diff --git a/src/lib.rs b/src/lib.rs
index 6f140e1..cf9c9c3 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -23,6 +23,7 @@ pub use contracts::{CoreError, CoreResult};
 #[cfg(test)]
 mod tests {
     use crate::{
+        CoreError,
         config::{
             AuthConfig, CacheConfig, EmbeddingProviderConfig, EmbeddingProviderKind,
             ExtractionProviderConfig, ExtractionProviderKind, IngestionPolicy, NextralConfig,
@@ -192,6 +193,45 @@ mod tests {
         );
     }
 
+    #[test]
+    fn ingest_rejects_cross_tenant_id_collision() {
+        let mut store = TestMemoryStore::new();
+        let policy = IngestionPolicy {
+            min_importance_score: 0.0,
+            min_confidence_score: 0.0,
+        };
+
+        let mut tenant_a = IngestMemoryRequest::new(
+            "tenant_a",
+            "usr_1",
+            "Tenant A fact",
+            ContentType::Fact,
+            MemoryType::Semantic,
+            SourceType::Manual,
+            policy.clone(),
+        );
+        tenant_a.id = Some("shared-id".to_string());
+        tenant_a.importance_score = 1.0;
+        assert_eq!(ingest_memory(&mut store, tenant_a).unwrap().status, IngestStatus::Accepted);
+
+        let mut tenant_b = IngestMemoryRequest::new(
+            "tenant_b",
+            "usr_1",
+            "Tenant B overwrite attempt",
+            ContentType::Fact,
+            MemoryType::Semantic,
+            SourceType::Manual,
+            policy,
+        );
+        tenant_b.id = Some("shared-id".to_string());
+        tenant_b.importance_score = 1.0;
+
+        let error = ingest_memory(&mut store, tenant_b).unwrap_err();
+        assert!(matches!(error, CoreError::Conflict(_)));
+        let original = store.get_memory("tenant_a", "usr_1", "shared-id").unwrap();
+        assert_eq!(original.unwrap().content, "Tenant A fact");
+    }
+
     #[test]
     fn all_seven_memory_types_have_runtime_topology() {
         let profiles = all_profiles();
diff --git a/src/testkit/mod.rs b/src/testkit/mod.rs
index 5e2ba40..dcb0cb0 100644
--- a/src/testkit/mod.rs
+++ b/src/testkit/mod.rs
@@ -81,10 +81,22 @@ impl TestMemoryStore {
 impl MemoryIndexStore for TestMemoryStore {
     fn upsert_memory(&mut self, record: MemoryRecord) -> CoreResult<()> {
         record.validate()?;
+        if self.memories.iter().any(|memory| {
+            memory.id == record.id
+                && (memory.tenant_id != record.tenant_id || memory.user_id != record.user_id)
+        }) {
+            return Err(CoreError::Conflict(
+                "memory id is already in use by a different tenant/user scope".to_string(),
+            ));
+        }
         if let Some(existing) = self
             .memories
             .iter_mut()
-            .find(|memory| memory.id == record.id)
+            .find(|memory| {
+                memory.id == record.id
+                    && memory.tenant_id == record.tenant_id
+                    && memory.user_id == record.user_id
+            })
         {
             *existing = record;
             return Ok(());