Top: Spectral energy distribution (SED) of S4 0954+65. Bottom: Spectral polarization distribution (SPD) of S4 0954+65. In both panels, the cyan data points show the median and standard deviation values at different frequencies during the IXPE exposure. The archival observations (non-simultaneous) are shown in black for reference. The different models are represented by the solid, dashed, and dotted colored lines, as indicated in the legend. In the bottom panel the black downward arrow indicates the 3σ upper limit for the entire exposure, and the magenta arrow the 3σ upper limit under the assumption that the X-ray polarization angle is aligned perpendicular to the 43 GHz jet position angle (see Table 1).