diff --git a/__tests__/stores/product-workspace/sprint-membership.test.ts b/__tests__/stores/product-workspace/sprint-membership.test.ts
new file mode 100644
index 0000000..6f271de
--- /dev/null
+++ b/__tests__/stores/product-workspace/sprint-membership.test.ts
@@ -0,0 +1,341 @@
+// @vitest-environment jsdom
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import { useProductWorkspaceStore } from '@/stores/product-workspace/store'
+import {
+  selectIsDirty,
+  selectPbiTriState,
+  selectPendingCount,
+  selectStoryEffectiveInSprint,
+  selectStoryIsBlocked,
+} from '@/stores/product-workspace/selectors'
+import type { BacklogStory } from '@/stores/product-workspace/types'
+
+function resetMembership() {
+  useProductWorkspaceStore.setState((s) => {
+    s.entities.storiesById = {}
+    s.relations.storyIdsByPbi = {}
+    s.sprintMembership = {
+      pbiSummary: {},
+      crossSprintBlocks: {},
+      pending: { adds: [], removes: [] },
+      loadedSummaryForSprintId: null,
+    }
+  })
+}
+
+function seedStory(id: string, pbiId: string, sprintId: string | null): BacklogStory {
+  return {
+    id,
+    code: id,
+    title: id,
+    description: null,
+    acceptance_criteria: null,
+    priority: 2,
+    sort_order: 1,
+    status: sprintId ? 'IN_SPRINT' : 'OPEN',
+    pbi_id: pbiId,
+    sprint_id: sprintId,
+    created_at: new Date('2026-01-01'),
+  }
+}
+
+beforeEach(() => {
+  resetMembership()
+})
+
+describe('toggleStorySprintMembership', () => {
+  it('adds storyId to pending.adds when currently not in sprint', () => {
+    useProductWorkspaceStore.getState().toggleStorySprintMembership('s1', false)
+    const pending = useProductWorkspaceStore.getState().sprintMembership.pending
+    expect(pending.adds).toEqual(['s1'])
+    expect(pending.removes).toEqual([])
+  })
+
+  it('adds storyId to pending.removes when currently in sprint', () => {
+    useProductWorkspaceStore.getState().toggleStorySprintMembership('s1', true)
+    const pending = useProductWorkspaceStore.getState().sprintMembership.pending
+    expect(pending.removes).toEqual(['s1'])
+    expect(pending.adds).toEqual([])
+  })
+
+  it('cancels out: toggle add → toggle remove same story (in-sprint) clears pending', () => {
+    const store = useProductWorkspaceStore.getState()
+    store.toggleStorySprintMembership('s1', false) // adds
+    // Story now appears to be "in sprint" via pending; calling with true should cancel
+    store.toggleStorySprintMembership('s1', false) // second click with same baseline
+    const pending = useProductWorkspaceStore.getState().sprintMembership.pending
+    expect(pending.adds).toEqual([])
+    expect(pending.removes).toEqual([])
+  })
+
+  it('removes from pending.removes when toggled back', () => {
+    const store = useProductWorkspaceStore.getState()
+    store.toggleStorySprintMembership('s1', true)
+    store.toggleStorySprintMembership('s1', true)
+    const pending = useProductWorkspaceStore.getState().sprintMembership.pending
+    expect(pending.removes).toEqual([])
+    expect(pending.adds).toEqual([])
+  })
+
+  it('resetSprintMembershipPending empties both arrays', () => {
+    const store = useProductWorkspaceStore.getState()
+    store.toggleStorySprintMembership('s1', false)
+    store.toggleStorySprintMembership('s2', true)
+    store.resetSprintMembershipPending()
+    const pending = useProductWorkspaceStore.getState().sprintMembership.pending
+    expect(pending.adds).toEqual([])
+    expect(pending.removes).toEqual([])
+  })
+})
+
