diff --git a/README.md b/README.md
index a659173..22decbe 100644
--- a/README.md
+++ b/README.md
@@ -212,6 +212,7 @@ python3 ci/validate-fixtures.py
 | `test-case-null-operator-flag.json` | Flag using IS_NULL operator |
 | `test-case-null-shard-range-isolation.json` | A flag with a null shard range is removed without poisoning valid flags |
 | `test-case-null-targeting-key.json` | Evaluations with an explicit null targeting key |
+| `test-case-optional-targeting-key.json` | Empty-key sharding, null versus empty `id` fallback, and explicit `id` precedence |
 | `test-case-numeric-flag.json` | Numeric flag evaluation |
 | `test-case-numeric-one-of.json` | Numeric ONE_OF operator matching |
 | `test-case-of-7-empty-targeting-key.json` | Evaluation with empty targeting key |
diff --git a/evaluation-cases/test-case-optional-targeting-key.json b/evaluation-cases/test-case-optional-targeting-key.json
new file mode 100644
index 0000000..9d4c7d3
--- /dev/null
+++ b/evaluation-cases/test-case-optional-targeting-key.json
@@ -0,0 +1,73 @@
+[
+  {
+    "attributes": {},
+    "defaultValue": "fallback",
+    "flag": "optional-targeting-key-sharded",
+    "result": {
+      "reason": "SPLIT",
+      "value": "sharded-value"
+    },
+    "targetingKey": "",
+    "variationType": "STRING"
+  },
+  {
+    "attributes": {},
+    "defaultValue": "fallback",
+    "flag": "optional-targeting-key-sharded",
+    "result": {
+      "reason": "ERROR",
+      "value": "fallback",
+      "errorCode": "TARGETING_KEY_MISSING"
+    },
+    "targetingKey": null,
+    "variationType": "STRING"
+  },
+  {
+    "attributes": {},
+    "defaultValue": "fallback",
+    "flag": "optional-targeting-key-id-rule",
+    "result": {
+      "reason": "TARGETING_MATCH",
+      "value": "empty-id-value"
+    },
+    "targetingKey": "",
+    "variationType": "STRING"
+  },
+  {
+    "attributes": {},
+    "defaultValue": "fallback",
+    "flag": "optional-targeting-key-id-rule",
+    "result": {
+      "reason": "DEFAULT",
+      "value": "fallback"
+    },
+    "targetingKey": null,
+    "variationType": "STRING"
+  },
+  {
+    "attributes": {
+      "id": ""
+    },
+    "defaultValue": "fallback",
+    "flag": "optional-targeting-key-id-rule",
+    "result": {
+      "reason": "TARGETING_MATCH",
+      "value": "empty-id-value"
+    },
+    "targetingKey": null,
+    "variationType": "STRING"
+  },
+  {
+    "attributes": {
+      "id": "explicit-user"
+    },
+    "defaultValue": "fallback",
+    "flag": "optional-targeting-key-id-rule",
+    "result": {
+      "reason": "DEFAULT",
+      "value": "fallback"
+    },
+    "targetingKey": "",
+    "variationType": "STRING"
+  }
+]
diff --git a/ufc-config.json b/ufc-config.json
index ece892b..26a48f3 100644
--- a/ufc-config.json
+++ b/ufc-config.json
@@ -5092,6 +5092,75 @@
         }
       ]
     },
+    "optional-targeting-key-sharded": {
+      "key": "optional-targeting-key-sharded",
+      "enabled": true,
+      "variationType": "STRING",
+      "variations": {
+        "assigned": {
+          "key": "assigned",
+          "value": "sharded-value"
+        }
+      },
+      "allocations": [
+        {
+          "key": "sharded-allocation",
+          "rules": [],
+          "splits": [
+            {
+              "variationKey": "assigned",
+              "shards": [
+                {
+                  "salt": "optional-targeting-key",
+                  "totalShards": 10000,
+                  "ranges": [
+                    {
+                      "start": 0,
+                      "end": 10000
+                    }
+                  ]
+                }
+              ]
+            }
+          ],
+          "doLog": false
+        }
+      ]
+    },
+    "optional-targeting-key-id-rule": {
+      "key": "optional-targeting-key-id-rule",
+      "enabled": true,
+      "variationType": "STRING",
+      "variations": {
+        "matched": {
+          "key": "matched",
+          "value": "empty-id-value"
+        }
+      },
+      "allocations": [
+        {
+          "key": "empty-id-allocation",
+          "rules": [
+            {
+              "conditions": [
+                {
+                  "attribute": "id",
+                  "operator": "MATCHES",
+                  "value": "^$"
+                }
+              ]
+            }
+          ],
+          "splits": [
+            {
+              "variationKey": "matched",
+              "shards": []
+            }
+          ],
+          "doLog": false
+        }
+      ]
+    },
     "semver-invalid-comparand-non-string-test": {
       "key": "semver-invalid-comparand-non-string-test",
       "enabled": true,