diff --git a/src/google/adk/plugins/bigquery_agent_analytics_plugin.py b/src/google/adk/plugins/bigquery_agent_analytics_plugin.py
index 50eb72ffdb..50ad3e62b4 100644
--- a/src/google/adk/plugins/bigquery_agent_analytics_plugin.py
+++ b/src/google/adk/plugins/bigquery_agent_analytics_plugin.py
@@ -2653,7 +2653,14 @@ async def __aexit__(self, exc_type, exc_val, exc_tb) -> None:
 
   async def _ensure_started(self, **kwargs) -> None:
     """Ensures that the plugin is started and initialized."""
-    if os.getpid() != self._init_pid:
+    # _init_pid == 0 means the plugin was unpickled and has never been
+    # initialized in this process (the pickle sentinel set by
+    # __getstate__).  Skip the fork reset in that case — no fork
+    # happened, and _started is already False so _lazy_setup will run.
+    # Real forks are caught by os.register_at_fork (line 108) and by
+    # this check when _init_pid is a real (non-zero) PID from a
+    # different process.
+    if self._init_pid != 0 and os.getpid() != self._init_pid:
       self._reset_runtime_state()
     if not self._started:
       # Kept original lock name as it was not explicitly changed.
@@ -2665,6 +2672,12 @@ async def _ensure_started(self, **kwargs) -> None:
             await self._lazy_setup(**kwargs)
             self._started = True
             self._startup_error = None
+            # Record the current PID so fork detection works for
+            # the rest of this instance's lifetime.  Without this,
+            # an unpickled plugin would keep _init_pid == 0 forever,
+            # disabling the PID-based fork check.
+            if self._init_pid == 0:
+              self._init_pid = os.getpid()
           except Exception as e:
             self._startup_error = e
             logger.error("Failed to initialize BigQuery Plugin: %s", e)
diff --git a/tests/unittests/plugins/test_bigquery_agent_analytics_plugin.py b/tests/unittests/plugins/test_bigquery_agent_analytics_plugin.py
index 8a05392bec..e2c5160468 100644
--- a/tests/unittests/plugins/test_bigquery_agent_analytics_plugin.py
+++ b/tests/unittests/plugins/test_bigquery_agent_analytics_plugin.py
@@ -7408,3 +7408,78 @@ async def test_view_error_still_logged(
       ) as plugin:
         await plugin._ensure_started()
         assert plugin._started
+
+
+# ================================================================
+# TEST CLASS: Fork detection after pickle (Issue #86)
+# ================================================================
+class TestForkDetectionAfterPickle:
+  """Tests that unpickled plugins do not false-positive fork detection."""
+
+  @pytest.mark.asyncio
+  async def test_no_reset_after_unpickle(
+      self,
+      mock_auth_default,
+      mock_bq_client,
+      mock_write_client,
+      mock_to_arrow_schema,
+      mock_asyncio_to_thread,
+  ):
+    """Unpickled plugin does not trigger _reset_runtime_state."""
+    import pickle
+
+    config = bigquery_agent_analytics_plugin.BigQueryLoggerConfig(
+        create_views=False,
+    )
+    plugin = bigquery_agent_analytics_plugin.BigQueryAgentAnalyticsPlugin(
+        PROJECT_ID, DATASET_ID, table_id=TABLE_ID, config=config
+    )
+    # Pickle round-trip simulates Vertex AI Agent Engine deployment
+    pickled = pickle.dumps(plugin)
+    unpickled = pickle.loads(pickled)
+
+    assert unpickled._init_pid == 0  # pickle sentinel
+
+    with mock.patch.object(unpickled, "_reset_runtime_state") as mock_reset:
+      await unpickled._ensure_started()
+      # Should NOT have called _reset_runtime_state because
+      # _init_pid == 0 means "unpickled, never initialized"
+      mock_reset.assert_not_called()
+
+    assert unpickled._started
+    # After successful startup, _init_pid should be recorded so
+    # fork detection works for the rest of this instance's lifetime.
+    assert unpickled._init_pid == os.getpid()
+    await unpickled.shutdown()
+
+  @pytest.mark.asyncio
+  async def test_reset_on_real_fork(
+      self,
+      mock_auth_default,
+      mock_bq_client,
+      mock_write_client,
+      mock_to_arrow_schema,
+      mock_asyncio_to_thread,
+  ):
+    """Plugin detects real fork when _init_pid is a real non-zero PID."""
+    config = bigquery_agent_analytics_plugin.BigQueryLoggerConfig(
+        create_views=False,
+    )
+    async with managed_plugin(
+        project_id=PROJECT_ID,
+        dataset_id=DATASET_ID,
+        table_id=TABLE_ID,
+        config=config,
+    ) as plugin:
+      await plugin._ensure_started()
+      # Simulate a fork: set _init_pid to a different real PID
+      plugin._init_pid = max(os.getpid() - 1, 1)
+      plugin._started = True  # pretend it was started in parent
+
+      with mock.patch.object(
+          plugin, "_reset_runtime_state", wraps=plugin._reset_runtime_state
+      ) as mock_reset:
+        await plugin._ensure_started()
+        # Should have called _reset_runtime_state because
+        # _init_pid is a real PID different from os.getpid()
+        mock_reset.assert_called_once()