Use AWS_NEW_RETRIES_2026 during mode resolution

core/sdk-core/src/main/java/software/amazon/awssdk/core/retry/RetryMode.java

@@ -130,8 +130,6 @@ public static Resolver resolver() {
      * Allows customizing the variables used during determination of a {@link RetryMode}. Created via {@link #resolver()}.
      */
     public static class Resolver {
-        private static final RetryMode SDK_DEFAULT_RETRY_MODE = LEGACY;
-
         private Supplier<ProfileFile> profileFile;
         private String profileName;
         private RetryMode defaultRetryMode;
@@ -204,7 +202,14 @@ private static Optional<RetryMode> fromString(String string) {
         }
 
         private RetryMode fromDefaultMode() {
-            return defaultRetryMode != null ? defaultRetryMode : SDK_DEFAULT_RETRY_MODE;
+            return defaultRetryMode != null ? defaultRetryMode : sdkDefaultRetryMode();
+        }
+
+        /**
+         * Resolves the SDK default retry mode dynamically based on the {@code AWS_NEW_RETRIES_2026} gate.
+         */
+        private static RetryMode sdkDefaultRetryMode() {
+            return SdkSystemSetting.AWS_NEW_RETRIES_2026.getBooleanValue().orElse(false) ? STANDARD : LEGACY;
         }
     }
 }

core/sdk-core/src/test/java/software/amazon/awssdk/core/SdkSystemSettingNewRetriesTest.java

@@ -0,0 +1,89 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License").
+ * You may not use this file except in compliance with the License.
+ * A copy of the License is located at
+ *
+ *  http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed
+ * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+package software.amazon.awssdk.core;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+import software.amazon.awssdk.testutils.EnvironmentVariableHelper;
+
+/**
+ * Tests for the {@link SdkSystemSetting#AWS_NEW_RETRIES_2026} system setting.
+ */
+class SdkSystemSettingNewRetriesTest {
+
+    @AfterEach
+    void cleanup() {
+        System.clearProperty(SdkSystemSetting.AWS_NEW_RETRIES_2026.property());
+    }
+
+    @Test
+    void defaultsToFalse_whenUnset() {
+        assertThat(SdkSystemSetting.AWS_NEW_RETRIES_2026.getBooleanValue()).hasValue(false);
+    }
+
+    @ParameterizedTest(name = "systemProperty=\"{0}\" -> {1}")
+    @CsvSource({
+        "false, false",
+        "true,  true"
+    })
+    void getBooleanValue_reflectsSystemProperty(String propertyValue, boolean expected) {
+        System.setProperty(SdkSystemSetting.AWS_NEW_RETRIES_2026.property(), propertyValue);
+        assertThat(SdkSystemSetting.AWS_NEW_RETRIES_2026.getBooleanValue()).hasValue(expected);
+    }
+
+    @ParameterizedTest(name = "envVar=\"{0}\" -> {1}")
+    @CsvSource({
+        "false, false",
+        "true,  true"
+    })
+    void getBooleanValue_reflectsEnvVar(String envVarValue, boolean expected) {
+        EnvironmentVariableHelper.run(helper -> {
+            helper.set(SdkSystemSetting.AWS_NEW_RETRIES_2026, envVarValue);
+            assertThat(SdkSystemSetting.AWS_NEW_RETRIES_2026.getBooleanValue()).hasValue(expected);
+        });
+    }
+
+    @Test
+    void systemPropertyTakesPrecedenceOverEnvVar() {
+        EnvironmentVariableHelper.run(helper -> {
+            System.setProperty(SdkSystemSetting.AWS_NEW_RETRIES_2026.property(), "false");
+            helper.set(SdkSystemSetting.AWS_NEW_RETRIES_2026, "true");
+            assertThat(SdkSystemSetting.AWS_NEW_RETRIES_2026.getBooleanValue()).hasValue(false);
+        });
+    }
+
+    @Test
+    void environmentVariable_isCorrectName() {
+        assertThat(SdkSystemSetting.AWS_NEW_RETRIES_2026.environmentVariable())
+            .isEqualTo("AWS_NEW_RETRIES_2026");
+    }
+
+    @Test
+    void systemProperty_isCorrectName() {
+        assertThat(SdkSystemSetting.AWS_NEW_RETRIES_2026.property())
+            .isEqualTo("aws.newRetries2026");
+    }
+
+    @Test
+    void defaultValue_isFalse() {
+        assertThat(SdkSystemSetting.AWS_NEW_RETRIES_2026.defaultValue())
+            .isEqualTo("false");
+    }
+}

core/sdk-core/src/test/java/software/amazon/awssdk/core/retry/RetryModeGatedDefaultTest.java

