Module Debug Matrix

The cross-configuration debug surface uses a fixed replay and changes exactly one factor at a time. It compares two edges of the same frozen rollout:

1. training score versus rollout prefill;
2. rollout prefill versus rollout decode.

The source of truth is rl_engine.alignment.cross_config.debug_matrix.module_debug_matrix. The manifest is reporting metadata, not a second collection of runtime flags.

Comparison gates

Before reporting numerical drift, the run must prove that the selected tokens, active mask, model state, and logical ownership map are unchanged. A failed gate records comparable=false; it does not produce a drift number. This keeps an input or sharding error from being misdiagnosed as kernel rounding.

First-line axes

Module	Baseline	Gate rows	Diagnostic rows	Invariant control
Attention	A0	A4 head/sequence ownership	A1 position/RoPE, A2 Q/K preprocessing, A3 mask boundary, A5 KV state, A6 precision/rounding, A7 block plan/merge	C0-C2
FFN / GEMM	F0	F1 TP weight ownership	F2 SwiGLU rounding, F3 K-reduction/Split-K, F4 token collective	FC0 batch-invariant row replay
Selected-token logp	L0	L1 vocabulary ownership/padding domain, L2 selected token/active mask	L3 vocabulary LSE tile/merge	LC0 batch-invariant row replay

The logp wrapper receives local logits and therefore owns the vocabulary log-sum-exp, selected-token gather, mask, padding, and TP merge. It does not own the language-model-head GEMM. The FFN/GEMM wrapper owns the K reduction and SwiGLU rounding; both must be diagnosed before attributing a logp difference to the vocabulary merge.

Numerical program

For each diagnostic row, record the same inputs and constants, intermediate precision and final write, reduction grouping and logical order, and every state handoff. Scheduler choices that do not change the numerical program or logical ownership are intentionally not exposed as mismatch axes.

Use the compact row first. The operator-specific secondary probes remain available for a focused follow-up only after the representative row reproduces the mismatch.

Report artifacts

The matrix reporter is owned by RL-Kernel because it consumes the sealed cross-configuration attempt contract. It reads actual.json, comparison.json, and token_diffs.pt only after COMPLETE validation, so the report displays actual selected operators and the materialized topology, not merely requested flags. See Cross-Configuration Drift Report for the .rlk-drift bundle, desktop viewer, static image, and trace entry points.