+describe('selectPbiTriState', () => {
+  function seedSummary(pbiId: string, total: number, inSprint: number) {
+    useProductWorkspaceStore.setState((s) => {
+      s.sprintMembership.pbiSummary[pbiId] = {
+        totalStoryCount: total,
+        inActiveSprintStoryCount: inSprint,
+      }
+    })
+  }
+
+  it('returns empty for PBI without summary', () => {
+    expect(
+      selectPbiTriState(useProductWorkspaceStore.getState(), 'pbi-1'),
+    ).toBe('empty')
+  })
+
+  it('returns empty when totalStoryCount == 0', () => {
+    seedSummary('pbi-1', 0, 0)
+    expect(
+      selectPbiTriState(useProductWorkspaceStore.getState(), 'pbi-1'),
+    ).toBe('empty')
+  })
+
+  it('returns full when all stories in sprint (no pending)', () => {
+    seedSummary('pbi-1', 3, 3)
+    expect(
+      selectPbiTriState(useProductWorkspaceStore.getState(), 'pbi-1'),
+    ).toBe('full')
+  })
+
+  it('returns partial when some stories in sprint', () => {
+    seedSummary('pbi-1', 3, 2)
+    expect(
+      selectPbiTriState(useProductWorkspaceStore.getState(), 'pbi-1'),
+    ).toBe('partial')
+  })
+
+  it('returns empty when inSprint == 0', () => {
+    seedSummary('pbi-1', 3, 0)
+    expect(
+      selectPbiTriState(useProductWorkspaceStore.getState(), 'pbi-1'),
+    ).toBe('empty')
+  })
+
+  it('applies pending adds when stories are loaded for the PBI', () => {
+    seedSummary('pbi-1', 3, 1)
+    useProductWorkspaceStore.setState((s) => {
+      s.relations.storyIdsByPbi['pbi-1'] = ['s1', 's2', 's3']
+      s.entities.storiesById['s1'] = seedStory('s1', 'pbi-1', 'sprint-1')
+      s.entities.storiesById['s2'] = seedStory('s2', 'pbi-1', null)
+      s.entities.storiesById['s3'] = seedStory('s3', 'pbi-1', null)
+      s.sprintMembership.pending.adds = ['s2', 's3']
+    })
+    expect(
+      selectPbiTriState(useProductWorkspaceStore.getState(), 'pbi-1'),
+    ).toBe('full')
+  })
+
+  it('applies pending removes when stories are loaded for the PBI', () => {
+    seedSummary('pbi-1', 3, 3)
+    useProductWorkspaceStore.setState((s) => {
+      s.relations.storyIdsByPbi['pbi-1'] = ['s1', 's2', 's3']
+      s.sprintMembership.pending.removes = ['s2']
+    })
+    expect(
+      selectPbiTriState(useProductWorkspaceStore.getState(), 'pbi-1'),
+    ).toBe('partial')
+  })
+
+  it('ignores pending entries for stories of other PBIs', () => {
+    seedSummary('pbi-1', 3, 3)
+    useProductWorkspaceStore.setState((s) => {
+      s.relations.storyIdsByPbi['pbi-1'] = ['s1', 's2', 's3']
+      s.sprintMembership.pending.removes = ['s99'] // not in pbi-1
+    })
+    expect(
+      selectPbiTriState(useProductWorkspaceStore.getState(), 'pbi-1'),
+    ).toBe('full')
+  })
+})
+
+describe('selectStoryEffectiveInSprint', () => {
+  it('returns true when story.sprint_id matches activeSprintId and no pending', () => {
+    useProductWorkspaceStore.setState((s) => {
+      s.entities.storiesById['s1'] = seedStory('s1', 'pbi-1', 'sprint-A')
+    })
+    expect(
+      selectStoryEffectiveInSprint(
+        useProductWorkspaceStore.getState(),
+        's1',
+        'sprint-A',
+      ),
+    ).toBe(true)
+  })
+
+  it('returns false when story.sprint_id is null', () => {
+    useProductWorkspaceStore.setState((s) => {
+      s.entities.storiesById['s1'] = seedStory('s1', 'pbi-1', null)
+    })
+    expect(
+      selectStoryEffectiveInSprint(
+        useProductWorkspaceStore.getState(),
+        's1',
+        'sprint-A',
+      ),
+    ).toBe(false)
+  })
+
+  it('returns true when story in pending.adds even if DB says no', () => {
