diff --git a/gax-java/gax/src/main/java/com/google/api/gax/rpc/ApiClientHeaderProvider.java b/gax-java/gax/src/main/java/com/google/api/gax/rpc/ApiClientHeaderProvider.java
index ae27404335..a5e80e10b1 100644
--- a/gax-java/gax/src/main/java/com/google/api/gax/rpc/ApiClientHeaderProvider.java
+++ b/gax-java/gax/src/main/java/com/google/api/gax/rpc/ApiClientHeaderProvider.java
@@ -42,7 +42,7 @@ public class ApiClientHeaderProvider implements HeaderProvider, Serializable {
   private static final long serialVersionUID = -8876627296793342119L;
   static final String QUOTA_PROJECT_ID_HEADER_KEY = "x-goog-user-project";
 
-  static final String API_VERSION_HEADER_KEY = "x-goog-api-version";
+  public static final String API_VERSION_HEADER_KEY = "x-goog-api-version";
 
   private final Map<String, String> headers;
 
diff --git a/showcase/gapic-showcase/pom.xml b/showcase/gapic-showcase/pom.xml
index 5bdc31a945..9e1b331ceb 100644
--- a/showcase/gapic-showcase/pom.xml
+++ b/showcase/gapic-showcase/pom.xml
@@ -19,7 +19,7 @@
   </parent>
 
   <properties>
-    <gapic-showcase.version>0.33.0</gapic-showcase.version>
+    <gapic-showcase.version>0.35.0</gapic-showcase.version>
   </properties>
 
   <build>
diff --git a/showcase/gapic-showcase/src/test/java/com/google/showcase/v1beta1/it/ITApiVersionHeaders.java b/showcase/gapic-showcase/src/test/java/com/google/showcase/v1beta1/it/ITApiVersionHeaders.java
new file mode 100644
index 0000000000..7ef4019975
--- /dev/null
+++ b/showcase/gapic-showcase/src/test/java/com/google/showcase/v1beta1/it/ITApiVersionHeaders.java
@@ -0,0 +1,324 @@
+/*
+ * Copyright 2024 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License 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 com.google.showcase.v1beta1.it;
+
+import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertThrows;
+
+import com.google.api.gax.httpjson.*;
+import com.google.api.gax.rpc.ApiClientHeaderProvider;
+import com.google.api.gax.rpc.FixedHeaderProvider;
+import com.google.api.gax.rpc.StubSettings;
+import com.google.common.collect.ImmutableList;
+import com.google.showcase.v1beta1.*;
+import com.google.showcase.v1beta1.it.util.TestClientInitializer;
+import com.google.showcase.v1beta1.stub.ComplianceStubSettings;
+import com.google.showcase.v1beta1.stub.EchoStubSettings;
+import io.grpc.*;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.concurrent.TimeUnit;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+// TODO: add testing on error responses once feat is implemented in showcase.
+//  https://github.com/googleapis/gapic-showcase/pull/1456
+// TODO: watch for showcase gRPC trailer changes suggested in
+// https://github.com/googleapis/gapic-showcase/pull/1509#issuecomment-2089147103
+public class ITApiVersionHeaders {
+  private static final String HTTP_RESPONSE_HEADER_STRING =
+      "x-showcase-request-" + ApiClientHeaderProvider.API_VERSION_HEADER_KEY;
+  private static final Metadata.Key<String> API_VERSION_HEADER_KEY =
+      Metadata.Key.of(
+          ApiClientHeaderProvider.API_VERSION_HEADER_KEY, Metadata.ASCII_STRING_MARSHALLER);
+
+  private static final String EXPECTED_ECHO_API_VERSION = "v1_20240408";
+  private static final String CUSTOM_API_VERSION = "user-supplied-version";
+  private static final String EXPECTED_EXCEPTION_MESSAGE =
+      "Header provider can't override the header: "
+          + ApiClientHeaderProvider.API_VERSION_HEADER_KEY;
+  private static final int DEFAULT_AWAIT_TERMINATION_SEC = 10;
+
+  // Implement a client interceptor to retrieve the trailing metadata from response.
