diff --git a/app/api/v1/companies/__tests__/route.test.ts b/app/api/v1/companies/__tests__/route.test.ts
new file mode 100644
index 00000000..4088dc9d
--- /dev/null
+++ b/app/api/v1/companies/__tests__/route.test.ts
@@ -0,0 +1,175 @@
+/**
+ * Integration tests for GET /api/v1/companies — the list-companies endpoint.
+ *
+ * Regression coverage for #781: this is the only AUTHENTICATED static route on
+ * the v1 surface (no `[companyId]` segment), so Next.js 16 invokes its handler
+ * with `{ params: undefined }` rather than `{ params: Promise<{}> }`. The v1
+ * wrapper used to `await params.params` blindly and null-deref, returning 500
+ * for every valid key. These tests invoke the real handler with that exact
+ * static-route context.
+ */
+import { beforeAll, beforeEach, describe, expect, it, vi } from 'vitest'
+
+beforeAll(() => {
+  if (process.env.NODE_ENV !== 'test') {
+    throw new Error(
+      `companies route tests require NODE_ENV=test (got ${process.env.NODE_ENV ?? 'undefined'})`,
+    )
+  }
+  process.env.NEXT_PUBLIC_SUPABASE_URL ||= 'http://localhost:54321'
+  process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY ||= 'test-anon-key'
+})
+
+vi.mock('@/lib/auth/api-keys', async () => {
+  const actual = await vi.importActual<typeof import('@/lib/auth/api-keys')>('@/lib/auth/api-keys')
+  return {
+    ...actual,
+    validateApiKey: vi.fn(),
+    createServiceClientNoCookies: vi.fn(),
+  }
+})
+
+vi.mock('@supabase/supabase-js', async () => {
+  const actual = await vi.importActual<typeof import('@supabase/supabase-js')>('@supabase/supabase-js')
+  return { ...actual, createClient: vi.fn().mockReturnValue({}) }
+})
+
+import { validateApiKey, createServiceClientNoCookies } from '@/lib/auth/api-keys'
+import { GET as listCompanies } from '../route'
+
+const mockValidate = validateApiKey as ReturnType<typeof vi.fn>
+const mockServiceClient = createServiceClientNoCookies as ReturnType<typeof vi.fn>
+
+const USER_ID = '930abb54-c5ef-4ae0-b274-30fb16e9a295'
+
+// A Proxy-based supabase stub: every chained builder method returns the chain,
+// and awaiting it resolves to the configured { data, error } for the table.
+function makeFlexibleSupabase(byTable: Record<string, { data?: unknown; error?: unknown }>) {
+  const buildChain = (table: string): unknown => {
+    const handler: ProxyHandler<object> = {
+      get(_t, prop) {
+        if (prop === 'then') {
+          return (resolve: (v: unknown) => void) =>
+            resolve(byTable[table] ?? { data: null, error: null })
+        }
+        return (..._args: unknown[]) => buildChain(table)
+      },
+    }
+    return new Proxy({}, handler)
+  }
+  return { from: vi.fn((table: string) => buildChain(table)) }
+}
+
+function makeRequest(url = 'https://x.test/api/v1/companies'): Request {
+  return new Request(url, {
+    headers: { Authorization: 'Bearer test-fixture-not-a-real-key' },
+  })
+}
+
+// THE crux of the regression: Next.js 16 passes `{ params: undefined }` to the
+// handler of a static (non-dynamic) route. Reproduce that exact shape.
+type GetCtx = Parameters<typeof listCompanies>[1]
+function staticRouteContext(): GetCtx {
+  return { params: undefined } as unknown as GetCtx
+}
+
+function membershipRow(over: Partial<Record<string, unknown>> = {}) {
+  return {
+    id: '11111111-1111-4111-8111-111111111111',
+    role: 'owner',
+    joined_at: '2025-01-04T08:00:00.000Z',
+    companies: {
+      id: '8fd5b1f4-0000-4000-8000-000000000001',
+      name: 'Acme AB',
+      org_number: '556677-8899',
+      entity_type: 'aktiebolag',
+      archived_at: null,
+      created_at: '2025-01-04T08:00:00.000Z',
+    },
+    ...over,
+  }
+}
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  mockValidate.mockResolvedValue({
+    userId: USER_ID,
+    companyId: undefined,
+    apiKeyId: 'ak_1',
+    apiKeyName: 'CI key',
+    scopes: ['companies:read'],
+    mode: 'live',
+  })
+  mockServiceClient.mockReturnValue(makeFlexibleSupabase({ company_members: { data: [], error: null } }))
+})
+
+describe('GET /api/v1/companies', () => {
+  it('returns 200 (not 500) for a static-route context where params is undefined — regression #781', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({ company_members: { data: [membershipRow()], error: null } }),
+    )
+
+    const res = await listCompanies(makeRequest(), staticRouteContext())
+
+    expect(res.status).toBe(200)
+    const body = await res.json()
+    expect(body.error).toBeUndefined()
+    expect(Array.isArray(body.data)).toBe(true)
+    expect(body.meta.request_id).toMatch(/^req_/)
+  })
+
+  it('maps each membership row to a company summary in the { data, meta } envelope', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({ company_members: { data: [membershipRow()], error: null } }),
+    )
+
+    const res = await listCompanies(makeRequest(), staticRouteContext())
+    const body = await res.json()
+
+    expect(body.data).toEqual([
+      {
+        id: '8fd5b1f4-0000-4000-8000-000000000001',
+        name: 'Acme AB',
+        org_number: '556677-8899',
+        entity_type: 'aktiebolag',
+        role: 'owner',
+        created_at: '2025-01-04T08:00:00.000Z',
+      },
+    ])
+  })
+
+  it('drops rows whose joined company was filtered out (archived → null embed)', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({
+        company_members: {
+          data: [membershipRow(), membershipRow({ id: '22222222-2222-4222-8222-222222222222', companies: null })],
+          error: null,
+        },
+      }),
+    )
+
+    const res = await listCompanies(makeRequest(), staticRouteContext())
+    const body = await res.json()
+    expect(res.status).toBe(200)
+    expect(body.data).toHaveLength(1)
+  })
+
+  it('returns an empty list (200) when the key user has no memberships', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({ company_members: { data: [], error: null } }),
+    )
+
+    const res = await listCompanies(makeRequest(), staticRouteContext())
+    const body = await res.json()
+    expect(res.status).toBe(200)
+    expect(body.data).toEqual([])
+  })
+
+  it('returns 401 for a missing bearer token (auth still enforced)', async () => {
+    const res = await listCompanies(
+      new Request('https://x.test/api/v1/companies'),
+      staticRouteContext(),
+    )
+    expect(res.status).toBe(401)
+  })
+})
diff --git a/lib/api/v1/__tests__/with-api-v1.test.ts b/lib/api/v1/__tests__/with-api-v1.test.ts
index 5a8cdf8a..7969f275 100644
--- a/lib/api/v1/__tests__/with-api-v1.test.ts
+++ b/lib/api/v1/__tests__/with-api-v1.test.ts
@@ -83,6 +83,14 @@ function emptyParams() {
   return { params: Promise.resolve({}) }
 }
 
