diff --git a/MIGRATION-2.0.md b/MIGRATION-2.0.md
index 51369c387..92d38b1cc 100644
--- a/MIGRATION-2.0.md
+++ b/MIGRATION-2.0.md
@@ -213,7 +213,7 @@ Schemas that explicitly declare a different dialect via `$schema` are accepted w
 
 The deprecated `Builder.customizeRequest(Consumer<HttpRequest.Builder>)` method on `HttpClientSseClientTransport` and `HttpClientStreamableHttpTransport` has been removed.
 
-**Action:** Use `requestBuilder(HttpRequest.Builder)` for static request setup, or `httpRequestCustomizer(McpSyncHttpClientRequestCustomizer)` for per-request customization.
+**Action:** Use `requestBuilder(HttpRequest.Builder)` for static request setup, or `addHttpRequestCustomizer(McpSyncHttpClientRequestCustomizer)` for per-request customization.
 
 ### `protocolVersions()` default now advertises all known versions
 
diff --git a/conformance-tests/client-spring-http-client/src/main/java/io/modelcontextprotocol/conformance/client/scenario/PreRegistrationScenario.java b/conformance-tests/client-spring-http-client/src/main/java/io/modelcontextprotocol/conformance/client/scenario/PreRegistrationScenario.java
index e783a9197..ecfd572ce 100644
--- a/conformance-tests/client-spring-http-client/src/main/java/io/modelcontextprotocol/conformance/client/scenario/PreRegistrationScenario.java
+++ b/conformance-tests/client-spring-http-client/src/main/java/io/modelcontextprotocol/conformance/client/scenario/PreRegistrationScenario.java
@@ -55,7 +55,7 @@ public void execute(String serverUrl) {
 
 		var customizer = new OAuth2ClientCredentialsSyncHttpRequestCustomizer(authorizedClientManager, REGISTRATION_ID);
 		HttpClientStreamableHttpTransport transport = HttpClientStreamableHttpTransport.builder(serverUrl)
-			.httpRequestCustomizer(customizer)
+			.addHttpRequestCustomizer(customizer)
 			.build();
 
 		var client = McpClient.sync(transport)
diff --git a/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java b/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
index 9ed5c5cd4..6bf1a2424 100644
--- a/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
+++ b/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
@@ -8,14 +8,15 @@
 import java.net.URI;
 import java.net.http.HttpClient;
 import java.net.http.HttpRequest;
-import java.net.http.HttpResponse;
 import java.time.Duration;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Optional;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Consumer;
 import java.util.function.Function;
 
+import io.modelcontextprotocol.client.transport.customizer.DelegatingMcpAsyncHttpClientRequestCustomizer;
 import io.modelcontextprotocol.client.transport.customizer.McpAsyncHttpClientRequestCustomizer;
 import io.modelcontextprotocol.client.transport.customizer.McpSyncHttpClientRequestCustomizer;
 import io.modelcontextprotocol.common.McpTransportContext;
@@ -203,7 +204,7 @@ public static class Builder {
 
 		private HttpRequest.Builder requestBuilder = HttpRequest.newBuilder();
 
-		private McpAsyncHttpClientRequestCustomizer httpRequestCustomizer = McpAsyncHttpClientRequestCustomizer.NOOP;
+		private final List<McpAsyncHttpClientRequestCustomizer> httpRequestCustomizers = new ArrayList<>();
 
 		private Duration connectTimeout = Duration.ofSeconds(10);
 
@@ -286,10 +287,10 @@ public Builder jsonMapper(McpJsonMapper jsonMapper) {
 
 		/**
 		 * Sets the customizer for {@link HttpRequest.Builder}, to modify requests before
-		 * executing them.
+		 * executing them. The customizer is internally converted to
+		 * {@link McpAsyncHttpClientRequestCustomizer}.
 		 * <p>
-		 * This overrides the customizer from
-		 * {@link #asyncHttpRequestCustomizer(McpAsyncHttpClientRequestCustomizer)}.
+		 * This replaces all customizers previously registered on this builder.
 		 * <p>
 		 * Do NOT use a blocking {@link McpSyncHttpClientRequestCustomizer} in a
 		 * non-blocking context. Use
@@ -297,25 +298,81 @@ public Builder jsonMapper(McpJsonMapper jsonMapper) {
 		 * instead.
 		 * @param syncHttpRequestCustomizer the request customizer
 		 * @return this builder
+		 * @deprecated Use {@link #addHttpRequestCustomizer} instead.
 		 */
+		@Deprecated
 		public Builder httpRequestCustomizer(McpSyncHttpClientRequestCustomizer syncHttpRequestCustomizer) {
-			this.httpRequestCustomizer = McpAsyncHttpClientRequestCustomizer.fromSync(syncHttpRequestCustomizer);
-			return this;
+			Assert.notNull(syncHttpRequestCustomizer, "syncHttpRequestCustomizer must not be null");
+			this.httpRequestCustomizers.clear();
+			return addHttpRequestCustomizer(syncHttpRequestCustomizer);
 		}
 
