diff --git a/langfuse/_client/span_processor.py b/langfuse/_client/span_processor.py
index b6ebbbce6..c86c345e1 100644
--- a/langfuse/_client/span_processor.py
+++ b/langfuse/_client/span_processor.py
@@ -225,10 +225,19 @@ def __init__(
                 mask_otel_spans=mask_otel_spans,
             )
 
+        # OpenTelemetry rejects a batch size larger than the queue size, and
+        # flush_at maps onto the batch size. The queue keeps OpenTelemetry's own
+        # default -- which is configurable through OTEL_BSP_MAX_QUEUE_SIZE -- and
+        # is only widened when a larger batch was asked for.
+        default_queue_size = self._default_max_queue_size()
+
         super().__init__(
             span_exporter=span_exporter,
             export_timeout_millis=timeout * 1_000 if timeout else None,
             max_export_batch_size=flush_at,
+            max_queue_size=max(flush_at, default_queue_size)
+            if flush_at is not None
+            else None,
             schedule_delay_millis=flush_interval * 1_000
             if flush_interval is not None
             else None,
diff --git a/tests/unit/test_flush_at_batch_size.py b/tests/unit/test_flush_at_batch_size.py
new file mode 100644
index 000000000..53f095d9b
--- /dev/null
+++ b/tests/unit/test_flush_at_batch_size.py
@@ -0,0 +1,135 @@
+"""A flush_at larger than the OpenTelemetry default queue size must be usable."""
+
+from typing import Optional, Sequence
+
+import pytest
+from opentelemetry.sdk.trace import ReadableSpan, TracerProvider
+from opentelemetry.sdk.trace.export import SpanExporter, SpanExportResult
+from opentelemetry.sdk.trace.export.in_memory_span_exporter import InMemorySpanExporter
+
+from langfuse import Langfuse
+from langfuse._client.resource_manager import LangfuseResourceManager
+from langfuse._client.span_processor import LangfuseSpanProcessor
+
+
+class NoOpSpanExporter(SpanExporter):
+    def export(self, spans: Sequence[ReadableSpan]) -> SpanExportResult:
+        return SpanExportResult.SUCCESS
+
+    def shutdown(self) -> None:
+        pass
+
+
+def _build_client(**kwargs) -> Langfuse:
+    """Build a client with its own TracerProvider.
+
+    Without this the processor is added to the globally registered provider, where
+    processors from earlier clients are still attached.
+    """
+    with LangfuseResourceManager._lock:
+        LangfuseResourceManager._instances.clear()
+
+    return Langfuse(
+        public_key="pk-flush-at",
+        secret_key="sk-flush-at",
+        tracer_provider=TracerProvider(),
+        **kwargs,
+    )
+
+
+def _queue_capacity(client: Langfuse) -> Optional[int]:
+    """Read the queue capacity of this client's span processor.
+
+    Returns None when the OpenTelemetry internals this relies on are not present,
+    so the caller can skip instead of failing on an upstream refactor.
+    """
+    provider = client._resources.tracer_provider
+    processors = [
+        processor
+        for processor in provider._active_span_processor._span_processors
+        if isinstance(processor, LangfuseSpanProcessor)
+    ]
+
+    assert len(processors) == 1, "the client must own its TracerProvider"
+
+    queue = getattr(processors[0], "_queue", None) or getattr(
+        processors[0], "_batch_processor", None
+    )
+    target = getattr(queue, "_queue", queue)
+    capacity = getattr(target, "maxlen", None) or getattr(target, "capacity", None)
+
+    return capacity if isinstance(capacity, int) else None
+
+
+@pytest.fixture
+def isolated_client():
+    client = _build_client(span_exporter=NoOpSpanExporter(), flush_at=5000)
+
+    yield client
+
+    client.shutdown()
+
+
+@pytest.mark.unit
+def test_flush_at_above_the_default_queue_size_is_accepted(isolated_client):
+    """Configuring a large flush_at must not fail client construction."""
+    assert isolated_client is not None
+
+
+@pytest.mark.unit
+def test_client_with_large_flush_at_can_flush(isolated_client):
+    isolated_client.flush()
+
+
+@pytest.mark.unit
+def test_flush_at_uses_the_effective_default_queue_size(monkeypatch):
+    """A smaller OTEL_BSP_MAX_QUEUE_SIZE must still be widened when flush_at needs it."""
+    monkeypatch.setenv("OTEL_BSP_MAX_QUEUE_SIZE", "1000")
+
+    client = _build_client(span_exporter=NoOpSpanExporter(), flush_at=1500)
+
+    try:
+        capacity = _queue_capacity(client)
+
+        if capacity is None:
+            pytest.skip("OpenTelemetry no longer exposes the queue capacity")
+
+        assert capacity == 1500
+    finally:
+        client.shutdown()
+
+
+@pytest.mark.unit
+def test_a_larger_default_queue_size_is_not_shrunk(monkeypatch):
+    """Widening the batch must not quietly narrow an already larger queue."""
+    monkeypatch.setenv("OTEL_BSP_MAX_QUEUE_SIZE", "5000")
+
+    client = _build_client(span_exporter=NoOpSpanExporter(), flush_at=3000)
+
+    try:
+        capacity = _queue_capacity(client)
+
+        if capacity is None:
+            pytest.skip("OpenTelemetry no longer exposes the queue capacity")
+
+        assert capacity == 5000
+    finally:
+        client.shutdown()
+
+
+@pytest.mark.unit
+def test_spans_are_still_exported_with_a_large_flush_at():
+    """A large flush_at must not silently drop or hold back finished spans."""
+    exporter = InMemorySpanExporter()
+
+    client = _build_client(span_exporter=exporter, flush_at=5000)
+
+    try:
+        client.start_observation(name="flush-at-span").end()
+
+        client.flush()
+
+        span_names = [span.name for span in exporter.get_finished_spans()]
+        assert "flush-at-span" in span_names
+    finally:
+        client.shutdown()