diff --git a/lib/mixpanel-ruby/flags/local_flags_provider.rb b/lib/mixpanel-ruby/flags/local_flags_provider.rb
index 9bf1bb0..fbe203e 100644
--- a/lib/mixpanel-ruby/flags/local_flags_provider.rb
+++ b/lib/mixpanel-ruby/flags/local_flags_provider.rb
@@ -31,7 +31,15 @@ def initialize(token, config, tracker_callback, error_handler)
 
         @flag_definitions = {}
         @polling_thread = nil
-        @stop_polling = false
+        # Per-thread stop signal. Each polling thread captures its own
+        # `[false]` cell by closure, so a subsequent start can't clear
+        # the previous thread's stop signal.
+        @polling_stop = nil
+        # Guards the polling-thread lifecycle. Without this, two concurrent
+        # start_polling_for_definitions! callers could both pass the
+        # !@polling_thread check before either commits and spawn duplicate
+        # threads — the earlier one becomes orphaned but keeps running.
+        @polling_mutex = Mutex.new
       end
 
       # Start polling for flag definitions
@@ -39,29 +47,42 @@ def initialize(token, config, tracker_callback, error_handler)
       def start_polling_for_definitions!
         fetch_flag_definitions
 
-        if @config[:enable_polling] && !@polling_thread
-          @stop_polling = false
+        return unless @config[:enable_polling]
+
+        @polling_mutex.synchronize do
+          return if @polling_thread
+
+          stop_flag = [false]
+          @polling_stop = stop_flag
           @polling_thread = Thread.new do
+            Thread.current.name = 'mixpanel-flags-poller'
             loop do
               sleep @config[:polling_interval_in_seconds]
-              break if @stop_polling
+              break if stop_flag[0]
 
               begin
                 fetch_flag_definitions
               rescue StandardError => e
-                @error_handler.handle(e) if @error_handler
+                log_polling_error(e)
               end
             end
           end
         end
       rescue StandardError => e
-        @error_handler.handle(e) if @error_handler
+        log_polling_error(e)
       end
 
       def stop_polling_for_definitions!
-        @stop_polling = true
-        @polling_thread&.join
-        @polling_thread = nil
+        thread_to_join = @polling_mutex.synchronize do
+          @polling_stop[0] = true if @polling_stop
+          @polling_stop = nil
+          t = @polling_thread
+          @polling_thread = nil
+          t
+        end
+        # Join outside the mutex so a long-running join can't block a
+        # concurrent start_polling_for_definitions! for the full poll interval.
+        thread_to_join&.join
       end
 
       def shutdown
@@ -141,6 +162,19 @@ def get_all_variants(context)
 
       private
 
+      # Surface polling-loop failures unconditionally. The default
+      # Mixpanel::ErrorHandler is a no-op, so dispatching only via
+      # @error_handler swallows schema drift (NoMethodError,
+      # JSON::ParserError, etc.) — the loop runs forever undetected.
+      # Always warn so the failure is visible without an error_handler
+      # being configured. Matches the convention in mixpanel-python /
+      # mixpanel-java / mixpanel-go / mixpanel-node, all of which log
+      # unconditionally and keep polling.
+      def log_polling_error(error)
+        warn "[Mixpanel] Failed to fetch flag definitions: #{error.class}: #{error.message}"
+        @error_handler.handle(error) if @error_handler
+      end
+
       def fetch_flag_definitions
         response = call_flags_endpoint
 
