Many Circuits, One Mechanism: Input Variation and Evaluation Granularity in Circuit Discovery

Phantom specialization in circuit discovery: structural differences don't imply distinct mechanisms

A new arXiv preprint challenges a core assumption in mechanistic interpretability: that structurally different circuits discovered for the same task imply distinct computational mechanisms. Using Literal Sequence Copying across token-frequency bands in five Pythia models (70M–1.4B), the authors extract 75 circuits and show that structurally distinct circuits implement the same computation, with band-specific edges transferring broadly and a shared core recovering ≥99% of circuit performance. The paper introduces the term 'phantom specialization' for this pattern and argues that standard source-level evaluation inflates apparent faithfulness, while edge-level evaluation and cross-condition transfer tests are needed to detect the many-to-one mapping from structure to function.