diff --git a/app/build.gradle.kts b/app/build.gradle.kts
index 13cf4bec..ceaafcaa 100644
--- a/app/build.gradle.kts
+++ b/app/build.gradle.kts
@@ -109,6 +109,12 @@ dependencies {
     // Encrypted storage
     implementation("androidx.security:security-crypto:1.0.0")
 
+    // Play Integrity API — runtime device integrity check
+    implementation("com.google.android.play:integrity:1.4.0")
+
+    // Firebase Functions — callable for server-side integrity token verification
+    implementation("com.google.firebase:firebase-functions-ktx")
+
     // RevenueCat native Android SDK (group: com.revenuecat.purchases, artifact: purchases)
     implementation("com.revenuecat.purchases:purchases:8.20.0")
 
diff --git a/app/src/main/java/app/closer/CloserApp.kt b/app/src/main/java/app/closer/CloserApp.kt
index 7fb6b51f..21dd7665 100644
--- a/app/src/main/java/app/closer/CloserApp.kt
+++ b/app/src/main/java/app/closer/CloserApp.kt
@@ -3,17 +3,26 @@ package app.closer
 import android.app.Application
 import app.closer.core.firebase.FirebaseInitializer
 import app.closer.data.repository.ActivityProvider
+import app.closer.domain.security.DeviceIntegrityChecker
 import dagger.hilt.android.HiltAndroidApp
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.SupervisorJob
+import kotlinx.coroutines.launch
 import javax.inject.Inject
 
 @HiltAndroidApp
 class CloserApp : Application() {
 
     @Inject lateinit var firebaseInitializer: FirebaseInitializer
+    @Inject lateinit var deviceIntegrityChecker: DeviceIntegrityChecker
+
+    private val appScope = CoroutineScope(SupervisorJob() + Dispatchers.IO)
 
     override fun onCreate() {
         super.onCreate()
         ActivityProvider.register(this)
         firebaseInitializer.initialize()
+        appScope.launch { deviceIntegrityChecker.runCheck() }
     }
 }
diff --git a/app/src/main/java/app/closer/data/security/PlayIntegrityChecker.kt b/app/src/main/java/app/closer/data/security/PlayIntegrityChecker.kt
new file mode 100644
index 00000000..f02834c8
--- /dev/null
+++ b/app/src/main/java/app/closer/data/security/PlayIntegrityChecker.kt
@@ -0,0 +1,96 @@
+package app.closer.data.security
+
+import android.content.Context
+import android.util.Base64
+import app.closer.BuildConfig
+import app.closer.domain.security.DeviceIntegrityChecker
+import app.closer.domain.security.DeviceIntegrityResult
+import com.google.android.play.core.integrity.IntegrityManagerFactory
+import com.google.android.play.core.integrity.IntegrityTokenRequest
+import com.google.firebase.functions.FirebaseFunctions
+import dagger.hilt.android.qualifiers.ApplicationContext
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.suspendCancellableCoroutine
+import java.util.UUID
+import javax.inject.Inject
+import javax.inject.Singleton
+import kotlin.coroutines.resume
+import kotlin.coroutines.resumeWithException
+
+/**
+ * Checks device integrity using the Play Integrity API.
+ *
+ * Flow:
+ * 1. Requests a signed integrity token from the Play Integrity API on-device.
+ * 2. Sends the token to the [checkDeviceIntegrity] Cloud Function for server-side
+ *    verification via the Google Play Integrity API.
+ * 3. Exposes the verdict through [state].
+ *
+ * The check runs once at app startup. A [COMPROMISED] result does not hard-block
+ * the app — it surfaces a warning so users on unintentionally modified devices
+ * can investigate. Firebase App Check (Play Integrity) already blocks Firebase
+ * API calls from compromised environments; this provides an in-app signal.
+ *
+ * In debug builds the check is skipped (many dev devices have unlocked bootloaders).
+ *
+ * Setup required:
+ * - Enable the Play Integrity API in your Google Cloud project.
+ * - Ensure the Cloud Functions service account has the "Play Integrity API User" role,
+ *   or link the Cloud project to your Play Console app in the Play Console settings.
