\textit{Copy-paste-modify} is a widespread and pragmatic practice in software development, where developers adapt reused code snippets—sourced from platforms such as Stack Overflow, GitHub, or LLM outputs—into their local codebase. A critical yet underexplored aspect of this adaptation is \textit{code wiring}, which involves substituting unresolved variables in the pasted code with suitable ones from the surrounding context. Existing solutions either rely on heuristic rules or historical templates, often failing to effectively utilize contextual information, despite studies showing that over half of adaptation cases are context-dependent. In this paper, we introduce \emph{WIRL}, an LLM-based agent for code wiring framed as a Retrieval-Augmented Generation (RAG) infilling task. \emph{WIRL} combines an LLM, a customized toolkit, and an orchestration module to identify unresolved variables, retrieve context, and perform context-aware substitutions. To balance efficiency and autonomy, the agent adopts a mixed strategy: deterministic rule-based steps for common patterns, and a state-machine-guided decision process for intelligent exploration. We evaluate \emph{WIRL} on a carefully curated, high-quality dataset consisting of real-world code adaptation scenarios. Our approach achieves an exact match precision of 91.7% and a recall of 90.0%, outperforming advanced LLMs by 22.6 and 13.7 percentage points in precision and recall, respectively, and surpassing IntelliJ IDEA by 54.3 and 49.9 percentage points. These results underscore its practical utility, particularly in contexts with complex variable dependencies or multiple unresolved variables. We believe \emph{WIRL} paves the way for more intelligent and context-aware developer assistance in modern IDEs.