v2.40.0 (16 September 2026)

• The same .stanfunctions file can be included multiple times, with later times being ignored. This is particularly useful when .stanfunctions files include other .stanfunctions files. (#1639)
• Added student_t_qf. (#1620)
• Added the poisson_binomial distribution. (#1690)
• Added new variadic functions integrate_1d_gauss_kronrod, integrate_1d_gauss_kronrod_tol, integrate_1d_double_exponential, integrate_1d_double_exponential_tol. (#1626)
• Added new overloads to softmax and log_softmax. In addition to vectors, they now accept row vectors, arraws of vectors, and arrays of row vectors. (#1629)
• Support row-vector and vector inputs for x in normal_id_glm_lpdf. (#1698)
• Added laplace_latent_solve(_tol), which return tuple(vector, matrix) (posterior mean and Cholesky factor). (#1649)
• Added to_vector_array(matrix), to_row_vector_array(matrix), and new overloads for to_matrix. (#1693)
• Updated the parser to reject invalid function definitions earlier, allowing for nicer error messages in some cases. (#1630)
• Improved the error message for using an unnormalized function in an invalid context. (#1636)
• Improved error messages for when a variable has a size which depends on an illegal expression. (#1647)
• Improved startup time of the compiler by deferring loading of the math library signatures. (#1623)
• Improved handling of some edge cases when the same variable is used on both left- and right- hand side of an assignment. (#1627)
• The Stan compiler is now built with OCaml 5.5.0 by default. (#1631)
• Formated .mll and .mly files with menhirformat. (#1634)
• Update ocamlformat to 0.29.0. (#1653)
• Added debug flag --debug-print-factor-graph to print the compiler's conservative over-approximation of the factor graph for the model(s) implemented in the Stan program. (#1673)
• Reduced dependencies by leveraging new OCaml features. (#1644, #1650, #1651,
  #1652, #1655, #1656, #1661, #1665, #1676)
• Update ICE handling with new Format functions. (#1645)
• Added a new vectorize_loops optimization (enabled at --Oexperimental).
  (#1666, #1672, #1683)
• Optimized map operations in dataflow analyses. (#1682)
• Optimized the partial evaluated by avoiding a re-typecheck of unchanged functions. (#1700)
• Fixed the inferred type of multi-indexed expressions in the compiler's middle intermediate representation. (#1667)
• Fixed an issue where compiler optimizations could treat unreachable code as valid and generate problematic results. (#1687)
• Fixed an incorrect optimization which could rewrite the categorical distribution in incorrect situations. (#1689)
• Fixed an issue where --name could be used to generated invalid C++. (#1701)
• Fixed an issue with --Oexperimental which would change the meaning of for loops with unusual expressions in their lower bound. (#1702)
• Fixed several issues where --Oexperimental would generate invalid C++. (#1712, #1713, #1715, #1716, #1717)
• Improved code-generation of local variable dimensions. (#1691)