diff --git a/src/schematic/event_buffer.py b/src/schematic/event_buffer.py
index cccf564..ebcd5d7 100644
--- a/src/schematic/event_buffer.py
+++ b/src/schematic/event_buffer.py
@@ -9,6 +9,10 @@
 from .types import CreateEventRequestBody
 
 DEFAULT_MAX_EVENTS = 100  # Default maximum number of events
+# The capture service rejects any batch larger than this with
+# `400 {"error": "batch too large", "max_size": 100}`, so a flush is split into
+# chunks of at most this many events regardless of how many are buffered.
+MAX_EVENTS_PER_REQUEST = 100
 DEFAULT_EVENT_BUFFER_PERIOD = 5  # 5 seconds
 DEFAULT_MAX_RETRIES = 3  # Default maximum number of retry attempts
 DEFAULT_INITIAL_RETRY_DELAY = 1  # Initial retry delay in seconds
@@ -47,8 +51,14 @@ def _flush(self):
             events_to_process = [event for event in self.events if event is not None]
             self.events.clear()
 
-        if events_to_process:
-            self._process_events(events_to_process)
+        # The buffer can hold more than one request's worth of events. push()
+        # appends unconditionally after its own flush, so concurrent producers
+        # can drive the backlog past max_events. Send in chunks so an oversized
+        # buffer is never turned into an oversized request. Each chunk retries
+        # on its own, since retrying the whole drained set would resend chunks
+        # that already succeeded.
+        for i in range(0, len(events_to_process), MAX_EVENTS_PER_REQUEST):
+            self._process_events(events_to_process[i:i + MAX_EVENTS_PER_REQUEST])
 
     def _process_events(self, events_to_process):
         """Process events with retry logic - called without holding lock"""
@@ -153,8 +163,10 @@ async def _flush(self):
             events_to_process = [event for event in self.events if event is not None]
             self.events.clear()
 
-        if events_to_process:
-            await self._process_events_async(events_to_process)
+        # See EventBuffer._flush: the buffer can exceed max_events, so cap the
+        # size of each request rather than sending the whole drained backlog.
+        for i in range(0, len(events_to_process), MAX_EVENTS_PER_REQUEST):
+            await self._process_events_async(events_to_process[i:i + MAX_EVENTS_PER_REQUEST])
 
     async def _process_events_async(self, events_to_process):
         """Process events with retry logic - called without holding lock"""
diff --git a/tests/custom/test_event_buffer.py b/tests/custom/test_event_buffer.py
index 64407bd..39abbe4 100644
--- a/tests/custom/test_event_buffer.py
+++ b/tests/custom/test_event_buffer.py
@@ -321,3 +321,45 @@ async def test_push_after_shutdown_rejected(self):
 
 if __name__ == "__main__":
     unittest.main()
+
+
+class TestEventBufferBatchSizeCap(unittest.TestCase):
+    """The capture service rejects batches over 100 events, so no single
+    send_batch call may exceed that no matter how deep the backlog is."""
+
+    def setUp(self):
+        self.mock_sender = MagicMock()
+        self.mock_logger = MagicMock()
+
+    def _batch_sizes(self):
+        return [len(call.args[0]) for call in self.mock_sender.send_batch.call_args_list]
+
+    def test_flush_splits_backlog_into_capped_requests(self):
+        buffer = EventBuffer(
+            event_sender=self.mock_sender, logger=self.mock_logger, period=3600, max_events=1000
+        )
+        try:
+            buffer.events = [MagicMock(spec=CreateEventRequestBody) for _ in range(250)]
+            buffer._flush()
+        finally:
+            buffer.stop()
+
+        self.assertEqual(self._batch_sizes(), [100, 100, 50])
+
+
+class TestAsyncEventBufferBatchSizeCap(unittest.TestCase):
+
+    def test_flush_splits_backlog_into_capped_requests(self):
+        async def run():
+            mock_sender = AsyncMock()
+            buffer = AsyncEventBuffer(
+                event_sender=mock_sender, logger=MagicMock(), period=3600, max_events=1000
+            )
+            try:
+                buffer.events = [MagicMock(spec=CreateEventRequestBody) for _ in range(250)]
+                await buffer._flush()
+            finally:
+                await buffer.stop()
+            return [len(call.args[0]) for call in mock_sender.send_batch.call_args_list]
+
+        self.assertEqual(asyncio.run(run()), [100, 100, 50])