),
+    ).rejects.toBeInstanceOf(ValidationError);
+  });
+
+  it('uses entity primary email + address when none provided in the request', async () => {
+    const port = await makePort();
+    const client = await makeClient({
+      portId: port.id,
+      overrides: { fullName: 'Bob Bosun' },
+    });
+
+    // Add a primary email contact and a primary address
+    await db.insert(clientContacts).values({
+      clientId: client.id,
+      channel: 'email',
+      value: 'bob@example.com',
+      isPrimary: true,
+    });
+    await db.insert(clientAddresses).values({
+      clientId: client.id,
+      portId: port.id,
+      label: 'Primary',
+      streetAddress: '1 Pier Road',
+      city: 'Harbor City',
+      stateProvince: 'CA',
+      postalCode: '90000',
+      country: 'USA',
+      isPrimary: true,
+    });
+
+    const invoice = await createInvoice(
+      port.id,
+      {
+        billingEntity: { type: 'client', id: client.id },
+        dueDate: '2026-12-31',
+        paymentTerms: 'net30',
+        currency: 'USD',
+        lineItems: [{ description: 'Dockage', quantity: 1, unitPrice: 100 }],
+      },
+      makeAuditMeta({ portId: port.id }),
+    );
+
+    expect(invoice.billingEmail).toBe('bob@example.com');
+    expect(invoice.billingAddress).toBe('1 Pier Road, Harbor City, CA, 90000, USA');
+  });
+
+  it('allows caller to override billingEmail and billingAddress', async () => {
+    const port = await makePort();
+    const company = await makeCompany({
+      portId: port.id,
+      overrides: { name: 'Nautical Corp', billingEmail: 'billing@nautical.example' },
+    });
+
+    await db.insert(companyAddresses).values({
+      companyId: company.id,
+      portId: port.id,
+      label: 'Primary',
+      streetAddress: '2 Ocean Blvd',
+      city: 'Portville',
+      stateProvince: 'FL',
+      postalCode: '33101',
+      country: 'USA',
+      isPrimary: true,
+    });
+
+    const invoice = await createInvoice(
+      port.id,
+      {
+        billingEntity: { type: 'company', id: company.id },
+        billingEmail: 'override@example.com',
+        billingAddress: 'Custom address line',
+        dueDate: '2026-12-31',
+        paymentTerms: 'net30',
+        currency: 'USD',
+        lineItems: [{ description: 'Dockage', quantity: 1, unitPrice: 100 }],
+      },
+      makeAuditMeta({ portId: port.id }),
+    );
+
+    expect(invoice.billingEmail).toBe('override@example.com');
+    expect(invoice.billingAddress).toBe('Custom address line');
+    // clientName snapshot is still entity-derived (not overridable on create)
+    expect(invoice.clientName).toBe('Nautical Corp');
+  });
+});
diff --git a/tests/unit/validators.test.ts b/tests/unit/validators.test.ts
index 8cdb716..62e0a52 100644
--- a/tests/unit/validators.test.ts
+++ b/tests/unit/validators.test.ts
@@ -194,7 +194,7 @@ describe('updateBerthStatusSchema', () => {
 
 describe('createInvoiceSchema', () => {
   const validInvoice = {
-    clientName: 'Bob',
+    billingEntity: { type: 'client' as const, id: 'client-123' },
     dueDate: '2026-06-01',
     lineItems: [{ description: 'Berth fee', quantity: 1, unitPrice: 5000 }],
   };
@@ -203,9 +203,17 @@ describe('createInvoiceSchema', () => {
     expect(createInvoiceSchema.safeParse(validInvoice).success).toBe(true);
   });
 
+  it('accepts a valid invoice with billingEntity type=company', () => {
+    const result = createInvoiceSchema.safeParse({
+      ...validInvoice,
+      billingEntity: { type: 'company' as const, id: 'company-123' },
+    });
+    expect(result.success).toBe(true);
+  });
+
   it('accepts invoice with only expenseIds', () => {
     const result = createInvoiceSchema.safeParse({
-      clientName: 'Bob',
+      billingEntity: { type: 'client' as const, id: 'client-123' },
       dueDate: '2026-06-01',
       expenseIds: ['exp-1'],
     });
@@ -213,12 +221,34 @@ describe('createInvoiceSchema', () => {
   });
 
   it('rejects invoice with neither lineItems nor expenseIds', () => {
-    const result = createInvoiceSchema.safeParse({ clientName: 'Bob', dueDate: '2026-06-01' });
+    const result = createInvoiceSchema.safeParse({
+      billingEntity: { type: 'client' as const, id: 'client-123' },
+      dueDate: '2026-06-01',
+    });
     expect(result.success).toBe(false);
   });
 
-  it('rejects empty clientName', () => {
-    const result = createInvoiceSchema.safeParse({ ...validInvoice, clientName: '' });
+  it('rejects missing billingEntity', () => {
+    const result = createInvoiceSchema.safeParse({
+      dueDate: '2026-06-01',
+      lineItems: [{ description: 'Fee', quantity: 1, unitPrice: 1 }],
+    });
+    expect(result.success).toBe(false);
+  });
+
+  it('rejects billingEntity with invalid type', () => {
+    const result = createInvoiceSchema.safeParse({
+      ...validInvoice,
+      billingEntity: { type: 'unknown', id: 'id-1' },
+    });
+    expect(result.success).toBe(false);
+  });
+
+  it('rejects billingEntity with empty id', () => {
+    const result = createInvoiceSchema.safeParse({
+      ...validInvoice,
+      billingEntity: { type: 'client', id: '' },
+    });
     expect(result.success).toBe(false);
   });