diff --git a/src/renderer/inbox/selectFilteredNotes.ts b/src/renderer/inbox/selectFilteredNotes.ts
new file mode 100644
index 0000000..7e679f7
--- /dev/null
+++ b/src/renderer/inbox/selectFilteredNotes.ts
@@ -0,0 +1,17 @@
+import type { Note } from '@shared/types';
+
+/**
+ * Pure selector for tag-based filtering.
+ *
+ * Extracted out of `store.ts` so unit tests can import it under the
+ * vitest `node` environment without pulling in `api.ts`, which touches
+ * `window.inkling` at module-load time.
+ */
+export function selectFilteredNotes(state: {
+  notes: Note[];
+  tagFilter: string | null;
+}): Note[] {
+  if (state.tagFilter === null) return state.notes;
+  const target = state.tagFilter;
+  return state.notes.filter((n) => n.tags.some((t) => t.name === target));
+}
diff --git a/src/renderer/inbox/store.ts b/src/renderer/inbox/store.ts
index 2581906..65641c4 100644
--- a/src/renderer/inbox/store.ts
+++ b/src/renderer/inbox/store.ts
@@ -2,16 +2,20 @@ import { create } from 'zustand';
 import type { Note, WeeklyContinuity } from '@shared/types';
 import { inboxApi } from './api.js';
 
+export { selectFilteredNotes } from './selectFilteredNotes.js';
+
 interface InboxState {
   notes: Note[];
   continuity: WeeklyContinuity;
   pendingCount: number;
   ollamaStatus: { ok: boolean; reason?: string };
   loading: boolean;
+  tagFilter: string | null;
   loadInitial: () => Promise<void>;
   refreshMeta: () => Promise<void>;
   upsertNote: (note: Note) => void;
   removeNote: (id: string) => void;
+  setTagFilter: (tag: string | null) => void;
 }
 
 const emptyContinuity: WeeklyContinuity = {
@@ -25,6 +29,7 @@ export const useInbox = create<InboxState>((set, get) => ({
   pendingCount: 0,
   ollamaStatus: { ok: true },
   loading: false,
+  tagFilter: null,
   async loadInitial() {
     set({ loading: true });
     const [notes, continuity, pendingCount, ollamaStatus] = await Promise.all([
@@ -55,5 +60,8 @@ export const useInbox = create<InboxState>((set, get) => ({
   },
   removeNote(id) {
     set({ notes: get().notes.filter((n) => n.id !== id) });
+  },
+  setTagFilter(tag) {
+    set({ tagFilter: tag });
   }
 }));
diff --git a/tests/unit/store.tagFilter.test.ts b/tests/unit/store.tagFilter.test.ts
new file mode 100644
index 0000000..e4f6765
--- /dev/null
+++ b/tests/unit/store.tagFilter.test.ts
@@ -0,0 +1,50 @@
+import { describe, it, expect } from 'vitest';
+import { selectFilteredNotes } from '../../src/renderer/inbox/selectFilteredNotes.js';
+import type { Note } from '@shared/types';
+
+function sample(id: string, tags: string[]): Note {
+  return {
+    id,
+    rawText: 'x',
+    aiTitle: null,
+    aiSummary: null,
+    aiStatus: 'done',
+    aiError: null,
+    aiProvider: null,
+    aiGeneratedAt: null,
+    titleEditedByUser: false,
+    summaryEditedByUser: false,
+    userIntent: null,
+    intentPromptedAt: null,
+    dueDate: null,
+    dueDateEditedByUser: false,
+    createdAt: '2026-04-26T00:00:00Z',
+    updatedAt: '2026-04-26T00:00:00Z',
+    tags: tags.map((name) => ({ name, source: 'ai' as const })),
+    media: []
+  };
+}
+
+describe('selectFilteredNotes', () => {
+  it('returns all notes when filter is null', () => {
+    const notes = [sample('a', ['x']), sample('b', ['y'])];
+    expect(selectFilteredNotes({ notes, tagFilter: null }).length).toBe(2);
+  });
+
+  it('filters to notes containing the tag', () => {
+    const notes = [sample('a', ['pr', 'review']), sample('b', ['demo'])];
+    const out = selectFilteredNotes({ notes, tagFilter: 'pr' });
+    expect(out.length).toBe(1);
+    expect(out[0]!.id).toBe('a');
+  });
+
+  it('empty filter result when tag not present', () => {
+    const notes = [sample('a', ['x'])];
+    expect(selectFilteredNotes({ notes, tagFilter: 'missing' }).length).toBe(0);
+  });
+
+  it('matches against any tag of the note', () => {
+    const notes = [sample('a', ['alpha', 'beta', 'gamma'])];
+    expect(selectFilteredNotes({ notes, tagFilter: 'gamma' }).length).toBe(1);
+  });
+});