+ */
+@Singleton
+class PlayIntegrityChecker @Inject constructor(
+    @ApplicationContext private val context: Context,
+    private val functions: FirebaseFunctions,
+) : DeviceIntegrityChecker {
+
+    private val _state = MutableStateFlow(DeviceIntegrityResult.UNKNOWN)
+    override val state: StateFlow<DeviceIntegrityResult> = _state.asStateFlow()
+
+    override suspend fun runCheck() {
+        if (BuildConfig.DEBUG) {
+            _state.value = DeviceIntegrityResult.UNAVAILABLE
+            return
+        }
+
+        val token = try {
+            requestToken()
+        } catch (_: Exception) {
+            _state.value = DeviceIntegrityResult.UNAVAILABLE
+            return
+        }
+
+        val passed = verifyWithServer(token)
+        _state.value = if (passed) DeviceIntegrityResult.PASSED else DeviceIntegrityResult.COMPROMISED
+    }
+
+    private suspend fun requestToken(): String = suspendCancellableCoroutine { cont ->
+        val nonce = Base64.encodeToString(
+            "${UUID.randomUUID()}-${System.currentTimeMillis()}".toByteArray(Charsets.UTF_8),
+            Base64.URL_SAFE or Base64.NO_WRAP,
+        )
+        IntegrityManagerFactory.create(context)
+            .requestIntegrityToken(IntegrityTokenRequest.builder().setNonce(nonce).build())
+            .addOnSuccessListener { cont.resume(it.token()) }
+            .addOnFailureListener { cont.resumeWithException(it) }
+    }
+
+    private suspend fun verifyWithServer(token: String): Boolean =
+        suspendCancellableCoroutine { cont ->
+            functions
+                .getHttpsCallable("checkDeviceIntegrity")
+                .call(mapOf("token" to token))
+                .addOnSuccessListener { result ->
+                    @Suppress("UNCHECKED_CAST")
+                    val passed = (result.getData() as? Map<*, *>)?.get("passed") as? Boolean ?: true
+                    cont.resume(passed)
+                }
+                .addOnFailureListener {
+                    // Fail-open: if server verification is unavailable, don't penalise the user.
+                    // Firebase App Check remains the server-side gatekeeper.
+                    cont.resume(true)
+                }
+        }
+}
diff --git a/app/src/main/java/app/closer/di/SecurityModule.kt b/app/src/main/java/app/closer/di/SecurityModule.kt
new file mode 100644
index 00000000..e993d3af
--- /dev/null
+++ b/app/src/main/java/app/closer/di/SecurityModule.kt
@@ -0,0 +1,24 @@
+package app.closer.di
+
+import app.closer.data.security.PlayIntegrityChecker
+import app.closer.domain.security.DeviceIntegrityChecker
+import com.google.firebase.functions.FirebaseFunctions
+import dagger.Binds
+import dagger.Module
+import dagger.Provides
+import dagger.hilt.InstallIn
+import dagger.hilt.components.SingletonComponent
+import javax.inject.Singleton
+
+@Module
+@InstallIn(SingletonComponent::class)
+abstract class SecurityModule {
+
+    @Binds @Singleton
+    abstract fun bindDeviceIntegrityChecker(impl: PlayIntegrityChecker): DeviceIntegrityChecker
+
+    companion object {
+        @Provides @Singleton
+        fun provideFirebaseFunctions(): FirebaseFunctions = FirebaseFunctions.getInstance()
+    }
+}
diff --git a/app/src/main/java/app/closer/domain/security/DeviceIntegrityChecker.kt b/app/src/main/java/app/closer/domain/security/DeviceIntegrityChecker.kt
new file mode 100644
index 00000000..e5b0a309
--- /dev/null
+++ b/app/src/main/java/app/closer/domain/security/DeviceIntegrityChecker.kt
@@ -0,0 +1,15 @@
+package app.closer.domain.security
+
+import kotlinx.coroutines.flow.StateFlow
+
+enum class DeviceIntegrityResult {
+    UNKNOWN,        // check not yet run
+    PASSED,         // device meets basic integrity requirements
+    COMPROMISED,    // device is rooted, unlocked, or otherwise modified
+    UNAVAILABLE,    // Play Integrity API not available (old device, no Play Services, etc.)
