diff --git a/hindsight-api-slim/hindsight_api/api/http.py b/hindsight-api-slim/hindsight_api/api/http.py
index c357f4116..e931da08c 100644
--- a/hindsight-api-slim/hindsight_api/api/http.py
+++ b/hindsight-api-slim/hindsight_api/api/http.py
@@ -466,6 +466,13 @@ class MemoryItem(BaseModel):
         description="Optional tags for visibility scoping. Memories with tags can be filtered during recall.",
     )
 
+    @field_validator("content")
+    @classmethod
+    def validate_content(cls, v: str) -> str:
+        if not v.strip():
+            raise ValueError("content cannot be empty")
+        return v
+
     @field_validator("tags", mode="before")
     @classmethod
     def coerce_tags(cls, v):
diff --git a/hindsight-api-slim/tests/test_none_llm_provider.py b/hindsight-api-slim/tests/test_none_llm_provider.py
index 3122348f4..034868c7b 100644
--- a/hindsight-api-slim/tests/test_none_llm_provider.py
+++ b/hindsight-api-slim/tests/test_none_llm_provider.py
@@ -248,6 +248,21 @@ async def test_http_retain_works(none_api_client):
     assert response.status_code == 200
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("content", ["", "   \n\t"])
+async def test_http_retain_rejects_blank_content(none_api_client, content):
+    """Retain endpoint should reject empty or whitespace-only content."""
+    bank_id = f"test_none_http_retain_blank_{datetime.now(timezone.utc).timestamp()}"
+
+    response = await none_api_client.post(
+        f"/v1/default/banks/{bank_id}/memories",
+        json={"items": [{"content": content, "context": "test"}]},
+    )
+
+    assert response.status_code == 422
+    assert "content cannot be empty" in str(response.json()["detail"])
+
+
 @pytest.mark.asyncio
 async def test_http_recall_works(none_api_client):
     """Recall endpoint should work with provider=none."""