diff --git a/__tests__/equipment-reserve.test.ts b/__tests__/equipment-reserve.test.ts
new file mode 100644
index 0000000..cf82381
--- /dev/null
+++ b/__tests__/equipment-reserve.test.ts
@@ -0,0 +1,130 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+
+/**
+ * Cross-tenant guard for reserveEquipment's manager "book on behalf" path.
+ *
+ * When a manager passes a target `memberId`, the action must verify that member
+ * belongs to the caller's space (mirroring signUpForClass) BEFORE inserting a
+ * reservation stamped with the target's membership. Without the guard a manager
+ * could pass a member id from another space and create a reservation there.
+ *
+ * These drive the ACTUAL action through recording mock Supabase clients: one
+ * RLS client (createClient) whose results are scripted in call order, and a
+ * separate admin client (createAdminClient) for the space_members lookup.
+ */
+
+type QueueItem = { data?: unknown; error?: unknown }
+
+function makeClient(opts: { user?: { id: string; email?: string } | null; queue?: QueueItem[] }) {
+  const calls: Array<{ method: string; args: unknown[] }> = []
+  const queue = [...(opts.queue ?? [])]
+  const next = (): QueueItem => (queue.length ? queue.shift()! : { data: null, error: null })
+
+  const builder: Record<string, unknown> = new Proxy(
+    {},
+    {
+      get(_t, prop: string) {
+        if (prop === 'then') {
+          return (resolve: (v: QueueItem) => void) => resolve(next())
+        }
+        if (prop === 'single' || prop === 'maybeSingle') {
+          return (...args: unknown[]) => {
+            calls.push({ method: prop, args })
+            return Promise.resolve(next())
+          }
+        }
+        return (...args: unknown[]) => {
+          calls.push({ method: prop, args })
+          return builder
+        }
+      },
+    },
+  )
+
+  const client = {
+    auth: {
+      getUser: vi.fn(() => Promise.resolve({ data: { user: opts.user ?? null } })),
+    },
+    from: (...args: unknown[]) => {
+      calls.push({ method: 'from', args })
+      return builder
+    },
+    // rpc returns the (thenable) builder, so `await rpc(...)` pulls the next
+    // queued result — used for the user_has_permission manager check.
+    rpc: (...args: unknown[]) => {
+      calls.push({ method: 'rpc', args })
+      return builder
+    },
+    _calls: calls,
+  }
+  return client
+}
+
+let currentClient: ReturnType<typeof makeClient>
+let currentAdmin: ReturnType<typeof makeClient>
+
+vi.mock('@/lib/supabase/server', () => ({
+  createClient: () => Promise.resolve(currentClient),
+}))
+vi.mock('@/lib/supabase/admin', () => ({
+  createAdminClient: () => currentAdmin,
+}))
+vi.mock('next/cache', () => ({ revalidatePath: vi.fn() }))
+
+import { reserveEquipment } from '@/lib/actions/equipment'
+
+const user = { id: 'user-A', email: 'a@example.com' }
+const managerRow = {
+  id: 'member-A',
+  space_id: 'space-A',
+  user_id: user.id,
+  role: 'manager',
+  status: 'current',
+  display_name: 'Manager A',
+  handle: 'manager-a',
+}
+const equipRow = {
+  id: '22222222-2222-2222-2222-222222222222',
+  name: 'Laser Cutter',
+  location: 'Lab',
+  space_id: 'space-A',
+  status: 'available',
+  is_active: true,
+  required_certification_id: null,
+}
+const FOREIGN_MEMBER = '33333333-3333-3333-3333-333333333333'
+const baseInput = {
+  equipmentId: equipRow.id,
+  starts_at: '2026-09-01T10:00:00Z',
+  ends_at: '2026-09-01T11:00:00Z',
+}
+
+beforeEach(() => {
+  vi.clearAllMocks()
+})
+
+describe('reserveEquipment cross-space target guard', () => {
+  it('rejects a manager booking for a member from another space', async () => {
+    currentClient = makeClient({
+      user,
+      queue: [
+        { data: managerRow }, // requireMember (space_members .single)
+        { data: true }, // user_has_permission rpc -> manager
+        { data: equipRow }, // equipment .maybeSingle
+      ],
+    })
+    // Admin client's space_members lookup finds nothing: the target belongs to
+    // another space (or does not exist).
+    currentAdmin = makeClient({ user: null, queue: [{ data: null }] })
+
+    const res = await reserveEquipment({ ...baseInput, memberId: FOREIGN_MEMBER })
+
+    expect(res).toEqual({ error: 'That member was not found in this space.' })
+    // The guard ran: a space_members lookup pinned to the caller's space.
+    const eqArgs = currentAdmin._calls.filter(c => c.method === 'eq').map(c => c.args)
+    expect(eqArgs).toContainEqual(['id', FOREIGN_MEMBER])
+    expect(eqArgs).toContainEqual(['space_id', 'space-A'])
+    // No reservation was inserted.
+    expect(currentAdmin._calls.some(c => c.method === 'insert')).toBe(false)
+  })
+})
diff --git a/lib/actions/equipment.ts b/lib/actions/equipment.ts
index 9e1a0ed..19d0993 100644
--- a/lib/actions/equipment.ts
+++ b/lib/actions/equipment.ts
@@ -257,6 +257,20 @@ export async function reserveEquipment(input: unknown) {
   if (!equip) return { error: 'Equipment not found' }
 
   const admin = createAdminClient()
+
+  // When booking on someone else's behalf, the target must belong to the
+  // caller's space, mirroring signUpForClass. Without this a manager could
+  // pass a member id from another space and stamp a reservation there.
+  if (targetMemberId !== member.id) {
+    const { data: tgt } = await admin
+      .from('space_members')
+      .select('id')
+      .eq('id', targetMemberId)
+      .eq('space_id', member.space_id)
+      .maybeSingle()
+    if (!tgt) return { error: 'That member was not found in this space.' }
+  }
+
   const { data: existing } = await admin
     .from('equipment_reservations')
     .select('starts_at, ends_at, status')