+}
+
+interface DeviceIntegrityChecker {
+    val state: StateFlow<DeviceIntegrityResult>
+    suspend fun runCheck()
+}
diff --git a/functions/dist/index.js b/functions/dist/index.js
index ca7934cc..e739c6fd 100644
--- a/functions/dist/index.js
+++ b/functions/dist/index.js
@@ -33,7 +33,7 @@ var __importStar = (this && this.__importStar) || (function () {
     };
 })();
 Object.defineProperty(exports, "__esModule", { value: true });
-exports.health = exports.sendPartnerAnsweredNotification = exports.sendDailyQuestionReminder = exports.syncEntitlement = exports.revenueCatWebhook = void 0;
+exports.health = exports.checkDeviceIntegrity = exports.sendPartnerAnsweredNotification = exports.sendDailyQuestionReminder = exports.syncEntitlement = exports.revenueCatWebhook = void 0;
 const functions = __importStar(require("firebase-functions"));
 var revenueCatWebhook_1 = require("./billing/revenueCatWebhook");
 Object.defineProperty(exports, "revenueCatWebhook", { enumerable: true, get: function () { return revenueCatWebhook_1.revenueCatWebhook; } });
@@ -42,6 +42,8 @@ Object.defineProperty(exports, "syncEntitlement", { enumerable: true, get: funct
 var reminders_1 = require("./notifications/reminders");
 Object.defineProperty(exports, "sendDailyQuestionReminder", { enumerable: true, get: function () { return reminders_1.sendDailyQuestionReminder; } });
 Object.defineProperty(exports, "sendPartnerAnsweredNotification", { enumerable: true, get: function () { return reminders_1.sendPartnerAnsweredNotification; } });
+var checkDeviceIntegrity_1 = require("./security/checkDeviceIntegrity");
+Object.defineProperty(exports, "checkDeviceIntegrity", { enumerable: true, get: function () { return checkDeviceIntegrity_1.checkDeviceIntegrity; } });
 /**
  * Basic health check callable.
  * Useful for verifying function deployment and firebase-tools wiring.
diff --git a/functions/dist/index.js.map b/functions/dist/index.js.map
index 99a62658..a30d7395 100644
--- a/functions/dist/index.js.map
+++ b/functions/dist/index.js.map
@@ -1 +1 @@
-{"version":3,"file":"index.js","sourceRoot":"","sources":["../src/index.ts"],"names":[],"mappings":";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;AAAA,8DAA+C;AAE/C,iEAA+D;AAAtD,sHAAA,iBAAiB,OAAA;AAC1B,6DAA2D;AAAlD,kHAAA,eAAe,OAAA;AACxB,uDAGkC;AAFhC,sHAAA,yBAAyB,OAAA;AACzB,4HAAA,+BAA+B,OAAA;AAGjC;;;GAGG;AACU,QAAA,MAAM,GAAG,SAAS,CAAC,KAAK,CAAC,SAAS,CAAC,CAAC,GAAG,EAAE,GAAG,EAAE,EAAE;IAC3D,GAAG,CAAC,MAAM,CAAC,GAAG,CAAC,CAAC,IAAI,CAAC,EAAE,MAAM,EAAE,IAAI,EAAE,CAAC,CAAA;AACxC,CAAC,CAAC,CAAA"}
\ No newline at end of file
+{"version":3,"file":"index.js","sourceRoot":"","sources":["../src/index.ts"],"names":[],"mappings":";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;AAAA,8DAA+C;AAE/C,iEAA+D;AAAtD,sHAAA,iBAAiB,OAAA;AAC1B,6DAA2D;AAAlD,kHAAA,eAAe,OAAA;AACxB,uDAGkC;AAFhC,sHAAA,yBAAyB,OAAA;AACzB,4HAAA,+BAA+B,OAAA;AAEjC,wEAAsE;AAA7D,4HAAA,oBAAoB,OAAA;AAE7B;;;GAGG;AACU,QAAA,MAAM,GAAG,SAAS,CAAC,KAAK,CAAC,SAAS,CAAC,CAAC,GAAG,EAAE,GAAG,EAAE,EAAE;IAC3D,GAAG,CAAC,MAAM,CAAC,GAAG,CAAC,CAAC,IAAI,CAAC,EAAE,MAAM,EAAE,IAAI,EAAE,CAAC,CAAA;AACxC,CAAC,CAAC,CAAA"}
\ No newline at end of file
diff --git a/functions/dist/security/checkDeviceIntegrity.js b/functions/dist/security/checkDeviceIntegrity.js
new file mode 100644
index 00000000..d661782e
--- /dev/null
+++ b/functions/dist/security/checkDeviceIntegrity.js
@@ -0,0 +1,94 @@
+"use strict";
+var __createBinding = (this && this.__createBinding) || (Object.create ? (function(o, m, k, k2) {
