diff --git a/src/aphront/requeststream/AphrontRequestStream.php b/src/aphront/requeststream/AphrontRequestStream.php
--- a/src/aphront/requeststream/AphrontRequestStream.php
+++ b/src/aphront/requeststream/AphrontRequestStream.php
@@ -101,8 +101,9 @@
 
     $filters = stream_get_filters();
     foreach ($filters as $filter) {
-      if (preg_match('/^zlib\\./', $filter)) {
+      if (!strncasecmp($filter, 'zlib.', strlen('zlib.'))) {
         $has_zlib = true;
+        break;
       }
     }
 
diff --git a/src/aphront/response/AphrontJSONResponse.php b/src/aphront/response/AphrontJSONResponse.php
--- a/src/aphront/response/AphrontJSONResponse.php
+++ b/src/aphront/response/AphrontJSONResponse.php
@@ -31,10 +31,10 @@
   }
 
   public function getHeaders() {
-    $headers = array(
-      array('Content-Type', 'application/json'),
-    );
-    $headers = array_merge(parent::getHeaders(), $headers);
+    $headers = parent::getHeaders();
+
+    $headers[] = array('Content-Type', 'application/json');
+
     return $headers;
   }
 
diff --git a/src/aphront/response/AphrontResponse.php b/src/aphront/response/AphrontResponse.php
--- a/src/aphront/response/AphrontResponse.php
+++ b/src/aphront/response/AphrontResponse.php
@@ -10,7 +10,7 @@
   private $contentSecurityPolicyURIs;
   private $disableContentSecurityPolicy;
   protected $frameable;
-
+  private $headers = array();
 
   public function setRequest($request) {
     $this->request = $request;
@@ -49,6 +49,11 @@
     return $this;
   }
 
+  final public function addHeader($key, $value) {
+    $this->headers[] = array($key, $value);
+    return $this;
+  }
+
 
 /* -(  Content  )------------------------------------------------------------ */
 
@@ -105,6 +110,10 @@
 
     $headers[] = array('Referrer-Policy', 'no-referrer');
 
+    foreach ($this->headers as $header) {
+      $headers[] = $header;
+    }
+
     return $headers;
   }
 
diff --git a/src/applications/conduit/controller/PhabricatorConduitAPIController.php b/src/applications/conduit/controller/PhabricatorConduitAPIController.php
--- a/src/applications/conduit/controller/PhabricatorConduitAPIController.php
+++ b/src/applications/conduit/controller/PhabricatorConduitAPIController.php
@@ -134,9 +134,17 @@
           $method_implementation);
       case 'json':
       default:
-        return id(new AphrontJSONResponse())
+        $response = id(new AphrontJSONResponse())
           ->setAddJSONShield(false)
           ->setContent($response->toDictionary());
+
+        $capabilities = $this->getConduitCapabilities();
+        if ($capabilities) {
+          $capabilities = implode(' ', $capabilities);
+          $response->addHeader('X-Conduit-Capabilities', $capabilities);
+        }
+
+        return $response;
     }
   }
 
@@ -716,5 +724,14 @@
     return false;
   }
 
+  private function getConduitCapabilities() {
+    $capabilities = array();
+
+    if (AphrontRequestStream::supportsGzip()) {
+      $capabilities[] = 'gzip';
+    }
+
+    return $capabilities;
+  }
 
 }