 		/**
 		 * Sets the customizer for {@link HttpRequest.Builder}, to modify requests before
 		 * executing them.
 		 * <p>
-		 * This overrides the customizer from
-		 * {@link #httpRequestCustomizer(McpSyncHttpClientRequestCustomizer)}.
+		 * This replaces all customizers previously registered on this builder.
 		 * <p>
 		 * Do NOT use a blocking implementation in a non-blocking context.
 		 * @param asyncHttpRequestCustomizer the request customizer
 		 * @return this builder
+		 * @deprecated Use {@link #addAsyncHttpRequestCustomizer} instead.
 		 */
+		@Deprecated
 		public Builder asyncHttpRequestCustomizer(McpAsyncHttpClientRequestCustomizer asyncHttpRequestCustomizer) {
-			this.httpRequestCustomizer = asyncHttpRequestCustomizer;
+			Assert.notNull(asyncHttpRequestCustomizer, "asyncHttpRequestCustomizer must not be null");
+			this.httpRequestCustomizers.clear();
+			return addAsyncHttpRequestCustomizer(asyncHttpRequestCustomizer);
+		}
+
+		/**
+		 * Adds a customizer for {@link HttpRequest.Builder}, to modify requests before
+		 * executing them. Customizers are applied in the order they are added, after
+		 * those already registered on this builder. The customizer is internally
+		 * converted to {@link McpAsyncHttpClientRequestCustomizer}.
+		 * <p>
+		 * Do NOT use a blocking {@link McpSyncHttpClientRequestCustomizer} in a
+		 * non-blocking context. Use
+		 * {@link #addAsyncHttpRequestCustomizer(McpAsyncHttpClientRequestCustomizer)}
+		 * instead.
+		 * @param syncHttpRequestCustomizer the request customizer
+		 * @return this builder
+		 */
+		public Builder addHttpRequestCustomizer(McpSyncHttpClientRequestCustomizer syncHttpRequestCustomizer) {
+			Assert.notNull(syncHttpRequestCustomizer, "syncHttpRequestCustomizer must not be null");
+			this.httpRequestCustomizers.add(McpAsyncHttpClientRequestCustomizer.fromSync(syncHttpRequestCustomizer));
+			return this;
+		}
+
+		/**
+		 * Adds a customizer for {@link HttpRequest.Builder}, to modify requests before
+		 * executing them. Customizers are applied in the order they are added, after
+		 * those already registered on this builder.
+		 * <p>
+		 * Do NOT use a blocking implementation in a non-blocking context.
+		 * @param asyncHttpRequestCustomizer the request customizer
+		 * @return this builder
+		 */
+		public Builder addAsyncHttpRequestCustomizer(McpAsyncHttpClientRequestCustomizer asyncHttpRequestCustomizer) {
+			Assert.notNull(asyncHttpRequestCustomizer, "asyncHttpRequestCustomizer must not be null");
+			this.httpRequestCustomizers.add(asyncHttpRequestCustomizer);
+			return this;
+		}
+
+		/**
+		 * Provides access to the mutable list of request customizers registered on this
+		 * builder, so they can be inspected, reordered, added or removed. Customizers are
+		 * applied in list order. Synchronous customizers registered through
+		 * {@link #httpRequestCustomizer(McpSyncHttpClientRequestCustomizer)} or
+		 * {@link #addHttpRequestCustomizer(McpSyncHttpClientRequestCustomizer)} appear in
+		 * the list wrapped as {@link McpAsyncHttpClientRequestCustomizer}.
+		 * @param customizersConsumer a consumer of the list of customizers
+		 * @return this builder
+		 */
+		public Builder asyncHttpRequestCustomizers(
+				Consumer<List<McpAsyncHttpClientRequestCustomizer>> customizersConsumer) {
+			Assert.notNull(customizersConsumer, "customizersConsumer must not be null");
+			customizersConsumer.accept(this.httpRequestCustomizers);
 			return this;
 		}
 
@@ -369,10 +426,19 @@ public Builder maxResponseSize(int maxResponseSize) {
 		public HttpClientSseClientTransport build() {
 			HttpClient httpClient = this.clientBuilder.connectTimeout(this.connectTimeout).build();
 			return new HttpClientSseClientTransport(httpClient, requestBuilder, baseUri, sseEndpoint,
-					jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper, httpRequestCustomizer,
+					jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper, httpRequestCustomizer(),
 					messageEndpointValidator, maxResponseSize);
 		}
 
+		private McpAsyncHttpClientRequestCustomizer httpRequestCustomizer() {
+			Assert.noNullElements(this.httpRequestCustomizers, "httpRequestCustomizers must not contain null elements");
+			return switch (this.httpRequestCustomizers.size()) {
+				case 0 -> McpAsyncHttpClientRequestCustomizer.NOOP;
+				case 1 -> this.httpRequestCustomizers.get(0);
+				default -> new DelegatingMcpAsyncHttpClientRequestCustomizer(List.copyOf(this.httpRequestCustomizers));
+			};
+		}
+
 	}
 