+    if (k2 === undefined) k2 = k;
+    var desc = Object.getOwnPropertyDescriptor(m, k);
+    if (!desc || ("get" in desc ? !m.__esModule : desc.writable || desc.configurable)) {
+      desc = { enumerable: true, get: function() { return m[k]; } };
+    }
+    Object.defineProperty(o, k2, desc);
+}) : (function(o, m, k, k2) {
+    if (k2 === undefined) k2 = k;
+    o[k2] = m[k];
+}));
+var __setModuleDefault = (this && this.__setModuleDefault) || (Object.create ? (function(o, v) {
+    Object.defineProperty(o, "default", { enumerable: true, value: v });
+}) : function(o, v) {
+    o["default"] = v;
+});
+var __importStar = (this && this.__importStar) || (function () {
+    var ownKeys = function(o) {
+        ownKeys = Object.getOwnPropertyNames || function (o) {
+            var ar = [];
+            for (var k in o) if (Object.prototype.hasOwnProperty.call(o, k)) ar[ar.length] = k;
+            return ar;
+        };
+        return ownKeys(o);
+    };
+    return function (mod) {
+        if (mod && mod.__esModule) return mod;
+        var result = {};
+        if (mod != null) for (var k = ownKeys(mod), i = 0; i < k.length; i++) if (k[i] !== "default") __createBinding(result, mod, k[i]);
+        __setModuleDefault(result, mod);
+        return result;
+    };
+})();
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.checkDeviceIntegrity = void 0;
+const functions = __importStar(require("firebase-functions"));
+const google_auth_library_1 = require("google-auth-library");
+const PACKAGE_NAME = 'app.closer';
+const PLAY_INTEGRITY_URL = `https://playintegrity.googleapis.com/v1/${PACKAGE_NAME}:decodeIntegrityToken`;
+/**
+ * Verifies a Play Integrity API token server-side and returns whether the
+ * device meets basic integrity requirements.
+ *
+ * Called by [PlayIntegrityChecker] on app startup. Requires the caller to be
+ * authenticated — the Firebase Auth token is verified automatically by the
+ * Functions runtime.
+ *
+ * Setup required before this function works in production:
+ * 1. Enable the Play Integrity API in your Google Cloud project.
+ * 2. Link the Cloud project to your Play Console app (Play Console →
+ *    Setup → API access → Link to Cloud project).
+ * 3. Grant the Cloud Functions service account the "Play Integrity API User"
+ *    role in IAM, or use a dedicated service account key via
+ *    GOOGLE_APPLICATION_CREDENTIALS.
+ *
+ * If the API is not yet configured the function fails-open (returns
+ * passed: true) and logs the error, so unconfigured environments don't
+ * block users. Firebase App Check remains the server-side gatekeeper.
+ */
+exports.checkDeviceIntegrity = functions.https.onCall(async (data, context) => {
+    if (!context.auth) {
+        throw new functions.https.HttpsError('unauthenticated', 'Caller must be authenticated.');
+    }
+    const token = data === null || data === void 0 ? void 0 : data.token;
+    if (!token || typeof token !== 'string') {
+        throw new functions.https.HttpsError('invalid-argument', 'token is required.');
+    }
+    try {
+        const verdicts = await decodeIntegrityToken(token);
+        const passed = verdicts.includes('MEETS_DEVICE_INTEGRITY') ||
+            verdicts.includes('MEETS_STRONG_INTEGRITY');
+        return { passed, verdicts };
+    }
+    catch (err) {
+        console.error('[checkDeviceIntegrity] verification unavailable:', err);
+        // Fail-open: treat as passed if the API is not yet configured.
