diff --git a/__tests__/api/cross-sprint-blocks.test.ts b/__tests__/api/cross-sprint-blocks.test.ts
new file mode 100644
index 0000000..5447900
--- /dev/null
+++ b/__tests__/api/cross-sprint-blocks.test.ts
@@ -0,0 +1,120 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+
+vi.mock('@/lib/prisma', () => ({
+  prisma: {
+    product: { findFirst: vi.fn() },
+    story: { findMany: vi.fn() },
+  },
+}))
+
+vi.mock('@/lib/api-auth', () => ({
+  authenticateApiRequest: vi.fn(),
+}))
+
+vi.mock('@/lib/product-access', () => ({
+  productAccessFilter: vi.fn().mockReturnValue({}),
+}))
+
+import { prisma } from '@/lib/prisma'
+import { authenticateApiRequest } from '@/lib/api-auth'
+import { GET } from '@/app/api/products/[id]/cross-sprint-blocks/route'
+
+const mockPrisma = prisma as unknown as {
+  product: { findFirst: ReturnType<typeof vi.fn> }
+  story: { findMany: ReturnType<typeof vi.fn> }
+}
+const mockAuth = authenticateApiRequest as unknown as ReturnType<typeof vi.fn>
+
+function makeRequest(url: string) {
+  return new Request(url)
+}
+
+describe('GET /api/products/[id]/cross-sprint-blocks', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    mockPrisma.product.findFirst.mockReset()
+    mockPrisma.story.findMany.mockReset()
+    mockAuth.mockReset().mockResolvedValue({ userId: 'user-1' })
+  })
+
+  it('returns blocking sprint info per story for happy path', async () => {
+    mockPrisma.product.findFirst.mockResolvedValue({ id: 'p1' })
+    mockPrisma.story.findMany.mockResolvedValue([
+      {
+        id: 'story-1',
+        sprint: { id: 'sprint-x', code: 'SP-X' },
+      },
+      {
+        id: 'story-2',
+        sprint: { id: 'sprint-y', code: 'SP-Y' },
+      },
+    ])
+
+    const req = makeRequest(
+      'http://localhost/api/products/p1/cross-sprint-blocks?excludeSprintId=sp-1&pbiIds=pbiA',
+    )
+    const res = await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+
+    expect(res.status).toBe(200)
+    const body = await res.json()
+    expect(body).toEqual({
+      'story-1': { sprintId: 'sprint-x', sprintName: 'SP-X' },
+      'story-2': { sprintId: 'sprint-y', sprintName: 'SP-Y' },
+    })
+  })
+
+  it('rejects when pbiIds is missing', async () => {
+    const req = makeRequest(
+      'http://localhost/api/products/p1/cross-sprint-blocks?excludeSprintId=sp-1',
+    )
+    const res = await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+    expect(res.status).toBe(400)
+  })
+
+  it('rejects when pbiIds is empty', async () => {
+    const req = makeRequest(
+      'http://localhost/api/products/p1/cross-sprint-blocks?pbiIds=',
+    )
+    const res = await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+    expect(res.status).toBe(400)
+  })
+
+  it('returns 404 when product is not accessible', async () => {
+    mockPrisma.product.findFirst.mockResolvedValue(null)
+    const req = makeRequest(
+      'http://localhost/api/products/p1/cross-sprint-blocks?pbiIds=pbiA',
+    )
+    const res = await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+    expect(res.status).toBe(404)
+  })
+
+  it('returns auth error when authenticate fails', async () => {
+    mockAuth.mockResolvedValue({ error: 'Niet ingelogd', status: 401 })
+    const req = makeRequest(
+      'http://localhost/api/products/p1/cross-sprint-blocks?pbiIds=pbiA',
+    )
+    const res = await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+    expect(res.status).toBe(401)
+  })
+
+  it('passes NOT excludeSprintId to prisma when provided', async () => {
+    mockPrisma.product.findFirst.mockResolvedValue({ id: 'p1' })
+    mockPrisma.story.findMany.mockResolvedValue([])
+
+    const req = makeRequest(
+      'http://localhost/api/products/p1/cross-sprint-blocks?excludeSprintId=sp-active&pbiIds=pbiA',
+    )
+    await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+
