@@ -28,6 +28,21 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   Micronaut client configuration hard-coded the three as not advertised, so a Micronaut client
   could not advertise elicitation or boolean config options from its settings.
 
+- **Headers on the remote client transports: `WebSocketAcpClientTransport.webSocketCustomizer(...)`
+  and `StreamableHttpAcpClientTransport.requestCustomizer(...)`.** The JDK's `HttpClient` has no
+  default headers, so neither transport could send an `Authorization` header, an API key or any
+  other header an agent's endpoint requires; an application had to write its own transport.
+  `goose serve`, for one, refuses every connection without its `X-Secret-Key`. The WebSocket
+  customizer receives the `WebSocket.Builder` of each connect attempt; the HTTP one receives the
+  `HttpRequest.Builder` of every request the transport sends (the cleartext probe, `initialize`,
+  each POST, every SSE stream it opens or reopens, and the closing `DELETE`), so a token that
+  expires is read again for each. The HTTP transport keeps the method, body, URI and its own
+  headers (Content-Type, Accept, Acp-Connection-Id, Acp-Session-Id): a customizer's values for
+  them are dropped. A customizer that throws, or sets a header the JDK restricts, fails that
+  connect or that request's Mono rather than the caller, and a failed connect or `initialize` may
+  be tried again. The HTTP transport now also builds each request inside its Mono, so a failure
+  building one is reported there too.
+
 - **Transport constructors without a JSON mapper:** `StreamableHttpAcpClientTransport(URI)`,
   `WebSocketAcpClientTransport(URI)`, `StreamableHttpAcpAgentTransport(int port, AcpAgentFactory)`
   and `StreamableHttpAcpServlet(AcpAgentFactory)` use `AcpJsonMapper.createDefault()`, as the stdio
 
@@ -61,7 +61,16 @@
  * the server does not. The default client keeps cookies in a cookie manager of its own and
  * runs on a bounded pool of daemon threads; {@link StreamableHttpAcpClientTransportOptions}
  * sets its sizes and the number of SSE streams. Pass an {@link HttpClient} of your own for TLS,
- * proxy or authentication settings.
+ * proxy or cookie settings.
+ *
+ * <p>An endpoint that requires authentication, such as one that expects an API key or a
+ * bearer token in a header, gets it through {@link #requestCustomizer}, which every request
+ * the transport sends passes through:
+ *
+ * <pre>{@code
+ * var transport = new StreamableHttpAcpClientTransport(URI.create("https://agents.example.com/acp"))
+ *     .requestCustomizer(builder -> builder.header("Authorization", "Bearer " + tokens.current()));
+ * }</pre>
  *
  * <p>{@link #closeGracefully()} closes the streams and sends {@code DELETE} for the connection,
  * waiting at most five seconds for the answer; {@link #close()} does the same and blocks for up
@@ -201,6 +210,31 @@ private static HttpClientBundle createDefaultHttpClient(StreamableHttpAcpClientT
 		return HttpClientBundle.createDefault(options);
 	}
 
+	/**
+	 * Customizes every HTTP request the transport sends, typically to add the headers the
+	 * endpoint requires: an {@code Authorization} header, an API key, a tenant. The JDK's
+	 * {@link HttpClient} has no default headers, so this is the only way to send one. It is
+	 * applied to each request as it is built, the cleartext probe, {@code initialize}, every
+	 * POST, every SSE stream (re)opened and the closing {@code DELETE}, so a header whose
+	 * value changes, such as a token that expires, is read again for each one. It runs on the
+	 * thread that sends the request; keep it quick, and refresh a token elsewhere.
+	 *
+	 * <p>The transport owns the method, the body, the URI and its own headers (Content-Type,
+	 * Accept, Acp-Connection-Id, Acp-Session-Id): values the customizer sets for any of them
+	 * are dropped or replaced, so a customizer cannot break the protocol by accident. Other
+	 * settings, such as a per-request timeout, are kept. The JDK refuses restricted headers
+	 * such as {@code Host} or {@code Connection}; setting one, or any exception the customizer
+	 * throws, fails that request's Mono. Call it before the first message is sent.
+	 * @param customizer applied to the builder of each request
+	 * @return this transport
+	 * @throws IllegalArgumentException if {@code customizer} is null
+	 */
+	public StreamableHttpAcpClientTransport requestCustomizer(Consumer<HttpRequest.Builder> customizer) {
+		Assert.notNull(customizer, "The request customizer can not be null");
+		this.requests.requestCustomizer(customizer);
+		return this;
+	}
+
 	/**
 	 * {@inheritDoc}
 	 * <p>It contacts nothing: it registers the handler and completes at once. The connection
@@ -249,17 +283,21 @@ private Mono<Void> initialize(AcpSchema.JSONRPCRequest request) {
 			return Mono.error(new IllegalStateException("Transport is already initialized"));
 		}
 
