diff --git a/python/sglang/srt/mem_cache/kv_cache_configurator.py b/python/sglang/srt/mem_cache/kv_cache_configurator.py
index cd6b55ec4..3835f5fc7 100644
--- a/python/sglang/srt/mem_cache/kv_cache_configurator.py
+++ b/python/sglang/srt/mem_cache/kv_cache_configurator.py
@@ -1717,7 +1717,7 @@ class KVCacheConfigurator:
                 max_mamba_cache_size=server_args.max_mamba_cache_size
                 // self.ps.attn_dp_size,
             )
-            # Reserve intermediate memory based on capped max_num_reqs
+            # Reserve intermediate memory based on capped max_num_reqs (+1 padding slot)
             if has_spec_dec:
                 ratio = self._calculate_mamba_ratio()
                 capped_reqs = min(
@@ -1726,7 +1726,7 @@ class KVCacheConfigurator:
                 )
                 intermediate_size = (
                     config.mamba2_cache_params.mamba_cache_per_req
-                    * capped_reqs
+                    * (capped_reqs + 1)
                     * server_args.speculative_num_draft_tokens
                 )
                 total_rest_memory = total_rest_memory - (intermediate_size / (1 << 30))
@@ -1740,11 +1740,11 @@ class KVCacheConfigurator:
                 max_mamba_cache_size=server_args.max_running_requests
                 // self.ps.attn_dp_size,
             )
-            # Reserve intermediate memory based on capped max_num_reqs
+            # Reserve intermediate memory based on capped max_num_reqs (+1 padding slot)
             if has_spec_dec:
                 intermediate_size = (
                     config.mamba2_cache_params.mamba_cache_per_req
-                    * server_args.max_mamba_cache_size
+                    * (server_args.max_mamba_cache_size + 1)
                     * server_args.speculative_num_draft_tokens
                 )
                 total_rest_memory = total_rest_memory - (intermediate_size / (1 << 30))
@@ -1753,11 +1753,9 @@ class KVCacheConfigurator:
             assert config.mamba2_cache_params.mamba_cache_per_req > 0
             per_req = config.mamba2_cache_params.mamba_cache_per_req
 
-            # Solve jointly for max_mamba_cache_size accounting for intermediate memory.
-            # The mamba budget (from the ratio split) must cover both:
-            #   1. main mamba state: max_mamba_cache_size * per_req
-            #   2. intermediate states: (max_mamba_cache_size / ratio) * D * per_req
-            # So: max_mamba_cache_size * per_req * (1 + D/ratio) = mamba_budget_bytes
+            # Solve jointly for max_mamba_cache_size (K), including the pool's
+            # +1 padding slot on both buffers (see memory_pool.py):
+            #   (K + 1) * per_req + (K / ratio + 1) * D * per_req = mamba_budget_bytes
             mamba_budget = (
                 total_rest_memory
                 * server_args.mamba_full_memory_ratio
@@ -1772,7 +1770,8 @@ class KVCacheConfigurator:
                 server_args.override(
                     "mamba_pool.memory_budget_spec",
                     max_mamba_cache_size=int(
-                        mamba_budget_bytes // (per_req * (1 + D / ratio))
+                        (mamba_budget_bytes - per_req * (1 + D))
+                        // (per_req * (1 + D / ratio))
                     ),
                 )
                 # Intermediate memory is included in mamba_budget, subtract it
@@ -1781,12 +1780,12 @@ class KVCacheConfigurator:
                     server_args.max_running_requests // self.ps.attn_dp_size,
                     server_args.max_mamba_cache_size // ratio,
                 )
-                intermediate_size = per_req * capped_reqs * D
+                intermediate_size = per_req * (capped_reqs + 1) * D
                 total_rest_memory = total_rest_memory - (intermediate_size / (1 << 30))
             else:
                 server_args.override(
                     "mamba_pool.memory_budget",
-                    max_mamba_cache_size=int(mamba_budget_bytes // per_req),
+                    max_mamba_cache_size=int((mamba_budget_bytes - per_req) // per_req),
                 )
 
         # Validate: max_mamba_cache_size must be positive after memory allocation.
@@ -1805,8 +1804,9 @@ class KVCacheConfigurator:
                 f"(4) use GPUs with more memory."
             )
 
+        # +1: the pool's padding slot
         mamba_state_memory = (
-            server_args.max_mamba_cache_size
+            (server_args.max_mamba_cache_size + 1)
             * config.mamba2_cache_params.mamba_cache_per_req
             / (1 << 30)
         )