nnsight is a Python library for interpreting and intervening in neural network models during execution. It provides a tracing API that allows users to access and modify internal activations, compute gradients, and perform interventions like activation patching and steering. The system uses a deferred execution model with thread-based synchronization to run user intervention code alongside model forward passes. Key features include support for Hugging Face models, vLLM integration for high-performance inference, remote execution via NDIF infrastructure, and persistent model editing. The architecture centers around Envoy wrappers for modules, a tracer/interleaver system for coordinating execution, and a batching mechanism for efficient multi-input processing.