Understanding Robustness of Model Editing in Code LLMs

Large language models (LLMs) for code are increasingly used in software development, but they remain static after pretraining while APIs and software libraries continue to evolve. Model editing offers a lightweight alternative to retraining for incorporating API updates, yet it remains unclear whether existing editing methods can induce correct API migration, generalize that behavior to unseen tasks, and preserve performance on tasks involving unmodified APIs. We present a controlled benchmark for evaluating model editing under API updates in code LLMs, built from HumanEval, MBPP, and APPS, with 2,040 problems spanning 140 unique synthetic API modifications, together with an execution sandbox that enforces edited APIs under standard Python semantics. We evaluate several state-of-the-art editing methods on three code LLMs under both single-edit and successive-edit regimes using execution-based metrics that distinguish successful API adoption from workaround-based task completion. Under single edits, edited models generalize poorly to unseen uses of the modified API, and many apparent successes are workaround-based rather than true API migrations. Performance on tasks involving unmodified APIs also degrades, although memory-based methods and fine-tuning preserve specificity better than locate-then-edit methods. Under successive edits, most method-model combinations collapse to near-zero Pass@k on both generalization and specificity, revealing substantial interference beyond the target edits. A two-factor Shapley decomposition further shows that single-edit failures on generalization include a substantial compilation component, whereas specificity failures are more often post-compilation. Under successive edits, failures become predominantly compilation-driven.