+    useProductWorkspaceStore.setState((s) => {
+      s.entities.storiesById['s1'] = seedStory('s1', 'pbi-1', null)
+      s.sprintMembership.pending.adds = ['s1']
+    })
+    expect(
+      selectStoryEffectiveInSprint(
+        useProductWorkspaceStore.getState(),
+        's1',
+        'sprint-A',
+      ),
+    ).toBe(true)
+  })
+
+  it('returns false when story in pending.removes even if DB says yes', () => {
+    useProductWorkspaceStore.setState((s) => {
+      s.entities.storiesById['s1'] = seedStory('s1', 'pbi-1', 'sprint-A')
+      s.sprintMembership.pending.removes = ['s1']
+    })
+    expect(
+      selectStoryEffectiveInSprint(
+        useProductWorkspaceStore.getState(),
+        's1',
+        'sprint-A',
+      ),
+    ).toBe(false)
+  })
+
+  it('returns false when activeSprintId is null', () => {
+    useProductWorkspaceStore.setState((s) => {
+      s.entities.storiesById['s1'] = seedStory('s1', 'pbi-1', 'sprint-A')
+    })
+    expect(
+      selectStoryEffectiveInSprint(
+        useProductWorkspaceStore.getState(),
+        's1',
+        null,
+      ),
+    ).toBe(false)
+  })
+})
+
+describe('selectStoryIsBlocked', () => {
+  it('returns null when no block', () => {
+    expect(
+      selectStoryIsBlocked(useProductWorkspaceStore.getState(), 's1'),
+    ).toBeNull()
+  })
+
+  it('returns block info when story is in another sprint', () => {
+    useProductWorkspaceStore.setState((s) => {
+      s.sprintMembership.crossSprintBlocks['s1'] = {
+        sprintId: 'sprint-x',
+        sprintName: 'SP-X',
+      }
+    })
+    expect(
+      selectStoryIsBlocked(useProductWorkspaceStore.getState(), 's1'),
+    ).toEqual({ sprintId: 'sprint-x', sprintName: 'SP-X' })
+  })
+})
+
+describe('selectIsDirty + selectPendingCount', () => {
+  it('clean by default', () => {
+    expect(selectIsDirty(useProductWorkspaceStore.getState())).toBe(false)
+    expect(selectPendingCount(useProductWorkspaceStore.getState())).toBe(0)
+  })
+
+  it('counts adds + removes', () => {
+    useProductWorkspaceStore.setState((s) => {
+      s.sprintMembership.pending = {
+        adds: ['a1', 'a2'],
+        removes: ['r1'],
+      }
+    })
+    expect(selectIsDirty(useProductWorkspaceStore.getState())).toBe(true)
+    expect(selectPendingCount(useProductWorkspaceStore.getState())).toBe(3)
+  })
+})
+
+describe('fetch helpers', () => {
+  it('fetchSprintMembershipSummary populates store and gates by sprintId', async () => {
+    const originalFetch = globalThis.fetch
+    const responseBody = {
+      pbiA: { totalStoryCount: 5, inActiveSprintStoryCount: 2 },
+    }
+    globalThis.fetch = vi.fn().mockResolvedValue(
+      new Response(JSON.stringify(responseBody), { status: 200 }),
+    ) as unknown as typeof fetch
+    try {
+      await useProductWorkspaceStore
+        .getState()
+        .fetchSprintMembershipSummary('prod-1', 'sprint-A', ['pbiA'])
+
+      const slice = useProductWorkspaceStore.getState().sprintMembership
+      expect(slice.pbiSummary.pbiA).toEqual({
+        totalStoryCount: 5,
+        inActiveSprintStoryCount: 2,
+      })
+      expect(slice.loadedSummaryForSprintId).toBe('sprint-A')
+    } finally {
+      globalThis.fetch = originalFetch
+    }
+  })
+
+  it('fetchCrossSprintBlocks populates store', async () => {
+    const originalFetch = globalThis.fetch
+    const responseBody = {
+      's1': { sprintId: 'sprint-x', sprintName: 'SP-X' },
+    }
+    globalThis.fetch = vi.fn().mockResolvedValue(