+        return { passed: true, verdicts: [], error: 'verification_unavailable' };
+    }
+});
+async function decodeIntegrityToken(token) {
+    var _a, _b, _c;
+    const auth = new google_auth_library_1.GoogleAuth({
+        scopes: ['https://www.googleapis.com/auth/playintegrity'],
+    });
+    const client = await auth.getClient();
+    const response = await client.request({
+        url: PLAY_INTEGRITY_URL,
+        method: 'POST',
+        data: { integrity_token: token },
+    });
+    return ((_c = (_b = (_a = response.data.tokenPayloadExternal) === null || _a === void 0 ? void 0 : _a.deviceIntegrity) === null || _b === void 0 ? void 0 : _b.deviceRecognitionVerdict) !== null && _c !== void 0 ? _c : []);
+}
+//# sourceMappingURL=checkDeviceIntegrity.js.map
\ No newline at end of file
diff --git a/functions/dist/security/checkDeviceIntegrity.js.map b/functions/dist/security/checkDeviceIntegrity.js.map
new file mode 100644
index 00000000..a4880234
--- /dev/null
+++ b/functions/dist/security/checkDeviceIntegrity.js.map
@@ -0,0 +1 @@
+{"version":3,"file":"checkDeviceIntegrity.js","sourceRoot":"","sources":["../../src/security/checkDeviceIntegrity.ts"],"names":[],"mappings":";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;AAAA,8DAA+C;AAC/C,6DAAgD;AAEhD,MAAM,YAAY,GAAG,YAAY,CAAA;AACjC,MAAM,kBAAkB,GACtB,2CAA2C,YAAY,uBAAuB,CAAA;AAEhF;;;;;;;;;;;;;;;;;;;GAmBG;AACU,QAAA,oBAAoB,GAAG,SAAS,CAAC,KAAK,CAAC,MAAM,CACxD,KAAK,EAAE,IAAwB,EAAE,OAAO,EAAE,EAAE;IAC1C,IAAI,CAAC,OAAO,CAAC,IAAI,EAAE,CAAC;QAClB,MAAM,IAAI,SAAS,CAAC,KAAK,CAAC,UAAU,CAClC,iBAAiB,EACjB,+BAA+B,CAChC,CAAA;IACH,CAAC;IAED,MAAM,KAAK,GAAG,IAAI,aAAJ,IAAI,uBAAJ,IAAI,CAAE,KAAK,CAAA;IACzB,IAAI,CAAC,KAAK,IAAI,OAAO,KAAK,KAAK,QAAQ,EAAE,CAAC;QACxC,MAAM,IAAI,SAAS,CAAC,KAAK,CAAC,UAAU,CAClC,kBAAkB,EAClB,oBAAoB,CACrB,CAAA;IACH,CAAC;IAED,IAAI,CAAC;QACH,MAAM,QAAQ,GAAG,MAAM,oBAAoB,CAAC,KAAK,CAAC,CAAA;QAClD,MAAM,MAAM,GACV,QAAQ,CAAC,QAAQ,CAAC,wBAAwB,CAAC;YAC3C,QAAQ,CAAC,QAAQ,CAAC,wBAAwB,CAAC,CAAA;QAC7C,OAAO,EAAE,MAAM,EAAE,QAAQ,EAAE,CAAA;IAC7B,CAAC;IAAC,OAAO,GAAG,EAAE,CAAC;QACb,OAAO,CAAC,KAAK,CAAC,kDAAkD,EAAE,GAAG,CAAC,CAAA;QACtE,+DAA+D;QAC/D,OAAO,EAAE,MAAM,EAAE,IAAI,EAAE,QAAQ,EAAE,EAAE,EAAE,KAAK,EAAE,0BAA0B,EAAE,CAAA;IAC1E,CAAC;AACH,CAAC,CACF,CAAA;AAUD,KAAK,UAAU,oBAAoB,CAAC,KAAa;;IAC/C,MAAM,IAAI,GAAG,IAAI,gCAAU,CAAC;QAC1B,MAAM,EAAE,CAAC,+CAA+C,CAAC;KAC1D,CAAC,CAAA;IACF,MAAM,MAAM,GAAG,MAAM,IAAI,CAAC,SAAS,EAAE,CAAA;IACrC,MAAM,QAAQ,GAAG,MAAM,MAAM,CAAC,OAAO,CAAoB;QACvD,GAAG,EAAE,kBAAkB;QACvB,MAAM,EAAE,MAAM;QACd,IAAI,EAAE,EAAE,eAAe,EAAE,KAAK,EAAE;KACjC,CAAC,CAAA;IACF,OAAO,CACL,MAAA,MAAA,MAAA,QAAQ,CAAC,IAAI,CAAC,oBAAoB,0CAAE,eAAe,0CAC/C,wBAAwB,mCAAI,EAAE,CACnC,CAAA;AACH,CAAC"}
\ No newline at end of file
diff --git a/functions/package-lock.json b/functions/package-lock.json
index 4f143abc..dfb5b939 100644
--- a/functions/package-lock.json
+++ b/functions/package-lock.json
@@ -9,7 +9,8 @@
       "version": "1.0.0",
       "dependencies": {
         "firebase-admin": "^12.2.0",
-        "firebase-functions": "^5.1.0"
+        "firebase-functions": "^5.1.0",
+        "google-auth-library": "^9.0.0"
       },
       "devDependencies": {
         "typescript": "^5.4.5"
@@ -517,7 +518,6 @@
       "resolved": "https://registry.npmjs.org/agent-base/-/agent-base-7.1.4.tgz",
       "integrity": "sha512-MnA+YT8fwfJPgBx3m60MNqakm30XOkyIoH1y6huTQvC0PwZG7ki8NacLBcrPbNoo8vEZy7Jpuk7+jMO+CUovTQ==",
       "license": "MIT",
-      "optional": true,
       "engines": {
         "node": ">= 14"
       }
@@ -612,15 +612,13 @@
