What problem does it solve? JAX code fails in non-obvious ways: stale APIs from outdated training data, silent retracing that makes training 100x slower, NaN gradients from jnp.where on singular functions, and OOM jobs with no clear cause. This Skill encodes the idioms, anti-patterns, and current API surface needed to write correct and fast JAX programs. ## Core Features & Use Cases - JAX Idioms & Transformations: Enforces pure functions, explicit PRNG key splitting, pytree discipline, and correct use of jit, vmap, scan, and grad. - Stale API Detection: Maintains a verified table of removed or renamed APIs (jax.tree_map, PRNGKey, jnp.trapz, jax.experimental.optimizers) against the installed JAX version. - Performance & Memory Diagnosis: Provides an ordered debugging workflow for slow or OOMing jobs, covering retracing, dispatch overhead, and allocation measurement before tuning. - Use Case: A training loop calls bare jax.grad plus optax.update in a Python for loop and runs 100x slower than expected; the Skill directs you to jit the combined grad-and-update step and verify with a cache-size check. ## Quick Start Use the dev-jax skill to review my JAX training script for retracing issues and outdated API calls.