+    const callArg = mockPrisma.story.findMany.mock.calls[0][0] as {
+      where: Record<string, unknown>
+    }
+    expect(callArg.where).toMatchObject({
+      pbi_id: { in: ['pbiA'] },
+      product_id: 'p1',
+      sprint_id: { not: null },
+      NOT: { sprint_id: 'sp-active' },
+      sprint: { status: 'OPEN' },
+    })
+  })
+})
diff --git a/__tests__/api/sprint-membership-summary.test.ts b/__tests__/api/sprint-membership-summary.test.ts
new file mode 100644
index 0000000..c526210
--- /dev/null
+++ b/__tests__/api/sprint-membership-summary.test.ts
@@ -0,0 +1,121 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+
+vi.mock('@/lib/prisma', () => ({
+  prisma: {
+    product: { findFirst: vi.fn() },
+    story: { groupBy: vi.fn() },
+  },
+}))
+
+vi.mock('@/lib/api-auth', () => ({
+  authenticateApiRequest: vi.fn(),
+}))
+
+vi.mock('@/lib/product-access', () => ({
+  productAccessFilter: vi.fn().mockReturnValue({}),
+}))
+
+import { prisma } from '@/lib/prisma'
+import { authenticateApiRequest } from '@/lib/api-auth'
+import { GET } from '@/app/api/products/[id]/sprint-membership-summary/route'
+
+const mockPrisma = prisma as unknown as {
+  product: { findFirst: ReturnType<typeof vi.fn> }
+  story: { groupBy: ReturnType<typeof vi.fn> }
+}
+const mockAuth = authenticateApiRequest as unknown as ReturnType<typeof vi.fn>
+
+function makeRequest(url: string) {
+  return new Request(url)
+}
+
+describe('GET /api/products/[id]/sprint-membership-summary', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    mockPrisma.product.findFirst.mockReset()
+    mockPrisma.story.groupBy.mockReset()
+    mockAuth.mockReset().mockResolvedValue({ userId: 'user-1' })
+  })
+
+  it('returns counts per PBI for happy path', async () => {
+    mockPrisma.product.findFirst.mockResolvedValue({ id: 'p1' })
+    mockPrisma.story.groupBy
+      .mockResolvedValueOnce([
+        { pbi_id: 'pbiA', _count: { _all: 5 } },
+        { pbi_id: 'pbiB', _count: { _all: 3 } },
+      ])
+      .mockResolvedValueOnce([{ pbi_id: 'pbiA', _count: { _all: 2 } }])
+
+    const req = makeRequest(
+      'http://localhost/api/products/p1/sprint-membership-summary?sprintId=sp-1&pbiIds=pbiA,pbiB',
+    )
+    const res = await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+
+    expect(res.status).toBe(200)
+    const body = await res.json()
+    expect(body).toEqual({
+      pbiA: { total: 5, inSprint: 2 },
+      pbiB: { total: 3, inSprint: 0 },
+    })
+  })
+
+  it('rejects when pbiIds is missing', async () => {
+    const req = makeRequest(
+      'http://localhost/api/products/p1/sprint-membership-summary?sprintId=sp-1',
+    )
+    const res = await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+    expect(res.status).toBe(400)
+  })
+
+  it('rejects when pbiIds is empty', async () => {
+    const req = makeRequest(
+      'http://localhost/api/products/p1/sprint-membership-summary?sprintId=sp-1&pbiIds=',
+    )
+    const res = await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+    expect(res.status).toBe(400)
+  })
+
+  it('rejects when sprintId is missing', async () => {
+    const req = makeRequest(
+      'http://localhost/api/products/p1/sprint-membership-summary?pbiIds=pbiA',
+    )
+    const res = await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+    expect(res.status).toBe(400)
+  })
+
+  it('returns 404 when product is not accessible', async () => {
+    mockPrisma.product.findFirst.mockResolvedValue(null)
+    const req = makeRequest(
+      'http://localhost/api/products/p1/sprint-membership-summary?sprintId=sp-1&pbiIds=pbiA',
+    )
+    const res = await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+    expect(res.status).toBe(404)
+  })
+
+  it('returns auth error when authenticate fails', async () => {
