In processing an external function for which source code is not available, such as an application program interface (API), a model is used to evaluate whether the external function is properly invoked. An error potentially resulting from improper invocation of the external function and at least one condition that will result in the potential error are identified. The model is created to test for the condition that will result in the potential error. When the condition is found, generation of an error message is triggered. The model is associated with a source code analyzer such that when the source code is analyzed by the source code analyzer, the model is applied to evaluate invocation of the external function. The potential error and conditions that result in the error, such as input/output parameter errors and dependency errors, may be derived from documentation describing the operation of the external function or API.