 	@Override
diff --git a/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java b/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
index 9d55e816c..e56a23f08 100644
--- a/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
+++ b/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
@@ -11,6 +11,7 @@
 import java.net.http.HttpResponse;
 import java.nio.ByteBuffer;
 import java.time.Duration;
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.List;
@@ -22,6 +23,7 @@
 import java.util.function.Function;
 
 import io.modelcontextprotocol.client.McpAsyncClient;
+import io.modelcontextprotocol.client.transport.customizer.DelegatingMcpAsyncHttpClientRequestCustomizer;
 import io.modelcontextprotocol.client.transport.customizer.McpAsyncHttpClientRequestCustomizer;
 import io.modelcontextprotocol.client.transport.customizer.McpHttpClientAuthorizationErrorHandler;
 import io.modelcontextprotocol.client.transport.customizer.McpHttpClientTransportAuthorizationErrorHandler;
@@ -672,7 +674,7 @@ public static class Builder {
 
 		private HttpRequest.Builder requestBuilder = HttpRequest.newBuilder();
 
-		private McpAsyncHttpClientRequestCustomizer httpRequestCustomizer = McpAsyncHttpClientRequestCustomizer.NOOP;
+		private final List<McpAsyncHttpClientRequestCustomizer> httpRequestCustomizers = new ArrayList<>();
 
 		private Duration connectTimeout = Duration.ofSeconds(10);
 
@@ -776,10 +778,10 @@ public Builder openConnectionOnStartup(boolean openConnectionOnStartup) {
 
 		/**
 		 * Sets the customizer for {@link HttpRequest.Builder}, to modify requests before
-		 * executing them.
+		 * executing them. The customizer is internally converted to
+		 * {@link McpAsyncHttpClientRequestCustomizer}.
 		 * <p>
-		 * This overrides the customizer from
-		 * {@link #asyncHttpRequestCustomizer(McpAsyncHttpClientRequestCustomizer)}.
+		 * This replaces all customizers previously registered on this builder.
 		 * <p>
 		 * Do NOT use a blocking {@link McpSyncHttpClientRequestCustomizer} in a
 		 * non-blocking context. Use
@@ -787,25 +789,81 @@ public Builder openConnectionOnStartup(boolean openConnectionOnStartup) {
 		 * instead.
 		 * @param syncHttpRequestCustomizer the request customizer
 		 * @return this builder
+		 * @deprecated Use {@link #addHttpRequestCustomizer} instead.
 		 */
+		@Deprecated
 		public Builder httpRequestCustomizer(McpSyncHttpClientRequestCustomizer syncHttpRequestCustomizer) {
-			this.httpRequestCustomizer = McpAsyncHttpClientRequestCustomizer.fromSync(syncHttpRequestCustomizer);
-			return this;
+			Assert.notNull(syncHttpRequestCustomizer, "syncHttpRequestCustomizer must not be null");
+			this.httpRequestCustomizers.clear();
+			return addHttpRequestCustomizer(syncHttpRequestCustomizer);
 		}
 
 		/**
 		 * Sets the customizer for {@link HttpRequest.Builder}, to modify requests before
 		 * executing them.
 		 * <p>
-		 * This overrides the customizer from
-		 * {@link #httpRequestCustomizer(McpSyncHttpClientRequestCustomizer)}.
+		 * This replaces all customizers previously registered on this builder.
 		 * <p>
 		 * Do NOT use a blocking implementation in a non-blocking context.
 		 * @param asyncHttpRequestCustomizer the request customizer
 		 * @return this builder
+		 * @deprecated Use {@link #addAsyncHttpRequestCustomizer} instead.
 		 */
+		@Deprecated
 		public Builder asyncHttpRequestCustomizer(McpAsyncHttpClientRequestCustomizer asyncHttpRequestCustomizer) {
-			this.httpRequestCustomizer = asyncHttpRequestCustomizer;
+			Assert.notNull(asyncHttpRequestCustomizer, "asyncHttpRequestCustomizer must not be null");
+			this.httpRequestCustomizers.clear();
+			return addAsyncHttpRequestCustomizer(asyncHttpRequestCustomizer);
+		}
+
+		/**
+		 * Adds a customizer for {@link HttpRequest.Builder}, to modify requests before
+		 * executing them. Customizers are applied in the order they are added, after
+		 * those already registered on this builder. The customizer is internally
+		 * converted to {@link McpAsyncHttpClientRequestCustomizer}.
+		 * <p>
+		 * Do NOT use a blocking {@link McpSyncHttpClientRequestCustomizer} in a
+		 * non-blocking context. Use
+		 * {@link #addAsyncHttpRequestCustomizer(McpAsyncHttpClientRequestCustomizer)}
+		 * instead.
+		 * @param syncHttpRequestCustomizer the request customizer
+		 * @return this builder
+		 */
+		public Builder addHttpRequestCustomizer(McpSyncHttpClientRequestCustomizer syncHttpRequestCustomizer) {
+			Assert.notNull(syncHttpRequestCustomizer, "syncHttpRequestCustomizer must not be null");
+			this.httpRequestCustomizers.add(McpAsyncHttpClientRequestCustomizer.fromSync(syncHttpRequestCustomizer));
+			return this;
+		}
+
+		/**
+		 * Adds a customizer for {@link HttpRequest.Builder}, to modify requests before
+		 * executing them. Customizers are applied in the order they are added, after
+		 * those already registered on this builder.
+		 * <p>
+		 * Do NOT use a blocking implementation in a non-blocking context.
+		 * @param asyncHttpRequestCustomizer the request customizer
+		 * @return this builder
+		 */
+		public Builder addAsyncHttpRequestCustomizer(McpAsyncHttpClientRequestCustomizer asyncHttpRequestCustomizer) {
+			Assert.notNull(asyncHttpRequestCustomizer, "asyncHttpRequestCustomizer must not be null");
+			this.httpRequestCustomizers.add(asyncHttpRequestCustomizer);
+			return this;
+		}
+
+		/**
+		 * Provides access to the mutable list of request customizers registered on this
+		 * builder, so they can be inspected, reordered, added or removed. Customizers are
+		 * applied in list order. Synchronous customizers registered through
+		 * {@link #httpRequestCustomizer(McpSyncHttpClientRequestCustomizer)} or
+		 * {@link #addHttpRequestCustomizer(McpSyncHttpClientRequestCustomizer)} appear in
+		 * the list wrapped as {@link McpAsyncHttpClientRequestCustomizer}.
+		 * @param customizersConsumer a consumer of the list of customizers
+		 * @return this builder
+		 */
+		public Builder asyncHttpRequestCustomizers(
+				Consumer<List<McpAsyncHttpClientRequestCustomizer>> customizersConsumer) {
+			Assert.notNull(customizersConsumer, "customizersConsumer must not be null");
+			customizersConsumer.accept(this.httpRequestCustomizers);
 			return this;
 		}
 