diff --git a/spec/mixpanel-ruby/flags/local_flags_spec.rb b/spec/mixpanel-ruby/flags/local_flags_spec.rb
index 6969521..040d6cf 100644
--- a/spec/mixpanel-ruby/flags/local_flags_spec.rb
+++ b/spec/mixpanel-ruby/flags/local_flags_spec.rb
@@ -755,5 +755,110 @@ def user_context_with_properties(properties)
         polling_provider.stop_polling_for_definitions!
       end
     end
+
+    # SDK-81: concurrent start! callers must not both pass the !@polling_thread
+    # check and spawn duplicate threads — the earlier one becomes orphaned.
+    it 'is idempotent under concurrent start calls and does not leak threads' do
+      stub_flag_definitions([create_test_flag])
+
+      polling_provider = Mixpanel::Flags::LocalFlagsProvider.new(
+        test_token,
+        # Short interval so stop_polling's join doesn't wait long for the
+        # poller thread to wake from sleep.
+        { enable_polling: true, polling_interval_in_seconds: 0.05 },
+        mock_tracker,
+        mock_error_handler
+      )
+
+      begin
+        gate = Queue.new
+        threads = Array.new(8) do
+          Thread.new do
+            gate.pop
+            polling_provider.start_polling_for_definitions!
+          end
+        end
+
+        # Release all contenders at once to maximize the chance of a race.
+        8.times { gate << :go }
+        threads.each(&:join)
+
+        # Exactly one poller thread should be alive, not eight. Without
+        # the mutex, each contender that passed the check would spawn
+        # its own. Filter by name so unrelated background threads (RSpec
+        # workers, GC finalizers, etc.) can't perturb the count.
+        poller_threads = Thread.list.select { |t| t.name == 'mixpanel-flags-poller' }
+        expect(poller_threads.size).to eq(1)
+      ensure
+        polling_provider.stop_polling_for_definitions!
+      end
+    end
+
+    # SDK-81: previously stop_polling_for_definitions! flipped a shared
+    # @stop_polling flag, so a subsequent start could reset it while the
+    # old thread was still asleep — the old thread would wake, see false,
+    # and keep polling forever. With per-thread stop signals the old
+    # thread breaks on its captured flag regardless of subsequent starts.
+    it 'stop+start does not leave the previous poller running' do
+      stub_flag_definitions([create_test_flag])
+
+      polling_provider = Mixpanel::Flags::LocalFlagsProvider.new(
+        test_token,
+        { enable_polling: true, polling_interval_in_seconds: 0.05 },
+        mock_tracker,
+        mock_error_handler
+      )
+
+      begin
+        polling_provider.start_polling_for_definitions!
+        polling_provider.stop_polling_for_definitions!
+        polling_provider.start_polling_for_definitions!
+
+        # Give the previous thread ample time to wake from its sleep and
+        # check its stop flag. With a per-thread flag it will break; with
+        # the old shared flag it would see the new start's `false` and
+        # keep going.
+        sleep 0.2
+
+        poller_threads = Thread.list.select { |t| t.name == 'mixpanel-flags-poller' && t.alive? }
+        expect(poller_threads.size).to eq(1)
+      ensure
+        polling_provider.stop_polling_for_definitions!
+      end
+    end
+
+    # SDK-78: the polling loop used to dispatch errors only to @error_handler,
+    # whose default Mixpanel::ErrorHandler#handle is a no-op — schema drift
+    # (NoMethodError, JSON::ParserError, etc.) looped forever undetected.
+    it 'warns to stderr when fetch raises and no error_handler is configured' do
+      stub_request(:get, endpoint_url_regex).to_return(status: 500, body: 'server down')
+
+      polling_provider = Mixpanel::Flags::LocalFlagsProvider.new(
+        test_token,
+        { enable_polling: false },
+        mock_tracker,
+        nil
+      )
+
+      expect { polling_provider.start_polling_for_definitions! }
+        .to output(/\[Mixpanel\] Failed to fetch flag definitions: Mixpanel::ServerError/).to_stderr
+    end
+
+    it 'surfaces unexpected errors (schema drift) instead of swallowing them silently' do
+      stub_flag_definitions([create_test_flag])
+
+      polling_provider = Mixpanel::Flags::LocalFlagsProvider.new(
+        test_token,
+        { enable_polling: false },
+        mock_tracker,
+        mock_error_handler
+      )
+
+      allow(polling_provider).to receive(:fetch_flag_definitions).and_raise(NoMethodError, "undefined method `[]' for nil:NilClass")
+
+      expect(mock_error_handler).to receive(:handle).with(an_instance_of(NoMethodError))
+      expect { polling_provider.start_polling_for_definitions! }
+        .to output(/\[Mixpanel\] Failed to fetch flag definitions: NoMethodError/).to_stderr
+    end
   end
 end