Release CmdStan 2.40

Feature freeze (Wednesday, September 9th 2026):

• Ensure all expiring language deprecations have been removed or given new removal dates.
• Create and merge version updating pull requests in Math/Stan/Cmdstan. These should be the last PRs accepted before the freeze.
• Create Math/Stan RC releases.
• Create Stanc3 RC binary.
• Create a release candidate tarball for x86. Make sure RC tarballs include stanc3 binaries.
• Check external links in docs (i.e. TBB docs link) - use https://github.com/tcort/markdown-link-check
• Create a release candidate feature/bugfix list (major features/bugfixes that need testing, link to new docs in Github)
• Run CmdStanR tests with the RC tarball.
• Run CmdStanPy tests with the RC tarball.
• Make a Discourse RC post.

Release (Wednesday, September 15th 2026):

• Create the Math Release notes.
• Create the Stan Release notes.
• Create the Cmdstan Release notes.
• Create the Stanc3 Release notes, include new deprecations and removals.
• Rebuild and publish docs for the new version.
• Check that docs for the previous release links correctly to the newest docs.
• Create the Math release.
• Create the Stan release.
• Create the Stanc3 release.
• Create x86 CmdStan tarballs (check version, check that the extracted folder is in the cmdstan-version format).
• Create non-x86 CmdStan tarballs.
• Run CmdStanR/Py tests with the release tarball.
• Make a Stan blog release announcement post (thank the sponsors and all contributors, mention new devs).
• Link to the blog post in a Discourse thread.

Start with the unchecked CmdStanR and CmdStanPy test workflows linked in the release checklist, using the published release tarball as their input. Confirm both workflows pass against that tarball; the release checklist is complete when this final test item is checked.