Solar System constraints on a general scalar-tensor theory with generic nonminimal coupling function, noncanonical kinetic function, and scalar potential are investigated in both the metric and Palatini formalisms. A unified post-Newtonian treatment is developed, yielding analytical expressions for the effective scalar mass, the effective gravitational coupling, and the parametrized post-Newtonian parameters $\gamma$ and $\beta$. The results show explicitly how the choice of variational principle affects the weak-field phenomenology. Comparison with Solar System observations, primarily the Cassini bound on $\gamma$, indicates that the observational impact of the formalism is strongly model dependent. Generic nonminimally coupled scalar fields may satisfy significantly weaker local bounds in the Palatini case because of stronger Yukawa suppression, whereas in Brans-Dicke gravity the differences are typically small and become appreciable only in restricted regions of parameter space. For the point-particle source considered here, Palatini $f(\widehat{R})$ gravity reproduces the general relativistic exterior post-Newtonian limit, unlike metric $f(R)$ gravity.

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• Alternative gravity theories
• Astrophysical studies of gravity
• General relativity