A real 3-LoRA merge (characters/concept, no style) oversaturated badly (energy
2.52x vs main's 1.02x): additive stacking is wrong for a balanced multi-LoRA blend.
The analyzer cannot tell 'preserve this style' from 'blend these characters' — both
are orthogonal — so additive must never be auto-selected; it has to be user intent.
- Revert _decide_prefix_mode: orthogonal groups blend by default again
(weighted_average / SLERP), exactly like main. No auto sum_preserve.
- Make the preserve flag a general overlay in _merge_diffs: blend the non-preserved
LoRAs with the chosen mode, then add each preserved LoRA's full delta on top
(works for weighted_average/slerp/ties; still exempt from sparsification + TIES
sign-election). Skip the exact-linear fast path when the stack has a preserve flag.
- Remove the standalone sum_preserve mode, its exact-linear handling,
_ORTHO_SUM_HEADROOM, and the sum+ report/legend/strategy-count display.
- Tests: SumPreserveMergeTests -> PrefixModeRoutingTests (asserts additive is never
auto-selected); add preserve-overlay-on-weighted_average tests. 299 pass.
- Docs (Merge-Algorithms, Tips) + design-doc postmortem updated; algo note reworded.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Real run showed the first cut didn't fix it by default: the applied config was
the 'full' strategy (SLERP, energy 1.22x), while the style-preserving sum_preserve
config (no_slerp/basic, energy 1.91x) ranked LOWER and wasn't applied. Cause: the
Phase-2 scorer (_score_merge_result) is effective_rank*0.4 + (1-norm_cv)*0.3 +
sparsity_fit*0.3 — no magnitude/energy term, and (1-norm_cv) rewards uniform norms,
which is exactly SLERP's magnitude-flattening. So the scorer structurally prefers
the style-flattening merge over the style-preserving one.
SLERP is strictly worse than sum_preserve for orthogonal groups anyway (it rotates
two independent directions into a 45-degree blend that loses both, vs keeping each
in its own subspace), so orthogonal non-opposing groups now use sum_preserve in ALL
strategy sets — the AutoTuner's top config preserves the style by default, no manual
selection. SLERP is retained for its real sweet spot: non-orthogonal, similar-
direction (cos ~0.25-0.5) low-conflict groups in the full set.
Tests: orthogonal/full -> sum_preserve; non-orthogonal-aligned/full -> slerp;
non-orthogonal-aligned/no_slerp -> weighted_average. 304 pass. Docs + algo note updated.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
