Marionette provides an asynchronous, parallel pipelining user-facing interface. Message sequencing limits chances of payload race conditions and provides a uniform way in which payloads are serialised.

Clients that deliver a blocking WebDriver interface are still expected to not send further command requests before the response from the last command has come back, but if they still happen to do so because of programming error, no harm will be done. This guards against mixing up responses.

Schematic flow of messages:

        client      server
          |            |
msgid=1    |----------->|
          |  command   |
          |            |
msgid=2    |<-----------|
          |  command   |
          |            |
msgid=2    |----------->|
          |  response  |
          |            |
msgid=1    |<-----------|
          |  response  |
          |            |

The protocol consists of a command message and the corresponding response message. A response message must always be sent in reply to a command message.

This means that the server implementation does not need to send the reply precisely in the order of the received commands: if it receives multiple messages, the server may even reply in random order. It is therefore strongly advised that clients take this into account when imlpementing the client end of this wire protocol.

This is required for pipelining messages. On the server side, some functions are fast, and some less so. If the server must reply in order, the slow functions delay the other replies even if its execution is already completed.


The request, or command message, is a four element JSON Array as shown below, that may originate from either the client- or server remote ends:

[type, message ID, command, parameters]
  • type must be 0 (integer). This indicates that the message is a command.

  • message ID is a 32-bit unsigned integer. This number is used as a sequencing number that uniquely identifies a pair of command and response messages. The other remote part will reply with a corresponding response with the same message ID.

  • command is a string identifying the RPC method or command to execute.

  • parameters is an arbitrary JSON serialisable object.


The response message is also a four element array as shown below, and must always be sent after receiving a command:

[type, message ID, error, result]
  • type must be 1 (integer). This indicates that the message is a response.

  • message ID is a 32-bit unsigned integer. This corresponds to the command’s message ID.

  • error is null if the command executed correctly. If the error occurred on the server-side, then this is an [error] object.

  • result is the result object from executing the command, if it executed correctly. If an error occurred on the server-side, this field is null.

The structure of the result field can vary, but is documented individually for each command.

Error object

An error object is a serialisation of JavaScript error types, and it is structured like this:

  "error": "invalid session id",
  "message": "No active session with ID 1234",
  "stacktrace": ""

All the fields of the error object are required, so the stacktrace and message fields may be empty strings. The error field is guaranteed to be one of the JSON error codes as laid out by the WebDriver standard.


Clients may be implemented in any language that is capable of writing and receiving data over TCP socket. A reference client is provided. Clients may be implemented both synchronously and asynchronously, although the latter is impossible in protocol levels 2 and earlier due to the lack of message sequencing.