-		HttpRequest httpRequest;
+		String json;
 		try {
-			httpRequest = requests.jsonPost(RouteScope.bootstrap(), jsonMapper.writeValueAsString(request));
+			json = jsonMapper.writeValueAsString(request);
 		}
 		catch (IOException e) {
 			initialized.set(false);
 			return Mono.error(new AcpConnectionException("Failed to serialize initialize request", e));
 		}
 
+		// Built after the probe, so it carries the HTTP version the probe settled on; built
+		// inside defer, so a request customizer that throws fails this Mono rather than the
+		// caller.
 		return requests.upgradeCleartextToHttp2()
-			.then(Mono.defer(() -> requests.sendAsync(requests.pinned(httpRequest), HttpResponse.BodyHandlers.ofString())))
+			.then(Mono.defer(() -> requests.sendAsync(requests.jsonPost(RouteScope.bootstrap(), json),
+					HttpResponse.BodyHandlers.ofString())))
 			.flatMap(response -> readInitializeResponse(request, response))
 			.flatMap(responseMessage -> streams.openConnectionStream().then(inbound.emit(responseMessage)))
 			.doOnError(error -> {
 
@@ -13,13 +13,15 @@
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
 import java.util.Locale;
+import java.util.Set;
 import java.util.concurrent.ArrayBlockingQueue;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
+import java.util.function.Consumer;
 
 import com.agentclientprotocol.sdk.error.AcpConnectionException;
 import com.agentclientprotocol.sdk.util.AcpSchedulers;
@@ -30,8 +32,8 @@
 
 /**
  * The HTTP exchanges of a Streamable HTTP client connection: the scope headers, the HTTP
- * version settled by the cleartext probe, and the POST, GET and DELETE requests with the
- * status and content type each must answer with. Completion signals are delivered on a
+ * version settled by the cleartext probe, the application's request customizer, and the
+ * POST, GET and DELETE requests with the status and content type each must answer with. Completion signals are delivered on a
  * bounded executor of their own, never on the HTTP client's.
  */
 final class StreamableHttpRequests {
@@ -48,6 +50,14 @@ final class StreamableHttpRequests {
 
 	static final Duration PROBE_TIMEOUT = Duration.ofSeconds(5);
 
+	/**
+	 * The headers this transport owns, lower-cased. A request customizer's values for them are
+	 * dropped, including on the bootstrap {@code initialize}, which carries no connection id
+	 * of the transport's own for one set by the customizer to be replaced by.
+	 */
+	private static final Set<String> PROTOCOL_HEADERS = Set.of("content-type", "accept",
+			HEADER_CONNECTION_ID.toLowerCase(Locale.ROOT), HEADER_SESSION_ID.toLowerCase(Locale.ROOT));
+
 	/** An HTTP client and the executor this transport created for it, if any. */
 	record HttpClientBundle(HttpClient httpClient, @Nullable ExecutorService ownedExecutor) {
 
@@ -75,6 +85,9 @@ static HttpClientBundle createDefault(StreamableHttpAcpClientTransportOptions op
 
 	private volatile @Nullable String connectionId;
 
+	private volatile Consumer<HttpRequest.Builder> requestCustomizer = builder -> {
+	};
+
 	/**
 	 * HTTP version pinned for every request after the cleartext probe, or {@code null} to
 	 * use the client's own setting. Set to HTTP/1.1 when an {@code http://} server does not
@@ -106,6 +119,10 @@ static ThreadFactory daemonThreadFactory(String threadName) {
 		};
 	}
 
+	void requestCustomizer(Consumer<HttpRequest.Builder> requestCustomizer) {
+		this.requestCustomizer = requestCustomizer;
+	}
+
 	@Nullable String connectionId() {
 		return connectionId;
 	}
@@ -129,9 +146,10 @@ Mono<Void> upgradeCleartextToHttp2() {
 		if (!"http".equalsIgnoreCase(endpointUri.getScheme()) || httpClient.version() != HttpClient.Version.HTTP_2) {
 			return Mono.empty();
 		}
-		HttpRequest probe = HttpRequest.newBuilder(endpointUri).GET().build();
-		// Cancelling the Mono on timeout cancels the HTTP exchange (see sendAsync).
-		return sendAsync(probe, HttpResponse.BodyHandlers.discarding())
+		// Customized like every other request: a gateway in front of the agent may refuse the
+		// probe without the application's credentials. Cancelling the Mono on timeout cancels
+		// the HTTP exchange (see sendAsync).
+		return Mono.defer(() -> sendAsync(customized().GET().build(), HttpResponse.BodyHandlers.discarding()))
 			.timeout(PROBE_TIMEOUT, AcpSchedulers.timeouts())
 			.doOnNext(response -> {
 				logger.debug("Cleartext probe to {} negotiated {}", endpointUri, response.version());
@@ -148,52 +166,66 @@ Mono<Void> upgradeCleartextToHttp2() {
 			});
 	}
 
