diff --git a/vizier/_src/algorithms/designers/harmonica.py b/vizier/_src/algorithms/designers/harmonica.py
index f9cf188a9..efcf4b072 100644
--- a/vizier/_src/algorithms/designers/harmonica.py
+++ b/vizier/_src/algorithms/designers/harmonica.py
@@ -272,8 +272,14 @@ def __init__(
           f'This designer {self} does not support conditional search.'
       )
     for p_config in problem_statement.search_space.parameters:
-      if p_config.external_type != vz.ExternalType.BOOLEAN:
-        raise ValueError('Only boolean search spaces are supported.')
+      if p_config.external_type != vz.ExternalType.BOOLEAN and (
+          p_config.type == vz.ParameterType.DOUBLE
+          or len(p_config.feasible_values) != 2
+      ):
+        raise ValueError(
+            'Only boolean search spaces are supported, bot got parameter'
+            f' config: f{p_config}'
+        )
 
     self._problem_statement = problem_statement
     self._metric_name = self._problem_statement.metric_information.item().name
@@ -345,5 +351,11 @@ def suggest(
 
     parameters = vz.ParameterDict()
     for i, p in enumerate(self._search_space.parameters):
-      parameters[p.name] = 'True' if x_new[i] == 1.0 else 'False'
+      if p.external_type == vz.ExternalType.BOOLEAN:
+        parameters[p.name] = 'True' if x_new[i] == 1.0 else 'False'
+      else:
+        parameters[p.name] = (
+            p.feasible_values[1] if x_new[i] == 1.0 else p.feasible_values[0]
+        )
+
     return [vz.TrialSuggestion(parameters=parameters)]
diff --git a/vizier/_src/algorithms/designers/harmonica_test.py b/vizier/_src/algorithms/designers/harmonica_test.py
index 79da37f0a..07986c4ba 100644
--- a/vizier/_src/algorithms/designers/harmonica_test.py
+++ b/vizier/_src/algorithms/designers/harmonica_test.py
@@ -15,9 +15,11 @@
 from __future__ import annotations
 
 """Tests for bocs."""
+from vizier import pyvizier as vz
 from vizier._src.algorithms.designers import harmonica
 from vizier._src.algorithms.testing import test_runners
 from vizier._src.benchmarks.experimenters import combo_experimenter
+from vizier.testing import test_studies
 
 from absl.testing import absltest
 
@@ -39,6 +41,89 @@ def test_make_suggestions(self):
         validate_parameters=True)
     self.assertLen(trials, num_trials)
 
+  def test_make_suggestions_with_two_values(self):
+    space = vz.SearchSpace()
+    root = space.root
+    root.add_int_param('integer_0', 11, 12)
+    root.add_discrete_param('discrete_0', feasible_values=[1001, 2034])
+    root.add_categorical_param('categorical_0', feasible_values=['a', 'b'])
+    root.add_bool_param('bool_0')
+    problem_statement = vz.ProblemStatement(space)
+    problem_statement.metric_information.append(
+        vz.MetricInformation(
+            name='main_objective', goal=vz.ObjectiveMetricGoal.MAXIMIZE
+        )
+    )
+    designer = harmonica.HarmonicaDesigner(
+        problem_statement, num_init_samples=1
+    )
+    num_trials = 10
+    trials = test_runners.run_with_random_metrics(
+        designer,
+        problem_statement,
+        iters=num_trials,
+        batch_size=1,
+        verbose=1,
+        validate_parameters=True,
+    )
+    self.assertLen(trials, num_trials)
+
+  def test_categorical_search_space_with_more_than_two_values_raises_error(
+      self,
+  ):
+    experimenter = combo_experimenter.CentroidExperimenter(centroid_n_choice=3)
+    with self.assertRaisesRegex(
+        ValueError, 'Only boolean search spaces are supported'
+    ):
+      _ = harmonica.HarmonicaDesigner(
+          experimenter.problem_statement(), num_init_samples=1
+      )
+
+  def test_integer_search_space_with_more_than_two_values_raises_error(
+      self,
+  ):
+    space = vz.SearchSpace()
+    root = space.root
+    root.add_int_param('integer_0', 11, 13)
+    problem_statement = vz.ProblemStatement(space)
+    problem_statement.metric_information.append(
+        vz.MetricInformation(
+            name='main_objective', goal=vz.ObjectiveMetricGoal.MAXIMIZE
+        )
+    )
+    with self.assertRaisesRegex(
+        ValueError, 'Only boolean search spaces are supported'
+    ):
+      _ = harmonica.HarmonicaDesigner(problem_statement, num_init_samples=1)
+
+  def test_discrete_search_space_with_more_than_two_values_raises_error(
+      self,
+  ):
+    space = vz.SearchSpace()
+    root = space.root
+    root.add_discrete_param('discrete_0', feasible_values=[1001, 2034, 3000])
+    problem_statement = vz.ProblemStatement(space)
+    problem_statement.metric_information.append(
+        vz.MetricInformation(
+            name='main_objective', goal=vz.ObjectiveMetricGoal.MAXIMIZE
+        )
+    )
+    with self.assertRaisesRegex(
+        ValueError, 'Only boolean search spaces are supported'
+    ):
+      _ = harmonica.HarmonicaDesigner(problem_statement, num_init_samples=1)
+
+  def test_continuous_search_space_raises_error(self):
+    with self.assertRaisesRegex(
+        ValueError, 'Only boolean search spaces are supported'
+    ):
+      _ = harmonica.HarmonicaDesigner(
+          vz.ProblemStatement(
+              test_studies.flat_continuous_space_with_scaling()
+          ),
+          num_init_samples=1,
+      )
+
 
 if __name__ == '__main__':
   absltest.main()