diff --git a/components/l4d2-host-lib/src/l4d2host/spec.py b/components/l4d2-host-lib/src/l4d2host/spec.py
new file mode 100644
index 0000000..5f16ccc
--- /dev/null
+++ b/components/l4d2-host-lib/src/l4d2host/spec.py
@@ -0,0 +1,22 @@
+from dataclasses import dataclass, field
+from pathlib import Path
+
+import yaml
+
+
+@dataclass(slots=True)
+class InstanceSpec:
+    port: int
+    overlays: list[str] = field(default_factory=list)
+    arguments: list[str] = field(default_factory=list)
+    config: list[str] = field(default_factory=list)
+
+
+def load_spec(path: Path) -> InstanceSpec:
+    raw = yaml.safe_load(path.read_text()) or {}
+    return InstanceSpec(
+        port=int(raw["port"]),
+        overlays=[str(item) for item in raw.get("overlays", [])],
+        arguments=[str(item) for item in raw.get("arguments", [])],
+        config=[str(item) for item in raw.get("config", [])],
+    )
diff --git a/components/l4d2-host-lib/tests/test_spec.py b/components/l4d2-host-lib/tests/test_spec.py
new file mode 100644
index 0000000..44dd13f
--- /dev/null
+++ b/components/l4d2-host-lib/tests/test_spec.py
@@ -0,0 +1,36 @@
+from pathlib import Path
+
+import pytest
+
+from l4d2host.spec import load_spec
+
+
+def test_minimal_spec_parses(tmp_path: Path) -> None:
+    path = tmp_path / "server.yaml"
+    path.write_text("port: 27015\noverlays: [standard]\n")
+    spec = load_spec(path)
+    assert spec.port == 27015
+    assert spec.overlays == ["standard"]
+
+
+def test_defaults_are_empty_lists(tmp_path: Path) -> None:
+    path = tmp_path / "server.yaml"
+    path.write_text("port: 27015\n")
+    spec = load_spec(path)
+    assert spec.overlays == []
+    assert spec.arguments == []
+    assert spec.config == []
+
+
+def test_missing_port_fails(tmp_path: Path) -> None:
+    path = tmp_path / "server.yaml"
+    path.write_text("overlays: [standard]\n")
+    with pytest.raises((KeyError, ValueError, TypeError)):
+        load_spec(path)
+
+
+def test_unknown_keys_ignored(tmp_path: Path) -> None:
+    path = tmp_path / "server.yaml"
+    path.write_text("port: 27015\nfoo: bar\n")
+    spec = load_spec(path)
+    assert spec.port == 27015