@@ -910,7 +968,16 @@ public HttpClientStreamableHttpTransport build() {
 			HttpClient httpClient = this.clientBuilder.connectTimeout(this.connectTimeout).build();
 			return new HttpClientStreamableHttpTransport(jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper,
 					httpClient, requestBuilder, baseUri, endpoint, resumableStreams, openConnectionOnStartup,
-					httpRequestCustomizer, authorizationErrorHandler, supportedProtocolVersions, maxResponseSize);
+					httpRequestCustomizer(), authorizationErrorHandler, supportedProtocolVersions, maxResponseSize);
+		}
+
+		private McpAsyncHttpClientRequestCustomizer httpRequestCustomizer() {
+			Assert.noNullElements(this.httpRequestCustomizers, "httpRequestCustomizers must not contain null elements");
+			return switch (this.httpRequestCustomizers.size()) {
+				case 0 -> McpAsyncHttpClientRequestCustomizer.NOOP;
+				case 1 -> this.httpRequestCustomizers.get(0);
+				default -> new DelegatingMcpAsyncHttpClientRequestCustomizer(List.copyOf(this.httpRequestCustomizers));
+			};
 		}
 
 	}
diff --git a/mcp-test/src/main/java/io/modelcontextprotocol/client/AbstractMcpAsyncClientTests.java b/mcp-test/src/main/java/io/modelcontextprotocol/client/AbstractMcpAsyncClientTests.java
index 71df07085..f422871b0 100644
--- a/mcp-test/src/main/java/io/modelcontextprotocol/client/AbstractMcpAsyncClientTests.java
+++ b/mcp-test/src/main/java/io/modelcontextprotocol/client/AbstractMcpAsyncClientTests.java
@@ -10,6 +10,7 @@
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.assertj.core.api.Assertions.fail;
 import static org.junit.jupiter.api.Assertions.assertInstanceOf;
+import static org.junit.jupiter.api.Assumptions.assumeTrue;
 
 import java.time.Duration;
 import java.util.ArrayList;
@@ -23,6 +24,7 @@
 import java.util.function.Consumer;
 import java.util.function.Function;
 
+import io.modelcontextprotocol.client.transport.customizer.McpAsyncHttpClientRequestCustomizer;
 import io.modelcontextprotocol.spec.McpSchema.ElicitFormRequest;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
@@ -67,6 +69,15 @@ public abstract class AbstractMcpAsyncClientTests {
 
 	abstract protected McpClientTransport createMcpTransport();
 
+	// Creates a client transport that applies request customizers, in the order they
+	// appear in the list. Subclasses whose transport supports
+	// McpAsyncHttpClientRequestCustomizer should override this; tests relying on it are
+	// skipped otherwise.
+	protected McpClientTransport createMcpTransport(
+			Consumer<List<McpAsyncHttpClientRequestCustomizer>> requestCustomizers) {
+		return null;
+	}
+
 	protected Duration getRequestTimeout() {
 		return Duration.ofSeconds(14);
 	}
@@ -126,6 +137,30 @@ <T> void verifyCallSucceedsWithImplicitInitialization(Function<McpAsyncClient, M
 		});
 	}
 
+	@Test
+	void testRequestCustomizersAreAppliedInOrder() {
+		var invocations = new CopyOnWriteArrayList<String>();
+		Function<String, McpAsyncHttpClientRequestCustomizer> recording = name -> (builder, method, endpoint, body,
+				context) -> {
+			invocations.add(name);
+			return Mono.just(builder);
+		};
+
+		var transport = createMcpTransport(customizers -> {
+			customizers.add(recording.apply("second"));
+			customizers.add(recording.apply("third"));
+			customizers.add(0, recording.apply("first"));
+		});
+		assumeTrue(transport != null, "Transport does not support request customizers");
+
+		withClient(transport, mcpAsyncClient -> {
+			StepVerifier.create(mcpAsyncClient.initialize()).expectNextCount(1).verifyComplete();
+			// Later requests may run concurrently, e.g. the Streamable HTTP GET
+			// stream, so only check the first one.
+			assertThat(invocations).startsWith("first", "second", "third");
+		});
+	}
+
 	@Test
 	void testConstructorWithInvalidArguments() {
 		assertThatThrownBy(() -> McpClient.async(null).build()).isInstanceOf(IllegalArgumentException.class)
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpClientStreamableHttpAsyncClientTests.java b/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpClientStreamableHttpAsyncClientTests.java
index a29ca16db..521f4397e 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpClientStreamableHttpAsyncClientTests.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpClientStreamableHttpAsyncClientTests.java
@@ -4,7 +4,11 @@
 
 package io.modelcontextprotocol.client;
 