+    mockAuth.mockResolvedValue({ error: 'Niet ingelogd', status: 401 })
+    const req = makeRequest(
+      'http://localhost/api/products/p1/sprint-membership-summary?sprintId=sp-1&pbiIds=pbiA',
+    )
+    const res = await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+    expect(res.status).toBe(401)
+  })
+
+  it('returns zero counts for PBIs without stories', async () => {
+    mockPrisma.product.findFirst.mockResolvedValue({ id: 'p1' })
+    mockPrisma.story.groupBy
+      .mockResolvedValueOnce([])
+      .mockResolvedValueOnce([])
+
+    const req = makeRequest(
+      'http://localhost/api/products/p1/sprint-membership-summary?sprintId=sp-1&pbiIds=pbiA,pbiB',
+    )
+    const res = await GET(req, { params: Promise.resolve({ id: 'p1' }) })
+
+    const body = await res.json()
+    expect(body).toEqual({
+      pbiA: { total: 0, inSprint: 0 },
+      pbiB: { total: 0, inSprint: 0 },
+    })
+  })
+})
diff --git a/app/api/products/[id]/cross-sprint-blocks/route.ts b/app/api/products/[id]/cross-sprint-blocks/route.ts
new file mode 100644
index 0000000..ba10da2
--- /dev/null
+++ b/app/api/products/[id]/cross-sprint-blocks/route.ts
@@ -0,0 +1,74 @@
+// PBI-79 / T-929: GET /api/products/:id/cross-sprint-blocks
+//
+// Lichte UX-hint voor disabled-vinkjes: welke stories binnen pbiIds zitten in
+// een andere OPEN sprint (excludeSprintId expliciet uitgesloten). Server-side
+// commit-actions blijven autoritatief — dit endpoint is alleen voor UI.
+import { authenticateApiRequest } from '@/lib/api-auth'
+import { prisma } from '@/lib/prisma'
+import { productAccessFilter } from '@/lib/product-access'
+
+export const dynamic = 'force-dynamic'
+
+function parsePbiIds(raw: string | null): string[] | null {
+  if (!raw) return null
+  const ids = raw
+    .split(',')
+    .map((s) => s.trim())
+    .filter(Boolean)
+  return ids.length === 0 ? null : ids
+}
+
+export async function GET(
+  request: Request,
+  { params }: { params: Promise<{ id: string }> },
+) {
+  const auth = await authenticateApiRequest(request)
+  if ('error' in auth) {
+    return Response.json({ error: auth.error }, { status: auth.status })
+  }
+
+  const { id: productId } = await params
+  const url = new URL(request.url)
+  const excludeSprintId = url.searchParams.get('excludeSprintId') ?? undefined
+  const pbiIds = parsePbiIds(url.searchParams.get('pbiIds'))
+
+  if (!pbiIds) {
+    return Response.json(
+      { error: 'pbiIds is verplicht (comma-separated)' },
+      { status: 400 },
+    )
+  }
+
+  const product = await prisma.product.findFirst({
+    where: { id: productId, ...productAccessFilter(auth.userId) },
+    select: { id: true },
+  })
+  if (!product) {
+    return Response.json({ error: 'Product niet gevonden' }, { status: 404 })
+  }
+
+  const stories = await prisma.story.findMany({
+    where: {
+      pbi_id: { in: pbiIds },
+      product_id: productId,
+      sprint_id: { not: null },
+      ...(excludeSprintId ? { NOT: { sprint_id: excludeSprintId } } : {}),
+      sprint: { status: 'OPEN' },
+    },
+    select: {
+      id: true,
+      sprint: { select: { id: true, code: true } },
+    },
+  })
+
+  const result: Record<string, { sprintId: string; sprintName: string }> = {}
+  for (const story of stories) {
+    if (!story.sprint) continue
+    result[story.id] = {
+      sprintId: story.sprint.id,
+      sprintName: story.sprint.code,
+    }
+  }
+
+  return Response.json(result)
+}
diff --git a/app/api/products/[id]/sprint-membership-summary/route.ts b/app/api/products/[id]/sprint-membership-summary/route.ts
new file mode 100644
index 0000000..16f6b6d
--- /dev/null
+++ b/app/api/products/[id]/sprint-membership-summary/route.ts
@@ -0,0 +1,87 @@
+// PBI-79 / T-928: GET /api/products/:id/sprint-membership-summary
+//
+// Levert per PBI {total, inSprint} counts, gescoped op de doorgegeven pbiIds.
