Fair enough. Yet, spiderweb was making the claim that the good thief’s act was a ‘work’ in the context of the good works that are necessary for salvation. I don’t think that his assertion or your explanation support that claim, for two reasons:
First off, the ‘works’ we talk about in this context – that is, the ‘good works’ that are necessary for salvation – are works of supernatural virtue, which proceed only in the process of sanctification as a result of the justification of baptism. (CCC 2010: “Since the initiative belongs to God in the order of grace, no one can merit the initial grace of forgiveness and justification, at the beginning of conversion. Moved by the Holy Spirit and by charity, we can then merit for ourselves and for others the graces needed for our sanctification, for the increase of grace and charity, and for the attainment of eternal life.”
CCC 1273: "“The baptismal seal enables and commits Christians to serve God by a vital participation in the holy liturgy of the Church and to exercise their baptismal priesthood by the witness of holy lives and practical charity.”)
The ‘acts’ of the good thief are neither post-justification acts nor are the result of sanctifying grace; therefore, they are not the acts whose merit enables us to attain to eternal life.
Secondly, though, there’s the assertion about an ‘act of faith’. That doesn’t quite hold, either. Faith is a “human act” (cf CCC 154), but it’s a “gift of God, a supernatural virtue infused by him” (CCC 153). An act of faith, then, is a cooperation with divine grace (cf CCC 155).
Therefore, the act of faith of the good thief has the character of ‘gift’ and ‘grace’, not the character of a good work which merits salvation.
On both counts, then, I don’t think that we can claim that the “act of faith” of the good thief is the kind of “good work” that merits eternal life.