fix: retry ACP websocket after plain-text 404

lib/runtime/gateway_acp_client.dart

@@ -321,7 +321,22 @@ class GatewayAcpClient {
         );
       } catch (error) {
         if (error is GatewayAcpException) {
-          rethrow;
+          if (!_shouldRetryViaWebSocketAfterHttpFailure(
+            error,
+            endpoint: resolvedEndpoint,
+          )) {
+            rethrow;
+          }
+          try {
+            return await _requestViaWebSocket(
+              request,
+              onNotification: onNotification,
+              endpointOverride: resolvedEndpoint,
+              authorizationOverride: authorizationOverride,
+            );
+          } catch (_) {
+            rethrow;
+          }
         }
         return _requestViaWebSocket(
           request,
@@ -544,6 +559,22 @@ class GatewayAcpClient {
     return base;
   }
 
+  bool _shouldRetryViaWebSocketAfterHttpFailure(
+    GatewayAcpException error, {
+    required Uri? endpoint,
+  }) {
+    if (resolveAcpWebSocketEndpoint(endpoint) == null) {
+      return false;
+    }
+    final details = asMap(error.details);
+    final statusCode = intValue(details['statusCode']);
+    final contentType = (stringValue(details['contentType']) ?? '')
+        .trim()
+        .toLowerCase();
+    return statusCode == HttpStatus.notFound &&
+        (contentType.isEmpty || contentType.contains('text/plain'));
+  }
+
   bool _contentTypeLooksJsonOrSse(String contentType) {
     return contentType.contains('application/json') ||
         contentType.contains('application/problem+json') ||

test/runtime/gateway_acp_client_suite.dart

@@ -117,6 +117,27 @@ void main() {
       },
     );
 
+    test(
+      'falls back to websocket when HTTP bridge returns plain-text 404',
+      () async {
+        final server = await _AcpFakeServer.start(
+          respondWithPlainTextNotFound: true,
+        );
+        addTearDown(server.close);
+
+        final client = GatewayAcpClient(
+          endpointResolver: () => server.baseHttpUri,
+        );
+
+        final capabilities = await client.loadCapabilities(forceRefresh: true);
+
+        expect(capabilities.singleAgent, isTrue);
+        expect(server.lastHttpRequestPath, '/acp/rpc');
+        expect(server.lastWebSocketRequestPath, '/acp');
+        expect(server.rpcMethods, contains('acp.capabilities'));
+      },
+    );
+
     test(
       'forwards ACP authorization resolver headers over websocket',
       () async {
@@ -234,12 +255,14 @@ class _AcpFakeServer {
     this._server, {
     required this.disableWebSocket,
     required this.respondWithHtmlError,
+    required this.respondWithPlainTextNotFound,
     required this.pathPrefix,
   });
 
   final HttpServer _server;
   final bool disableWebSocket;
   final bool respondWithHtmlError;
+  final bool respondWithPlainTextNotFound;
   final String pathPrefix;
   final List<String> rpcMethods = <String>[];
   String? lastWebSocketAuthorization;
@@ -253,6 +276,7 @@ class _AcpFakeServer {
   static Future<_AcpFakeServer> start({
     bool disableWebSocket = false,
     bool respondWithHtmlError = false,
+    bool respondWithPlainTextNotFound = false,
     String pathPrefix = '',
   }) async {
     final server = await HttpServer.bind(InternetAddress.loopbackIPv4, 0);
@@ -260,6 +284,7 @@ class _AcpFakeServer {
       server,
       disableWebSocket: disableWebSocket,
       respondWithHtmlError: respondWithHtmlError,
+      respondWithPlainTextNotFound: respondWithPlainTextNotFound,
       pathPrefix: _normalizePathPrefix(pathPrefix),
     );
     unawaited(fake._listen());
@@ -333,6 +358,13 @@ class _AcpFakeServer {
       await request.response.close();
       return;
     }
+    if (respondWithPlainTextNotFound) {
+      request.response.statusCode = HttpStatus.notFound;
+      request.response.headers.set(HttpHeaders.contentTypeHeader, 'text/plain');
+      request.response.write('not found');
+      await request.response.close();
+      return;
+    }
     final body = await utf8.decodeStream(request);
     final envelope = _decodeMap(body);
     final id = envelope['id'];