+// Endpoint weigert product-brede aanroepen (pbiIds is verplicht). Eén groupBy
+// + één count-by-sprint waar pbi_id IN (pbiIds).
+import { authenticateApiRequest } from '@/lib/api-auth'
+import { prisma } from '@/lib/prisma'
+import { productAccessFilter } from '@/lib/product-access'
+
+export const dynamic = 'force-dynamic'
+
+function parsePbiIds(raw: string | null): string[] | null {
+  if (!raw) return null
+  const ids = raw
+    .split(',')
+    .map((s) => s.trim())
+    .filter(Boolean)
+  return ids.length === 0 ? null : ids
+}
+
+export async function GET(
+  request: Request,
+  { params }: { params: Promise<{ id: string }> },
+) {
+  const auth = await authenticateApiRequest(request)
+  if ('error' in auth) {
+    return Response.json({ error: auth.error }, { status: auth.status })
+  }
+
+  const { id: productId } = await params
+  const url = new URL(request.url)
+  const sprintId = url.searchParams.get('sprintId')
+  const pbiIds = parsePbiIds(url.searchParams.get('pbiIds'))
+
+  if (!sprintId) {
+    return Response.json({ error: 'sprintId is verplicht' }, { status: 400 })
+  }
+  if (!pbiIds) {
+    return Response.json(
+      { error: 'pbiIds is verplicht (comma-separated)' },
+      { status: 400 },
+    )
+  }
+
+  const product = await prisma.product.findFirst({
+    where: { id: productId, ...productAccessFilter(auth.userId) },
+    select: { id: true },
+  })
+  if (!product) {
+    return Response.json({ error: 'Product niet gevonden' }, { status: 404 })
+  }
+
+  const [totals, inSprint] = await Promise.all([
+    prisma.story.groupBy({
+      by: ['pbi_id'],
+      where: { pbi_id: { in: pbiIds }, product_id: productId },
+      _count: { _all: true },
+    }),
+    prisma.story.groupBy({
+      by: ['pbi_id'],
+      where: {
+        pbi_id: { in: pbiIds },
+        product_id: productId,
+        sprint_id: sprintId,
+      },
+      _count: { _all: true },
+    }),
+  ])
+
+  const inSprintByPbi = new Map<string, number>()
+  for (const row of inSprint) {
+    inSprintByPbi.set(row.pbi_id, row._count._all)
+  }
+
+  const result: Record<string, { total: number; inSprint: number }> = {}
+  for (const pbiId of pbiIds) {
+    result[pbiId] = { total: 0, inSprint: inSprintByPbi.get(pbiId) ?? 0 }
+  }
+  for (const row of totals) {
+    result[row.pbi_id] = {
+      total: row._count._all,
+      inSprint: inSprintByPbi.get(row.pbi_id) ?? 0,
+    }
+  }
+
+  return Response.json(result)
+}