diff --git a/image_analysis/geom_refinement/GeometryRefiner.cpp b/image_analysis/geom_refinement/GeometryRefiner.cpp
index b10ca118..b7eb486e 100644
--- a/image_analysis/geom_refinement/GeometryRefiner.cpp
+++ b/image_analysis/geom_refinement/GeometryRefiner.cpp
@@ -174,6 +174,10 @@ GeometryRefinerResult RefineGlobalGeometry(const DiffractionGeometry &nominal_ge
             return 0.3 - (0.3 - 0.1) * t;
         };
 
+        // Whether the last solve that ran produced a usable solution; folded into result.ok below,
+        // which otherwise reports success on spot and frame counts alone.
+        bool solve_usable = false;
+
         for (int round = 0; round < settings.rounds; ++round) {
             EffectiveCell(system, cell_len, cell_ang, eff_len, eff_alpha, eff_beta, eff_gamma);
 
@@ -265,6 +269,7 @@ GeometryRefinerResult RefineGlobalGeometry(const DiffractionGeometry &nominal_ge
 
             ceres::Solver::Summary summary;
             ceres::Solve(options, &problem, &summary);
+            solve_usable = summary.IsSolutionUsable();
 
             result.frames_used = frames_used;
             result.spots_used = spots_used;
@@ -298,7 +303,7 @@ GeometryRefinerResult RefineGlobalGeometry(const DiffractionGeometry &nominal_ge
             result.median_residual_px = residuals[residuals.size() / 2];
         }
 
-        result.ok = result.spots_used >= settings.min_spots_total && result.frames_used > 0;
+        result.ok = solve_usable && result.spots_used >= settings.min_spots_total && result.frames_used > 0;
         result.beam_x_px = beam[0];
         result.beam_y_px = beam[1];
         result.distance_mm = distance_mm;
diff --git a/image_analysis/geom_refinement/RingOptimizer.cpp b/image_analysis/geom_refinement/RingOptimizer.cpp
index 626f66f8..1a120180 100644
--- a/image_analysis/geom_refinement/RingOptimizer.cpp
+++ b/image_analysis/geom_refinement/RingOptimizer.cpp
@@ -94,6 +94,12 @@ DiffractionGeometry RingOptimizer::Run(const std::vector<RingOptimizerInput> &in
     // Run optimization
     ceres::Solve(options, &problem, &summary);
 
+    // A failed fit must not move the detector geometry. Both callers assign the result straight
+    // back over the geometry they passed in, so handing back the reference leaves the calibration
+    // where it was instead of committing a diverged beam centre and distance.
+    if (!summary.IsSolutionUsable())
+        return reference;
+
     DiffractionGeometry refined_geom(reference);
     refined_geom.BeamX_pxl(center_x).BeamY_pxl(center_y).DetectorDistance_mm(distance)
                 .PoniRot1_rad(rot1).PoniRot2_rad(rot2);
diff --git a/image_analysis/geom_refinement/XtalOptimizer.cpp b/image_analysis/geom_refinement/XtalOptimizer.cpp
index c769e871..a2843d78 100644
--- a/image_analysis/geom_refinement/XtalOptimizer.cpp
+++ b/image_analysis/geom_refinement/XtalOptimizer.cpp
@@ -356,6 +356,13 @@ bool XtalOptimizerInternal(XtalOptimizerData &data,
         // Run optimization
         ceres::Solve(options, &problem, &summary);
 
+        // Only a genuine numerical failure is rejected here: a solve that ran out of iterations or
+        // out of time but still descended counts as usable, which is what the real-time caller
+        // relies on when it sets max_solver_time. Checked before anything is written back, so a
+        // failed refinement leaves data untouched rather than committing half a fit.
+        if (!summary.IsSolutionUsable())
+            return false;
+
         if (data.refine_beam_center) {
             data.beam_corr_x = data.geom.GetBeamX_pxl() - beam[0];
             data.beam_corr_y = data.geom.GetBeamY_pxl() - beam[1];
@@ -490,6 +497,9 @@ bool XtalOptimizerRotationOnly(XtalOptimizerData &data,
         ceres::Solver::Summary summary;
         ceres::Solve(options, &problem, &summary);
 
+        if (!summary.IsSolutionUsable())
+            return false;
+
         // Apply rotation to direct-lattice vectors.
         // ceres::AngleAxisToRotationMatrix writes a **row-major** 3×3 matrix,
         // and Eigen's << operator also fills row-by-row, so the assignment