fix: retain state checkpoints for block-aligned prompts

crates/core/src/engine/scheduler/vllm/core.rs

@@ -2598,7 +2598,8 @@ impl VllmCore {
                 end = aligned;
             }
         }
-        let last_full = known / alignment * alignment;
+        // Match the state cache's replay boundary, including prompts that end on a full block.
+        let last_full = prompt.saturating_sub(1) / alignment * alignment;
         let tail = prompt / unit * unit;
         let next = start.checked_add(alignment - start % alignment);
         for stop in [
@@ -3723,6 +3724,31 @@ mod state_cache_tests {
         core.try_execute_hidden_pass(0.0).unwrap()
     }
 
+    #[test]
+    fn aligned_prompt_retains_a_reusable_state_after_long_decode() {
+        let mut config = args(64, true, None);
+        config.prefix_match_unit = Some(4);
+        let mut core = VllmCore::new(config);
+        submit(&mut core, 93_030, 16, 16);
+        for _ in 0..32 {
+            if core.state.requests.is_empty() {
+                break;
+            }
+            step(&mut core);
+        }
+        assert!(core.state.requests.is_empty());
+        let second = submit(&mut core, 93_031, 16, 1);
+        let pass = step(&mut core);
+        assert_eq!(
+            pass.admissions
+                .iter()
+                .find(|admission| admission.uuid == second)
+                .unwrap()
+                .reused_input_tokens,
+            12
+        );
+    }
+
     #[test]
     fn kda_prefill_executes_aligned_chunks_tail_and_remaining_tokens() {
         let mut config = args(512, true, None);