+// What Next.js 16 ACTUALLY passes to a STATIC route's handler: `{ params:
+// undefined }` (app-route module: `params: context.params ? ... : undefined`).
+// `emptyParams()` above is NOT what the runtime hands a static route, so it
+// masked #781. Use this for the real static-route contract.
+function staticRouteContext() {
+  return { params: undefined } as unknown as { params: Promise<Record<string, never>> }
+}
+
 function companyParams(companyId: string) {
   return { params: Promise.resolve({ companyId }) }
 }
@@ -246,6 +254,39 @@ describe('withApiV1 — company membership', () => {
   })
 })
 
+describe('withApiV1 — static (non-dynamic) route params', () => {
+  // Regression for #781: GET /api/v1/companies is the only authenticated
+  // static route. Next.js 16 hands it `{ params: undefined }`. The wrapper
+  // must not null-deref on `params.params` after auth succeeds.
+  it('does not 500 when Next passes { params: undefined } for a static route', async () => {
+    mockValidate.mockResolvedValue({
+      userId: 'user-1',
+      companyId: undefined,
+      scopes: ['companies:read'],
+      mode: 'live',
+    })
+
+    let observedCompanyId: string | undefined = 'sentinel'
+    let handlerCalled = false
+    const handler = withApiV1('companies.list', async (_req, ctx) => {
+      handlerCalled = true
+      observedCompanyId = ctx.companyId
+      return ok({ ok: true }, { requestId: ctx.requestId })
+    })
+
+    const res = await handler(
+      makeRequest('https://x.test/api/v1/companies', {
+        headers: { Authorization: 'Bearer gnubok_sk_x' },
+      }),
+      staticRouteContext(),
+    )
+
+    expect(res.status).toBe(200)
+    expect(handlerCalled).toBe(true)
+    expect(observedCompanyId).toBeUndefined()
+  })
+})
+
 describe('withApiV1 — idempotency', () => {
   it('replays a cached response when the idempotency key matches', async () => {
     mockValidate.mockResolvedValue({
diff --git a/lib/api/v1/with-api-v1.ts b/lib/api/v1/with-api-v1.ts
index 03fbef34..2f1485fb 100644
--- a/lib/api/v1/with-api-v1.ts
+++ b/lib/api/v1/with-api-v1.ts
@@ -324,7 +324,19 @@ export function withApiV1<P extends DynamicParams = { params: Promise<Record<str
       }
 
       // 5. Resolve URL companyId and verify access.
-      const resolvedParams = (await params.params) as Record<string, string | string[] | undefined>
+      //
+      // Next.js 16 invokes a route handler with `{ params: undefined }` for a
+      // STATIC route (no `[segment]` in the path) — see app-route module.js
+      // `handlerContext = { params: context.params ? ... : undefined }`. The
+      // only authenticated static route on this surface is `/api/v1/companies`,
+      // so awaiting `params.params` blindly null-derefs there (`undefined` has
+      // no `.companyId`) and the catch below turns it into a 500 for every
+      // valid key. Dynamic routes still pass a real `Promise<{ companyId }>`.
+      // Guard the await and default to an empty param set.
+      const resolvedParams = ((await params?.params) ?? {}) as Record<
+        string,
+        string | string[] | undefined
+      >
       const rawCompanyId = resolvedParams.companyId
       const companyId = typeof rawCompanyId === 'string' ? rawCompanyId : undefined