Cross-simulator gate (Isaac Lab to MuJoCo) comes before any hardware attempt

Symptom

A policy that only ever ran in its training simulator carries untested dependencies on that simulator's solver, contact model, and defaults; the first place those dependencies surface should not be the real robot.

Context

Standing order of operations for the whole Lucen program, recorded as the opening line of the experience log: train in Isaac Lab, gate in MuJoCo, only then go to hardware. The MuJoCo side is the same plant used for evaluation batteries, so a sim2sim pass also validates the exported policy + contract (obs ordering, scales, defaults) outside the training stack.

Change

Pipeline rule adopted - every checkpoint must pass the MuJoCo evaluation battery (sim2sim) before it is considered for real deployment (sim2real).

Outcome

Used generation after generation as the cheap filter; hardware sessions only ever received policies that had already survived a second simulator.

Mechanism

Two simulators disagree exactly where a policy is overfit to simulator-specific artifacts (contact softness, integrator, default parameters, obs conventions); a cross-sim transfer catches contract bugs and solver overfitting at zero hardware risk, so real-robot failures that remain are attributable to genuine plant/actuator gaps.

Applies when

• planning the path from training to first hardware trial
• exported policy behaves differently outside the training framework
• triaging whether a real-robot failure is contract vs plant