Scope

Tiny docs-only change per #38 continuation agent's recommendation. No code change needed since default is already correct.

Change

In `configs/training/default.yaml`, add a comment near where `packing` could be added (or in a relevant note section):

```yaml

NOTE: packing defaults to False for the in-process LoRA path.

Pilot #38 (c1_evil_wrong_em, 2 seeds, 2 arms) showed packing=True on small

coupling datasets:

- slower (-10.5% tokens/sec)

- under-trains coupling signal (+28 pt alignment toward base, +0.27 train_loss)

- collapses optimizer steps 3.67× with same token budget

For distributed Tulu-scale SFT, packing=True is different — see configs/tulu/

configs. Revisit the LoRA default only for long-sequence realistic data.

```

Budget

5 min. No GPU.

Timeline · 1 event

epm:done2026-04-18T00:05:37.000Z· system

## Completion **Status:** SUCCESS **Commit:** `0a89fcf` (docs(configs): document packing default r…

<!-- epm:done v1 -->
## Completion

**Status:** SUCCESS
**Commit:** `0a89fcf` (docs(configs): document packing default rationale [refs #38 #44])
**Branch:** main
**Files:** `configs/training/default.yaml` (+9 lines)

### Verification
- `uv run ruff check configs/` → `All checks passed!` (no Python files under configs/, as expected)
- `git diff` → only the comment addition, no other changes

### Diff
```diff
+# NOTE: packing defaults to False for the in-process LoRA path.
+# Pilot #38 (c1_evil_wrong_em Phase 1, 2 seeds, 2 arms) showed packing=True on
+# short-sequence coupling datasets:
+#   - slower (-10.5% tokens/sec, +11.7% wall time)
+#   - under-trains coupling signal (+28 pt alignment toward base, +0.27 train_loss)
+#   - collapses optimizer steps 3.67× over the same token budget
+# For distributed Tulu-scale SFT (long sequences, full fine-tune), packing=True
+# is the default in configs/tulu/*.yaml — that's a different regime.
+# Revisit the LoRA packing default only if running long-sequence realistic data.
```

Closing per task spec.

[Infra] Update training/default.yaml comment citing #38 packing pilot