diff --git a/.changes/d4f09e32-8bcc-406e-96e0-2743280c13d9.json b/.changes/d4f09e32-8bcc-406e-96e0-2743280c13d9.json
new file mode 100644
index 000000000..2d09a6ce5
--- /dev/null
+++ b/.changes/d4f09e32-8bcc-406e-96e0-2743280c13d9.json
@@ -0,0 +1,8 @@
+{
+    "id": "d4f09e32-8bcc-406e-96e0-2743280c13d9",
+    "type": "bugfix",
+    "description": "Switch to always serialize defaults in requests. Previously fields were not serialized in requests if they weren't `@required` and hadn't been changed from the default value.",
+    "issues": [
+        "https://github.com/awslabs/aws-sdk-kotlin/issues/1608"
+    ]
+}
diff --git a/codegen/smithy-kotlin-codegen-testutils/src/main/kotlin/software/amazon/smithy/kotlin/codegen/test/ModelTestUtils.kt b/codegen/smithy-kotlin-codegen-testutils/src/main/kotlin/software/amazon/smithy/kotlin/codegen/test/ModelTestUtils.kt
index e857f91fe..d3b8960d4 100644
--- a/codegen/smithy-kotlin-codegen-testutils/src/main/kotlin/software/amazon/smithy/kotlin/codegen/test/ModelTestUtils.kt
+++ b/codegen/smithy-kotlin-codegen-testutils/src/main/kotlin/software/amazon/smithy/kotlin/codegen/test/ModelTestUtils.kt
@@ -173,7 +173,7 @@ fun Model.defaultSettings(
     sdkId: String = TestModelDefault.SDK_ID,
     generateDefaultBuildFiles: Boolean = false,
     nullabilityCheckMode: CheckMode = CheckMode.CLIENT_CAREFUL,
-    defaultValueSerializationMode: DefaultValueSerializationMode = DefaultValueSerializationMode.WHEN_DIFFERENT,
+    defaultValueSerializationMode: DefaultValueSerializationMode = DefaultValueSerializationMode.DEFAULT,
 ): KotlinSettings {
     val serviceId = if (serviceName == null) {
         this.inferService()
diff --git a/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/KotlinSettings.kt b/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/KotlinSettings.kt
index 3ee6e0633..e72712281 100644
--- a/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/KotlinSettings.kt
+++ b/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/KotlinSettings.kt
@@ -5,12 +5,7 @@
 
 package software.amazon.smithy.kotlin.codegen
 
-import software.amazon.smithy.aws.traits.protocols.AwsJson1_0Trait
-import software.amazon.smithy.aws.traits.protocols.AwsJson1_1Trait
-import software.amazon.smithy.aws.traits.protocols.AwsQueryTrait
-import software.amazon.smithy.aws.traits.protocols.Ec2QueryTrait
-import software.amazon.smithy.aws.traits.protocols.RestJson1Trait
-import software.amazon.smithy.aws.traits.protocols.RestXmlTrait
+import software.amazon.smithy.aws.traits.protocols.*
 import software.amazon.smithy.codegen.core.CodegenException
 import software.amazon.smithy.kotlin.codegen.lang.isValidPackageName
 import software.amazon.smithy.kotlin.codegen.utils.getOrNull
@@ -24,10 +19,9 @@ import software.amazon.smithy.model.shapes.ServiceShape
 import software.amazon.smithy.model.shapes.Shape
 import software.amazon.smithy.model.shapes.ShapeId
 import software.amazon.smithy.protocol.traits.Rpcv2CborTrait
-import java.util.Optional
+import java.util.*
 import java.util.logging.Logger
-import kotlin.IllegalArgumentException
-import kotlin.streams.toList
+import java.util.stream.Collectors
 
 // shapeId of service from which to generate an SDK
 private const val SERVICE = "service"
@@ -164,7 +158,7 @@ fun Model.inferService(): ShapeId {
     val services = shapes(ServiceShape::class.java)
         .map(Shape::getId)
         .sorted()
-        .toList()
+        .collect(Collectors.toList())
 
     return when {
         services.isEmpty() -> {
@@ -273,10 +267,15 @@ enum class DefaultValueSerializationMode(val value: String) {
 
     override fun toString(): String = value
     companion object {
+        /**
+         * The default value serialization mode, which is [ALWAYS]
+         */
+        val DEFAULT = ALWAYS
+
         fun fromValue(value: String): DefaultValueSerializationMode =
-            values().find {
-                it.value == value
-            } ?: throw IllegalArgumentException("$value is not a valid DefaultValueSerializationMode, expected one of ${values().map { it.value }}")
+            requireNotNull(entries.find { it.value.equals(value, ignoreCase = true) }) {
+                "$value is not a valid DefaultValueSerializationMode, expected one of ${values().map { it.value }}"
+            }
     }
 }
 
