diff --git a/crates/xserv-server/src/main.rs b/crates/xserv-server/src/main.rs
index aee0d53..497ca8d 100644
--- a/crates/xserv-server/src/main.rs
+++ b/crates/xserv-server/src/main.rs
@@ -87,11 +87,14 @@ async fn main() {
     let (tx, rx) = mpsc::channel::<GenerateRequest>();
 
     let model_dir_clone = model_dir.clone();
+    // gpt-oss is only implemented in the TP engine; route it there even at
+    // tp=1 (single-rank world) so quantized models can serve on one GPU.
+    let is_gpt_oss = model_config.model_type.as_deref() == Some("gpt_oss");
     std::thread::spawn(move || {
         if pp > 1 {
             // Pipeline-parallel path: stage-0 coordinator + worker stage threads.
             pp_engine::run_pp(&model_dir_clone, pp, max_seq_len, rx);
-        } else if tp <= 1 {
+        } else if tp <= 1 && !is_gpt_oss {
             let mut engine = engine::Engine::load_with_swap(&model_dir_clone, max_batch, max_seq_len, swap_space_gb);
             engine.run(rx);
         } else {
diff --git a/crates/xserv-server/src/tp_engine.rs b/crates/xserv-server/src/tp_engine.rs
index 36602e6..be6453e 100644
--- a/crates/xserv-server/src/tp_engine.rs
+++ b/crates/xserv-server/src/tp_engine.rs
@@ -120,7 +120,9 @@ fn worker_loop(
 /// Run the TP coordinator (rank 0) on the calling thread. Spawns worker ranks
 /// internally and consumes generation requests from `rx`.
 pub fn run_tp(model_dir: &Path, world: usize, max_seq_len: usize, rx: mpsc::Receiver<GenerateRequest>) {
-    assert!(world >= 2, "run_tp requires world >= 2");
+    // world=1 is a valid single-rank configuration (gpt-oss has no
+    // single-GPU engine path; NCCL init and all_reduce no-op at world=1).
+    assert!(world >= 1, "run_tp requires world >= 1");
     let config = ModelConfig::from_file(&model_dir.join("config.json"));
     assert!(
         config.num_kv_heads() % world == 0,
diff --git a/tools/eval_gsm8k_fast.py b/tools/eval_gsm8k_fast.py
index b37c5dd..268bdda 100644
--- a/tools/eval_gsm8k_fast.py
+++ b/tools/eval_gsm8k_fast.py
@@ -81,7 +81,8 @@ def main():
     parser.add_argument("--max-tokens", type=int, default=512, help="Max generation tokens")
     parser.add_argument("--tp", type=int, default=1, help="Tensor parallelism")
     parser.add_argument("--offset", type=int, default=0, help="Start from problem N")
-    parser.add_argument("--gpu", type=int, default=0, help="GPU device index")
+    parser.add_argument("--gpu", type=str, default="0",
+                        help="CUDA_VISIBLE_DEVICES value, e.g. '0' or '2,3' (must cover --tp ranks)")
     args = parser.parse_args()
 
     if not DATA_PATH.exists():