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In the updated APOGEE-Kepler catalog, we have asteroseismic and spectroscopicdata for over 3000 first ascent red giants. Given the size and accuracy of thissample, these data offer an unprecedented test of the accuracy of stellarmodels on the post-main-sequence. When we compare these data to theoreticalpredictions, we find a metallicity dependent temperature offset with a slope ofaround 100 K per dex in metallicity. We find that this effect is present in allmodel grids tested and that theoretical uncertainties in the models, correlatedspectroscopic errors, and shifts in the asteroseismic mass scale areinsufficient to explain this effect. Stellar models can be brought intoagreement with the data if a metallicity dependent convective mixing length isused, with $ \Delta\alpha_{\rm ML, YREC} \sim 0.2$ per dex in metallicity, atrend inconsistent with the predictions of three dimensional stellar convectionsimulations. If this effect is not taken into account, isochrone ages for redgiants from the Gaia data will be off by as much as a factor of 2 even atmodest deviations from solar metallicity ([Fe/H]=$-$0.5).

The Correlation Between Mixing Length and Metallicity on the Giant Branch: Implications for Ages in the Gaia Era