           "url": "https://feross.org/support"
         }
       ],
-      "license": "MIT",
-      "optional": true
+      "license": "MIT"
     },
     "node_modules/bignumber.js": {
       "version": "9.3.1",
       "resolved": "https://registry.npmjs.org/bignumber.js/-/bignumber.js-9.3.1.tgz",
       "integrity": "sha512-Ko0uX15oIUS7wJ3Rb30Fs6SkVbLmPBAKdlm7q9+ak9bbIeFf0MwuBsQV6z7+X768/cHsfg+WlysDWJcmthjsjQ==",
       "license": "MIT",
-      "optional": true,
       "engines": {
         "node": "*"
       }
@@ -1031,8 +1029,7 @@
       "version": "3.0.2",
       "resolved": "https://registry.npmjs.org/extend/-/extend-3.0.2.tgz",
       "integrity": "sha512-fjquC59cD7CyW6urNXK0FBufkZcoiGG80wTuPujX590cB5Ttln20E2UB4S/WARVqhXffZl2LNgS+gQdPIIim/g==",
-      "license": "MIT",
-      "optional": true
+      "license": "MIT"
     },
     "node_modules/farmhash-modern": {
       "version": "1.1.0",
@@ -1224,7 +1221,6 @@
       "resolved": "https://registry.npmjs.org/gaxios/-/gaxios-6.7.1.tgz",
       "integrity": "sha512-LDODD4TMYx7XXdpwxAVRAIAuB0bzv0s+ywFonY46k126qzQHT9ygyoa9tncmOiQmmDrik65UYsEkv3lbfqQ3yQ==",
       "license": "Apache-2.0",
-      "optional": true,
       "dependencies": {
         "extend": "^3.0.2",
         "https-proxy-agent": "^7.0.1",
@@ -1246,7 +1242,6 @@
         "https://github.com/sponsors/ctavan"
       ],
       "license": "MIT",
-      "optional": true,
       "bin": {
         "uuid": "dist/bin/uuid"
       }
@@ -1256,7 +1251,6 @@
       "resolved": "https://registry.npmjs.org/gcp-metadata/-/gcp-metadata-6.1.1.tgz",
       "integrity": "sha512-a4tiq7E0/5fTjxPAaH4jpjkSv/uCaU2p5KC6HVGrvl0cDjA8iBZv4vv1gyzlmK0ZUKqwpOyQMKzZQe3lTit77A==",
       "license": "Apache-2.0",
-      "optional": true,
       "dependencies": {
         "gaxios": "^6.1.1",
         "google-logging-utils": "^0.0.2",
@@ -1318,7 +1312,6 @@
       "resolved": "https://registry.npmjs.org/google-auth-library/-/google-auth-library-9.15.1.tgz",
       "integrity": "sha512-Jb6Z0+nvECVz+2lzSMt9u98UsoakXxA2HGHMCxh+so3n90XgYWkq5dur19JAJV7ONiJY22yBTyJB1TSkvPq9Ng==",
       "license": "Apache-2.0",
-      "optional": true,
       "dependencies": {
         "base64-js": "^1.3.0",
         "ecdsa-sig-formatter": "^1.0.11",
@@ -1375,7 +1368,6 @@
       "resolved": "https://registry.npmjs.org/google-logging-utils/-/google-logging-utils-0.0.2.tgz",
       "integrity": "sha512-NEgUnEcBiP5HrPzufUkBzJOD/Sxsco3rLNo1F1TNf7ieU8ryUzBhqba8r756CjLX7rn3fHl6iLEwPYuqpoKgQQ==",
       "license": "Apache-2.0",
-      "optional": true,
       "engines": {
         "node": ">=14"
       }
@@ -1397,7 +1389,6 @@
       "resolved": "https://registry.npmjs.org/gtoken/-/gtoken-7.1.0.tgz",
       "integrity": "sha512-pCcEwRi+TKpMlxAQObHDQ56KawURgyAf6jtIY046fJ5tIv3zDe/LEIubckAO8fj6JnAxLdmWkUfNyulQ2iKdEw==",
       "license": "MIT",
-      "optional": true,
       "dependencies": {
         "gaxios": "^6.0.0",
         "jws": "^4.0.0"
@@ -1547,7 +1538,6 @@
       "resolved": "https://registry.npmjs.org/https-proxy-agent/-/https-proxy-agent-7.0.6.tgz",
       "integrity": "sha512-vK9P5/iUfdl95AI+JVyUuIcVtd4ofvtrOr3HNtM2yxC9bnMbEdp3x01OhQNnjb8IJYi38VlTE3mBXwcfvywuSw==",
       "license": "MIT",
-      "optional": true,
       "dependencies": {
         "agent-base": "^7.1.2",
         "debug": "4"
@@ -1561,7 +1551,6 @@
       "resolved": "https://registry.npmjs.org/debug/-/debug-4.4.3.tgz",
       "integrity": "sha512-RGwwWnwQvkVfavKVt22FGLw+xYSdzARwm0ru6DhTVA3umU5hZc28V3kO4stgYryrTlLpuvgI9GiijltAjNbcqA==",
