diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index bef21e6a..1e5e9bb7 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -239,6 +239,25 @@ jobs:
       - name: Lint JS
         run: npm run lint:js
 
+  # ######################################
+  # Jest unit tests
+  # ######################################
+  jest-tests:
+    name: Jest
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-node@v6
+        with:
+          node-version-file: '.nvmrc'
+          cache: npm
+
+      - run: npm ci
+
+      - name: Run Jest unit tests
+        run: npm run test:unit
+
   # ######################################
   # Plugin Check
   # ######################################
diff --git a/src/settings/store/index.js b/src/settings/store/index.js
index ec6eb305..83bad5e5 100644
--- a/src/settings/store/index.js
+++ b/src/settings/store/index.js
@@ -4,34 +4,59 @@
 import apiFetch from '@wordpress/api-fetch';
 import { createReduxStore } from '@wordpress/data';
 
-// Shared editor state. Adding a key here gives you a `getKey` selector for
-// free; write a resolver below that returns `set( 'key', value )`.
+// Shared editor state, fetched lazily by the resolvers below; each key has a
+// matching selector. Most keys hold a single session-wide value. `voices`,
+// `videoSizes` and `project` are maps keyed by their resolver's argument:
+// @wordpress/data marks resolution finished per selector-args, so each
+// argument's result needs its own slot — with one shared slot, a later fetch
+// (say, another language's voices) would overwrite it while the resolution
+// cache keeps reporting the overwritten entry as fresh.
 export const DEFAULT_STATE = {
 	settings: {},
 	languages: [],
-	voices: [],
+	voices: {}, // languageCode → voice list
 	scriptTemplates: [],
 	videoTemplates: [],
-	videoSizes: [],
-	project: {},
+	videoSizes: {}, // projectId → size list
+	project: {}, // projectId → project record
 };
 
 const reducer = ( state = DEFAULT_STATE, action ) => {
+	// Replace a session-wide key.
 	if ( action.type === 'SET' && action.key in state ) {
 		const empty = Array.isArray( DEFAULT_STATE[ action.key ] ) ? [] : {};
 		return { ...state, [ action.key ]: action.value || empty };
 	}
+	// Merge one argument's result into a per-argument key, leaving the other
+	// arguments' entries untouched.
+	if ( action.type === 'SET_BY' && action.key in state ) {
+		return {
+			...state,
+			[ action.key ]: {
+				...state[ action.key ],
+				[ action.arg ]: action.value,
+			},
+		};
+	}
 	return state;
 };
 
-const selectors = Object.fromEntries(
-	Object.keys( DEFAULT_STATE ).map( ( key ) => [
-		`get${ key[ 0 ].toUpperCase() }${ key.slice( 1 ) }`,
-		( state ) => state[ key ],
-	] )
-);
+// The per-argument selectors (voices, video sizes, project) read the entry for
+// the requested argument only, with an empty value while it is unfetched.
+const selectors = {
+	getSettings: ( state ) => state.settings,
+	getLanguages: ( state ) => state.languages,
+	getVoices: ( state, languageCode ) => state.voices[ languageCode ] ?? [],
+	getScriptTemplates: ( state ) => state.scriptTemplates,
+	getVideoTemplates: ( state ) => state.videoTemplates,
+	getVideoSizes: ( state, projectId ) => state.videoSizes[ projectId ] ?? [],
+	getProject: ( state, projectId ) => state.project[ projectId ] ?? {},
+};
 
+// Both action shapes are private to this store — it registers no `actions`,
+// so the resolvers below are the only dispatchers.
 const set = ( key, value ) => ( { type: 'SET', key, value } );
+const setBy = ( key, arg, value ) => ( { type: 'SET_BY', key, arg, value } );
 
 const resolvers = {
 	async getSettings() {
@@ -46,7 +71,7 @@ const resolvers = {
 		const value = await apiFetch( {
 			path: `/beyondwords/v1/languages/${ languageCode }/voices`,
 		} );
-		return set( 'voices', value );
+		return setBy( 'voices', languageCode, value || [] );
 	},
 	async getScriptTemplates() {
 		const value = await apiFetch( {
@@ -62,23 +87,23 @@ const resolvers = {
 	},
 	async getVideoSizes( projectId ) {
 		if ( ! projectId ) {
-			return set( 'videoSizes', [] );
+			return setBy( 'videoSizes', projectId, [] );
 		}
 		const r = await apiFetch( {
 			path: `/beyondwords/v1/projects/${ projectId }/video-settings`,
 		} );
-		return set( 'videoSizes', r?.sizes ?? [] );
+		return setBy( 'videoSizes', projectId, r?.sizes ?? [] );
 	},
 	// The project carries the default `language` used to pre-select the Language
 	// dropdown when Customize is enabled. Fetched on demand (behind the toggle).
 	async getProject( projectId ) {
 		if ( ! projectId ) {
-			return set( 'project', {} );
+			return setBy( 'project', projectId, {} );
 		}
 		const value = await apiFetch( {
 			path: `/beyondwords/v1/projects/${ projectId }`,
 		} );
-		return set( 'project', value );
+		return setBy( 'project', projectId, value || {} );
 	},
 };
 