+      new Response(JSON.stringify(responseBody), { status: 200 }),
+    ) as unknown as typeof fetch
+    try {
+      await useProductWorkspaceStore
+        .getState()
+        .fetchCrossSprintBlocks('prod-1', 'sprint-A', ['pbiA'])
+
+      const slice = useProductWorkspaceStore.getState().sprintMembership
+      expect(slice.crossSprintBlocks['s1']).toEqual({
+        sprintId: 'sprint-x',
+        sprintName: 'SP-X',
+      })
+    } finally {
+      globalThis.fetch = originalFetch
+    }
+  })
+
+  it('fetchSprintMembershipSummary is a no-op for empty pbiIds', async () => {
+    const fetchSpy = vi.fn()
+    const originalFetch = globalThis.fetch
+    globalThis.fetch = fetchSpy as unknown as typeof fetch
+    try {
+      await useProductWorkspaceStore
+        .getState()
+        .fetchSprintMembershipSummary('prod-1', 'sprint-A', [])
+      expect(fetchSpy).not.toHaveBeenCalled()
+    } finally {
+      globalThis.fetch = originalFetch
+    }
+  })
+})
diff --git a/__tests__/stores/product-workspace/store.test.ts b/__tests__/stores/product-workspace/store.test.ts
index bfa6cfd..f2db43b 100644
--- a/__tests__/stores/product-workspace/store.test.ts
+++ b/__tests__/stores/product-workspace/store.test.ts
@@ -56,6 +56,12 @@ function resetStore() {
     s.sync.lastResyncAt = null
     s.sync.resyncReason = null
     s.pendingMutations = {}
+    s.sprintMembership = {
+      pbiSummary: {},
+      crossSprintBlocks: {},
+      pending: { adds: [], removes: [] },
+      loadedSummaryForSprintId: null,
+    }
     Object.assign(s, originalActions)
   })
 }
diff --git a/stores/product-workspace/selectors.ts b/stores/product-workspace/selectors.ts
index 9d9e364..8a40d53 100644
--- a/stores/product-workspace/selectors.ts
+++ b/stores/product-workspace/selectors.ts
@@ -1,5 +1,13 @@
 import type { ProductWorkspaceStore } from './store'
-import type { BacklogPbi, BacklogStory, BacklogTask, TaskDetail } from './types'
+import type {
+  BacklogPbi,
+  BacklogStory,
+  BacklogTask,
+  CrossSprintBlock,
+  TaskDetail,
+} from './types'
+
+export type PbiTriState = 'empty' | 'partial' | 'full'
 
 // G1: stable EMPTY-references zodat selectors geen nieuwe array per call retourneren.
 const EMPTY_PBIS: BacklogPbi[] = []
@@ -100,3 +108,72 @@ export function selectStoriesForPbi(
   }
   return out.length === 0 ? EMPTY_STORIES : out
 }
+
+// PBI-79 / ST-1336 — sprint-membership selectors.
+//
+// Tri-state PBI-vinkje. Werkt op counts uit het summary-endpoint zolang
+// de PBI dichtgeklapt is (relations.storyIdsByPbi leeg). Wanneer stories
+// geladen zijn rekenen we ook de pending-buffer mee per-story.
+export function selectPbiTriState(
+  s: ProductWorkspaceStore,
+  pbiId: string,
+): PbiTriState {
+  const summary = s.sprintMembership.pbiSummary[pbiId]
+  if (!summary || summary.totalStoryCount === 0) return 'empty'
+
+  const storyIds = s.relations.storyIdsByPbi[pbiId]
+  let inSprintAfterPending = summary.inActiveSprintStoryCount
+
+  if (storyIds && storyIds.length > 0) {
+    const idSet = new Set(storyIds)
+    const adds = s.sprintMembership.pending.adds
+    const removes = s.sprintMembership.pending.removes
+    for (const id of adds) if (idSet.has(id)) inSprintAfterPending++
+    for (const id of removes) if (idSet.has(id)) inSprintAfterPending--
+  }
+
+  if (inSprintAfterPending <= 0) return 'empty'
+  if (inSprintAfterPending >= summary.totalStoryCount) return 'full'
+  return 'partial'
+}
+
+/**
+ * Effectief membership van een story rekening houdend met de pending buffer.