@@ -291,7 +290,7 @@ enum class DefaultValueSerializationMode(val value: String) {
 data class ApiSettings(
     val visibility: Visibility = Visibility.PUBLIC,
     val nullabilityCheckMode: CheckMode = CheckMode.CLIENT_CAREFUL,
-    val defaultValueSerializationMode: DefaultValueSerializationMode = DefaultValueSerializationMode.WHEN_DIFFERENT,
+    val defaultValueSerializationMode: DefaultValueSerializationMode = DefaultValueSerializationMode.DEFAULT,
     val enableEndpointAuthProvider: Boolean = false,
     val protocolResolutionPriority: Set<ShapeId> = DEFAULT_PROTOCOL_RESOLUTION_PRIORITY,
 ) {
@@ -315,7 +314,7 @@ data class ApiSettings(
                 node.get()
                     .getStringMemberOrDefault(
                         DEFAULT_VALUE_SERIALIZATION_MODE,
-                        DefaultValueSerializationMode.WHEN_DIFFERENT.value,
+                        DefaultValueSerializationMode.DEFAULT.value,
                     ),
             )
             val enableEndpointAuthProvider = node.get().getBooleanMemberOrDefault(ENABLE_ENDPOINT_AUTH_PROVIDER, false)
diff --git a/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/core/KotlinDependency.kt b/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/core/KotlinDependency.kt
index b575efae6..de8f29413 100644
--- a/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/core/KotlinDependency.kt
+++ b/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/core/KotlinDependency.kt
@@ -134,6 +134,7 @@ data class KotlinDependency(
         // External third-party dependencies
         val KOTLIN_STDLIB = KotlinDependency(GradleConfiguration.Implementation, "kotlin", "org.jetbrains.kotlin", "kotlin-stdlib", KOTLIN_COMPILER_VERSION)
         val KOTLIN_TEST = KotlinDependency(GradleConfiguration.TestImplementation, "kotlin.test", "org.jetbrains.kotlin", "kotlin-test", KOTLIN_COMPILER_VERSION)
+        val KOTLIN_TEST_IMPL = KOTLIN_TEST.copy(config = GradleConfiguration.Implementation)
     }
 
     override fun getDependencies(): List<SymbolDependency> {
diff --git a/codegen/smithy-kotlin-codegen/src/test/kotlin/software/amazon/smithy/kotlin/codegen/rendering/protocol/HttpStringValuesMapSerializerTest.kt b/codegen/smithy-kotlin-codegen/src/test/kotlin/software/amazon/smithy/kotlin/codegen/rendering/protocol/HttpStringValuesMapSerializerTest.kt
index 95138ad52..097db4b4c 100644
--- a/codegen/smithy-kotlin-codegen/src/test/kotlin/software/amazon/smithy/kotlin/codegen/rendering/protocol/HttpStringValuesMapSerializerTest.kt
+++ b/codegen/smithy-kotlin-codegen/src/test/kotlin/software/amazon/smithy/kotlin/codegen/rendering/protocol/HttpStringValuesMapSerializerTest.kt
@@ -53,7 +53,8 @@ class HttpStringValuesMapSerializerTest {
 
     @Test
     fun `it handles primitive header shapes when different mode`() {
-        val contents = getTestContents(defaultModel, "com.test#PrimitiveShapesOperation", HttpBinding.Location.HEADER)
+        val settings = defaultModel.defaultSettings(defaultValueSerializationMode = DefaultValueSerializationMode.WHEN_DIFFERENT)
+        val contents = getTestContents(defaultModel, "com.test#PrimitiveShapesOperation", HttpBinding.Location.HEADER, settings)
         contents.assertBalancedBracesAndParens()
 
         val expectedContents = """
@@ -68,7 +69,8 @@ class HttpStringValuesMapSerializerTest {
 
     @Test
     fun `it handles primitive query shapes when different mode`() {
-        val contents = getTestContents(defaultModel, "com.test#PrimitiveShapesOperation", HttpBinding.Location.QUERY)
+        val settings = defaultModel.defaultSettings(defaultValueSerializationMode = DefaultValueSerializationMode.WHEN_DIFFERENT)
+        val contents = getTestContents(defaultModel, "com.test#PrimitiveShapesOperation", HttpBinding.Location.QUERY, settings)
         contents.assertBalancedBracesAndParens()
 
         val expectedContents = """
@@ -129,7 +131,8 @@ class HttpStringValuesMapSerializerTest {
             }
         """.prependNamespaceAndService(operations = listOf("Foo")).toSmithyModel()
 
-        val contents = getTestContents(model, "com.test#Foo", HttpBinding.Location.HEADER)
+        val settings = defaultModel.defaultSettings(defaultValueSerializationMode = DefaultValueSerializationMode.WHEN_DIFFERENT)
+        val contents = getTestContents(model, "com.test#Foo", HttpBinding.Location.HEADER, settings)
         contents.assertBalancedBracesAndParens()
 
         val expectedContents = """
diff --git a/codegen/smithy-kotlin-codegen/src/test/kotlin/software/amazon/smithy/kotlin/codegen/rendering/serde/SerializeStructGeneratorTest.kt b/codegen/smithy-kotlin-codegen/src/test/kotlin/software/amazon/smithy/kotlin/codegen/rendering/serde/SerializeStructGeneratorTest.kt
index 347da007c..c0eae1f05 100644
--- a/codegen/smithy-kotlin-codegen/src/test/kotlin/software/amazon/smithy/kotlin/codegen/rendering/serde/SerializeStructGeneratorTest.kt
+++ b/codegen/smithy-kotlin-codegen/src/test/kotlin/software/amazon/smithy/kotlin/codegen/rendering/serde/SerializeStructGeneratorTest.kt
@@ -72,7 +72,8 @@ class SerializeStructGeneratorTest {
             }
         """.trimIndent()
 
-        val actual = codegenSerializerForShape(model, "com.test#Foo")
+        val settings = model.defaultSettings(defaultValueSerializationMode = DefaultValueSerializationMode.WHEN_DIFFERENT)
+        val actual = codegenSerializerForShape(model, "com.test#Foo", settings = settings)
         actual.shouldContainOnlyOnceWithDiff(expected)
     }