diff --git a/core/connectors/sinks/mongodb_sink/README.md b/core/connectors/sinks/mongodb_sink/README.md
index 550612cc3..5e0dd4cfa 100644
--- a/core/connectors/sinks/mongodb_sink/README.md
+++ b/core/connectors/sinks/mongodb_sink/README.md
@@ -137,10 +137,11 @@ This connector provides **at-least-once** delivery semantics.
 ### Behavior
 
 - Messages may be delivered more than once on retry or restart
-- Uses Iggy message ID as MongoDB `_id` for document identity
-- **Insert-only mode**: duplicate key error is a hard failure (not upsert)
+- Uses a deterministic composite MongoDB `_id`: `stream:topic:partition:message_id`
+- Duplicate key collisions are treated as idempotent replay of already-written messages
+- The sink remains insert-only; it does not upsert existing documents
 
 ### Known Limitations
 
-- On network timeout during insert, retry may cause duplicate key error
-- Sink does not upsert on duplicate (future improvement)
+- On network timeout during insert, MongoDB may partially commit a batch before returning an error
+- The sink does not upsert on duplicate; replay safety relies on deterministic `_id` values and duplicate-key tolerance
diff --git a/core/integration/tests/connectors/mongodb/mongodb_sink.rs b/core/integration/tests/connectors/mongodb/mongodb_sink.rs
index 37c522e34..474b809ed 100644
--- a/core/integration/tests/connectors/mongodb/mongodb_sink.rs
+++ b/core/integration/tests/connectors/mongodb/mongodb_sink.rs
@@ -261,7 +261,7 @@ async fn large_batch_processed_correctly(harness: &TestHarness, fixture: MongoDb
     server(connectors_runtime(config_path = "tests/connectors/mongodb/sink.toml")),
     seed = seeds::connector_stream
 )]
-async fn duplicate_key_is_explicit_failure_and_not_silent_success(
+async fn duplicate_key_is_idempotent_replay_not_sink_error(
     harness: &TestHarness,
     fixture: MongoDbSinkFixture,
 ) {