@@ -0,0 +1,104 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License").
+ * You may not use this file except in compliance with the License.
+ * A copy of the License is located at
+ *
+ *  http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed
+ * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+package software.amazon.awssdk.core.retry;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+import software.amazon.awssdk.core.SdkSystemSetting;
+import software.amazon.awssdk.testutils.EnvironmentVariableHelper;
+
+/**
+ * Tests for the gated default {@link RetryMode} behavior controlled by
+ * {@link SdkSystemSetting#AWS_NEW_RETRIES_2026}.
+ */
+class RetryModeGatedDefaultTest {
+
+    @BeforeEach
+    @AfterEach
+    void cleanup() {
+        System.clearProperty(SdkSystemSetting.AWS_NEW_RETRIES_2026.property());
+        System.clearProperty(SdkSystemSetting.AWS_RETRY_MODE.property());
+    }
+
+    @Test
+    void defaultRetryMode_returnsLegacy_whenGateIsUnset() {
+        assertThat(RetryMode.defaultRetryMode()).isEqualTo(RetryMode.LEGACY);
+    }
+
+    @ParameterizedTest(name = "gate=\"{0}\" -> {1}")
+    @CsvSource({
+        "false, LEGACY",
+        "true,  STANDARD"
+    })
+    void defaultRetryMode_reflectsGate_whenSetViaSystemProperty(String gateValue, RetryMode expected) {
+        System.setProperty(SdkSystemSetting.AWS_NEW_RETRIES_2026.property(), gateValue);
+        assertThat(RetryMode.defaultRetryMode()).isEqualTo(expected);
+    }
+
+    @ParameterizedTest(name = "gate=\"{0}\" -> {1}")
+    @CsvSource({
+        "false, LEGACY",
+        "true,  STANDARD"
+    })
+    void defaultRetryMode_reflectsGate_whenSetViaEnvVar(String gateValue, RetryMode expected) {
+        EnvironmentVariableHelper.run(helper -> {
+            helper.set(SdkSystemSetting.AWS_NEW_RETRIES_2026, gateValue);
+            assertThat(RetryMode.defaultRetryMode()).isEqualTo(expected);
+        });
+    }
+
+    @Test
+    void defaultRetryMode_changesDynamically_whenGateSystemPropertyChangesAtRuntime() {
+        // Initially unset — should be LEGACY
+        assertThat(RetryMode.defaultRetryMode()).isEqualTo(RetryMode.LEGACY);
+
+        // Enable gate — should switch to STANDARD
+        System.setProperty(SdkSystemSetting.AWS_NEW_RETRIES_2026.property(), "true");
+        assertThat(RetryMode.defaultRetryMode()).isEqualTo(RetryMode.STANDARD);
+
+        // Disable gate — should revert to LEGACY
+        System.setProperty(SdkSystemSetting.AWS_NEW_RETRIES_2026.property(), "false");
+        assertThat(RetryMode.defaultRetryMode()).isEqualTo(RetryMode.LEGACY);
+
+        // Clear gate — should fall back to default (LEGACY)
+        System.clearProperty(SdkSystemSetting.AWS_NEW_RETRIES_2026.property());
+        assertThat(RetryMode.defaultRetryMode()).isEqualTo(RetryMode.LEGACY);
+    }
+
+    @ParameterizedTest(name = "gate=\"{0}\" retryMode=\"{1}\" -> {2}")
+    @CsvSource({
+        "true,  legacy,   LEGACY",
+        "false, standard, STANDARD",
+        "false, adaptive, ADAPTIVE_V2"
+    })
+    void resolve_honorsExplicitRetryMode_regardlessOfGate(String gateValue, String retryModeValue, RetryMode expected) {
+        System.setProperty(SdkSystemSetting.AWS_NEW_RETRIES_2026.property(), gateValue);
+        System.setProperty(SdkSystemSetting.AWS_RETRY_MODE.property(), retryModeValue);
+        assertThat(RetryMode.defaultRetryMode()).isEqualTo(expected);
+    }
+
+    @Test
+    void resolve_throwsIllegalStateException_whenInvalidRetryModeConfigured() {
+        System.setProperty(SdkSystemSetting.AWS_RETRY_MODE.property(), "invalid_mode");
+        assertThatThrownBy(RetryMode::defaultRetryMode).isInstanceOf(IllegalStateException.class);
+    }
+}