European Union (EU) agencies' relations with interest groups have received scarce research attention despite their relevance to bureaucratic autonomy and functioning. Such relations may be organised via advisory committees, which are durable organisational structures for regularised contact between an agency and those societal actors given membership by the agency. Advisory committees may be imposed by the legislator to control an agency or harnessed by the latter to build autonomy. The EU legislator leaves significant discretion to EU agencies regarding advisory committees. However, EU agencies' late emergence in already densely populated regulatory fields begs the question of whether the use of agency discretion is fully autonomous. A case in point is the Agency for the Cooperation of Energy Regulators (ACER), whose establishment and design of three European Stakeholder Committees are examined here. Advisory committees were needed by, but also imposed on, ACER, reflecting agency preferences to improve task performance as much as prior institutionalisation within the organisational field. Hence, factors beyond legal requirements can constrain agency discretion on how to organise the organisation of agency–interest group relations.