Agency can take on many different understandings in Le Carre's work. On one hand, agency can be viewed in a literal sense. In this regard, agency refers to the capacity for an individual to participate in espionage activities. This reality is quite present in the novel as all of the main characters are part of the agency in some form another. Either they are working for it or against it. They have agency because they are attached to it.
I would suggest that a more divergent understanding of viewed as the capacity to take action in the world. The capacity to act is a significant part of Le Carre's theory behind agency. It is in this regard where Le Carre's novel presents interesting aspects. Certainly, Smiley possesses a high degree of agency. Despite being forced into retirement, a capacity that denies agency, he takes it upon himself to display agency with his work in determining who the mole in the Circus is. Le Carre designed Smiley with a specific condition of agency embedded within him: "I wanted to create someone who didn’t belong to any social class but who did belong to a specific generation--the generation that believed you had to be politically committed either to one camp or the other, with no middle ground." Smiley's agency is one where action is seen as a foregone conclusion.
Smiley acts honorably because it is the right thing to do. Whether it is in his marriage that fails or in the agency that forces him into retirement, Smiley's agency is not dependent on external reality. The construction of Arthurian legend is reflective of agency in this regard. Smiley acts as the honorable King Arthur trying to restore that which has been distorted. He acts because of his commitment to that which is right and honorable. It is for this reason that he is uncomfortable with the position of retirement and recognizes the need to act in trying to ascertain the levels of deception in the Circus.
Haydon might be another example of a character that possesses agency. Revealed to be the mole who rises to a high rank in the Soviet army, Haydon uses his agency to facilitate espionage against England for the Russians. His belief that Britain will always lack agency in its dealings with America motivates his agency against them. Haydon represents agency in how he takes an active stance in what he believes.
Le Carre constructs agency as a concept critical to the identity of an international spy. The characters that Le Carre displays are individuals who recognize the presence of real and vital issues at hand. They are ones who actively move towards a specific agenda. The world of international espionage features an agency where individuals act in accordance to their own subjectivity. Their primary motivation is not commercial. For example, Smiley does not assume agency because he wants a raise. Rather, the world of international espionage is one in which agency is defined by what individuals believe and how their actions help to bring about greater success in those belief systems.