diff --git a/src/agentcore_rl_toolkit/backends/slime/runner.py b/src/agentcore_rl_toolkit/backends/slime/runner.py
index 3f7edbf..629a78b 100644
--- a/src/agentcore_rl_toolkit/backends/slime/runner.py
+++ b/src/agentcore_rl_toolkit/backends/slime/runner.py
@@ -87,6 +87,7 @@ class SlimeRunner:
     # Gateway use_sglang parsers (must match the served model) + cumulative mode.
     sglang_tool_call_parser: str = "qwen"
     sglang_reasoning_parser: str | None = None
+    sglang_data_parallel_size: int = 1
     cumulative_token_mode: bool = False
     renderer_family: str = "auto"
     gateway_log_level: str = "warning"
@@ -356,6 +357,8 @@ def _build_slime_flags(
         # Optional SGLang context length cap.
         if self.sglang_context_length is not None:
             flags.extend(["--sglang-context-length", str(self.sglang_context_length)])
+        # SGLang data parallel size (required by slime's sglang_utils validation).
+        flags.extend(["--sglang-data-parallel-size", str(self.sglang_data_parallel_size)])
         # CUDA pinning for the Megatron train actors: slime gives them their own
         # runtime_env that does NOT carry CUDA_HOME/LD_LIBRARY_PATH, so pass the
         # pinned paths through --train-env-vars too (mirrors train.sh). Only when
diff --git a/tests/test_slime_runner.py b/tests/test_slime_runner.py
index a84fc68..513536d 100644
--- a/tests/test_slime_runner.py
+++ b/tests/test_slime_runner.py
@@ -88,3 +88,20 @@ def test_toolkit_config_yaml_includes_acr_pointers():
     assert data["s3_bucket"] == REQUIRED_KWARGS["s3_bucket"]
     assert data["exp_id"] == REQUIRED_KWARGS["exp_id"]
     assert data["model_id"] == "qwen-served"
+
+
+def test_sglang_data_parallel_size_default_in_flags():
+    """--sglang-data-parallel-size is always emitted with default value 1."""
+    runner = SlimeRunner(**REQUIRED_KWARGS)
+    flags = runner._build_slime_flags(num_rollout=1, model_args=[], config_path="/tmp/cfg.yaml")
+
+    assert "--sglang-data-parallel-size" in flags
+    assert flags[flags.index("--sglang-data-parallel-size") + 1] == "1"
+
+
+def test_sglang_data_parallel_size_custom_value():
+    """--sglang-data-parallel-size respects a non-default value."""
+    runner = SlimeRunner(**REQUIRED_KWARGS, sglang_data_parallel_size=4)
+    flags = runner._build_slime_flags(num_rollout=1, model_args=[], config_path="/tmp/cfg.yaml")
+
+    assert flags[flags.index("--sglang-data-parallel-size") + 1] == "4"