+  private static class GrpcCapturingClientInterceptor implements ClientInterceptor {
+    private Metadata metadata;
+
+    @Override
+    public <RequestT, ResponseT> ClientCall<RequestT, ResponseT> interceptCall(
+        MethodDescriptor<RequestT, ResponseT> method, final CallOptions callOptions, Channel next) {
+      ClientCall<RequestT, ResponseT> call = next.newCall(method, callOptions);
+      return new ForwardingClientCall.SimpleForwardingClientCall<RequestT, ResponseT>(call) {
+        @Override
+        public void start(Listener<ResponseT> responseListener, Metadata headers) {
+          Listener<ResponseT> wrappedListener =
+              new SimpleForwardingClientCallListener<ResponseT>(responseListener) {
+                @Override
+                public void onClose(Status status, Metadata trailers) {
+                  if (status.isOk()) {
+                    metadata = trailers;
+                  }
+                  super.onClose(status, trailers);
+                }
+              };
+
+          super.start(wrappedListener, headers);
+        }
+      };
+    }
+  }
+
+  private static class SimpleForwardingClientCallListener<RespT>
+      extends ClientCall.Listener<RespT> {
+    private final ClientCall.Listener<RespT> delegate;
+
+    SimpleForwardingClientCallListener(ClientCall.Listener<RespT> delegate) {
+      this.delegate = delegate;
+    }
+
+    @Override
+    public void onHeaders(Metadata headers) {
+      delegate.onHeaders(headers);
+    }
+
+    @Override
+    public void onMessage(RespT message) {
+      delegate.onMessage(message);
+    }
+
+    @Override
+    public void onClose(Status status, Metadata trailers) {
+      delegate.onClose(status, trailers);
+    }
+
+    @Override
+    public void onReady() {
+      delegate.onReady();
+    }
+  }
+  // Implement a client interceptor to retrieve the response headers
+  private static class HttpJsonCapturingClientInterceptor implements HttpJsonClientInterceptor {
+    private HttpJsonMetadata metadata;
+
+    @Override
+    public <RequestT, ResponseT> HttpJsonClientCall<RequestT, ResponseT> interceptCall(
+        ApiMethodDescriptor<RequestT, ResponseT> method,
+        HttpJsonCallOptions callOptions,
+        HttpJsonChannel next) {
+      HttpJsonClientCall<RequestT, ResponseT> call = next.newCall(method, callOptions);
+      return new ForwardingHttpJsonClientCall.SimpleForwardingHttpJsonClientCall<
+          RequestT, ResponseT>(call) {
+        @Override
+        public void start(Listener<ResponseT> responseListener, HttpJsonMetadata requestHeaders) {
+          Listener<ResponseT> forwardingResponseListener =
+              new ForwardingHttpJsonClientCallListener.SimpleForwardingHttpJsonClientCallListener<
+                  ResponseT>(responseListener) {
+                @Override
+                public void onHeaders(HttpJsonMetadata responseHeaders) {
+                  metadata = responseHeaders;
+                  super.onHeaders(responseHeaders);
+                }
+
+                @Override
+                public void onMessage(ResponseT message) {
+                  super.onMessage(message);
+                }
+
+                @Override
+                public void onClose(int statusCode, HttpJsonMetadata trailers) {
+                  super.onClose(statusCode, trailers);
+                }
+              };
+
+          super.start(forwardingResponseListener, requestHeaders);
+        }
+      };
+    }
+  }
+
+  private HttpJsonCapturingClientInterceptor httpJsonInterceptor;
