Legacy COBOL mainframes resist APIfication due to monolithic structure and scattered logic. We present a novel pipeline that extracts user-selected code blocks and generates \emph{compilable API modules with request/response copybooks}.

Key innovations include: (1) \emph{five COBOL patterns} (P1–P5) guiding analysis selection, (2) \emph{WoCC}—a pattern-based call-chain approximation avoiding recursive traversal for organized monoliths, and (3) \emph{end-to-end code synthesis} handling REDEFINES aliasing and inter-program calls.

Unlike prior slicing (variable sets only) or ADDI (read/write reporting), we deliver deployable APIs. On GENAPP/CBSA/industry systems, signature inference drops from weeks to minutes; refactoring effort reduces 15–50%. Practitioners validated precision; modules deployed via z/OS Connect pass functional tests.