We reassess the realistic discovery reach of Solar-System experiments for dark energy (DE) and dark matter (DM), making explicit the bridge from cosmology-level linear responses to local, screened residuals. In scalar-tensor frameworks with a universal conformal coupling $A(\varphi )$ and chameleon/Vainshtein screening, we map cosmological responses $\{\mu (z,k),\mathrm{\Sigma }(z,k)\}$ inferred by DESI and Euclid to thin-shell or Vainshtein residuals in deep Solar potentials ${\mathrm{\Phi }}_N$. We emphasize a two-branch strategy. In a detection-first branch, a verified local anomaly—an Einstein equivalence principle (EEP) violation, a Shapiro-delay signal with $|\gamma -1|\sim \mathrm{few}\times {10}^{-6}$, an AU-scale Yukawa tail, or a narrowband ultralight DM (ULDM) line in clocks/atom interferometers in space (AIS)—triggers a joint refit of cosmology and Solar-System data under a common microphysical parameterization $\{V(\varphi ),A(\varphi )\}$. In a guardrail branch, Solar-System tests enforce universal constraints (EEP; PPN parameters $\gamma$, $\beta$; and $\dot{G}/G$) and close unscreened or weakly screened corners indicated by cosmology. Under realistic near-term conditions we forecast, per conjunction, $|\gamma -1|\lesssim (2-5)\times {10}^{-6}$ (Ka-/X-band or optical Shapiro), ${\eta }_{\mathrm{EEP}}\sim (1-10)\times {10}^{-17}$ (drag-free AIS), $|\dot{G}/G|\sim (3-5)\times {10}^{-15}\mathrm{y}{\mathrm{r}}^{-1}$ [sub-mm-class lunar laser ranging (LLR)], a uniform $\simeq 2\times$ tightening of AU-scale Yukawa/DM-density bounds, and $(3-10)\times$ improved ULDM-coupling reach from clock networks. For a conformal benchmark, ${\mu }_{\mathrm{lin},0}=0.10$ implies $\chi \simeq \sqrt{{\mu }_{\mathrm{lin},0}/2}$ and a Sun thin shell $\mathrm{\Delta }R/R\lesssim (1/3\chi )\sqrt{|\gamma -1|/2}=2.4\times {10}^{-3}$ at $|\gamma -1|=5\times {10}^{-6}$; Vainshtein screening at 1 AU yields $|\gamma -1|\lesssim {10}^{-11}$, naturally below near-term reach. We recommend a cost-effective $\mathrm{guardrail}+\mathrm{discovery}$ portfolio with explicit triggers for escalation to dedicated missions.

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• Dark energy
• Dark matter
• Experimental studies of gravity