Diagnosis of Plan Structure Violations Nico Roos ∗ , Cees Witteveen † Abstract Failures in plan execution can be attributed to errors in the execution of plan steps or violations of the plan structure. The structure of a plan prescribes which actions have to be performed and which precedence con- straints between them have to be respected. Especially in multi-agent envi- ronments violations of plan structure might easily occur as the consequence of synchronization errors. While in previous work we have concentrated on the first type of failures, in this paper we introduce the idea of diagnos- ing plan structure violations. Using a formal framework for plan diagnosis, we describe how Model-Based Diagnosis can applied to identify these vio- lations of plan structure specifications and we analyze their computational complexity. Keywords: Model-Based Diagnosis, Plan execution, Coordination errors. 1 Introduction Plan diagnosis deals with the identification of errors occurring during the exe- cution of a plan. In previous work, we have presented methods for identifying such errors as failed executions of plan steps in multi-agent plans [16, 18], equip- ment failures and malfunctioning agents causing the execution of plan steps to fail [6, 5], and methods for assigning responsibility to agents in case plan execution failed [5]. In all these papers, however, the tacit assumption was that the plan structure itself is not violated, we tacitly assumed that during plan execution the plan structure is not violated, i.e., all plan steps as specified in the plan are exe- cuted (correctly or incorrectly) and the order in which they are executed does not violate any precedence constraint. In reality, however, violations of the plan structure may easily occur and might result in plan failure. For instance, consider a plan for loading a truck that has to visit several places to deliver cargo. Often, such a plan contains a specific ordering ∗ Maastricht ICT Competence Center, Universiteit Maastricht, P.O.Box 616, NL-6200 MD Maastricht, roos@micc.unimaas.nl † Faculty EWI, Delft University of Technology, P.O.Box 5031, NL-2600 GA Delft, C.Witteveen@tudelft.nl 1