diff --git a/src/conduit/ConduitFuture.php b/src/conduit/ConduitFuture.php
index 5966f42e..09b5016b 100644
--- a/src/conduit/ConduitFuture.php
+++ b/src/conduit/ConduitFuture.php
@@ -54,10 +54,22 @@ final class ConduitFuture extends FutureProxy {
     try {
       $data = phutil_json_decode($raw);
     } catch (PhutilJSONParserException $ex) {
+      // Include what the host actually returned, so a proxy or filter that
+      // answers in place of Phabricator can be identified from the logs.
+      $header_lines = array();
+      foreach ($headers as $header) {
+        list($name, $value) = $header;
+        $header_lines[] = $name.': '.$value;
+      }
       throw new PhutilProxyException(
         pht(
-          'Host returned HTTP/200, but invalid JSON data in response to '.
-          'a Conduit method call.'),
+          "Host returned HTTP/200, but invalid JSON data in response to ".
+          "a Conduit method call (%s).\nResponse headers:\n%s\n".
+          "Response body (%s bytes, up to 4096 shown):\n%s",
+          $this->conduitMethod,
+          implode("\n", $header_lines),
+          new PhutilNumber(strlen($body)),
+          substr($body, 0, 4096)),
         $ex);
     }