diff --git a/evals/metrics.py b/evals/metrics.py
index f46b1261..22a8cf55 100644
--- a/evals/metrics.py
+++ b/evals/metrics.py
@@ -17,7 +17,13 @@ class CaseResult:
     versor_closure: bool
     versor_condition: float
     trace_hash: str
+    #: The SERVED surface — register-decorated, what the user reads.
     surface: str
+    #: The register-INVARIANT truth-path bytes that ``compute_trace_hash``
+    #: folds (ADR-0069 inv C). A different field with a different contract:
+    #: ``surface`` is *expected* to vary across the register axis, this is
+    #: *required* not to. Defaulted so older constructors stay valid.
+    hash_surface: str = ""
 
 
 @dataclass(slots=True)
diff --git a/evals/run_cognition_eval.py b/evals/run_cognition_eval.py
index 3ece4009..840f095a 100644
--- a/evals/run_cognition_eval.py
+++ b/evals/run_cognition_eval.py
@@ -65,6 +65,7 @@ def _run_case(case: dict, pipeline: CognitiveTurnPipeline) -> CaseResult:
         versor_condition=result.versor_condition,
         trace_hash=result.trace_hash,
         surface=result.surface,
+        hash_surface=result.hash_surface or result.surface,
     )