       "license": "MIT",
-      "optional": true,
       "dependencies": {
         "ms": "^2.1.3"
       },
@@ -1578,8 +1567,7 @@
       "version": "2.1.3",
       "resolved": "https://registry.npmjs.org/ms/-/ms-2.1.3.tgz",
       "integrity": "sha512-6FlzubTLZG3J2a/NVCAleEhjzq5oxgHyaCU9yYXvcLsvoVaHJq/s5xXI6/XXP6tz7R9xAOtHnSO/tXtF3WRTlA==",
-      "license": "MIT",
-      "optional": true
+      "license": "MIT"
     },
     "node_modules/iconv-lite": {
       "version": "0.4.24",
@@ -1623,7 +1611,6 @@
       "resolved": "https://registry.npmjs.org/is-stream/-/is-stream-2.0.1.tgz",
       "integrity": "sha512-hFoiJiTl63nn+kstHGBtewWSKnQLpyb155KHheA1l39uvtO9nWIop1p3udqPcUd/xbF1VLMO4n7OI6p7RbngDg==",
       "license": "MIT",
-      "optional": true,
       "engines": {
         "node": ">=8"
       },
@@ -1658,7 +1645,6 @@
       "resolved": "https://registry.npmjs.org/json-bigint/-/json-bigint-1.0.0.tgz",
       "integrity": "sha512-SiPv/8VpZuWbvLSMtTDU8hEfrZWg/mH/nV/b4o0CYbSxu1UIQPLdwKOCIyLQX+VIPO5vrLX3i8qtqFyhdPSUSQ==",
       "license": "MIT",
-      "optional": true,
       "dependencies": {
         "bignumber.js": "^9.0.0"
       }
@@ -1929,7 +1915,6 @@
       "resolved": "https://registry.npmjs.org/node-fetch/-/node-fetch-2.7.0.tgz",
       "integrity": "sha512-c4FRfUm/dbcWZ7U+1Wq0AwCyFL+3nt2bEw05wfxSz+DWpWsitgmSgYmy2dQdWyKC1694ELPqMs/YzUSNozLt8A==",
       "license": "MIT",
-      "optional": true,
       "dependencies": {
         "whatwg-url": "^5.0.0"
       },
@@ -2549,8 +2534,7 @@
       "version": "0.0.3",
       "resolved": "https://registry.npmjs.org/tr46/-/tr46-0.0.3.tgz",
       "integrity": "sha512-N3WMsuqV66lT30CrXNbEjx4GEwlow3v6rr4mCcv6prnfwhS01rkgyFdjPNBYd9br7LpXV1+Emh01fHnq2Gdgrw==",
-      "license": "MIT",
-      "optional": true
+      "license": "MIT"
     },
     "node_modules/tslib": {
       "version": "2.8.1",
@@ -2643,8 +2627,7 @@
       "version": "3.0.1",
       "resolved": "https://registry.npmjs.org/webidl-conversions/-/webidl-conversions-3.0.1.tgz",
       "integrity": "sha512-2JAn3z8AR6rjK8Sm8orRC0h/bcl/DqL7tRPdGZ4I1CjdF+EaMLmYxBHyXuKL849eucPFhvBoxMsflfOb8kxaeQ==",
-      "license": "BSD-2-Clause",
-      "optional": true
+      "license": "BSD-2-Clause"
     },
     "node_modules/websocket-driver": {
       "version": "0.7.5",
@@ -2674,7 +2657,6 @@
       "resolved": "https://registry.npmjs.org/whatwg-url/-/whatwg-url-5.0.0.tgz",
       "integrity": "sha512-saE57nupxk6v3HY35+jzBwYa0rKSy0XR8JSxZPwgLr7ys0IBzhGviA1/TUGJLmSVqs8pb9AnvICXEuOHLprYTw==",
       "license": "MIT",
-      "optional": true,
       "dependencies": {
         "tr46": "~0.0.3",
         "webidl-conversions": "^3.0.0"
diff --git a/functions/package.json b/functions/package.json
index 29ca634e..07792205 100644
--- a/functions/package.json
+++ b/functions/package.json
@@ -16,7 +16,8 @@
   },
   "dependencies": {
     "firebase-admin": "^12.2.0",
-    "firebase-functions": "^5.1.0"
+    "firebase-functions": "^5.1.0",
+    "google-auth-library": "^9.0.0"
   },
   "devDependencies": {
     "typescript": "^5.4.5"
diff --git a/functions/src/index.ts b/functions/src/index.ts
index c4c641e4..f63ccd4e 100644
--- a/functions/src/index.ts
+++ b/functions/src/index.ts
@@ -6,6 +6,7 @@ export {
   sendDailyQuestionReminder,
   sendPartnerAnsweredNotification,
 } from './notifications/reminders'
+export { checkDeviceIntegrity } from './security/checkDeviceIntegrity'
 
 /**
  * Basic health check callable.
