diff --git a/docs/HANDOVER_API_PROFILING.md b/docs/HANDOVER_API_PROFILING.md
index 7b76b07b..bb842098 100644
--- a/docs/HANDOVER_API_PROFILING.md
+++ b/docs/HANDOVER_API_PROFILING.md
@@ -13,16 +13,56 @@ deproven approaches + the meter/shower data-fidelity findings), and the earlier
 `energy_rating_current`. Headline gauge:
 `PYTHONPATH=/workspaces/model python scripts/eval_api_sap_accuracy.py`.
 
-| metric | session-3 (`a8e5563a`) | session-4 (`faf29942`) | session-5 (`19235d11`) | **session-6 (`a7761ea8`)** |
+| metric | session-4 (`faf29942`) | session-5 (`19235d11`) | session-6 (`a7761ea8`) | **session-7 (`3e05c95e`)** |
 |--------|------------------|------------------|------------------|------------------|
-| **% \|err\| < 0.5** | 45.1% | 47.6% | 53.1% | **53.14%** |
-| % \|err\| < 1.0 | 59.4% | 62.6% | 68.0% | **67.99%** |
-| % \|err\| < 2.0 | 77.7% | 79.6% | ~81% | **81.85%** |
-| mean \|err\| | 1.702 | 1.586 | 1.329 | **1.312** |
-| median \|err\| | — | — | 0.467 | **0.467** |
+| **% \|err\| < 0.5** | 47.6% | 53.1% | 53.14% | **54.24%** |
+| % \|err\| < 1.0 | 62.6% | 68.0% | 67.99% | **69.64%** |
+| % \|err\| < 2.0 | 79.6% | ~81% | 81.85% | **83.50%** |
+| mean \|err\| | 1.586 | 1.329 | 1.312 | **1.248** |
+| median \|err\| | — | 0.467 | 0.467 | **0.457** |
 | computed / raises | 909 / 0 | 909 / 0 | 909 / 0 | **909 / 0** |
 | unsupported_schema | 100 (deferred) | 100 (deferred) | 100 (deferred) | 100 (deferred) |
 
+### SESSION-7 — sheltered alternative walls (RdSAP Table 4 R=0.5), `3e05c95e`
+The headline-moving audit. User: 53% is poor enough to indicate a MAJOR error
+— audit again. The decisive diagnostic CHAIN (reusable):
+1. **Error by `dwelling_type`** → flats are the drag (houses 60-66% within 0.5,
+   flats 28-47%; top-floor flat −1.19, mid-floor 28% within 0.5).
+2. **Split flats by `mains_gas`** → ELECTRIC flats are the killer (gas flats
+   ~45%, electric flats 13-19%; top-floor electric mean|err| 3.62).
+3. **Invert the SAP equation** (ECF = 0.42·cost/(TFA+45) → `sap_rating`) to get
+   `our_cost / lodged_cost` → electric flats over-cost ~3% median (houses 1.00),
+   amplified by the 4× electric price + steep low-band log curve (+3% ≈ +1.5 SAP
+   at band 40).
+4. **Under-rate tracks space-heating kWh/m² precisely** — accurate certs 14-110,
+   under-rating certs 130-289 (cert 2021: 11 275 kWh for 39 m²) → over-stated
+   FABRIC, not tariff (storage flats on the correct 5.5p rate under-rate just as
+   much as room-heater flats at 13p).
+5. **Field-by-field on the worst** → cert 0340-2976 (band-A flat) computed wall
+   128 W/K though its main wall is a FILLED cavity (U 0.7); the excess was a
+   SECOND `u_wall` call — a `sap_alternative_wall_1` timber-frame sub-area at
+   U=2.5 lodging **`sheltered_wall="Y"`**.
+
+ROOT: the gov-EPC API lodges `sheltered_wall="Y"` per alt-wall, but it was
+DROPPED by the schema + domain dataclasses, so `_alt_wall_w_per_k` billed the
+sub-area at its full exposed U. RdSAP 10 Table 4 (PDF p.22) "Sheltered": added
+external resistance R=0.5 m²K/W → U_sheltered = 1/(1/U + 0.5) — the SAME
+adjustment the MAIN wall already applies for `gable_wall_type=2`
+(`gable_wall_sheltered`, `_SHELTERED_GABLE_ADDED_RESISTANCE_M2K_W`). Threaded
+end-to-end: schema (21.0.0/21.0.1) + domain `SapAlternativeWall.is_sheltered`
+(default False → Summary/Elmhurst path unchanged, goldens untouched) +
+`from_api_response` `"Y"→True` + `_alt_wall_w_per_k` applies the 0.5 resistance
+(lodged-U + basement alts return before it). 140 certs (15% of corpus) carry a
+sheltered alt-wall; they under-rated at median −0.82 / signed −1.33 / 23% within
+0.5. Eval: 102 improved / 38 regressed (offsetting-error cases — applied
+spec-uniformly per the determinism principle); +10 net within-0.5. Goldens +
+full regression green, pyright net-zero. **OPEN audit leftovers: electric-flat
+tail persists (2021 −3.9 genuine uninsulated solid brick = data-fidelity;
+top-floor "Flat, limited insulation" roofs); detached houses 45% (balanced
+bidirectional scatter); the 38 sheltering regressions = offsetting errors
+elsewhere; main-wall (non-gable) sheltering not audited; multi-element wall
+description joined-string-to-all-BPs (113 certs, mild).**
+
 ### SESSION-6 — community fuels 30/31/32 collide with electricity codes, `a7761ea8`
 Picked up the deferred "fuel-collision part 2". The profiler's strongly-biased
 `main_control=2306` bucket (n=11, signed −3.75, nearly uniform) was a PROXY: every