June 2026: Upstream CPython synchronization and interpreter robustness improvements across two repos with a focus on stability, security, and maintainability. Key features delivered: - facebookincubator/cinder: Imported CPython 3.14.5+ stable branch, delivering new features and security fixes; added native test files (Modules/_testcapi/weakref.c, Modules/_testlimitedcapi/weakref.c) and supporting changes across tests/docs/NEWS, with review and differential revision D108133995. - facebookincubator/cinderx: Synced with CPython 3.16.0a0, updating interpreter frame stack pointer validation to improve robustness of the Python execution model; added low-signal files (including NEWS.d/docs/.github) as part of upstream alignment, with review and differential revision D109107038. Major bugs fixed: - cinderx alignment with upstream CPython 3.16.0a0 introduces robust frame stack pointer validation, reducing edge-case execution risks. Overall impact and accomplishments: - Strengthened runtime stability, security posture, and compatibility with upstream CPython across two repos; improved test coverage and traceability through explicit reviews and differential revisions; positioned foundation for safer Python execution and smoother future upgrades. Technologies/skills demonstrated: - Upstream synchronization of CPython, interpreter internals, test ecosystem expansion, code reviews, diff-driven workflows, and cross-repo collaboration.