Although task complexity effects on L2 oral production have been widely studied in teaching contexts, their application to task-based language assessment and pragmatic language use remains underexplored. Since pragmatic competence is part of effective communication and interactional abilities and is context-dependent, an investigation of the relationships among task complexity, pragmatic language use, and paired speaking test performance is needed. This study examined the effect of the resource-directing and resource-dispersing dimensions (the number of elements, causal reasoning demand, and planning time) of Robinson’s task complexity construct on fifty-two intermediate-level English as a Second Language (ESL) learners’ rapport-building language use during two decision-making tasks as the achievement test in an EAP program, and the relationships between rapport language use and three dimensions of paired speaking test performance: collaboration, task completion, and style. The results showed that frequency and variety of rapport-building language use did not significantly differ between the two tasks. However, the study found that only in the simple task did different types of rapport-building language have statistically significant positive or negative relationships with different dimensions of paired speaking test scores. Specifically, greeting language use had a strong or close to strong positive relationship with collaboration and style scores, whereas agreeing language had a strong negative relationship with collaboration scores. Additionally, thanking language had a strong negative association with task completion scores. The findings further suggest that task complexity effects learners’ production of rapport-building language in terms of alignment with their peer interlocutors and formality of style, and also impacts raters’ perceptions of paired speaking task performance. The findings highlight the importance of task effects, paired oral assessment rubrics development, and rapport-building language instruction.