diff --git a/ink_stroke_modeler/stroke_modeler.cc b/ink_stroke_modeler/stroke_modeler.cc
index 54554f9..6458077 100644
--- a/ink_stroke_modeler/stroke_modeler.cc
+++ b/ink_stroke_modeler/stroke_modeler.cc
@@ -132,7 +132,7 @@ absl::Status StrokeModeler::Update(const Input &input,
   return absl::InvalidArgumentError("Invalid EventType.");
 }
 
-absl::Status StrokeModeler::Predict(std::vector<Result> &results) {
+absl::Status StrokeModeler::Predict(std::vector<Result> &results) const {
   results.clear();
 
   if (!stroke_model_params_.has_value()) {
diff --git a/ink_stroke_modeler/stroke_modeler.h b/ink_stroke_modeler/stroke_modeler.h
index 00b77b9..84624b0 100644
--- a/ink_stroke_modeler/stroke_modeler.h
+++ b/ink_stroke_modeler/stroke_modeler.h
@@ -89,7 +89,7 @@ class StrokeModeler {
   //
   // The output is limited to results where the predictor has sufficient
   // confidence.
-  absl::Status Predict(std::vector<Result>& results);
+  absl::Status Predict(std::vector<Result>& results) const;
 
   // Saves the current modeler state.
   //
@@ -121,7 +121,9 @@ class StrokeModeler {
   PositionModeler position_modeler_;
   StylusStateModeler stylus_state_modeler_;
 
-  std::vector<TipState> tip_state_buffer_;
+  // This buffer is used as optimization to avoid re-allocating the vector in
+  // the predictor but doesn't hold state between calls, so can be mutable.
+  mutable std::vector<TipState> tip_state_buffer_;
 
   struct InputAndCorrectedPosition {
     Input input;