+import java.util.List;
+import java.util.function.Consumer;
+
 import io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport;
+import io.modelcontextprotocol.client.transport.customizer.McpAsyncHttpClientRequestCustomizer;
 import io.modelcontextprotocol.spec.McpClientTransport;
 import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.BeforeAll;
@@ -29,6 +33,12 @@ protected McpClientTransport createMcpTransport() {
 		return HttpClientStreamableHttpTransport.builder(host).build();
 	}
 
+	@Override
+	protected McpClientTransport createMcpTransport(
+			Consumer<List<McpAsyncHttpClientRequestCustomizer>> requestCustomizers) {
+		return HttpClientStreamableHttpTransport.builder(host).asyncHttpRequestCustomizers(requestCustomizers).build();
+	}
+
 	@BeforeAll
 	static void startContainer() {
 		container.start();
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpClientStreamableHttpSyncClientTests.java b/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpClientStreamableHttpSyncClientTests.java
index ee5e5de05..13c9cf221 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpClientStreamableHttpSyncClientTests.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpClientStreamableHttpSyncClientTests.java
@@ -41,7 +41,7 @@ public class HttpClientStreamableHttpSyncClientTests extends AbstractMcpSyncClie
 
 	@Override
 	protected McpClientTransport createMcpTransport() {
-		return HttpClientStreamableHttpTransport.builder(host).httpRequestCustomizer(requestCustomizer).build();
+		return HttpClientStreamableHttpTransport.builder(host).addHttpRequestCustomizer(requestCustomizer).build();
 	}
 
 	@BeforeAll
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpSseMcpAsyncClientTests.java b/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpSseMcpAsyncClientTests.java
index 91a8b6c82..f6ada1503 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpSseMcpAsyncClientTests.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpSseMcpAsyncClientTests.java
@@ -4,6 +4,9 @@
 
 package io.modelcontextprotocol.client;
 
+import java.util.List;
+import java.util.function.Consumer;
+
 import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Timeout;
@@ -11,6 +14,7 @@
 import org.testcontainers.containers.wait.strategy.Wait;
 
 import io.modelcontextprotocol.client.transport.HttpClientSseClientTransport;
+import io.modelcontextprotocol.client.transport.customizer.McpAsyncHttpClientRequestCustomizer;
 import io.modelcontextprotocol.spec.McpClientTransport;
 
 /**
@@ -35,6 +39,12 @@ protected McpClientTransport createMcpTransport() {
 		return HttpClientSseClientTransport.builder(host).build();
 	}
 
+	@Override
+	protected McpClientTransport createMcpTransport(
+			Consumer<List<McpAsyncHttpClientRequestCustomizer>> requestCustomizers) {
+		return HttpClientSseClientTransport.builder(host).asyncHttpRequestCustomizers(requestCustomizers).build();
+	}
+
 	@BeforeAll
 	static void startContainer() {
 		container.start();
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpSseMcpSyncClientTests.java b/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpSseMcpSyncClientTests.java
index d903b3b3c..90860892d 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpSseMcpSyncClientTests.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/client/HttpSseMcpSyncClientTests.java
@@ -47,7 +47,7 @@ class HttpSseMcpSyncClientTests extends AbstractMcpSyncClientTests {
 
 	@Override
 	protected McpClientTransport createMcpTransport() {
-		return HttpClientSseClientTransport.builder(host).httpRequestCustomizer(requestCustomizer).build();
+		return HttpClientSseClientTransport.builder(host).addHttpRequestCustomizer(requestCustomizer).build();
 	}
 
 	@BeforeAll
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransportTests.java b/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransportTests.java
index 8735aaaf1..35b13815e 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransportTests.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransportTests.java
@@ -38,13 +38,16 @@
 import static io.modelcontextprotocol.util.McpJsonMapperUtils.JSON_MAPPER;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatCode;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.ArgumentMatchers.isNull;
 import static org.mockito.ArgumentMatchers.matches;
 import static org.mockito.Mockito.clearInvocations;
+import static org.mockito.Mockito.inOrder;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoInteractions;
 import static org.mockito.Mockito.when;
 
 /**
@@ -335,7 +338,7 @@ void testRequestCustomizer() {
 
 		// Create a transport with the customizer
 		var customizedTransport = HttpClientSseClientTransport.builder(host)
-			.httpRequestCustomizer(mockCustomizer)
+			.addHttpRequestCustomizer(mockCustomizer)
 			.build();
 
