Whether an antiferromagnet can host direction-robust spin-polarized transport without a conventional spin-selective band gap remains a central challenge in antiferromagnetic spintronics. Here, we establish a gapless, direction-robust spin-filtering mechanism in a compensated two-dimensional altermagnetic Weyl semimetal that requires neither a spin-selective band gap nor a large velocity contrast between spin projections. Using Janus monolayer ${\mathrm{Ta}}_2\mathrm{TeSeO}$ as a realistic platform, we combine symmetry analysis with first-principles calculations, full-Brillouin-zone Wannier interpolation, and semiclassical transport. Rotating the Néel vector removes a unitary-mirror constraint and shifts one Weyl-cone pair away from its parent high-symmetry line. For an in-plane Néel vector, the residual $C_{2z}\mathcal{T}$ symmetry forbids the independent ${\sigma }_y$ mass that would open a local gap, allowing the reconstructed cones to shift in momentum while remaining gapless. Breaking unitary $C_{2z}$ simultaneously lifts the energy equivalence of the remaining mirror-pinned Weyl cones. The resulting coexistence of a metallic spin-projected manifold and a low-density-of-states (DOS) Weyl-derived manifold produces a predominantly DOS-driven conductance imbalance. At charge neutrality and 20 K, the longitudinal conductivity polarization for $\mathbf{n}\parallel x$ remains positive for every in-plane current direction and ranges from $76.4\%$ to $82.0\%$. The symmetry-related in-plane Néel orientations provide two distinct spin-filtering states. This Néel-vector-driven symmetry mechanism provides a general route to direction-robust gapless spin filtering in compensated altermagnets.

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• Altermagnetism
• Altermagnets
• Antiferromagnets
• Half-metals
• First-principles calculations