fix(server): clamp piecewise_cuda_graph_max_tokens to context_length (#22516)

Co-authored-by: XueLiang Yang <102161631+xueliangyang-oeuler@users.noreply.github.com>

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()