diff --git a/functions/src/security/checkDeviceIntegrity.ts b/functions/src/security/checkDeviceIntegrity.ts
new file mode 100644
index 00000000..662bfd68
--- /dev/null
+++ b/functions/src/security/checkDeviceIntegrity.ts
@@ -0,0 +1,81 @@
+import * as functions from 'firebase-functions'
+import { GoogleAuth } from 'google-auth-library'
+
+const PACKAGE_NAME = 'app.closer'
+const PLAY_INTEGRITY_URL =
+  `https://playintegrity.googleapis.com/v1/${PACKAGE_NAME}:decodeIntegrityToken`
+
+/**
+ * Verifies a Play Integrity API token server-side and returns whether the
+ * device meets basic integrity requirements.
+ *
+ * Called by [PlayIntegrityChecker] on app startup. Requires the caller to be
+ * authenticated — the Firebase Auth token is verified automatically by the
+ * Functions runtime.
+ *
+ * Setup required before this function works in production:
+ * 1. Enable the Play Integrity API in your Google Cloud project.
+ * 2. Link the Cloud project to your Play Console app (Play Console →
+ *    Setup → API access → Link to Cloud project).
+ * 3. Grant the Cloud Functions service account the "Play Integrity API User"
+ *    role in IAM, or use a dedicated service account key via
+ *    GOOGLE_APPLICATION_CREDENTIALS.
+ *
+ * If the API is not yet configured the function fails-open (returns
+ * passed: true) and logs the error, so unconfigured environments don't
+ * block users. Firebase App Check remains the server-side gatekeeper.
+ */
+export const checkDeviceIntegrity = functions.https.onCall(
+  async (data: { token?: string }, context) => {
+    if (!context.auth) {
+      throw new functions.https.HttpsError(
+        'unauthenticated',
+        'Caller must be authenticated.'
+      )
+    }
+
+    const token = data?.token
+    if (!token || typeof token !== 'string') {
+      throw new functions.https.HttpsError(
+        'invalid-argument',
+        'token is required.'
+      )
+    }
+
+    try {
+      const verdicts = await decodeIntegrityToken(token)
+      const passed =
+        verdicts.includes('MEETS_DEVICE_INTEGRITY') ||
+        verdicts.includes('MEETS_STRONG_INTEGRITY')
+      return { passed, verdicts }
+    } catch (err) {
+      console.error('[checkDeviceIntegrity] verification unavailable:', err)
+      // Fail-open: treat as passed if the API is not yet configured.
+      return { passed: true, verdicts: [], error: 'verification_unavailable' }
+    }
+  }
+)
+
+interface IntegrityResponse {
+  tokenPayloadExternal?: {
+    deviceIntegrity?: {
+      deviceRecognitionVerdict?: string[]
+    }
+  }
+}
+
+async function decodeIntegrityToken(token: string): Promise<string[]> {
+  const auth = new GoogleAuth({
+    scopes: ['https://www.googleapis.com/auth/playintegrity'],
+  })
+  const client = await auth.getClient()
+  const response = await client.request<IntegrityResponse>({
+    url: PLAY_INTEGRITY_URL,
+    method: 'POST',
+    data: { integrity_token: token },
+  })
+  return (
+    response.data.tokenPayloadExternal?.deviceIntegrity
+      ?.deviceRecognitionVerdict ?? []
+  )
+}