The presidency of the council of the EU rotates among member states every six months, with the incumbent state responsible for planning and chairing meetings in the council and its related bodies, and representing the council in relations with other EU institutions.
Bulgaria holds the presidency for the first six calendar months of next year - APCO's brief runs to the end of this period, with work beginning immediately after contracts were finalised this week.
A public tender was held earlier this year - tender documents indicate that the work is worth up to BGN500,000 (£228,000, $303,000), with APCO understood to have budgeted for just under that amount.
The work includes digital and creative elements, international media work and stakeholder engagement.
It is led by Brussels-based Nicholas Whyte, head of the firm's global solutions practice, and his team includes colleagues from other APCO members in locations including London, Germany, France and Italy.
That team also recently worked on Milan's narrowly unsucessful bid to become the new seat of the European Medicines Agency - in the event, Amsterdam edged the Italian city into second place. It also did PRWeek Global Awards-winning work with Ukraine's finance ministry in 2015.
APCO does not have an office in Bulgaria itself but has several Bulgarian natives working for the firm in London and elsewhere.
Claire Boussagol, who was promoted to chair of APCO Europe at the start of the year, and remains managing director of its Brussels office, said: "Bulgaria is the first of the countries who joined the EU in 2007 to hold the Presidency and, with the European Union facing such critical issues, we are delighted to assist the Ministry of Foreign Affairs at this significant juncture."