diff --git a/EvoScientist/model_registry/http_api.py b/EvoScientist/model_registry/http_api.py
index 758390d..41a844d 100644
--- a/EvoScientist/model_registry/http_api.py
+++ b/EvoScientist/model_registry/http_api.py
@@ -109,9 +109,13 @@ class GetModelRegistryResponse(BaseModel):
 
 
 class PutModelRegistryRequest(BaseModel):
-    """The section 9.2 registry save request."""
+    """The section 9.2 registry save request.
 
-    expected_revision: PositiveInt
+    ``expected_revision`` is deprecated: accepted for backward
+    compatibility but ignored — saves are last-write-wins.
+    """
+
+    expected_revision: PositiveInt | None = None
     registry: RegistryV4
     credential_writes: list[CredentialWrite] = Field(default_factory=list)
 
diff --git a/EvoScientist/model_registry/openapi.json b/EvoScientist/model_registry/openapi.json
index c20c699..e6b858f 100644
--- a/EvoScientist/model_registry/openapi.json
+++ b/EvoScientist/model_registry/openapi.json
@@ -1010,7 +1010,7 @@
         "type": "object"
       },
       "PutModelRegistryRequest": {
-        "description": "The section 9.2 registry save request.",
+        "description": "The section 9.2 registry save request.\n\n``expected_revision`` is deprecated: accepted for backward\ncompatibility but ignored \u2014 saves are last-write-wins.",
         "properties": {
           "credential_writes": {
             "items": {
@@ -1020,16 +1020,23 @@
             "type": "array"
           },
           "expected_revision": {
-            "exclusiveMinimum": 0,
-            "title": "Expected Revision",
-            "type": "integer"
+            "anyOf": [
+              {
+                "exclusiveMinimum": 0,
+                "type": "integer"
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "default": null,
+            "title": "Expected Revision"
           },
           "registry": {
             "$ref": "#/components/schemas/RegistryV4"
           }
         },
         "required": [
-          "expected_revision",
           "registry"
         ],
         "title": "PutModelRegistryRequest",
diff --git a/tests/test_model_registry_http.py b/tests/test_model_registry_http.py
index d4b9deb..1d01186 100644
--- a/tests/test_model_registry_http.py
+++ b/tests/test_model_registry_http.py
@@ -355,13 +355,22 @@ def test_put_registry_success_returns_full_response(active_client):
     assert states["glm-5.2"]["state"] == "enabled"
 
 
-def test_put_registry_revision_conflict(active_client):
+def test_put_registry_stale_revision_is_ignored(active_client):
     current = _current_registry(active_client)
     response = _put(active_client, current["registry"], expected_revision=99)
-    assert response.status_code == 409
+    assert response.status_code == 200
     body = response.json()
-    assert body["code"] == "REGISTRY_REVISION_CONFLICT"
-    assert set(body) == {"code", "message", "details", "request_id"}
+    assert body["revision"] == current["revision"] + 1
+
+
+def test_put_registry_without_expected_revision(active_client):
+    current = _current_registry(active_client)
+    payload = {"registry": current["registry"], "credential_writes": []}
+    response = active_client.put(
+        "/api/model-registry", headers=_admin_headers(), json=payload
+    )
+    assert response.status_code == 200
+    assert response.json()["revision"] == current["revision"] + 1
 
 
 def test_put_registry_rejects_ssrf_endpoint(active_client):