The Talmud (Shabbat 12b, Sotah 33a) states that one should not pray for his needs in Aramaic, as the ministering angels will be unhelpful, as they do not understand Aramaic. Tosafot question how this is possible, considering the fact that they know even the thoughts of men. The Ra’avad (T’mim Deim 184, cited in Gilyon HaShas) understands the Talmud’s statement somewhat differently, interpreting that the angels will not pay attention to the prayer to bring it before G-d.

The S’dei Chemed (ma’arekhet ha-mem, klal 202) cites the Chida as assuming that angels do not know the thoughts of humans, and thus explains the principle that sinful intent is not counted as action: G-d Himself will not testify against His children, and the Heavenly Beit Din [of angels] is unaware of thoughts [compare Maharsha, Sanhedrin 92b, that accusing angels do not speak Aramaic, but defending angels speak all languages]. The S’dei Chemed questions this opinion because of Tosafot, but concludes, against Tosafot, that only G-d knows the thoughts of man. Further, the S’dei Chemed quotes two opinions as to whether angels know the future.

The Klausenberger Rebbe (Resp. Divrei Yatziv, Y.D. 121:5) discusses the question of whether any obligation exists to follow the commands of an angel. He assumes that this is dependent on the question of whether angels are on a higher level than tzadikim, or vice versa. He concludes that this point varies, depending on the angel and on the individual.