 		// Connect
@@ -377,7 +380,7 @@ void testAsyncRequestCustomizer() {
 
 		// Create a transport with the customizer
 		var customizedTransport = HttpClientSseClientTransport.builder(host)
-			.asyncHttpRequestCustomizer(mockCustomizer)
+			.addAsyncHttpRequestCustomizer(mockCustomizer)
 			.build();
 
 		// Connect
@@ -411,6 +414,74 @@ void testAsyncRequestCustomizer() {
 		customizedTransport.closeGracefully().block();
 	}
 
+	@Test
+	void testRequestCustomizersAreAppliedInOrder() {
+		var first = mock(McpSyncHttpClientRequestCustomizer.class);
+		var second = mock(McpAsyncHttpClientRequestCustomizer.class);
+		when(second.customize(any(), any(), any(), any(), any()))
+			.thenAnswer(invocation -> Mono.just(invocation.getArguments()[0]));
+		var third = mock(McpSyncHttpClientRequestCustomizer.class);
+
+		var customizedTransport = HttpClientSseClientTransport.builder(host)
+			.addHttpRequestCustomizer(first)
+			.addAsyncHttpRequestCustomizer(second)
+			.addHttpRequestCustomizer(third)
+			.build();
+
+		StepVerifier.create(customizedTransport.connect(Function.identity())).verifyComplete();
+
+		var inOrder = inOrder(first, second, third);
+		inOrder.verify(first).customize(any(), eq("GET"), any(), isNull(), any());
+		inOrder.verify(second).customize(any(), eq("GET"), any(), isNull(), any());
+		inOrder.verify(third).customize(any(), eq("GET"), any(), isNull(), any());
+
+		customizedTransport.closeGracefully().block();
+	}
+
+	@Test
+	@SuppressWarnings("deprecation")
+	void testRequestCustomizerSettersReplacePreviousCustomizers() {
+		var replacedSync = mock(McpSyncHttpClientRequestCustomizer.class);
+		var replacedAsync = mock(McpAsyncHttpClientRequestCustomizer.class);
+		var syncCustomizer = mock(McpSyncHttpClientRequestCustomizer.class);
+		var asyncCustomizer = mock(McpAsyncHttpClientRequestCustomizer.class);
+		when(asyncCustomizer.customize(any(), any(), any(), any(), any()))
+			.thenAnswer(invocation -> Mono.just(invocation.getArguments()[0]));
+
+		var syncTransport = HttpClientSseClientTransport.builder(host)
+			.addHttpRequestCustomizer(replacedSync)
+			.addAsyncHttpRequestCustomizer(replacedAsync)
+			.httpRequestCustomizer(syncCustomizer)
+			.build();
+		var asyncTransport = HttpClientSseClientTransport.builder(host)
+			.addHttpRequestCustomizer(replacedSync)
+			.addAsyncHttpRequestCustomizer(replacedAsync)
+			.asyncHttpRequestCustomizer(asyncCustomizer)
+			.build();
+
+		StepVerifier.create(syncTransport.connect(Function.identity())).verifyComplete();
+		StepVerifier.create(asyncTransport.connect(Function.identity())).verifyComplete();
+
+		verify(syncCustomizer).customize(any(), eq("GET"), any(), isNull(), any());
+		verify(asyncCustomizer).customize(any(), eq("GET"), any(), isNull(), any());
+		verifyNoInteractions(replacedSync, replacedAsync);
+
+		syncTransport.closeGracefully().block();
+		asyncTransport.closeGracefully().block();
+	}
+
+	@Test
+	void testNullRequestCustomizerIsRejected() {
+		var builder = HttpClientSseClientTransport.builder(host);
+
+		assertThatIllegalArgumentException().isThrownBy(() -> builder.addHttpRequestCustomizer(null));
+		assertThatIllegalArgumentException().isThrownBy(() -> builder.addAsyncHttpRequestCustomizer(null));
+
+		builder.asyncHttpRequestCustomizers(customizers -> customizers.add(null));
+		assertThatIllegalArgumentException().isThrownBy(builder::build)
+			.withMessage("httpRequestCustomizers must not contain null elements");
+	}
+
 	@Test
 	void testMessageEndpointValidation() throws InvalidSseMessageEndpointException {
 		var uriCaptor = ArgumentCaptor.forClass(URI.class);
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportEmptyResponseTests.java b/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportEmptyResponseTests.java
index 5faf3ae4e..fb2e8b49f 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportEmptyResponseTests.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportEmptyResponseTests.java
@@ -108,7 +108,7 @@ void testNotificationInitialized() throws URISyntaxException {
 		var uri = new URI(host + "/mcp");
 		var mockRequestCustomizer = mock(McpSyncHttpClientRequestCustomizer.class);
 		var transport = HttpClientStreamableHttpTransport.builder(host)
-			.httpRequestCustomizer(mockRequestCustomizer)
+			.addHttpRequestCustomizer(mockRequestCustomizer)
 			.build();
 
 		// Some servers answer a notification with an empty JSON body rather than 202.
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportTest.java b/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportTest.java
index 002bf5f6d..ad14c70a1 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportTest.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportTest.java
@@ -24,11 +24,14 @@
 import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.atLeastOnce;
+import static org.mockito.Mockito.inOrder;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoInteractions;
 import static org.mockito.Mockito.when;
 
