Probe cleartext HTTP/2 with a bodiless GET instead of OPTIONS

The interop re-run showed the Python SDK's Hypercorn server upgrades a
GET to h2c but answers OPTIONS with 405 on HTTP/1.1, so the probe
pinned that pairing to HTTP/1.1 although the server speaks HTTP/2. The
probe is now a GET without Acp-Connection-Id: every server answers it
with a 4xx without opening a stream, and only the negotiated version is
used. Servers without h2c still pin the client to HTTP/1.1.

Author: Mark Pollack

CHANGELOG.md

@@ -36,7 +36,7 @@ building a second client on an already-connected transport now fails at construc
 - **HTTP/2 over plain `http://`.** The RFD requires HTTP/2, and localhost without TLS is a first-class
   deployment. Over cleartext the JDK client only offers the h2c upgrade on a request without a body, so
   `initialize`, a POST, went out on HTTP/1.1. `StreamableHttpAcpClientTransport` now sends a bodiless
-  OPTIONS first on `http://` endpoints, so against a server that speaks h2c (the SDK's own does) every
+  GET first on `http://` endpoints, so against a server that speaks h2c (the SDK's own does) every
   request, streams included, runs on HTTP/2. Against one that does not, the probe settles on HTTP/1.1
   and later requests stop offering the upgrade, which some servers route to their WebSocket handler.
 - **Client sessions learn that their transport died.** `AcpClientTransport.awaitTermination()` (default:

acp-core/src/main/java/com/agentclientprotocol/sdk/client/transport/StreamableHttpAcpClientTransport.java

@@ -314,16 +314,19 @@ public Mono<Void> sendMessage(JSONRPCMessage message) {
 	 * The transport requires HTTP/2 (RFD). Over {@code https} ALPN negotiates it. Over
 	 * cleartext {@code http} the JDK offers an h2c upgrade on every request, but servers
 	 * (Jetty among them) only honour it on a request without a body, and {@code initialize}
-	 * is a POST. A bodiless OPTIONS first upgrades the connection when the server speaks
+	 * is a POST. A bodiless GET first upgrades the connection when the server speaks
 	 * h2c; every later request reuses it over HTTP/2. When it does not (answer on HTTP/1.1,
 	 * an error, or no answer within five seconds), every later request is pinned to HTTP/1.1.
+	 * A GET rather than OPTIONS because some h2c servers (Hypercorn) upgrade a GET but answer
+	 * OPTIONS with 405 on HTTP/1.1. It carries no connection id, so servers answer it with a
+	 * 4xx without opening a stream; only the negotiated version is used.
 	 */
 	private Mono<Void> upgradeCleartextToHttp2() {
 		if (!"http".equalsIgnoreCase(endpointUri.getScheme()) || httpClient.version() != HttpClient.Version.HTTP_2) {
 			return Mono.empty();
 		}
 		HttpRequest probe = HttpRequest.newBuilder(endpointUri)
-			.method("OPTIONS", HttpRequest.BodyPublishers.noBody())
+			.GET()
 			.build();
 		// Cancelling the Mono on timeout cancels the HTTP exchange (see sendAsync).
 		return sendAsync(probe, HttpResponse.BodyHandlers.discarding())

acp-core/src/test/java/com/agentclientprotocol/sdk/client/transport/StreamableHttpAcpClientTransportTest.java

@@ -805,8 +805,8 @@ void cleartextServerWithoutH2cGetsPlainHttp11Requests() throws Exception {
 		when(httpClient.sendAsync(any(), any())).thenAnswer(invocation -> {
 			HttpRequest request = invocation.getArgument(0);
 			requests.add(request);
-			if ("OPTIONS".equals(request.method())) {
-				HttpResponse<Object> probe = response(405, Map.of(), null);
+			if ("GET".equals(request.method()) && request.headers().firstValue("Acp-Connection-Id").isEmpty()) {
+				HttpResponse<Object> probe = response(400, Map.of(), null);
 				when(probe.version()).thenReturn(HttpClient.Version.HTTP_1_1);
 				return CompletableFuture.completedFuture(probe);
 			}
@@ -828,7 +828,8 @@ void cleartextServerWithoutH2cGetsPlainHttp11Requests() throws Exception {
 			transport.sendMessage(AcpTestFixtures.createJsonRpcRequest(AcpSchema.METHOD_INITIALIZE, "init-1",
 					AcpTestFixtures.createInitializeRequest())).block();
 
-			assertThat(requests.get(0).method()).isEqualTo("OPTIONS");
+			assertThat(requests.get(0).method()).as("the probe").isEqualTo("GET");
+			assertThat(requests.get(0).headers().firstValue("Acp-Connection-Id")).isEmpty();
 			assertThat(requests.subList(1, requests.size()))
 				.as("every request after the probe is pinned to HTTP/1.1")
 				.isNotEmpty()