Do Skill Descriptions Tell the Truth? Detecting Undisclosed Security Behaviors in Code-Backed LLM Skills

Programmatic skills in LLM ecosystems consist of a natural-language description and executable implementation files. Users and LLMs rely on the description to understand the skill's scope. However, the implementation may perform security-relevant operations, such as credential access, network communication, or command execution, that the description does not state. We study this description--implementation inconsistency by asking whether the implementation stays within the security-relevant scope declared in the description. We manually analyze 920 real-world programmatic skills and construct an 11-category security property taxonomy. Based on this taxonomy, we build SKILLSCOPE, which constructs source-level security property graphs (SPGs) from implementations and performs LLM-assisted consistency checking. SPG nodes retain source-level code patterns rather than abstract taxonomy labels, preserving fine-grained evidence for checking. On 4,556 programmatic skills with double-blind human review, SKILLSCOPE achieves a precision of 84.8% and a recall of 96.5% for identifying inconsistency. Confirmed inconsistency affects 9.4% of skills, while cases of coarser description, in which implementation details remain within the declared scope, account for 24.3%. Ablation experiments confirm that both the SPG and the taxonomy contribute: removing the taxonomy reduces precision from 87.8% to 72.3%, while removing the SPG reduces recall from 94.7% to 79.0%.