diff --git a/python/sglang/srt/managers/scheduler.py b/python/sglang/srt/managers/scheduler.py
index a9719e5a4..8c7c63959 100644
--- a/python/sglang/srt/managers/scheduler.py
+++ b/python/sglang/srt/managers/scheduler.py
@@ -2575,9 +2575,16 @@ class Scheduler(
                         ) > 0 or (not self.running_batch.is_empty())
                     else:
                         self.running_batch.batch_is_full = True
-                # revert matched mamba idx to avoid memory leak, if req is not added
+                # revert matched mamba idx to avoid memory leak, if req is not added.
+                # Only free if the slot was freshly allocated in this batch (not
+                # pre-existing from a session). Session-held slots have their own
+                # lifecycle and freeing them here causes double-free.
                 added = len(adder.can_run_list) > 0 and req is adder.can_run_list[-1]
-                if not added and req.mamba_pool_idx is not None:
+                if (
+                    not added
+                    and req.mamba_pool_idx is not None
+                    and not getattr(req, "session", None)
+                ):
                     self.tree_cache.req_to_token_pool.mamba_pool.free(
                         req.mamba_pool_idx.unsqueeze(-1)
                     )