+ * `activeSprintId` is de gekozen sprint (state B); zonder die context valt de
+ * selector terug op de DB-waarde.
+ */
+export function selectStoryEffectiveInSprint(
+  s: ProductWorkspaceStore,
+  storyId: string,
+  activeSprintId: string | null,
+): boolean {
+  const story = s.entities.storiesById[storyId]
+  const inSprintDb = story?.sprint_id === activeSprintId && activeSprintId !== null
+  const inAdds = s.sprintMembership.pending.adds.includes(storyId)
+  const inRemoves = s.sprintMembership.pending.removes.includes(storyId)
+  if (inAdds) return true
+  if (inRemoves) return false
+  return inSprintDb
+}
+
+export function selectStoryIsBlocked(
+  s: ProductWorkspaceStore,
+  storyId: string,
+): CrossSprintBlock | null {
+  return s.sprintMembership.crossSprintBlocks[storyId] ?? null
+}
+
+export function selectIsDirty(s: ProductWorkspaceStore): boolean {
+  return (
+    s.sprintMembership.pending.adds.length +
+      s.sprintMembership.pending.removes.length >
+    0
+  )
+}
+
+export function selectPendingCount(s: ProductWorkspaceStore): number {
+  return (
+    s.sprintMembership.pending.adds.length +
+    s.sprintMembership.pending.removes.length
+  )
+}
diff --git a/stores/product-workspace/store.ts b/stores/product-workspace/store.ts
index 103eb28..54955bb 100644
--- a/stores/product-workspace/store.ts
+++ b/stores/product-workspace/store.ts
@@ -7,12 +7,15 @@ import {
   type BacklogPbi,
   type BacklogStory,
   type BacklogTask,
+  type CrossSprintBlock,
   type OptimisticMutation,
+  type PbiSummaryEntry,
   type PendingOptimisticMutation,
   type ProductBacklogSnapshot,
   type ProductRealtimeEvent,
   type RealtimeStatus,
   type ResyncReason,
+  type SprintMembershipSlice,
   type TaskDetail,
 } from './types'
 import {
@@ -73,6 +76,7 @@ interface State {
   loading: LoadingSlice
   sync: SyncSlice
   pendingMutations: Record<string, PendingOptimisticMutation>
+  sprintMembership: SprintMembershipSlice
 }
 
 interface Actions {
@@ -100,6 +104,22 @@ interface Actions {
   settleMutation(mutationId: string): void
 
   setRealtimeStatus(status: RealtimeStatus): void
+
+  // PBI-79 / ST-1336: sprint-membership acties.
+  setPbiSummary(summary: Record<string, PbiSummaryEntry>): void
+  setCrossSprintBlocks(blocks: Record<string, CrossSprintBlock>): void
+  toggleStorySprintMembership(storyId: string, currentlyInSprint: boolean): void
+  resetSprintMembershipPending(): void
+  fetchSprintMembershipSummary(
+    productId: string,
+    sprintId: string,
+    pbiIds: string[],
+  ): Promise<void>
+  fetchCrossSprintBlocks(
+    productId: string,
+    excludeSprintId: string | null,
+    pbiIds: string[],
+  ): Promise<void>
 }
 
 export type ProductWorkspaceStore = State & Actions
@@ -136,6 +156,12 @@ const initialState: State = {
     resyncReason: null,
   },
   pendingMutations: {},
+  sprintMembership: {
+    pbiSummary: {},
+    crossSprintBlocks: {},
+    pending: { adds: [], removes: [] },
+    loadedSummaryForSprintId: null,
+  },
 }
 
 function comparePbi(a: BacklogPbi, b: BacklogPbi): number {
@@ -194,6 +220,12 @@ export const useProductWorkspaceStore = create<ProductWorkspaceStore>()(
         s.entities.storiesById = {}
         s.entities.tasksById = {}
         s.relations.pbiIds = []
+        s.sprintMembership = {
+          pbiSummary: {},
+          crossSprintBlocks: {},
+          pending: { adds: [], removes: [] },
+          loadedSummaryForSprintId: null,
+        }
         s.relations.storyIdsByPbi = {}
