diff --git a/python/sglang/srt/server_args.py b/python/sglang/srt/server_args.py
index 5fdf50ab3..a93b26803 100644
--- a/python/sglang/srt/server_args.py
+++ b/python/sglang/srt/server_args.py
@@ -1576,6 +1576,14 @@ class ServerArgs:
                     self.piecewise_cuda_graph_max_tokens, 4096
                 )
 
+        # Clamp to context_length if explicitly set — prevents PCG warmup
+        # from compiling graphs with more tokens than the model buffers
+        # can hold, which causes illegal memory access (#21112)
+        if self.context_length is not None:
+            self.piecewise_cuda_graph_max_tokens = min(
+                self.piecewise_cuda_graph_max_tokens, self.context_length
+            )
+
         if self.piecewise_cuda_graph_tokens is None:
             self.piecewise_cuda_graph_tokens = (
                 self._generate_piecewise_cuda_graph_tokens()