 /**
@@ -77,7 +80,7 @@ void testRequestCustomizer() throws URISyntaxException {
 		var mockRequestCustomizer = mock(McpSyncHttpClientRequestCustomizer.class);
 
 		var transport = HttpClientStreamableHttpTransport.builder(host)
-			.httpRequestCustomizer(mockRequestCustomizer)
+			.addHttpRequestCustomizer(mockRequestCustomizer)
 			.build();
 
 		withTransport(transport, (t) -> {
@@ -107,7 +110,7 @@ void testAsyncRequestCustomizer() throws URISyntaxException {
 			.thenAnswer(invocation -> Mono.just(invocation.getArguments()[0]));
 
 		var transport = HttpClientStreamableHttpTransport.builder(host)
-			.asyncHttpRequestCustomizer(mockRequestCustomizer)
+			.addAsyncHttpRequestCustomizer(mockRequestCustomizer)
 			.build();
 
 		withTransport(transport, (t) -> {
@@ -129,6 +132,79 @@ void testAsyncRequestCustomizer() throws URISyntaxException {
 		});
 	}
 
+	@Test
+	void testRequestCustomizersAreAppliedInOrder() {
+		var first = mock(McpSyncHttpClientRequestCustomizer.class);
+		var second = mock(McpAsyncHttpClientRequestCustomizer.class);
+		when(second.customize(any(), any(), any(), any(), any()))
+			.thenAnswer(invocation -> Mono.just(invocation.getArguments()[0]));
+		var third = mock(McpSyncHttpClientRequestCustomizer.class);
+
+		var transport = HttpClientStreamableHttpTransport.builder(host)
+			.addHttpRequestCustomizer(first)
+			.addAsyncHttpRequestCustomizer(second)
+			.addHttpRequestCustomizer(third)
+			.build();
+
+		withTransport(transport, (t) -> {
+			StepVerifier.create(t.sendMessage(initializeMessage())).verifyComplete();
+
+			var inOrder = inOrder(first, second, third);
+			inOrder.verify(first).customize(any(), eq("POST"), any(), any(), any());
+			inOrder.verify(second).customize(any(), eq("POST"), any(), any(), any());
+			inOrder.verify(third).customize(any(), eq("POST"), any(), any(), any());
+		});
+	}
+
+	@Test
+	@SuppressWarnings("deprecation")
+	void testRequestCustomizerSettersReplacePreviousCustomizers() {
+		var replacedSync = mock(McpSyncHttpClientRequestCustomizer.class);
+		var replacedAsync = mock(McpAsyncHttpClientRequestCustomizer.class);
+		var syncCustomizer = mock(McpSyncHttpClientRequestCustomizer.class);
+		var asyncCustomizer = mock(McpAsyncHttpClientRequestCustomizer.class);
+		when(asyncCustomizer.customize(any(), any(), any(), any(), any()))
+			.thenAnswer(invocation -> Mono.just(invocation.getArguments()[0]));
+
+		var syncTransport = HttpClientStreamableHttpTransport.builder(host)
+			.addHttpRequestCustomizer(replacedSync)
+			.addAsyncHttpRequestCustomizer(replacedAsync)
+			.httpRequestCustomizer(syncCustomizer)
+			.build();
+		var asyncTransport = HttpClientStreamableHttpTransport.builder(host)
+			.addHttpRequestCustomizer(replacedSync)
+			.addAsyncHttpRequestCustomizer(replacedAsync)
+			.asyncHttpRequestCustomizer(asyncCustomizer)
+			.build();
+
+		withTransport(syncTransport, (t) -> StepVerifier.create(t.sendMessage(initializeMessage())).verifyComplete());
+		withTransport(asyncTransport, (t) -> StepVerifier.create(t.sendMessage(initializeMessage())).verifyComplete());
+
+		verify(syncCustomizer).customize(any(), eq("POST"), any(), any(), any());
+		verify(asyncCustomizer).customize(any(), eq("POST"), any(), any(), any());
+		verifyNoInteractions(replacedSync, replacedAsync);
+	}
+
+	@Test
+	void testNullRequestCustomizerIsRejected() {
+		var builder = HttpClientStreamableHttpTransport.builder(host);
+
+		assertThatIllegalArgumentException().isThrownBy(() -> builder.addHttpRequestCustomizer(null));
+		assertThatIllegalArgumentException().isThrownBy(() -> builder.addAsyncHttpRequestCustomizer(null));
+
+		builder.asyncHttpRequestCustomizers(customizers -> customizers.add(null));
+		assertThatIllegalArgumentException().isThrownBy(builder::build)
+			.withMessage("httpRequestCustomizers must not contain null elements");
+	}
+
+	private static McpSchema.JSONRPCRequest initializeMessage() {
+		var initializeRequest = McpSchema.InitializeRequest
+			.builder(ProtocolVersions.MCP_2025_11_25, McpSchema.ClientCapabilities.builder().roots(true).build(),
+					McpSchema.Implementation.builder("MCP Client", "0.3.1").build())
+			.build();
+		return new McpSchema.JSONRPCRequest(McpSchema.METHOD_INITIALIZE, "test-id", initializeRequest);
+	}
+
 	@Test
 	void testCloseUninitialized() {
 		var transport = HttpClientStreamableHttpTransport.builder(host).build();
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/common/AsyncServerMcpTransportContextIntegrationTests.java b/mcp-test/src/test/java/io/modelcontextprotocol/common/AsyncServerMcpTransportContextIntegrationTests.java
index 6979e0983..7460de65a 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/common/AsyncServerMcpTransportContextIntegrationTests.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/common/AsyncServerMcpTransportContextIntegrationTests.java
@@ -115,13 +115,13 @@ public class AsyncServerMcpTransportContextIntegrationTests {
 
