diff --git a/common/build.gradle b/common/build.gradle
--- a/common/build.gradle
+++ b/common/build.gradle
@@ -6,7 +6,7 @@ plugins {
 java {
     withJavadocJar()
     withSourcesJar()
-        toolchain {
+    toolchain {
         languageVersion = JavaLanguageVersion.of(17)
     }
 }
diff --git a/common/src/main/java/org/implab/gradle/common/dsl/TaskEnvSpecMixin.java b/common/src/main/java/org/implab/gradle/common/dsl/TaskEnvSpecMixin.java
--- a/common/src/main/java/org/implab/gradle/common/dsl/TaskEnvSpecMixin.java
+++ b/common/src/main/java/org/implab/gradle/common/dsl/TaskEnvSpecMixin.java
@@ -7,6 +7,7 @@ import org.gradle.api.Action;
 import org.gradle.api.file.DirectoryProperty;
 import org.gradle.api.provider.MapProperty;
 import org.gradle.api.provider.Property;
+import org.gradle.api.provider.Provider;
 import org.implab.gradle.common.utils.Closures;
 import org.implab.gradle.common.utils.Values;
 
@@ -31,19 +32,13 @@ public interface TaskEnvSpecMixin {
     /**
      * Configures the environment variable using the specified action. The
      * action is called when the value is calculated;
+     *
+     * <p>
+     * The configuration action is called immediately. To support lazy evaluation,
+     * properties may be assigned to providers.
      */
-    default void env(Action<Map<String, Object>> configure) {
-        var provider = getEnvironment()
-                .orElse(Map.of())
-                .map((base) -> {
-                    var props = new HashMap<String, Object>(base);
-
-                    configure.execute(props);
-
-                    return Values.mapValues(props, Values::toString);
-                });
-
-        getEnvironment().set(provider);
+    default void env(Action<Map<String, ?>> configure) {
+        Values.configureMap(getEnvironment(), configure, Object.class, Values::toString);
     }
 
     default void env(Closure<?> configure) {
diff --git a/common/src/main/java/org/implab/gradle/common/utils/ExtraProps.java b/common/src/main/java/org/implab/gradle/common/utils/ExtraProps.java
--- a/common/src/main/java/org/implab/gradle/common/utils/ExtraProps.java
+++ b/common/src/main/java/org/implab/gradle/common/utils/ExtraProps.java
@@ -1,16 +1,25 @@
 package org.implab.gradle.common.utils;
 
-import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
 
 import org.gradle.api.plugins.ExtensionAware;
 
-public class ExtraProps {
+public interface ExtraProps {
+
+    Set<String> keys();
+
+    boolean has(String name);
+
+    <T> Optional<T> get(String name, Class<T> clazz);
 