+  private GrpcCapturingClientInterceptor grpcInterceptor;
+  private HttpJsonCapturingClientInterceptor httpJsonComplianceInterceptor;
+  private GrpcCapturingClientInterceptor grpcComplianceInterceptor;
+  private EchoClient grpcClient;
+  private EchoClient httpJsonClient;
+  private ComplianceClient grpcComplianceClient;
+  private ComplianceClient httpJsonComplianceClient;
+
+  @Before
+  public void createClients() throws Exception {
+    // Create gRPC Interceptor and Client
+    grpcInterceptor = new GrpcCapturingClientInterceptor();
+    grpcClient = TestClientInitializer.createGrpcEchoClient(ImmutableList.of(grpcInterceptor));
+
+    // Create HttpJson Interceptor and Client
+    httpJsonInterceptor = new HttpJsonCapturingClientInterceptor();
+    httpJsonClient =
+        TestClientInitializer.createHttpJsonEchoClient(ImmutableList.of(httpJsonInterceptor));
+
+    // Create gRPC ComplianceClient and Interceptor
+    // Creating a compliance client to test case where api version is not set
+    grpcComplianceInterceptor = new GrpcCapturingClientInterceptor();
+    grpcComplianceClient =
+        TestClientInitializer.createGrpcComplianceClient(
+            ImmutableList.of(grpcComplianceInterceptor));
+
+    // Create HttpJson ComplianceClient and Interceptor
+    httpJsonComplianceInterceptor = new HttpJsonCapturingClientInterceptor();
+    httpJsonComplianceClient =
+        TestClientInitializer.createHttpJsonComplianceClient(
+            ImmutableList.of(httpJsonComplianceInterceptor));
+  }
+
+  @After
+  public void destroyClient() throws InterruptedException {
+    grpcClient.close();
+    httpJsonClient.close();
+    grpcComplianceClient.close();
+    httpJsonComplianceClient.close();
+
+    grpcClient.awaitTermination(DEFAULT_AWAIT_TERMINATION_SEC, TimeUnit.SECONDS);
+    httpJsonClient.awaitTermination(DEFAULT_AWAIT_TERMINATION_SEC, TimeUnit.SECONDS);
+    grpcComplianceClient.awaitTermination(DEFAULT_AWAIT_TERMINATION_SEC, TimeUnit.SECONDS);
+    httpJsonComplianceClient.awaitTermination(DEFAULT_AWAIT_TERMINATION_SEC, TimeUnit.SECONDS);
+  }
+
+  @Test
+  public void testGrpc_matchesApiVersion() {
+    grpcClient.echo(EchoRequest.newBuilder().build());
+    String headerValue = grpcInterceptor.metadata.get(API_VERSION_HEADER_KEY);
+    assertThat(headerValue).isEqualTo(EXPECTED_ECHO_API_VERSION);
+  }
+
+  @Test
+  public void testHttpJson_matchesHeaderName() {
+    httpJsonClient.echo(EchoRequest.newBuilder().build());
+    ArrayList headerValues =
+        (ArrayList) httpJsonInterceptor.metadata.getHeaders().get(HTTP_RESPONSE_HEADER_STRING);
+    String headerValue = (String) headerValues.get(0);
+    assertThat(headerValue).isEqualTo(EXPECTED_ECHO_API_VERSION);
+  }
+
+  @Test
+  public void testGrpc_noApiVersion() {
+    RepeatRequest request =
+        RepeatRequest.newBuilder().setInfo(ComplianceData.newBuilder().setFString("test")).build();
+    grpcComplianceClient.repeatDataSimplePath(request);
+    assertThat(API_VERSION_HEADER_KEY).isNotIn(grpcComplianceInterceptor.metadata.keys());
+  }
+
+  @Test
+  public void testHttpJson_noApiVersion() {
+    RepeatRequest request =
+        RepeatRequest.newBuilder().setInfo(ComplianceData.newBuilder().setFString("test")).build();
+    httpJsonComplianceClient.repeatDataSimplePath(request);
+    assertThat(API_VERSION_HEADER_KEY)
+        .isNotIn(httpJsonComplianceInterceptor.metadata.getHeaders().keySet());
+  }
+
+  @Test
+  public void testGrpcEcho_userApiVersionThrowsException() throws IOException {
+    StubSettings stubSettings =
