[DCP] Resolve --dcp-comm-backend to fi_a2a/a2a by default for every model (#39165)

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

test/registered/unit/server_args/test_server_args.py

@@ -3407,5 +3407,45 @@ class TestTpLmHeadAllToAllNcclGraphRegister(unittest.TestCase):
                 self.assertNotIn("NCCL_GRAPH_REGISTER", os.environ)
 
 
+class TestDcpCommBackendDefault(CustomTestCase):
+    def _resolved(self, **fields):
+        args = ServerArgs(model_path="dummy", tp_size=8, **fields)
+        parallel_hook.handle_decode_context_parallelism(args)
+        return resolution_result(args, "dcp_comm_backend")
+
+    def test_no_dcp_is_ag_rs(self):
+        self.assertEqual(self._resolved(dcp_size=1), "ag_rs")
+
+    @override_platform(is_cuda=True, is_hip=False)
+    def test_fi_a2a_where_supported(self):
+        with patch(
+            "sglang.srt.arg_groups.overrides.is_fi_a2a_supported", return_value=True
+        ):
+            self.assertEqual(self._resolved(dcp_size=4), "fi_a2a")
+
+    @override_platform(is_cuda=True, is_hip=False)
+    def test_a2a_on_cuda_without_mnnvl(self):
+        with patch(
+            "sglang.srt.arg_groups.overrides.is_fi_a2a_supported", return_value=False
+        ):
+            self.assertEqual(self._resolved(dcp_size=4), "a2a")
+
+    @override_platform(is_cuda=False, is_hip=False)
+    def test_ag_rs_off_cuda(self):
+        with patch(
+            "sglang.srt.arg_groups.overrides.is_fi_a2a_supported", return_value=False
+        ):
+            self.assertEqual(self._resolved(dcp_size=4), "ag_rs")
+
+    @override_platform(is_cuda=True, is_hip=False)
+    def test_explicit_value_wins(self):
+        with patch(
+            "sglang.srt.arg_groups.overrides.is_fi_a2a_supported", return_value=True
+        ):
+            self.assertEqual(
+                self._resolved(dcp_size=4, dcp_comm_backend="ag_rs"), "ag_rs"
+            )
+
+
 if __name__ == "__main__":
     unittest.main()