-	/** Re-stamps a request built before the probe with the version the probe settled on. */
-	HttpRequest pinned(HttpRequest request) {
-		HttpClient.Version version = this.pinnedVersion;
-		if (version == null) {
-			return request;
-		}
-		return HttpRequest.newBuilder(request, (name, value) -> true).version(version).build();
-	}
-
+	/** A customized builder for the endpoint, with the version the probe settled on. */
 	private HttpRequest.Builder newRequest() {
-		HttpRequest.Builder requestBuilder = HttpRequest.newBuilder(endpointUri);
+		HttpRequest.Builder requestBuilder = customized();
 		HttpClient.Version version = this.pinnedVersion;
 		if (version != null) {
 			requestBuilder.version(version);
 		}
 		return requestBuilder;
 	}
 
-	/** A JSON POST of {@code json} in {@code scope}, not yet sent. */
+	/**
+	 * A builder for the endpoint carrying what the application's customizer set, less the
+	 * protocol headers and with the endpoint's URI whatever the customizer did. The
+	 * customizer runs on a builder of its own and the result is copied, because a builder
+	 * can replace a header but never remove one. The transport sets the method, the body and
+	 * its own headers afterwards.
+	 */
+	private HttpRequest.Builder customized() {
+		HttpRequest.Builder scratch = HttpRequest.newBuilder(endpointUri);
+		requestCustomizer.accept(scratch);
+		return HttpRequest
+			.newBuilder(scratch.build(), (name, value) -> !PROTOCOL_HEADERS.contains(name.toLowerCase(Locale.ROOT)))
+			.uri(endpointUri);
+	}
+
+	/**
+	 * A JSON POST of {@code json} in {@code scope}, not yet sent. Builds the request, so it
+	 * throws whatever the request customizer throws; the Monos below build inside
+	 * {@code defer} and fail with it instead.
+	 */
 	HttpRequest jsonPost(RouteScope scope, String json) {
-		HttpRequest.Builder builder = newRequest().header("Content-Type", CONTENT_TYPE_JSON)
-			.header("Accept", CONTENT_TYPE_JSON);
+		HttpRequest.Builder builder = newRequest().setHeader("Content-Type", CONTENT_TYPE_JSON)
+			.setHeader("Accept", CONTENT_TYPE_JSON);
 		addScopeHeaders(builder, scope);
 		return builder.POST(HttpRequest.BodyPublishers.ofString(json, StandardCharsets.UTF_8)).build();
 	}
 
 	/** Posts a message that the server must accept with 202. */
 	Mono<Void> postAccepted(RouteScope scope, String json) {
-		return sendAsync(jsonPost(scope, json), HttpResponse.BodyHandlers.discarding())
+		return Mono.defer(() -> sendAsync(jsonPost(scope, json), HttpResponse.BodyHandlers.discarding()))
 			.flatMap(response -> expectStatus(response, 202, "for POST"));
 	}
 
 	/** Opens the SSE stream of {@code scope}; emits its body or an error, never completes empty. */
 	Mono<InputStream> openEventStream(RouteScope scope) {
-		HttpRequest.Builder builder = newRequest().GET().header("Accept", CONTENT_TYPE_EVENT_STREAM);
-		addScopeHeaders(builder, scope);
-		return sendAsync(builder.build(), HttpResponse.BodyHandlers.ofInputStream())
+		return Mono.defer(() -> {
+			HttpRequest.Builder builder = newRequest().GET().setHeader("Accept", CONTENT_TYPE_EVENT_STREAM);
+			addScopeHeaders(builder, scope);
+			return sendAsync(builder.build(), HttpResponse.BodyHandlers.ofInputStream());
+		})
 			.flatMap(response -> expectStatus(response, 200, "when opening SSE stream")
 				.then(expectContentType(response, CONTENT_TYPE_EVENT_STREAM, "response"))
 				.then(Mono.fromSupplier(response::body)));
 	}
 