-    public static Map<String, Object> of(Object target) {
-        if (target instanceof ExtensionAware ext)
-            return ext.getExtensions().getExtraProperties().getProperties();
-        else
-            return Map.of();
+    void put(String name, Object value);
+
+    void remove(String name);
+
+    public static ExtraProps of(Object target) {
+        return target instanceof ExtensionAware ext
+                ? new ExtraPropsExtension(ext.getExtensions().getExtraProperties())
+                : new ExtraPropsNone();
     }
-
 }
diff --git a/common/src/main/java/org/implab/gradle/common/utils/ExtraPropsExtension.java b/common/src/main/java/org/implab/gradle/common/utils/ExtraPropsExtension.java
new file mode 100644
--- /dev/null
+++ b/common/src/main/java/org/implab/gradle/common/utils/ExtraPropsExtension.java
@@ -0,0 +1,43 @@
+package org.implab.gradle.common.utils;
+
+import java.util.Optional;
+import java.util.Set;
+
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.gradle.api.plugins.ExtraPropertiesExtension;
+
+@NonNullByDefault
+class ExtraPropsExtension implements ExtraProps {
+
+    private final ExtraPropertiesExtension ext;
+
+    public ExtraPropsExtension(ExtraPropertiesExtension ext) {
+        this.ext = ext;
+    }
+
+    @Override
+    public Set<String> keys() {
+        return ext.getProperties().keySet();
+    }
+
+    @Override
+    public boolean has(String name) {
+        return ext.has(name);
+    }
+
+    @Override
+    public <T> Optional<T> get(String name, Class<T> clazz) {
+        return Optional.ofNullable(ext.get(name)).map(clazz::cast);
+    }
+
+    @Override
+    public void put(String name, Object value) {
+        ext.set(name, value);
+    }
+
+    @Override
+    public void remove(String name) {
+        ext.set(name, null);
+    }
+    
+}
diff --git a/common/src/main/java/org/implab/gradle/common/utils/ExtraPropsNone.java b/common/src/main/java/org/implab/gradle/common/utils/ExtraPropsNone.java
new file mode 100644
--- /dev/null
+++ b/common/src/main/java/org/implab/gradle/common/utils/ExtraPropsNone.java
@@ -0,0 +1,31 @@
+package org.implab.gradle.common.utils;
+
+import java.util.Optional;
+import java.util.Set;
+
+public class ExtraPropsNone implements ExtraProps {
+
+    @Override
+    public Set<String> keys() {
+        return Set.of();
+    }
+
+    @Override
+    public boolean has(String name) {
+        return false;
+    }
+
+    @Override
+    public <T> Optional<T> get(String name, Class<T> clazz) {
+        return Optional.empty();
+    }
+
+    @Override
+    public void put(String name, Object value) {
+    }
+
+    @Override
+    public void remove(String name) {
+    }
+    
+}
diff --git a/common/src/main/java/org/implab/gradle/common/utils/Values.java b/common/src/main/java/org/implab/gradle/common/utils/Values.java
--- a/common/src/main/java/org/implab/gradle/common/utils/Values.java
+++ b/common/src/main/java/org/implab/gradle/common/utils/Values.java
@@ -1,5 +1,6 @@
 package org.implab.gradle.common.utils;
 
+import java.util.HashMap;
 import java.util.Iterator;
 import java.util.Map;
 import java.util.Spliterators;
@@ -11,6 +12,8 @@ import java.util.stream.Collectors;
 import java.util.stream.Stream;
 import java.util.stream.StreamSupport;
 
+import org.gradle.api.Action;
+import org.gradle.api.provider.MapProperty;
 import org.gradle.api.provider.Provider;
 
 public final class Values {
@@ -20,7 +23,7 @@ public final class Values {
 
     /**
      * Converts values in the specified map
-     * 
+     *
      * @param <K>
      * @param <V>
      * @param <U>
@@ -35,6 +38,33 @@ public final class Values {
                 .collect(Collectors.toMap(Entry::getKey, getter.andThen(mapper)));
     }
 
+    public static <K, V> void mergeMap(MapProperty<K,V> prop, Map<K, ? > map, Class<V> clazz) {
+        map.forEach((k,v) -> {
+            if(v instanceof Provider<?>)
+                prop.put(k, ((Provider<?>)v).map(clazz::cast));
+            else
+                prop.put(k, clazz.cast(v));
+        });
+    }
+
+    public static <K,V,U> void mergeMap(MapProperty<K,V> prop, Map<K, ?> map, Class<U> clazz, Function<U,V> mapper) {
+        Function<Object, U> caster = clazz::cast;
+        var castMap = caster.andThen(mapper);
+
+        map.forEach((k,v) -> {
+            if(v instanceof Provider<?>)
+                prop.put(k, ((Provider<?>)v).map(x -> castMap.apply(x)));
+            else
+                prop.put(k, castMap.apply(v));
+        });
+    }
+
+    public static <K,V,U> void configureMap(MapProperty<K,V> prop, Action<Map<K, ?>> configure, Class<U> clazz, Function<U,V> mapper)  {
+        var map = new HashMap<K,V>();
+        configure.execute(map);
+        mergeMap(prop, map, clazz, mapper);
+    }
+
     /**
      * Converts the supplied value to a string.
      */