diff --git a/src/main/java/it/cnr/isti/workflow/manager/executions/ExecutionContext.java b/src/main/java/it/cnr/isti/workflow/manager/executions/ExecutionContext.java
index 359abc8..670dd82 100644
--- a/src/main/java/it/cnr/isti/workflow/manager/executions/ExecutionContext.java
+++ b/src/main/java/it/cnr/isti/workflow/manager/executions/ExecutionContext.java
@@ -899,8 +899,7 @@ public class ExecutionContext implements ExecutionListener {
     }
 
     private void refreshRuntimeExecutionVariables() {
-        this.runtimeExecutionVariables.clear();
-        this.runtimeExecutionVariables.putAll(ExecutionRuntimeContextSupport.runtimeValues(
+        replaceContents(this.runtimeExecutionVariables, ExecutionRuntimeContextSupport.runtimeValues(
                 this.executionVariables,
                 this.globalInputs,
                 this.projectContext,
@@ -908,13 +907,29 @@ public class ExecutionContext implements ExecutionListener {
     }
 
     private void synchronizeExecutionVariablesFromDescriptors() {
-        this.executionVariables.clear();
+        Map<String, Object> declared = new HashMap<>();
         this.executionVariableDescriptors.values().stream()
                 .map(ExecutionVariableRegistry::normalize)
-                .forEach(descriptor -> this.executionVariables.put(descriptor.getName(), descriptor.getValue()));
+                .forEach(descriptor -> declared.put(descriptor.getName(), descriptor.getValue()));
+        replaceContents(this.executionVariables, declared);
         refreshRuntimeExecutionVariables();
     }
 
+    /**
+     * Brings {@code target} to {@code desired} touching only what differs. Steps running on other
+     * threads read these maps while this runs on every step's completion: emptied and refilled,
+     * they would read a map with nothing in it, or fail copying one that changed under them. With
+     * the same keys - the usual case, variables being declared up front - nothing is written at all.
+     */
+    private static void replaceContents(Map<String, Object> target, Map<String, Object> desired) {
+        target.keySet().removeIf(key -> !desired.containsKey(key));
+        desired.forEach((key, value) -> {
+            if (!target.containsKey(key) || !java.util.Objects.equals(target.get(key), value)) {
+                target.put(key, value);
+            }
+        });
+    }
+
     private synchronized void abortAfterFailure(String failedStepId) {
         this.waitingSteps.clear();
         this.steps.values().stream()