 	/** Deletes the connection; the server must accept with 202. */
 	Mono<Void> deleteConnection(String id) {
-		HttpRequest request = newRequest().DELETE().header(HEADER_CONNECTION_ID, id).build();
-		return sendAsync(request, HttpResponse.BodyHandlers.discarding())
+		return Mono
+			.defer(() -> sendAsync(newRequest().DELETE().setHeader(HEADER_CONNECTION_ID, id).build(),
+					HttpResponse.BodyHandlers.discarding()))
 			.flatMap(response -> expectStatus(response, 202, "for DELETE"));
 	}
 
@@ -215,10 +247,10 @@ static Mono<Void> expectContentType(HttpResponse<?> response, String expected, S
 
 	private void addScopeHeaders(HttpRequest.Builder builder, RouteScope scope) {
 		if (!scope.isBootstrap()) {
-			builder.header(HEADER_CONNECTION_ID, requireConnectionId());
+			builder.setHeader(HEADER_CONNECTION_ID, requireConnectionId());
 		}
 		if (scope.isSession()) {
-			builder.header(HEADER_SESSION_ID, scope.boundSessionId());
+			builder.setHeader(HEADER_SESSION_ID, scope.boundSessionId());
 		}
 	}
 
 
@@ -56,6 +56,14 @@
  * skipped. When the agent closes the connection, or it fails, {@link #awaitTermination()} ends
  * and the client's pending requests fail.
  *
+ * <p>An endpoint that requires authentication, such as one that expects an API key or a
+ * bearer token in a header, gets it through {@link #webSocketCustomizer}:
+ *
+ * <pre>{@code
+ * var transport = new WebSocketAcpClientTransport(URI.create("wss://agents.example.com/acp"))
+ *     .webSocketCustomizer(builder -> builder.header("Authorization", "Bearer " + token));
+ * }</pre>
+ *
  * <p>The transport is thread-safe: messages may be sent from any thread, and one daemon thread
  * of its own ({@code acp-ws-client-outbound}) sends them one frame at a time. The default
  * HTTP client runs on a pool of daemon threads named {@code acp-ws-client}.
@@ -108,6 +116,9 @@ public class WebSocketAcpClientTransport implements AcpClientTransport {
 
 	private Duration connectTimeout = Duration.ofSeconds(30);
 
+	private Consumer<WebSocket.Builder> webSocketCustomizer = builder -> {
+	};
+
 	/**
 	 * Creates a transport for the WebSocket endpoint at {@code serverUri}, with an HTTP client
 	 * of its own and the default JSON mapper ({@link AcpJsonMapper#createDefault()}).
@@ -186,6 +197,26 @@ public WebSocketAcpClientTransport connectTimeout(Duration timeout) {
 		return this;
 	}
 
+	/**
+	 * Customizes the WebSocket handshake before it is sent, typically to add the headers the
+	 * endpoint requires: an {@code Authorization} header, an API key, a tenant. The JDK's
+	 * {@link java.net.http.HttpClient} has no default headers, so this is the only way to send
+	 * one. Runs on every {@link #connect} attempt, after the transport has set its connect
+	 * timeout, so the customizer may replace that too; a header whose value changes, such as
+	 * a token that expires, is read again by each attempt. The JDK refuses headers that belong
+	 * to the handshake itself ({@code Connection}, {@code Upgrade}, {@code Host},
+	 * {@code Sec-WebSocket-*}): setting one, or any exception the customizer throws, fails
+	 * that connect, which may then be tried again. Call it before connecting.
+	 * @param customizer applied to the builder of each handshake
+	 * @return this transport
+	 * @throws IllegalArgumentException if {@code customizer} is null
+	 */
+	public WebSocketAcpClientTransport webSocketCustomizer(Consumer<WebSocket.Builder> customizer) {
+		Assert.notNull(customizer, "The WebSocket customizer can not be null");
+		this.webSocketCustomizer = customizer;
+		return this;
+	}
+
 	/**
 	 * {@inheritDoc}
 	 * <p>Opens the WebSocket connection when the returned Mono is subscribed, and completes
@@ -204,9 +235,9 @@ public Mono<Void> connect(Function<Mono<JSONRPCMessage>, Mono<JSONRPCMessage>> h
 
 			// Build WebSocket connection with listener; frames that arrive before the handshake
 			// completes wait in the inbound sink.
-			return httpClient.newWebSocketBuilder()
-				.connectTimeout(connectTimeout)
-				.buildAsync(serverUri, new AcpWebSocketListener());
+			WebSocket.Builder builder = httpClient.newWebSocketBuilder().connectTimeout(connectTimeout);
+			webSocketCustomizer.accept(builder);
+			return builder.buildAsync(serverUri, new AcpWebSocketListener());
 		}).doOnSuccess(ws -> {
 			this.webSocket = ws;
 			// Only an open connection takes the inbound sink's one subscriber, so a connect that