firebase · jrodiz · May 22, 2026
diff --git a/firebase-perf/CHANGELOG.md b/firebase-perf/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Unreleased
 
+- [fixed] Fixed `_app_start` traces being suppressed on API 34+ devices for typical
+  real-world apps. The previous timing-window heuristic has been replaced on API 34+ by
+  `RunningAppProcessInfo.importance` at first capture, which indicates whether the
+  process was forked to launch an activity. Pre-API-34 behavior is unchanged. [#8103]
+
 # 22.0.5
 
 - [changed] Bumped internal dependencies.

diff --git a/firebase-perf/src/main/java/com/google/firebase/perf/FirebasePerfEarly.java b/firebase-perf/src/main/java/com/google/firebase/perf/FirebasePerfEarly.java
@@ -15,6 +15,7 @@
 package com.google.firebase.perf;
 
 import android.content.Context;
+import android.os.Build;
 import androidx.annotation.Nullable;
 import com.google.firebase.FirebaseApp;
 import com.google.firebase.StartupTime;
@@ -48,7 +49,12 @@ public FirebasePerfEarly(
     if (startupTime != null) {
       AppStartTrace appStartTrace = AppStartTrace.getInstance();
       appStartTrace.registerActivityLifecycleCallbacks(context);
-      uiExecutor.execute(new AppStartTrace.StartFromBackgroundRunnable(appStartTrace));
+      // The posted runnable feeds AppStartTrace's pre-API-34 background-start check.
+      // On API 34+ the runnable's output is unused (the causal signal owns the
+      // decision), so we skip the main-thread post.
+      if (Build.VERSION.SDK_INT < 34) {
+        uiExecutor.execute(new AppStartTrace.StartFromBackgroundRunnable(appStartTrace));
+      }
     }
 
     // TODO: Bring back Firebase Sessions dependency to watch for updates to sessions.

diff --git a/firebase-perf/src/main/java/com/google/firebase/perf/metrics/AppStartTrace.java b/firebase-perf/src/main/java/com/google/firebase/perf/metrics/AppStartTrace.java
@@ -74,11 +74,6 @@ public class AppStartTrace implements ActivityLifecycleCallbacks, LifecycleObser
   private static final @NonNull Timer PERF_CLASS_LOAD_TIME = new Clock().getTime();
   private static final long MAX_LATENCY_BEFORE_UI_INIT = TimeUnit.MINUTES.toMicros(1);
 
-  // If the `mainThreadRunnableTime` was set within this duration, the assumption
-  // is that it was called immediately before `onActivityCreated` in foreground starts on API 34+.
-  // See b/339891952.
-  private static final long MAX_BACKGROUND_RUNNABLE_DELAY = TimeUnit.MILLISECONDS.toMicros(50);
-
   // Core pool size 0 allows threads to shut down if they're idle
   private static final int CORE_POOL_SIZE = 0;
   private static final int MAX_POOL_SIZE = 1; // Only need single thread
@@ -134,6 +129,11 @@ public class AppStartTrace implements ActivityLifecycleCallbacks, LifecycleObser
   private final DrawCounter onDrawCounterListener = new DrawCounter();
   private boolean systemForegroundCheck = false;
 
+  // OS-reported reason this process was forked. Captured once during
+  // registerActivityLifecycleCallbacks; consulted by resolveIsStartedFromBackground on
+  // API 34+.
+  private @Nullable ProcessStartCause processStartCause = null;
+
   /**
    * Called from onCreate() method of an activity by instrumented byte code.
    *
@@ -224,6 +224,9 @@ public synchronized void registerActivityLifecycleCallbacks(@NonNull Context con
     if (appContext instanceof Application) {
       ((Application) appContext).registerActivityLifecycleCallbacks(this);
       systemForegroundCheck = systemForegroundCheck || isAnyAppProcessInForeground(appContext);
+      // Capture the OS-reported start cause as early as possible (this method runs from
+      // FirebasePerfEarly during the ContentProvider init chain).
+      processStartCause = ProcessStartCause.capture(appContext);
       isRegisteredForLifecycleCallbacks = true;
       this.appContext = appContext;
     }
@@ -327,37 +330,30 @@ private void recordOnDrawFrontOfQueue() {
   }
 
