diff --git a/sound_algo/data_only/matrix.scd b/sound_algo/data_only/matrix.scd
index 2bab630..beef03c 100644
--- a/sound_algo/data_only/matrix.scd
+++ b/sound_algo/data_only/matrix.scd
@@ -407,10 +407,15 @@ File.exists(~matDir).not.if({ ("mkdir -p " ++ ~matDir.quote).systemCmd });
     });
 };
 
-// one mutation from a base snapshot + re-source (the evolve one-shot)
+// one mutation + re-source (the evolve one-shot). When the evolve toggle is
+// OFF, snapshot/mutate/discard so ~matBaseGrid (owned by the toggle: non-nil
+// iff evolve active) is never leaked across a preset load. When ON, keep the
+// persistent base and just re-mutate from it.
 ~matRelanceEvolve = {
-    ~matBaseGrid.isNil.if({ ~matBaseGrid = ~lp[\matrix].collect({ |row| row.copy }) });
+    var hadBase = ~matBaseGrid.notNil;
+    hadBase.not.if({ ~matBaseGrid = ~lp[\matrix].collect({ |row| row.copy }) });
     ~matEvolveStep.();
+    hadBase.not.if({ ~matBaseGrid = nil });
     ~matVoices.size.do { |i| ~matLastColor[i] = -1 };
     (~lp[\matPlaying] == true).if({ ~matApplyBar.(~lp[\matBar] ? 0) });
 };
diff --git a/sound_algo/data_only/matrix_presets/test_global_actions.scd b/sound_algo/data_only/matrix_presets/test_global_actions.scd
index 6d1278c..193d816 100644
--- a/sound_algo/data_only/matrix_presets/test_global_actions.scd
+++ b/sound_algo/data_only/matrix_presets/test_global_actions.scd
@@ -67,6 +67,19 @@ feat.load;
 (~matGlobalActions == [\evolve, \breakdown, \fill, \drop]).not.if({ pass = false; "FAIL: globalActions not round-tripped".postln });
 ("rm -f '" ++ (~matDir +/+ "zz_gact_test.matrix") ++ "'").systemCmd;
 
+// -- I: evolve one-shot must not leak ~matBaseGrid when the toggle is off,
+//       and must mutate from the CURRENT grid (not a stale snapshot) --
+~matEvolve = false; ~matBaseGrid = nil;
+~lp[\matrix] = Array.fill(~matVoices.size, { Array.fill(~matBars, 0) });
+~lp[\matrix][0][0] = 3;                                  // gridA: kick bar0 active
+~matRelanceEvolve.();
+(~matBaseGrid.isNil).not.if({ pass = false; "FAIL: evolve one-shot leaked baseGrid".postln });
+~lp[\matrix] = Array.fill(~matVoices.size, { Array.fill(~matBars, 0) });
+~lp[\matrix][1][5] = 2;                                  // gridB: hats bar5 active
+~matRelanceEvolve.();
+(~lp[\matrix][0][0] == 0).not.if({ pass = false; "FAIL: stale gridA cell resurrected by evolve".postln });
+(~lp[\matrix][1][5] > 0).not.if({ pass = false; "FAIL: gridB active cell lost by evolve".postln });
+
 pass.if({ "GLOBALACT PASS".postln }, { "GLOBALACT FAIL".postln });
 0.exit;
 )