[Bugfix][Rust Frontend] Fix startup with config-only model (#58643)

Signed-off-by: louie-tsai <louie.tsai@intel.com>

tests/entrypoints/launchers/api_server/test_api_server_process_manager.py

@@ -489,3 +489,49 @@ def test_rust_frontend_launch_log_redacts_credentials(monkeypatch, caplog):
     assert hf_token not in message
     assert api_key not in message
     assert '"hf_token": "***"' in message
+
+
+def test_rust_frontend_uses_config_model_as_model_tag(monkeypatch, caplog, tmp_path):
+    """Config-only model selection must be forwarded to the Rust frontend."""
+    import subprocess as subprocess_mod
+
+    from vllm.entrypoints.launchers.cli_args import make_arg_parser
+    from vllm.utils.argparse_utils import FlexibleArgumentParser
+    from vllm.v1.utils import RustFrontendProcessManager
+
+    config_path = tmp_path / "config.yml"
+    config_path.write_text("model: org/model\n")
+    args = make_arg_parser(FlexibleArgumentParser()).parse_args(
+        ["--config", str(config_path)]
+    )
+    assert args.model == "org/model"
+    assert args.model_tag is None
+
+    class _FakeProc:
+        pid = 4321
+        returncode = 0
+
+        def poll(self):
+            return 0
+
+        def wait(self, timeout=None):
+            return 0
+
+    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    try:
+        monkeypatch.setattr(subprocess_mod, "Popen", lambda *a, **kw: _FakeProc())
+        with caplog.at_level("INFO", logger="vllm.v1.utils"):
+            RustFrontendProcessManager(
+                binary_path="/nonexistent/vllm-rs",
+                sock=sock,
+                args=args,
+                input_address="ipc:///tmp/in",
+                output_address="ipc:///tmp/out",
+                engine_start_index=0,
+                engine_count=1,
+                data_parallel_size=1,
+            )
+    finally:
+        sock.close()
+
+    assert '"model_tag": "org/model"' in caplog.text

vllm/v1/utils.py

@@ -391,6 +391,16 @@ class RustFrontendProcessManager:
                 "data_parallel_hybrid_lb",
             },
         )
+
+        # `model_tag` is the positional `vllm serve` model argument. When the
+        # model is supplied only through `--config`, argparse populates `model`
+        # while leaving `model_tag` unset. The Rust frontend requires
+        # `model_tag` in its JSON bootstrap payload, so use the resolved model
+        # as a fallback.
+        model_tag = getattr(args, "model_tag", None) or getattr(args, "model", None)
+        if model_tag is not None:
+            args_dict["model_tag"] = model_tag
+
         # The Rust `frontend` subcommand parses --args-json via serde_json,
         # which bypasses clap and therefore ignores any `#[arg(env = ...)]`
         # declarations on SharedRuntimeArgs fields. Forward the env-driven