diff --git a/cirq-google/cirq_google/api/v2/sweeps.py b/cirq-google/cirq_google/api/v2/sweeps.py
index 5ce24719fcc..001c49d3de2 100644
--- a/cirq-google/cirq_google/api/v2/sweeps.py
+++ b/cirq-google/cirq_google/api/v2/sweeps.py
@@ -30,36 +30,38 @@
     from cirq.study import sweeps
 
 
-def _build_sweep_const(value: Any, use_float64: bool = False) -> run_context_pb2.ConstValue:
+def _add_sweep_const(
+    sweep: run_context_pb2.SingleSweep, value: Any, use_float64: bool = False
+) -> None:
     """Build the sweep const message from a value."""
     if isinstance(value, float):
         # comparing to float is ~5x than testing numbers.Real
         # if modifying the below, also modify the block below for numbers.Real
         if use_float64:
-            return run_context_pb2.ConstValue(double_value=value)
+            sweep.const_value.double_value = value
         else:
             # Note: A loss of precision for floating-point numbers may occur here.
-            return run_context_pb2.ConstValue(float_value=value)
+            sweep.const_value.float_value = value
     elif isinstance(value, int):
         # comparing to int is ~5x than testing numbers.Integral
         # if modifying the below, also modify the block below for numbers.Integral
-        return run_context_pb2.ConstValue(int_value=value)
+        sweep.const_value.int_value = value
     elif value is None:
-        return run_context_pb2.ConstValue(is_none=True)
+        sweep.const_value.is_none = True
     elif isinstance(value, str):
-        return run_context_pb2.ConstValue(string_value=value)
+        sweep.const_value.string_value = value
     elif isinstance(value, numbers.Integral):
         # more general than isinstance(int) but also slower
-        return run_context_pb2.ConstValue(int_value=int(value))
+        sweep.const_value.int_value = int(value)
     elif isinstance(value, numbers.Real):
         # more general than isinstance(float) but also slower
         if use_float64:
-            return run_context_pb2.ConstValue(double_value=float(value))
+            sweep.const_value.double_value = float(value)  # pragma: no cover
         else:
             # Note: A loss of precision for floating-point numbers may occur here.
-            return run_context_pb2.ConstValue(float_value=float(value))
+            sweep.const_value.float_value = float(value)
     elif isinstance(value, tunits.Value):
-        return run_context_pb2.ConstValue(with_unit_value=value.to_proto())
+        value.to_proto(sweep.const_value.with_unit_value)
     else:
         raise ValueError(
             f"Unsupported type for serializing const sweep: {value=} and {type(value)=}"
@@ -190,7 +192,7 @@ def sweep_to_proto(
         sweep = cast(cirq.Points, sweep_transformer(sweep))
         out.single_sweep.parameter_key = sweep.key
         if len(sweep.points) == 1:
-            out.single_sweep.const_value.MergeFrom(_build_sweep_const(sweep.points[0], use_float64))
+            _add_sweep_const(out.single_sweep, sweep.points[0], use_float64)
         else:
             if isinstance(sweep.points[0], tunits.Value):
                 unit = sweep.points[0].unit
@@ -402,7 +404,7 @@ def sweepable_to_proto(
         for key, val in sweepable.items():
             single_sweep = zip_proto.sweeps.add().single_sweep
             single_sweep.parameter_key = key
-            single_sweep.const_value.MergeFrom(_build_sweep_const(val, use_float64))
+            _add_sweep_const(single_sweep, val, use_float64)
         return out
     if isinstance(sweepable, Iterable):
         for sweepable_element in sweepable:
diff --git a/cirq-google/cirq_google/api/v2/sweeps_test.py b/cirq-google/cirq_google/api/v2/sweeps_test.py
index 3c42a2b56b8..86258051f3a 100644
--- a/cirq-google/cirq_google/api/v2/sweeps_test.py
+++ b/cirq-google/cirq_google/api/v2/sweeps_test.py
@@ -164,7 +164,9 @@ def test_sweep_to_proto_linspace():
 
 @pytest.mark.parametrize("val", [None, 1, 1.5, 's'])
 def test_build_recover_const(val):
-    val2 = v2.sweeps._recover_sweep_const(v2.sweeps._build_sweep_const(val))
+    sweep = v2.run_context_pb2.SingleSweep()
+    v2.sweeps._add_sweep_const(sweep, val)
+    val2 = v2.sweeps._recover_sweep_const(sweep.const_value)
     if isinstance(val, float):
         assert math.isclose(val, val2)  # avoid the floating precision issue.
     else:
@@ -179,7 +181,7 @@ def test_build_covert_const_double():
 
 def test_build_const_unsupported_type():
     with pytest.raises(ValueError, match='Unsupported type for serializing const sweep'):
-        v2.sweeps._build_sweep_const((1, 2))
+        v2.sweeps._add_sweep_const(v2.run_context_pb2.SingleSweep(), (1, 2))
 
 
 def test_list_sweep_bad_expression():