diff --git a/frontend/src/stores/gallery.js b/frontend/src/stores/gallery.js
index 10d3208..0c4ca77 100644
--- a/frontend/src/stores/gallery.js
+++ b/frontend/src/stores/gallery.js
@@ -10,7 +10,7 @@ export const useGalleryStore = defineStore('gallery', () => {
   const nextCursor = ref(null)
   const loading = ref(false)
   const error = ref(null)
-  const filter = ref({ tag_id: null })
+  const filter = ref({ tag_id: null, post_id: null })
 
   const timelineBuckets = ref([])
   const timelineLoading = ref(false)
@@ -30,8 +30,7 @@ export const useGalleryStore = defineStore('gallery', () => {
     error.value = null
     const myId = ++inflightId
     try {
-      const params = { limit: 50 }
-      if (filter.value.tag_id) params.tag_id = filter.value.tag_id
+      const params = { limit: 50, ...activeFilterParam() }
       if (nextCursor.value) params.cursor = nextCursor.value
       const body = await api.get('/api/gallery/scroll', { params })
       if (myId !== inflightId) return  // stale response
@@ -48,7 +47,7 @@ export const useGalleryStore = defineStore('gallery', () => {
   async function loadTimeline() {
     timelineLoading.value = true
     try {
-      const params = filter.value.tag_id ? { tag_id: filter.value.tag_id } : {}
+      const params = activeFilterParam()
       timelineBuckets.value = await api.get('/api/gallery/timeline', { params })
     } finally {
       timelineLoading.value = false
@@ -56,8 +55,7 @@ export const useGalleryStore = defineStore('gallery', () => {
   }
 
   async function jumpTo(year, month) {
-    const params = { year, month }
-    if (filter.value.tag_id) params.tag_id = filter.value.tag_id
+    const params = { year, month, ...activeFilterParam() }
     const body = await api.get('/api/gallery/jump', { params })
     if (body.cursor) {
       images.value = []
@@ -67,8 +65,22 @@ export const useGalleryStore = defineStore('gallery', () => {
     }
   }
 
+  function activeFilterParam() {
+    if (filter.value.tag_id) return { tag_id: filter.value.tag_id }
+    if (filter.value.post_id) return { post_id: filter.value.post_id }
+    return {}
+  }
+
   function setTagFilter(tagId) {
     filter.value.tag_id = tagId
+    filter.value.post_id = null
+    loadInitial()
+    loadTimeline()
+  }
+
+  function setPostFilter(postId) {
+    filter.value.post_id = postId
+    filter.value.tag_id = null
     loadInitial()
     loadTimeline()
   }
@@ -79,7 +91,7 @@ export const useGalleryStore = defineStore('gallery', () => {
   return {
     images, dateGroups, hasMore, isEmpty, loading, error,
     filter, timelineBuckets, timelineLoading,
-    loadInitial, loadMore, loadTimeline, jumpTo, setTagFilter
+    loadInitial, loadMore, loadTimeline, jumpTo, setTagFilter, setPostFilter
   }
 })
 
diff --git a/frontend/test/gallery.spec.js b/frontend/test/gallery.spec.js
new file mode 100644
index 0000000..4fb866a
--- /dev/null
+++ b/frontend/test/gallery.spec.js
@@ -0,0 +1,52 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
+import { setActivePinia, createPinia } from 'pinia'
+import { useGalleryStore } from '../src/stores/gallery.js'
+
+function stubFetch(handler) {
+  globalThis.fetch = vi.fn(async (url, init) => {
+    const { status, body } = handler(url, init)
+    return {
+      ok: status >= 200 && status < 300,
+      status,
+      statusText: String(status),
+      text: async () => (body == null ? '' : JSON.stringify(body))
+    }
+  })
+}
+
+const EMPTY = { images: [], date_groups: [], next_cursor: null }
+
+describe('gallery store: tag/post filter exclusivity', () => {
+  beforeEach(() => setActivePinia(createPinia()))
+  afterEach(() => vi.restoreAllMocks())
+
+  it('setPostFilter sets post_id and clears tag_id', () => {
+    const s = useGalleryStore()
+    stubFetch(() => ({ status: 200, body: EMPTY }))
+    s.setTagFilter(3)
+    expect(s.filter.tag_id).toBe(3)
+    s.setPostFilter(7)
+    expect(s.filter.post_id).toBe(7)
+    expect(s.filter.tag_id).toBe(null)
+  })
+
+  it('setTagFilter clears post_id', () => {
+    const s = useGalleryStore()
+    stubFetch(() => ({ status: 200, body: EMPTY }))
+    s.setPostFilter(7)
+    s.setTagFilter(3)
+    expect(s.filter.tag_id).toBe(3)
+    expect(s.filter.post_id).toBe(null)
+  })
+
+  it('loadMore sends exactly the active filter param', async () => {
+    const s = useGalleryStore()
+    const urls = []
+    stubFetch((url) => { urls.push(url); return { status: 200, body: EMPTY } })
+    s.setPostFilter(7)
+    await s.loadMore()
+    const scrollCall = urls.filter(u => u.includes('/api/gallery/scroll')).pop()
+    expect(scrollCall).toContain('post_id=7')
+    expect(scrollCall).not.toContain('tag_id=')
+  })
+})