Cinder is Meta's internal fork of CPython, developed specifically to handle the performance demands of Instagram's massive Django-based server infrastructure. Unlike Pyston or Unladen Swallow, Cinder was not created primarily as a community project — it was Meta's production runtime, open-sourced in 2021 to share innovations with the Python community.

Cinder introduced several significant optimizations: immortal objects (objects that are never reference-counted or garbage-collected, reducing overhead for long-lived objects), shadowcode (Meta's term for inline caching and bytecode quickening), Static Python (an optional strict typing mode that enables ahead-of-time compilation), and Strict Modules (a module system that enforces import-time side-effect restrictions). The JIT compiler includes a function inliner for additional optimization.

Rather than maintaining a permanent fork, Meta has increasingly worked to upstream Cinder's innovations into CPython itself. The immortal objects concept was adopted in CPython 3.12 (PEP 683). Starting with Python 3.10, Meta restructured its approach, extracting the performance extensions into a separate 'cinderx' package that can be installed as a Python extension rather than requiring a full runtime replacement.

Cinder represents a more mature approach to corporate Python forking: rather than trying to replace CPython, Meta has focused on contributing innovations upstream while maintaining a production fork for its immediate needs.

Immortal objects concept adopted into CPython 3.12. Demonstrated that corporate forks can successfully upstream innovations. Influenced the broader Faster CPython initiative. Powers Instagram's production Python infrastructure at massive scale.