         s.relations.taskIdsByStory = {}
 
@@ -566,6 +598,75 @@ export const useProductWorkspaceStore = create<ProductWorkspaceStore>()(
         s.sync.realtimeStatus = status
       })
     },
+
+    setPbiSummary(summary) {
+      set((s) => {
+        s.sprintMembership.pbiSummary = summary
+      })
+    },
+
+    setCrossSprintBlocks(blocks) {
+      set((s) => {
+        s.sprintMembership.crossSprintBlocks = blocks
+      })
+    },
+
+    toggleStorySprintMembership(storyId, currentlyInSprint) {
+      set((s) => {
+        const pending = s.sprintMembership.pending
+        if (currentlyInSprint) {
+          const inRemoves = pending.removes.indexOf(storyId)
+          if (inRemoves >= 0) {
+            pending.removes.splice(inRemoves, 1)
+          } else {
+            const inAdds = pending.adds.indexOf(storyId)
+            if (inAdds >= 0) pending.adds.splice(inAdds, 1)
+            pending.removes.push(storyId)
+          }
+        } else {
+          const inAdds = pending.adds.indexOf(storyId)
+          if (inAdds >= 0) {
+            pending.adds.splice(inAdds, 1)
+          } else {
+            const inRemoves = pending.removes.indexOf(storyId)
+            if (inRemoves >= 0) pending.removes.splice(inRemoves, 1)
+            pending.adds.push(storyId)
+          }
+        }
+      })
+    },
+
+    resetSprintMembershipPending() {
+      set((s) => {
+        s.sprintMembership.pending = { adds: [], removes: [] }
+      })
+    },
+
+    async fetchSprintMembershipSummary(productId, sprintId, pbiIds) {
+      if (pbiIds.length === 0) return
+      const url = `/api/products/${productId}/sprint-membership-summary?sprintId=${encodeURIComponent(sprintId)}&pbiIds=${pbiIds.map(encodeURIComponent).join(',')}`
+      const summary = await fetchJson<Record<string, PbiSummaryEntry>>(url)
+      set((s) => {
+        for (const [pbiId, entry] of Object.entries(summary)) {
+          s.sprintMembership.pbiSummary[pbiId] = entry
+        }
+        s.sprintMembership.loadedSummaryForSprintId = sprintId
+      })
+    },
+
+    async fetchCrossSprintBlocks(productId, excludeSprintId, pbiIds) {
+      if (pbiIds.length === 0) return
+      const params = new URLSearchParams()
+      if (excludeSprintId) params.set('excludeSprintId', excludeSprintId)
+      params.set('pbiIds', pbiIds.join(','))
+      const url = `/api/products/${productId}/cross-sprint-blocks?${params.toString()}`
+      const blocks = await fetchJson<Record<string, CrossSprintBlock>>(url)
+      set((s) => {
+        for (const [storyId, info] of Object.entries(blocks)) {
+          s.sprintMembership.crossSprintBlocks[storyId] = info
+        }
+      })
+    },
   })),
 )
 
diff --git a/stores/product-workspace/types.ts b/stores/product-workspace/types.ts
index d261316..1407a95 100644
--- a/stores/product-workspace/types.ts
+++ b/stores/product-workspace/types.ts
@@ -138,3 +138,21 @@ export interface PendingOptimisticMutation {
   mutation: OptimisticMutation
   createdAt: number
 }
+
+// PBI-79 / ST-1336: sprint-membership state voor backlog-page.
+export interface PbiSummaryEntry {
+  totalStoryCount: number
+  inActiveSprintStoryCount: number
+}
+
+export interface CrossSprintBlock {
+  sprintId: string
+  sprintName: string
+}
+
+export interface SprintMembershipSlice {
+  pbiSummary: Record<string, PbiSummaryEntry>
+  crossSprintBlocks: Record<string, CrossSprintBlock>
+  pending: { adds: string[]; removes: string[] }
+  loadedSummaryForSprintId: string | null
+}