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Foolscap is an RPC/RMI (Remote Procedure Call / Remote Method Invocation) protocol for use with Twisted, derived/inspired by Twisted's built-in "Perspective Broker" package.

If you have control of both ends of the wire, and are thus not constrained to use some other protocol like HTTP/XMLRPC/CORBA/etc, you might consider using Foolscap.

Fundamentally, Foolscap allows you to make a python object in one process available to code in other processes, which means you can invoke its methods remotely. This includes a data serialization layer to convey the object graphs for the arguments and the eventual response, and an object reference system to keep track of which objects you are connecting to. It uses a capability-based security model, such that once you create a non-public object, it is only accessible to clients to whom you've given the (unguessable) FURL. You can of course publish world-visible objects that have well-known FURLs.

Full documentation and examples are in the doc/ directory.


  • Python 3.8 or higher
  • Twisted 16.0.0 or later
  • PyOpenSSL (tested against 16.0.0)


To install foolscap into your system's normal python library directory, just run the following (you will probably have to do this as root):

pip install .

You can also just add the foolscap source tree to your PYTHONPATH, since there are no compile steps or .so/.dll files involved.


Foolscap's wire protocol is unlikely to change in the near future.

Foolscap has a built-in version-negotiation mechanism that allows the two processes to determine how to best communicate with each other. The two ends will agree upon the highest mutually-supported version for all their traffic. If they do not have any versions in common, the connection will fail with a NegotiationError.

Please check the NEWS file for announcements of compatibility-breaking changes in any given release.

As of Foolscap-0.14.0, this library is mostly compatible with Python 3 (specifically 3.5 or higher), and is tested against 3.5, 3.6, 3.7, and 3.8. It will retain compatibility with Python 2.7 for a little while longer, to ease the transition, but since Python 2 was marked End-Of-Life in January 2020, this compatibility will not be maintained forever.


Foolscap is a rewrite of the Perspective Broker protocol provided by Twisted (in the twisted.pb package), with the goal of improving serialization flexibility and connection security. It also adds features to assist application development, such as distributed/incident-triggered logging, Service management, persistent object naming, and debugging tools.

For a brief while, it was intended to replace Perspective Broker, so it had a name of "newpb" or "pb2". However we no longer expect Foolscap to ever be added to the Twisted source tree.

A "foolscap" is a size of paper, probably measuring 17 by 13.5 inches. A twisted foolscap of paper makes a good fool's cap. Also, "cap" implies capabilities, and Foolscap is a protocol to implement a distributed object-capabilities model in python.


Brian Warner is responsible for this thing. Please discuss it on the twisted-python mailing list.

The Foolscap home page is a Trac instance at It contains pointers to the latest release, bug reports, patches, documentation, and other resources.

Foolscap is distributed under the same license as Twisted itself, namely the MIT license. Details are in the LICENSE file.