diff --git a/src/settings/store/index.test.js b/src/settings/store/index.test.js
new file mode 100644
index 00000000..d60a74bb
--- /dev/null
+++ b/src/settings/store/index.test.js
@@ -0,0 +1,125 @@
+/* global jest, describe, it, expect, beforeEach */
+
+/**
+ * WordPress dependencies
+ */
+import apiFetch from '@wordpress/api-fetch';
+import { createRegistry } from '@wordpress/data';
+
+/**
+ * Internal dependencies
+ */
+import store from './index';
+
+jest.mock( '@wordpress/api-fetch' );
+
+const STORE = 'beyondwords/settings';
+
+// Distinct data per resolver argument, so a value leaking from one argument's
+// fetch into another's slot is observable in the assertions below.
+const VOICES = {
+	en_US: [ { id: 1, name: 'Amy' } ],
+	fr_FR: [ { id: 2, name: 'Céline' } ],
+};
+
+const PROJECT = {
+	11111: { id: 11111, language: 'en_US' },
+	22222: { id: 22222, language: 'fr_FR' },
+};
+
+const VIDEO_SIZES = {
+	11111: [ { name: 'square', enabled: true } ],
+	22222: [ { name: 'portrait', enabled: true } ],
+};
+
+describe( 'beyondwords/settings store', () => {
+	let registry;
+
+	beforeEach( () => {
+		apiFetch.mockReset();
+		apiFetch.mockImplementation( ( { path } ) => {
+			let m = path.match( /languages\/([^/]+)\/voices$/ );
+			if ( m ) {
+				return Promise.resolve( VOICES[ m[ 1 ] ] ?? [] );
+			}
+			m = path.match( /projects\/([^/]+)\/video-settings$/ );
+			if ( m ) {
+				return Promise.resolve( {
+					sizes: VIDEO_SIZES[ m[ 1 ] ] ?? [],
+				} );
+			}
+			m = path.match( /projects\/([^/]+)$/ );
+			if ( m ) {
+				return Promise.resolve( PROJECT[ m[ 1 ] ] ?? {} );
+			}
+			return Promise.resolve( [] );
+		} );
+
+		registry = createRegistry();
+		registry.register( store );
+	} );
+
+	describe( 'getVoices', () => {
+		it( 'keeps each language in its own slot after another is fetched', async () => {
+			const resolve = registry.resolveSelect( STORE );
+
+			// Fetch A, then B. With the old single shared `voices` slot, B's
+			// response overwrote the value A's already-finished resolution read.
+			expect( await resolve.getVoices( 'en_US' ) ).toEqual(
+				VOICES.en_US
+			);
+			expect( await resolve.getVoices( 'fr_FR' ) ).toEqual(
+				VOICES.fr_FR
+			);
+
+			// Re-reading A must still return A's voices, not B's — no refetch is
+			// dispatched because resolution for [ 'en_US' ] is already finished.
+			const select = registry.select( STORE );
+			expect( select.getVoices( 'en_US' ) ).toEqual( VOICES.en_US );
+			expect( select.getVoices( 'fr_FR' ) ).toEqual( VOICES.fr_FR );
+		} );
+
+		it( 'resolves each language once and reuses the cached result', async () => {
+			const resolve = registry.resolveSelect( STORE );
+			await resolve.getVoices( 'en_US' );
+			await resolve.getVoices( 'en_US' );
+			await resolve.getVoices( 'fr_FR' );
+
+			// en_US fetched once (second call is a cache hit), fr_FR once.
+			expect( apiFetch ).toHaveBeenCalledTimes( 2 );
+		} );
+
+		it( 'returns an empty list for a language that has not been fetched', () => {
+			expect( registry.select( STORE ).getVoices( 'de_DE' ) ).toEqual(
+				[]
+			);
+		} );
+	} );
+
+	describe( 'getProject / getVideoSizes', () => {
+		it( 'keeps each project id in its own slot', async () => {
+			const resolve = registry.resolveSelect( STORE );
+			await resolve.getProject( '11111' );
+			await resolve.getProject( '22222' );
+			await resolve.getVideoSizes( '11111' );
+			await resolve.getVideoSizes( '22222' );
+
+			const select = registry.select( STORE );
+			expect( select.getProject( '11111' ) ).toEqual( PROJECT[ 11111 ] );
+			expect( select.getProject( '22222' ) ).toEqual( PROJECT[ 22222 ] );
+			expect( select.getVideoSizes( '11111' ) ).toEqual(
+				VIDEO_SIZES[ 11111 ]
+			);
+			expect( select.getVideoSizes( '22222' ) ).toEqual(
+				VIDEO_SIZES[ 22222 ]
+			);
+		} );
+
+		it( 'defaults to {} / [] and skips the fetch for a falsy id', async () => {
+			const resolve = registry.resolveSelect( STORE );
+			expect( await resolve.getProject( '' ) ).toEqual( {} );
+			expect( await resolve.getVideoSizes( '' ) ).toEqual( [] );
+			expect( apiFetch ).not.toHaveBeenCalled();
+		} );
+	} );
+} );