From d94132a598cb8ce3c930cb3155f1bad8208512c9 Mon Sep 17 00:00:00 2001
From: Cohen Karnell <7363269+ckarnell@users.noreply.github.com>
Date: Sat, 15 Aug 2026 15:53:16 -0400
Subject: [PATCH] fix(BufferedConsumer): persist drained buffer per batch so a
 partial flush does not resend delivered events

---
 mixpanel/__init__.py |  2 +-
 test_mixpanel.py     | 24 ++++++++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/mixpanel/__init__.py b/mixpanel/__init__.py
index 4cc23f5..2712d36 100644
--- a/mixpanel/__init__.py
+++ b/mixpanel/__init__.py
@@ -969,4 +969,4 @@ def _flush_endpoint(self, endpoint):
                 mp_e.endpoint = endpoint
                 raise mp_e from orig_e
             buf = buf[self._max_size :]
-        self._buffers[endpoint] = buf
+            self._buffers[endpoint] = buf
diff --git a/test_mixpanel.py b/test_mixpanel.py
index 4cffaae..cbad72d 100644
--- a/test_mixpanel.py
+++ b/test_mixpanel.py
@@ -843,6 +843,30 @@ def test_useful_reraise_in_flush_endpoint(self):
             assert excinfo.value.message == f"[{broken_json}]"
             assert excinfo.value.endpoint == "events"
 
+    def test_partial_flush_does_not_resend_delivered_batches(self):
+        """A batch the server accepted must not be re-sent when a later one fails."""
+
+        class FlakyConsumer:
+            def __init__(self):
+                self.log = []
+                self.calls = 0
+
+            def send(self, endpoint, event, api_key=None):
+                self.calls += 1
+                if self.calls == 2:
+                    raise mixpanel.MixpanelException("network error")
+                self.log.append(json.loads(event))
+
+        consumer = mixpanel.BufferedConsumer(2)
+        consumer._consumer = FlakyConsumer()
+        consumer._buffers["events"] = ['"a"', '"b"', '"c"', '"d"']
+
+        with pytest.raises(mixpanel.MixpanelException):
+            consumer.flush()
+
+        assert consumer._consumer.log == [["a", "b"]]
+        assert consumer._buffers["events"] == ['"c"', '"d"']
+
     def test_send_remembers_api_key(self):
         self.consumer.send("imports", '"Event"', api_key="MY_API_KEY")
         assert len(self.log) == 0