   /**
-   * Sets the `isStartedFromBackground` flag to `true` if the `mainThreadRunnableTime` was set
-   * from the `StartFromBackgroundRunnable`.
-   * <p>
-   * If it's prior to API 34, it's always set to true if `mainThreadRunnableTime` was set.
-   * <p>
-   * If it's on or after API 34, and it was called less than `MAX_BACKGROUND_RUNNABLE_DELAY`
-   * before `onActivityCreated`, the
-   * assumption is that it was called immediately before the activity lifecycle callbacks in a
-   * foreground start.
-   * See b/339891952.
+   * Decide whether this process was background-only and, if so, set
+   * {@link #isStartedFromBackground} so the activity-lifecycle callbacks suppress the
+   * {@code _app_start} trace.
+   *
+   * API < 34: legacy pre-bug ordering. If {@link StartFromBackgroundRunnable} fired
+   *   before the first {@code onActivityCreated}, suppress.
+   *
+   * API 34+: {@link ProcessStartCause} owns the decision. {@code FOREGROUND} lets the
+   *   trace through; {@code UNKNOWN} or null suppresses.
+   *
+   * See b/339891952 and https://github.com/firebase/firebase-android-sdk/issues/8103.
    */
   private void resolveIsStartedFromBackground() {
-    // If the mainThreadRunnableTime is null, either the runnable hasn't run, or this check has
-    // already been made.
-    if (mainThreadRunnableTime == null) {
+    if (Build.VERSION.SDK_INT < 34) {
+      if (mainThreadRunnableTime != null) {
+        isStartedFromBackground = true;
+        mainThreadRunnableTime = null;
+      }
       return;
     }
-
-    // If the `mainThreadRunnableTime` was set prior to API 34, it's always assumed that's it's
-    // a background start.
-    // Otherwise it's assumed to be a background start if the runnable was set more than
-    // `MAX_BACKGROUND_RUNNABLE_DELAY`
-    // before the first `onActivityCreated` call.
-    // TODO(b/339891952): Investigate removing the API check.
-    if ((Build.VERSION.SDK_INT < 34)
-        || (mainThreadRunnableTime.getDurationMicros() > MAX_BACKGROUND_RUNNABLE_DELAY)) {
+    if (processStartCause == null
+        || processStartCause.cause != ProcessStartCause.Cause.FOREGROUND) {
       isStartedFromBackground = true;
     }
-
-    // Set this to null to prevent additional checks.
-    mainThreadRunnableTime = null;
   }
 
   @Override
@@ -633,4 +629,15 @@ Timer getOnResumeTime() {
   void setMainThreadRunnableTime(Timer timer) {
     mainThreadRunnableTime = timer;
   }
+
+  @VisibleForTesting
+  void setProcessStartCauseForTest(@Nullable ProcessStartCause cause) {
+    this.processStartCause = cause;
+  }
+
+  @VisibleForTesting
+  @Nullable
+  ProcessStartCause getProcessStartCauseForTest() {
+    return processStartCause;
+  }
 }
diff --git a/firebase-perf/src/main/java/com/google/firebase/perf/metrics/ProcessStartCause.java b/firebase-perf/src/main/java/com/google/firebase/perf/metrics/ProcessStartCause.java
@@ -0,0 +1,102 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+//
+// You may obtain a copy of the License at
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.firebase.perf.metrics;
+
+import android.app.ActivityManager;
+import android.content.Context;
+import android.os.Build;
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+import androidx.annotation.VisibleForTesting;
+
+/**
+ * OS-reported reason this process was forked, used by {@link AppStartTrace} to decide
+ * whether to emit the {@code _app_start} trace.
+ *
+ * API 34+: {@link ActivityManager#getMyMemoryState} importance.
+ *   {@code IMPORTANCE_FOREGROUND} at first capture indicates an activity-driven start.
+ *
+ * API < 34: returns {@link Cause#UNKNOWN}; legacy logic in {@link AppStartTrace} owns
+ *   the decision on these versions.
+ *
+ * @hide
+ */
+final class ProcessStartCause {
+
+  /** Classification of why the process was forked. */
+  enum Cause {
+    /** Process forked to satisfy an activity launch. */
+    FOREGROUND,
+    /** Couldn't decide — caller falls back to its own heuristic. */
+    UNKNOWN
+  }
+
+  /** OS classification. Never null. */
+  final @NonNull Cause cause;
+
+  /** {@code RunningAppProcessInfo.importance} at capture, or {@code -1} if unread. */
+  final int importance;
+
+  /** {@link Build.VERSION#SDK_INT} at capture. */
+  final int apiLevel;
+
+  @VisibleForTesting
+  ProcessStartCause(@NonNull Cause cause, int importance, int apiLevel) {
+    this.cause = cause;
+    this.importance = importance;
+    this.apiLevel = apiLevel;
+  }
+
+  /**
+   * Capture the cause for the current process. Call as early as possible (during
+   * {@code AppStartTrace.registerActivityLifecycleCallbacks}) so the OS-set values still
+   * reflect the original fork reason rather than transient state mid-init.
+   */
+  static @NonNull ProcessStartCause capture(@Nullable Context appContext) {
+    final int apiLevel = Build.VERSION.SDK_INT;
+    if (appContext == null) {
+      return new ProcessStartCause(Cause.UNKNOWN, -1, apiLevel);
+    }
+
+    final ActivityManager activityManager =
+        (ActivityManager) appContext.getSystemService(Context.ACTIVITY_SERVICE);
+    if (activityManager == null) {
+      return new ProcessStartCause(Cause.UNKNOWN, -1, apiLevel);
+    }
+
+    final int importance = readImportance();
+
+    if (apiLevel >= 34) {
+      Cause cause =
+          importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND
+              ? Cause.FOREGROUND
+              : Cause.UNKNOWN;
+      return new ProcessStartCause(cause, importance, apiLevel);
+    }
+
+    // API < 34: legacy AppStartTrace logic owns the decision.
+    return new ProcessStartCause(Cause.UNKNOWN, importance, apiLevel);
+  }
+
+  private static int readImportance() {
+    try {
+      ActivityManager.RunningAppProcessInfo info = new ActivityManager.RunningAppProcessInfo();
+      ActivityManager.getMyMemoryState(info);
+      return info.importance;
+    } catch (Throwable t) {
+      return -1;
+    }
+  }
+}
diff --git a/firebase-perf/src/test/java/com/google/firebase/perf/metrics/AppStartTraceTest.java b/firebase-perf/src/test/java/com/google/firebase/perf/metrics/AppStartTraceTest.java
@@ -238,55 +238,129 @@ public void testDelayedAppStart() {
             ArgumentMatchers.nullable(ApplicationProcessState.class));
   }
 
