diff --git a/python/sglang/srt/models/falcon_h1.py b/python/sglang/srt/models/falcon_h1.py
index 21e9c324428d..4767135a0273 100644
--- a/python/sglang/srt/models/falcon_h1.py
+++ b/python/sglang/srt/models/falcon_h1.py
@@ -187,6 +187,9 @@ def __init__(
             rms_norm_eps=config.rms_norm_eps,
             activation=config.hidden_act,
             use_rms_norm=config.mamba_rms_norm,
+            # Like the attention's o_proj: prepare_mlp completes the sum of the
+            # two mixers' partial outputs.
+            reduce_results=False,
             prefix=f"{prefix}.mixer",
         )
 
diff --git a/test/registered/unit/models/test_falcon_h1_mixer_sums.py b/test/registered/unit/models/test_falcon_h1_mixer_sums.py
new file mode 100644
index 000000000000..d1ac200a2d62
--- /dev/null
+++ b/test/registered/unit/models/test_falcon_h1_mixer_sums.py
@@ -0,0 +1,71 @@
+import unittest
+from types import SimpleNamespace
+from unittest.mock import MagicMock, patch
+
+from sglang.srt.models import falcon_h1
+from sglang.srt.runtime_context import get_parallel
+from sglang.test.ci.ci_register import register_cpu_ci
+from sglang.test.test_utils import CustomTestCase
+
+register_cpu_ci(est_time=2, suite="base-a-test-cpu")
+
+
+class _Built(Exception):
+    """Stops the layer's construction once its mixers are built."""
+
+
+class TestFalconH1MixersLeavePartialSums(CustomTestCase):
+    """The attention and Mamba mixers read the same input and their outputs are
+    summed before the FFN; prepare_mlp completes that sum over attention TP.
+    Neither mixer may reduce its own output first, or its share of the sum is
+    counted tp times."""
+
+    def test_neither_mixer_reduces_its_output(self):
+        config = SimpleNamespace(
+            hidden_size=64,
+            num_attention_heads=4,
+            num_key_value_heads=2,
+            head_dim=16,
+            rope_parameters={"rope_theta": 10000.0},
+            max_position_embeddings=128,
+            mamba_d_ssm=None,
+            mamba_expand=2,
+            mamba2_cache_params=None,
+            mamba_conv_bias=True,
+            mamba_proj_bias=False,
+            mamba_n_groups=1,
+            rms_norm_eps=1e-5,
+            hidden_act="silu",
+            mamba_rms_norm=False,
+        )
+        linears = []
+
+        def row_parallel(*args, **kwargs):
+            linears.append(kwargs)
+            return MagicMock()
+
+        mixer = MagicMock(side_effect=_Built)
+        for tp in (1, 2):
+            with self.subTest(tp=tp):
+                linears.clear()
+                mixer.reset_mock()
+                with (
+                    get_parallel().override(
+                        attn_tp_rank=0, attn_tp_size=tp, tp_size=tp
+                    ),
+                    patch.object(falcon_h1, "get_rope", MagicMock()),
+                    patch.object(falcon_h1, "QKVParallelLinear", MagicMock()),
+                    patch.object(falcon_h1, "RowParallelLinear", row_parallel),
+                    patch.object(falcon_h1, "RadixAttention", MagicMock()),
+                    patch.object(falcon_h1, "MambaMixer2", mixer),
+                ):
+                    with self.assertRaises(_Built):
+                        falcon_h1.FalconH1HybridAttentionDecoderLayer(
+                            config=config, layer_id=0
+                        )
+                self.assertEqual([k.get("reduce_results") for k in linears], [False])
+                self.assertIs(mixer.call_args.kwargs.get("reduce_results"), False)
+
+
+if __name__ == "__main__":
+    unittest.main()