diff --git a/docs/implementation-identity.md b/docs/implementation-identity.md
new file mode 100644
index 0000000..76aba0c
--- /dev/null
+++ b/docs/implementation-identity.md
@@ -0,0 +1,32 @@
+# Implementation identity is data, not policy
+
+This is development guidance for MNCDS consumers. It does not change MNCDS 0.1
+record schemas.
+
+## Principle
+
+An implementation identity — a model tag, worker id, runtime name, or provider
+namespace — may be recorded as data. It must not silently define capability or
+architecture.
+
+Brand or family substrings in an identity are not capability proof.
+
+## Required distinctions
+
+- **Capability claim**: a fact reported by a provider or runtime.
+- **Observed capability**: a capability demonstrated by bounded execution and
+  recorded as evidence.
+- **Policy preference**: an operator or system ranking choice.
+- **Exact pin**: an operator request for one implementation identity. Pins fail
+  closed when the identity is absent unless an explicit fallback policy says
+  otherwise.
+- **Compatibility mechanism**: an unusual but bounded path used only when
+  evidence is missing. It must remain visible, testable, and removable.
+
+Unknown remains unknown. Missing evidence must not be converted into PASS.
+
+## Unconventional mechanisms
+
+A workaround is acceptable only when its boundary, assumptions, invariant,
+validation, and evidence are explicit. Accidental shortcuts around validators
+are not architecture.
diff --git a/docs/mncds-decisions.md b/docs/mncds-decisions.md
index e7fe6de..05830a8 100644
--- a/docs/mncds-decisions.md
+++ b/docs/mncds-decisions.md
@@ -33,6 +33,13 @@ D1 requires one aggregate development record containing:
 Separate ledgers and role records may be introduced later, but the aggregate record is the
 minimum interoperable unit for 0.1.
 
+## Implementation identity
+
+Development systems that select generators or evaluators should treat
+implementation identity as data. Capability claims, observed evidence, and
+policy preferences remain distinct. See
+[implementation-identity.md](implementation-identity.md).
+
 ## Stochastic reproducibility
 
 MNCDS uses five declared classes: