Many animal groups routinely make consensus decisions jointly with all group members. This paper builds a novel model merging the locally neighboring reciprocal action and alignment together to investigate the mechanisms of consensus decision-making and its robustness. Our model reveals that the shapes of the coherent .ocks are limited in a common narrow interval for different group sizes and information structures. Moreover, the coherent groups display a surprising degree of tolerance against errors, however, they simultaneously show an extremely fragile to attacks. Our model and approach discover some novel phenomena and also reveal some underlying mechanisms of the consensus decision-making and its robustness in biological systems.