In conjunction with the idea of "author death," and the removal of his being from the process of "writing," the idea of Agent also quickly fades away. If the author is no longer thought to bear the unique power of imposing ultimate meaning on the text, or at the very least, thought to be the fountain from which the text springs forth, then his central position as the Agent of text, or language, has disappeared; perhaps even realized as never existing.
So where can Agent appear? For Foucault, it rested in the author's transition to the sociallly based, "Author-function," and Barthes, although not explicit, the Agent seemed to rest in language itself. However, what seems to be problematic in the location of the contemporary Agent, lies not in its new position, but in the failure of its conception as something that could be pluralistic.
Similar to the idea of government. When we say government, we do not conjure up one place of origin, one has to consider its multiple parts: the judicial branch, the executive branch, the legislation; and even this could be further fragmented down to sub-institutions, and from there, to individual persons.
So then, while Foucault and Barthes seems to unconsciously cling to this idea of Agent as singular, perhaps there could be a better understanding if placed in a postmodern, fragmented identity. Perhaps "Agent" refers to all the multiplicity of agency interactions in conjunction with a text; What discourse does it fall under? Who is the audience? How are the readers naturalizing the reading? What/which episteme(s) does it reflect? How are the significations playing/justifying themselves on all of these fronts?
Perhaps Agent, along with the self, has evolved to become stuck in between worlds of agency; pressed and cooked by the sheer pressure of literary ambiguity.