diff --git a/rslib/src/scheduler/fsrs/simulator.rs b/rslib/src/scheduler/fsrs/simulator.rs
index 13da583bd56..e83b0938392 100644
--- a/rslib/src/scheduler/fsrs/simulator.rs
+++ b/rslib/src/scheduler/fsrs/simulator.rs
@@ -363,3 +363,127 @@ impl Card {
         }
     }
 }
+
+#[cfg(test)]
+mod test {
+    use anki_proto::scheduler::SimulateFsrsReviewRequest;
+
+    use super::*;
+    use crate::services::NotesService;
+
+    fn base_request() -> SimulateFsrsReviewRequest {
+        SimulateFsrsReviewRequest {
+            params: vec![], // leave empty so fsrs fills defaults in tests
+            desired_retention: 0.85f32,
+            deck_size: 50u32,
+            days_to_simulate: 20u32,
+            new_limit: 5u32,
+            review_limit: 1000u32,
+            max_interval: 365u32,
+            search: "".to_string(),
+            new_cards_ignore_review_limit: false,
+            easy_days_percentages: vec![0.0f32, 0.0f32, 0.0f32, 0.0f32, 0.0f32, 0.0f32, 0.0f32],
+            review_order: ReviewCardOrder::Random as i32,
+            suspend_after_lapse_count: Some(3u32),
+            historical_retention: 0.9f32,
+            learning_step_count: 3u32,
+            relearning_step_count: 2u32,
+        }
+    }
+
+    #[test]
+    fn simulate_workload_varies() {
+        let mut col = Collection::new();
+
+        let resp = col.simulate_workload(base_request()).unwrap();
+
+        // Regression guard for #5101: the workload simulation must apply the per-card
+        // desired retention, so cost has to grow with the target retention. A plain
+        // "the values differ" check is not enough here: even with the bug present the
+        // costs vary due to simulation noise, so we assert the actual trend instead by
+        // comparing the low- and high-retention buckets.
+        let cost = |dr: u32| resp.cost[&dr];
+        let low_avg = (70..=79).map(cost).sum::<f32>() / 10.0;
+        let high_avg = (90..=99).map(cost).sum::<f32>() / 10.0;
+
+        assert!(
+            high_avg > low_avg * 1.2,
+            "expected workload cost to grow with desired retention, instead got \
+             low_avg={low_avg}, high_avg={high_avg}, cost={:?}",
+            resp.cost
+        );
+    }
+
+    #[test]
+    fn request_config_uses_requested_retention() -> Result<()> {
+        // Regression guard for the other half of #5101 ("for normal simulator too"):
+        // simulate_request_to_config must apply the *requested* desired retention to
+        // every card, ignoring each card's individually stored value. This helper feeds
+        // the normal simulator, the workload simulator and optimal-retention
+        // computation, so pinning it here covers all three callers without simulation
+        // noise.
+        let mut col = Collection::new();
+        let nt = col.get_notetype_by_name("Basic")?.unwrap();
+        let mut note = nt.new_note();
+        col.add_note(&mut note, DeckId(1))?;
+        let mut card = col
+            .storage
+            .all_cards_of_note(note.id)?
+            .into_iter()
+            .next()
+            .unwrap();
+        card.ctype = CardType::Review;
+        card.queue = CardQueue::Review;
+        card.interval = 100;
+        card.memory_state = Some(FsrsMemoryState {
+            stability: 100.0,
+            difficulty: 5.0,
+        });
+        // Deliberately differs from the requested retention below.
+        card.desired_retention = Some(0.70);
+        card.decay = Some(0.2);
+        col.storage.update_card(&card)?;
+
+        let req = SimulateFsrsReviewRequest {
+            desired_retention: 0.95f32,
+            // Isolate the existing review card: no synthetic new cards.
+            deck_size: 0u32,
+            new_limit: 0u32,
+            ..base_request()
+        };
+
+        let (_config, cards) = col.simulate_request_to_config(&req)?;
+
+        assert!(
+            !cards.is_empty(),
+            "expected the review card to be converted"
+        );
+        assert!(
+            cards
+                .iter()
+                .all(|c| (c.desired_retention - 0.95).abs() < 1e-6),
+            "converted cards must use the requested retention (0.95), instead got {:?}",
+            cards.iter().map(|c| c.desired_retention).collect_vec()
+        );
+
+        Ok(())
+    }
+
+    #[test]
+    fn simulate_uses_dr() {
+        let mut col = Collection::new();
+        col.set_config_bool(BoolKey::Fsrs, true, false).unwrap();
+
+        let note = crate::tests::NoteAdder::basic(&mut col).add(&mut col);
+        let cids = col.cards_of_note(note.id.into()).unwrap().cids;
+        let cids = cids.into_iter().map(CardId).collect_vec();
+        col.grade_now(&cids, 3).unwrap();
+
+        let mut req = base_request();
+        req.deck_size = 0;
+
+        let (_, cards) = col.simulate_request_to_config(&req).unwrap();
+        assert_eq!(cards.len(), 1);
+        assert_eq!(cards[0].desired_retention, 0.85f32)
+    }
+}