+  // --- Pre-API-34 regression tests for the legacy pre-bug-ordering path ---
+  //
+  // Pre-API-34 still detects background-only starts via the StartFromBackgroundRunnable
+  // firing before any activity. These tests pin that behavior on the still-active code path.
+
   @Test
-  public void testStartFromBackground_within50ms() {
+  @Config(sdk = 33)
+  public void preApi34_runnableFiredBeforeActivity_marksAsBackground() {
     FakeScheduledExecutorService fakeExecutorService = new FakeScheduledExecutorService();
-    Timer fakeTimer = spy(new Timer(currentTime));
     AppStartTrace trace =
         new AppStartTrace(transportManager, clock, configResolver, fakeExecutorService);
     trace.registerActivityLifecycleCallbacks(appContext);
-    trace.setMainThreadRunnableTime(fakeTimer);
+    // Simulate StartFromBackgroundRunnable having fired before any activity was created.
+    trace.setMainThreadRunnableTime(spy(new Timer(currentTime)));
 
-    // See AppStartTrace.MAX_BACKGROUND_RUNNABLE_DELAY.
-    when(fakeTimer.getDurationMicros()).thenReturn(TimeUnit.MILLISECONDS.toMicros(50) - 1);
     trace.onActivityCreated(activity1, bundle);
-    Assert.assertNotNull(trace.getOnCreateTime());
+    Assert.assertNull(trace.getOnCreateTime());
     ++currentTime;
     trace.onActivityStarted(activity1);
-    Assert.assertNotNull(trace.getOnStartTime());
+    Assert.assertNull(trace.getOnStartTime());
     ++currentTime;
     trace.onActivityResumed(activity1);
-    Assert.assertNotNull(trace.getOnResumeTime());
+    Assert.assertNull(trace.getOnResumeTime());
     fakeExecutorService.runAll();
-    // There should be a trace sent since the delay between the main thread and onActivityCreated
-    // is limited.
-    verify(transportManager, times(1))
+
+    // Trace suppressed — pre-bug ordering says background.
+    verify(transportManager, times(0))
         .log(
             traceArgumentCaptor.capture(),
             ArgumentMatchers.nullable(ApplicationProcessState.class));
   }
 
   @Test
-  public void testStartFromBackground_moreThan50ms() {
+  @Config(sdk = 33)
+  public void preApi34_runnableNotFired_traceLogs() {
     FakeScheduledExecutorService fakeExecutorService = new FakeScheduledExecutorService();
-    Timer fakeTimer = spy(new Timer(currentTime));
     AppStartTrace trace =
         new AppStartTrace(transportManager, clock, configResolver, fakeExecutorService);
     trace.registerActivityLifecycleCallbacks(appContext);
-    trace.setMainThreadRunnableTime(fakeTimer);
+    // mainThreadRunnableTime is NOT set — i.e., the runnable hasn't fired yet, which is
+    // the normal pre-bug-ordering state on a cold foreground start.
 