+        grpcClient
+            .getSettings()
+            .getStubSettings()
+            .toBuilder()
+            .setHeaderProvider(
+                FixedHeaderProvider.create(
+                    ApiClientHeaderProvider.API_VERSION_HEADER_KEY, CUSTOM_API_VERSION))
+            .build();
+
+    IllegalArgumentException exception =
+        assertThrows(
+            IllegalArgumentException.class,
+            () -> EchoClient.create(EchoSettings.create((EchoStubSettings) stubSettings)));
+    assertThat(exception.getMessage()).isEqualTo(EXPECTED_EXCEPTION_MESSAGE);
+  }
+
+  @Test
+  public void testHttpJsonEcho_userApiVersionThrowsException() throws IOException {
+    StubSettings stubSettings =
+        httpJsonClient
+            .getSettings()
+            .getStubSettings()
+            .toBuilder()
+            .setHeaderProvider(
+                FixedHeaderProvider.create(
+                    ApiClientHeaderProvider.API_VERSION_HEADER_KEY, CUSTOM_API_VERSION))
+            .build();
+
+    IllegalArgumentException exception =
+        assertThrows(
+            IllegalArgumentException.class,
+            () -> EchoClient.create(EchoSettings.create((EchoStubSettings) stubSettings)));
+    assertThat(exception.getMessage()).isEqualTo(EXPECTED_EXCEPTION_MESSAGE);
+  }
+
+  @Test
+  public void testGrpcCompliance_userApiVersionSetSuccess() throws IOException {
+    StubSettings stubSettingsWithApiVersionHeader =
+        grpcComplianceClient
+            .getSettings()
+            .getStubSettings()
+            .toBuilder()
+            .setHeaderProvider(
+                FixedHeaderProvider.create(
+                    ApiClientHeaderProvider.API_VERSION_HEADER_KEY, CUSTOM_API_VERSION))
+            .build();
+    try (ComplianceClient customComplianceClient =
+        ComplianceClient.create(
+            ComplianceSettings.create((ComplianceStubSettings) stubSettingsWithApiVersionHeader))) {
+
+      RepeatRequest request =
+          RepeatRequest.newBuilder()
+              .setInfo(ComplianceData.newBuilder().setFString("test"))
+              .build();
+      customComplianceClient.repeatDataSimplePath(request);
+      String headerValue = grpcComplianceInterceptor.metadata.get(API_VERSION_HEADER_KEY);
+      assertThat(headerValue).isEqualTo(CUSTOM_API_VERSION);
+    }
+  }
+
+  @Test
+  public void testHttpJsonCompliance_userApiVersionSetSuccess() throws IOException {
+    StubSettings httpJsonStubSettingsWithApiVersionHeader =
+        httpJsonComplianceClient
+            .getSettings()
+            .getStubSettings()
+            .toBuilder()
+            .setHeaderProvider(
+                FixedHeaderProvider.create(
+                    ApiClientHeaderProvider.API_VERSION_HEADER_KEY, CUSTOM_API_VERSION))
+            .build();
+    try (ComplianceClient customComplianceClient =
+        ComplianceClient.create(
+            ComplianceSettings.create(
+                (ComplianceStubSettings) httpJsonStubSettingsWithApiVersionHeader))) {
+
+      RepeatRequest request =
+          RepeatRequest.newBuilder()
+              .setInfo(ComplianceData.newBuilder().setFString("test"))
+              .build();
+      customComplianceClient.repeatDataSimplePath(request);
+
+      ArrayList headerValues =
+          (ArrayList)
+              httpJsonComplianceInterceptor.metadata.getHeaders().get(HTTP_RESPONSE_HEADER_STRING);
+      String headerValue = (String) headerValues.get(0);
+      assertThat(headerValue).isEqualTo(CUSTOM_API_VERSION);
+    }
+  }
+}