diff --git a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
index 43436f48..fa0671f4 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -1729,8 +1729,16 @@ _API_9285_JSON = (
     / "9285-3062-0205-7766-7200.json"
 )
 
-_ASHP_COHORT_CHAIN_TOLERANCE: float = 0.07
-"""SAP-precision floor for the 7-cert ASHP cohort — see handover."""
+_ASHP_COHORT_CHAIN_TOLERANCE: float = 0.04
+"""ASHP-cohort chain-test tolerance after S0380.28 (reciprocal-linear
+PSR interpolation per SAP 10.2 Appendix N footnote 43). Pre-slice the
+cohort sat at +0.03..+0.06 SAP and this constant was 0.07. Post-slice:
+  Summary path: 6 of 7 certs hit < 1e-4; cert 2636 -0.015 (cantilever)
+  API path:     all 6 certs hit +0.014..+0.031 (cohort-residual cluster
+                — open thread; Summary EPC ≠ API EPC for a load-bearing
+                field — see [[project-api-to-sap-residual-test]])
+0.04 gives ~30% headroom over the API path's worst residual (cert 0350
++0.031) and fires loudly on regression beyond the documented residuals."""
 
 
 def test_api_0380_full_chain_sap_within_spec_floor_of_worksheet() -> None: