diff --git a/python/PyQt6/core/auto_generated/processing/qgsprocessingparameters.sip.in b/python/PyQt6/core/auto_generated/processing/qgsprocessingparameters.sip.in
index 62c3328742d1..ef7742de2b7e 100644
--- a/python/PyQt6/core/auto_generated/processing/qgsprocessingparameters.sip.in
+++ b/python/PyQt6/core/auto_generated/processing/qgsprocessingparameters.sip.in
@@ -1189,10 +1189,13 @@ need to handle deletion of the returned layer.
 Evaluates the parameter with matching ``definition`` to a output layer destination.
 %End
 
-    static QString parameterAsOutputLayer( const QgsProcessingParameterDefinition *definition, const QVariant &value, QgsProcessingContext &context );
+    static QString parameterAsOutputLayer( const QgsProcessingParameterDefinition *definition, const QVariant &value, QgsProcessingContext &context, bool testOnly = false );
 %Docstring
 Evaluates the parameter with matching ``definition`` and ``value`` to a output layer destination.
 
+Since QGIS 3.38 the ``testOnly`` argument can be set to ``True`` to evaluate the parameter to an output layer destination for advance testing only. This
+prevents default behavior such as output post-processing which would otherwise occur.
+
 .. versionadded:: 3.4
 %End
 
diff --git a/python/core/auto_generated/processing/qgsprocessingparameters.sip.in b/python/core/auto_generated/processing/qgsprocessingparameters.sip.in
index 62c3328742d1..ef7742de2b7e 100644
--- a/python/core/auto_generated/processing/qgsprocessingparameters.sip.in
+++ b/python/core/auto_generated/processing/qgsprocessingparameters.sip.in
@@ -1189,10 +1189,13 @@ need to handle deletion of the returned layer.
 Evaluates the parameter with matching ``definition`` to a output layer destination.
 %End
 
-    static QString parameterAsOutputLayer( const QgsProcessingParameterDefinition *definition, const QVariant &value, QgsProcessingContext &context );
+    static QString parameterAsOutputLayer( const QgsProcessingParameterDefinition *definition, const QVariant &value, QgsProcessingContext &context, bool testOnly = false );
 %Docstring
 Evaluates the parameter with matching ``definition`` and ``value`` to a output layer destination.
 
+Since QGIS 3.38 the ``testOnly`` argument can be set to ``True`` to evaluate the parameter to an output layer destination for advance testing only. This
+prevents default behavior such as output post-processing which would otherwise occur.
+
 .. versionadded:: 3.4
 %End
 
diff --git a/src/core/processing/qgsprocessingparameters.cpp b/src/core/processing/qgsprocessingparameters.cpp
index b58234ee8c6e..ca013fcd74d5 100644
--- a/src/core/processing/qgsprocessingparameters.cpp
+++ b/src/core/processing/qgsprocessingparameters.cpp
@@ -937,7 +937,7 @@ QString QgsProcessingParameters::parameterAsOutputLayer( const QgsProcessingPara
   return parameterAsOutputLayer( definition, val, context );
 }
 
-QString QgsProcessingParameters::parameterAsOutputLayer( const QgsProcessingParameterDefinition *definition, const QVariant &value, QgsProcessingContext &context )
+QString QgsProcessingParameters::parameterAsOutputLayer( const QgsProcessingParameterDefinition *definition, const QVariant &value, QgsProcessingContext &context, bool testOnly )
 {
   QVariant val = value;
 
@@ -992,7 +992,8 @@ QString QgsProcessingParameters::parameterAsOutputLayer( const QgsProcessingPara
     else if ( definition && definition->type() == QgsProcessingParameterVectorTileDestination::typeName() )
       layerTypeHint = QgsProcessingUtils::LayerHint::VectorTile;
 
-    context.addLayerToLoadOnCompletion( dest, QgsProcessingContext::LayerDetails( destName, destinationProject, outputName, layerTypeHint ) );
+    if ( !testOnly )
+      context.addLayerToLoadOnCompletion( dest, QgsProcessingContext::LayerDetails( destName, destinationProject, outputName, layerTypeHint ) );
   }
 
   return dest;
diff --git a/src/core/processing/qgsprocessingparameters.h b/src/core/processing/qgsprocessingparameters.h
index f9131c869f57..a3475602696d 100644
--- a/src/core/processing/qgsprocessingparameters.h
+++ b/src/core/processing/qgsprocessingparameters.h
@@ -1303,9 +1303,13 @@ class CORE_EXPORT QgsProcessingParameters
 
     /**
      * Evaluates the parameter with matching \a definition and \a value to a output layer destination.
+     *
+     * Since QGIS 3.38 the \a testOnly argument can be set to TRUE to evaluate the parameter to an output layer destination for advance testing only. This
+     * prevents default behavior such as output post-processing which would otherwise occur.
+     *
      * \since QGIS 3.4
      */
-    static QString parameterAsOutputLayer( const QgsProcessingParameterDefinition *definition, const QVariant &value, QgsProcessingContext &context );
+    static QString parameterAsOutputLayer( const QgsProcessingParameterDefinition *definition, const QVariant &value, QgsProcessingContext &context, bool testOnly = false );
 
     /**
      * Evaluates the parameter with matching \a definition to a file based output destination.
diff --git a/tests/src/analysis/testqgsprocessing.cpp b/tests/src/analysis/testqgsprocessing.cpp
index 99e7aeb61c9d..39b9baa29280 100644
--- a/tests/src/analysis/testqgsprocessing.cpp
+++ b/tests/src/analysis/testqgsprocessing.cpp
@@ -8090,6 +8090,11 @@ void TestQgsProcessing::parameterVectorOut()
   QCOMPARE( context.layersToLoadOnCompletion().values().at( 0 ).name, QStringLiteral( "desc" ) );
   QCOMPARE( context.layersToLoadOnCompletion().values().at( 0 ).layerTypeHint, QgsProcessingUtils::LayerHint::Vector );
 
+  // if we set testOnly = true, then layer should not be loaded on completion
+  context.setLayersToLoadOnCompletion( {} );
+  QCOMPARE( QgsProcessingParameters::parameterAsOutputLayer( def.get(), QVariant::fromValue( fs ), context, true ), QStringLiteral( "test.shp" ) );
+  QCOMPARE( context.layersToLoadOnCompletion().size(), 0 );
+
   // with name overloading
   QgsProcessingContext context2;
   fs = QgsProcessingOutputLayerDefinition( QStringLiteral( "test.shp" ) );