diff --git a/src/llama-memory-hybrid-idx.cpp b/src/llama-memory-hybrid-idx.cpp
index 38e3e61c57..5cef7a032e 100644
--- a/src/llama-memory-hybrid-idx.cpp
+++ b/src/llama-memory-hybrid-idx.cpp
@@ -557,7 +557,14 @@ llama_memory_hybrid_idx_context::llama_memory_hybrid_idx_context(llama_memory_st
 
 llama_memory_hybrid_idx_context::llama_memory_hybrid_idx_context(llama_memory_hybrid_idx * mem) :
     llama_memory_hybrid_context(mem),
-    mem(mem) {}
+    mem(mem),
+    // graph reservation walks a full context, and qwen4exp builds the sparse attention only when
+    // this is set. without it the reserved worst case is the smaller dense graph, so ggml-alloc
+    // must grow the compute buffer on the first decode
+    ns_ubatch(mem->get_mem_idx() == nullptr ?
+        std::vector<uint32_t>() : std::vector<uint32_t>{ mem->get_mem_idx()->get_n_stream() }),
+    ctx_idx(mem->get_mem_idx() == nullptr ? nullptr :
+        new llama_kv_cache_context(mem->get_mem_idx())) {}
 
 llama_memory_hybrid_idx_context::llama_memory_hybrid_idx_context(
         llama_memory_hybrid_idx * mem,
diff --git a/src/llama-memory-hybrid-idx.h b/src/llama-memory-hybrid-idx.h
index 8c867b56eb..c16fe4eeb0 100644
--- a/src/llama-memory-hybrid-idx.h
+++ b/src/llama-memory-hybrid-idx.h
@@ -157,7 +157,7 @@ public:
     // llama_memory_hybrid_idx_context specific API
     //
 
-    // nullptr with no indexer, and for the full and update contexts, which build no sparse graph
+    // nullptr with no indexer, and for the update context, which builds no sparse graph
     const llama_kv_cache_context * get_idx() const;
 
     // streams in the current slot info, the `ns` of get_k/get_v; 1 if unified
@@ -182,7 +182,7 @@ private:
     // declared first, so it is initialised while sinfos_idx is still intact
     const std::vector<uint32_t> ns_ubatch;
 
-    // null unless the model has an indexer and this is a batch context
+    // null unless the model has an indexer and this is a batch or full context
     const llama_memory_context_ptr ctx_idx;
 
     // mirrors the base class's ubatch cursor, which is private there