-    // See AppStartTrace.MAX_BACKGROUND_RUNNABLE_DELAY.
-    when(fakeTimer.getDurationMicros()).thenReturn(TimeUnit.MILLISECONDS.toMicros(50) + 1);
+    currentTime = 1;
     trace.onActivityCreated(activity1, bundle);
-    Assert.assertNull(trace.getOnCreateTime());
-    ++currentTime;
+    currentTime = 2;
     trace.onActivityStarted(activity1);
-    Assert.assertNull(trace.getOnStartTime());
-    ++currentTime;
+    currentTime = 3;
     trace.onActivityResumed(activity1);
-    Assert.assertNull(trace.getOnResumeTime());
-    // There should be no trace sent.
     fakeExecutorService.runAll();
+
+    // Trace logs — runnable-before-activity didn't happen.
+    verify(transportManager, times(1))
+        .log(
+            traceArgumentCaptor.capture(),
+            ArgumentMatchers.nullable(ApplicationProcessState.class));
+  }
+
+  // --- API 34+ causal-signal decision tests ---
+  // ProcessStartCause is the only decision input on API 34+; exercise each Cause value.
+
+  /** Builds an {@link AppStartTrace} and registers callbacks. */
+  private AppStartTrace newTrace(FakeScheduledExecutorService executor) {
+    AppStartTrace trace = new AppStartTrace(transportManager, clock, configResolver, executor);
+    trace.registerActivityLifecycleCallbacks(appContext);
+    return trace;
+  }
+
+  @Test
+  public void api34Plus_foregroundCause_traceLogs() {
+    FakeScheduledExecutorService executor = new FakeScheduledExecutorService();
+    AppStartTrace trace = newTrace(executor);
+    trace.setProcessStartCauseForTest(
+        new ProcessStartCause(ProcessStartCause.Cause.FOREGROUND, 100, 35));
+
+    currentTime = 1;
+    trace.onActivityCreated(activity1, bundle);
+    Assert.assertNotNull(trace.getOnCreateTime());
+    currentTime = 2;
+    trace.onActivityStarted(activity1);
+    Assert.assertNotNull(trace.getOnStartTime());
+    currentTime = 3;
+    trace.onActivityResumed(activity1);
+    Assert.assertNotNull(trace.getOnResumeTime());
+    executor.runAll();
+
+    verify(transportManager, times(1))
+        .log(
+            traceArgumentCaptor.capture(),
+            ArgumentMatchers.nullable(ApplicationProcessState.class));
+  }
+
+  @Test
+  public void api34Plus_unknownCause_traceSuppressed() {
+    // UNKNOWN means importance != FOREGROUND at capture — typically a warm-start
+    // scenario. Suppress to keep _app_start measuring real cold foreground launches.
+    FakeScheduledExecutorService executor = new FakeScheduledExecutorService();
+    AppStartTrace trace = newTrace(executor);
+    trace.setProcessStartCauseForTest(
+        new ProcessStartCause(ProcessStartCause.Cause.UNKNOWN, 200, 34));
+
+    trace.onActivityCreated(activity1, bundle);
+
+    Assert.assertNull(trace.getOnCreateTime());
+    executor.runAll();
+    verify(transportManager, times(0))
+        .log(
+            traceArgumentCaptor.capture(),
+            ArgumentMatchers.nullable(ApplicationProcessState.class));
+  }
+
+  @Test
+  public void api34Plus_nullProcessStartCause_traceSuppressed() {
+    // Defensive: if processStartCause is somehow null at decision time (e.g. the
+    // capture didn't run), suppress — better to miss a trace than emit one with no
+    // provenance.
+    FakeScheduledExecutorService executor = new FakeScheduledExecutorService();
+    AppStartTrace trace = newTrace(executor);
+    trace.setProcessStartCauseForTest(null);
+
+    trace.onActivityCreated(activity1, bundle);
+
+    Assert.assertNull(trace.getOnCreateTime());
+    executor.runAll();
     verify(transportManager, times(0))
         .log(
             traceArgumentCaptor.capture(),