 	private final McpAsyncClient asyncStreamableClient = McpClient
 		.async(HttpClientStreamableHttpTransport.builder("http://localhost:" + PORT)
-			.asyncHttpRequestCustomizer(asyncClientRequestCustomizer)
+			.addAsyncHttpRequestCustomizer(asyncClientRequestCustomizer)
 			.build())
 		.build();
 
 	private final McpAsyncClient asyncSseClient = McpClient
 		.async(HttpClientSseClientTransport.builder("http://localhost:" + PORT)
-			.asyncHttpRequestCustomizer(asyncClientRequestCustomizer)
+			.addAsyncHttpRequestCustomizer(asyncClientRequestCustomizer)
 			.build())
 		.build();
 
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/common/SyncServerMcpTransportContextIntegrationTests.java b/mcp-test/src/test/java/io/modelcontextprotocol/common/SyncServerMcpTransportContextIntegrationTests.java
index 876f6c44d..6371d7d11 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/common/SyncServerMcpTransportContextIntegrationTests.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/common/SyncServerMcpTransportContextIntegrationTests.java
@@ -104,14 +104,14 @@ public class SyncServerMcpTransportContextIntegrationTests {
 
 	private final McpSyncClient streamableClient = McpClient
 		.sync(HttpClientStreamableHttpTransport.builder("http://localhost:" + PORT)
-			.httpRequestCustomizer(clientRequestCustomizer)
+			.addHttpRequestCustomizer(clientRequestCustomizer)
 			.build())
 		.transportContextProvider(clientContextProvider)
 		.build();
 
 	private final McpSyncClient sseClient = McpClient
 		.sync(HttpClientSseClientTransport.builder("http://localhost:" + PORT)
-			.httpRequestCustomizer(clientRequestCustomizer)
+			.addHttpRequestCustomizer(clientRequestCustomizer)
 			.build())
 		.transportContextProvider(clientContextProvider)
 		.build();
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/server/McpSyncListFilteringIntegrationTests.java b/mcp-test/src/test/java/io/modelcontextprotocol/server/McpSyncListFilteringIntegrationTests.java
index a4cbdee44..14b2c0f48 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/server/McpSyncListFilteringIntegrationTests.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/server/McpSyncListFilteringIntegrationTests.java
@@ -78,7 +78,7 @@ static void createTransportAndStartTomcat(ServerFactory serverFactory) {
 	void setUp() {
 		var clientTransport = HttpClientStreamableHttpTransport.builder(baseUrl)
 			.jsonMapper(McpJsonDefaults.getMapper())
-			.httpRequestCustomizer((builder, method, endpoint, body, context) -> requestCustomizer.customize(builder,
+			.addHttpRequestCustomizer((builder, method, endpoint, body, context) -> requestCustomizer.customize(builder,
 					method, endpoint, body, context))
 			.openConnectionOnStartup(true)
 			.build();
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/server/transport/ServerTransportSecurityIntegrationTests.java b/mcp-test/src/test/java/io/modelcontextprotocol/server/transport/ServerTransportSecurityIntegrationTests.java
index fefce73ef..d5141f229 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/server/transport/ServerTransportSecurityIntegrationTests.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/server/transport/ServerTransportSecurityIntegrationTests.java
@@ -223,7 +223,7 @@ public Sse() {
 		@Override
 		public McpSyncClient createMcpClient(String baseUrl, TestRequestCustomizer requestCustomizer) {
 			var transport = HttpClientSseClientTransport.builder(baseUrl)
-				.httpRequestCustomizer(requestCustomizer)
+				.addHttpRequestCustomizer(requestCustomizer)
 				.jsonMapper(McpJsonDefaults.getMapper())
 				.build();
 			return McpClient.sync(transport).initializationTimeout(Duration.ofMillis(500)).build();
@@ -256,7 +256,7 @@ public StreamableHttp() {
 		@Override
 		public McpSyncClient createMcpClient(String baseUrl, TestRequestCustomizer requestCustomizer) {
 			var transport = HttpClientStreamableHttpTransport.builder(baseUrl)
-				.httpRequestCustomizer(requestCustomizer)
+				.addHttpRequestCustomizer(requestCustomizer)
 				.jsonMapper(McpJsonDefaults.getMapper())
 				.openConnectionOnStartup(true)
 				.build();
@@ -290,7 +290,7 @@ public Stateless() {
 		@Override
 		public McpSyncClient createMcpClient(String baseUrl, TestRequestCustomizer requestCustomizer) {
 			var transport = HttpClientStreamableHttpTransport.builder(baseUrl)
-				.httpRequestCustomizer(requestCustomizer)
+				.addHttpRequestCustomizer(requestCustomizer)
 				.jsonMapper(McpJsonDefaults.getMapper())
 				.openConnectionOnStartup(true)
 				.build();