diff --git a/tests/test_nodes.py b/tests/test_nodes.py
index a475286..858724a 100644
--- a/tests/test_nodes.py
+++ b/tests/test_nodes.py
@@ -198,3 +198,150 @@ def test_get_profile_500_on_malformed_yaml(client, tmp_path):
 
     r = client.get("/api/nodes-mgmt/nodes/bad/profile")
     assert r.status_code == 500
+
+
+# ── POST /api/nodes-mgmt/nodes/{node_id}/gpu/{gpu_id}/services ─────────────────
+
+import os as _os
+
+_BASE_PROFILE = {
+    "services": {
+        "cf-text": {"min_compute_cap": 7.0, "max_mb": 8192, "priority": 1,
+                    "catalog": {"llama3": {"vram_mb": 6144, "path": "/m/llama3",
+                                           "description": "", "multi_gpu": False, "env": {}}}},
+        "ollama":  {"min_compute_cap": 0.0, "max_mb": 2048, "priority": 2, "catalog": {}},
+    },
+    "nodes": {
+        "heimdall": {
+            "gpus": [{"id": 0, "vram_mb": 24576, "compute_cap": 8.6,
+                       "services": [], "role": "primary", "card": "RTX 3090",
+                       "always_on": True}],
+            "agent_url": "http://10.1.10.71:7701",
+        }
+    }
+}
+
+
+def _setup_profile(tmp_path, profile=None):
+    profiles_dir = tmp_path / "profiles"
+    _write_config(tmp_path, {
+        "coordinator_url": "http://fake-coord:7700",
+        "profiles_dir": str(profiles_dir),
+    })
+    _write_profile(profiles_dir, "heimdall", profile or _BASE_PROFILE)
+    return profiles_dir
+
+
+def test_update_services_compatible_writes_and_reloads(client, tmp_path):
+    profiles_dir = _setup_profile(tmp_path)
+
+    mock_reload = MagicMock()
+    mock_reload.status_code = 200
+
+    with patch("httpx.post", return_value=mock_reload):
+        r = client.post(
+            "/api/nodes-mgmt/nodes/heimdall/gpu/0/services",
+            json={"services": ["cf-text"]},
+        )
+
+    assert r.status_code == 200
+    data = r.json()
+    assert data["ok"] is True
+    assert data["reloaded"] is True
+
+    saved = yaml.safe_load((profiles_dir / "heimdall.yaml").read_text())
+    assert saved["nodes"]["heimdall"]["gpus"][0]["services"] == ["cf-text"]
+
+
+def test_update_services_atomic_write_uses_tmp_file(client, tmp_path):
+    """YAML must be written to .tmp then renamed — never written directly."""
+    profiles_dir = _setup_profile(tmp_path)
+    renamed_pairs: list[tuple] = []
+
+    original_replace = _os.replace
+
+    def capture(src, dst):
+        renamed_pairs.append((str(src), str(dst)))
+        original_replace(src, dst)
+
+    with patch("os.replace", side_effect=capture), \
+         patch("httpx.post", return_value=MagicMock(status_code=200)):
+        client.post(
+            "/api/nodes-mgmt/nodes/heimdall/gpu/0/services",
+            json={"services": ["ollama"]},
+        )
+
+    assert any(src.endswith(".tmp") for src, dst in renamed_pairs), \
+        "Expected atomic write via .tmp rename"
+
+
+def test_update_services_incompatible_compute_cap_returns_422(client, tmp_path):
+    low_cap_profile = {
+        **_BASE_PROFILE,
+        "nodes": {
+            "heimdall": {
+                "gpus": [{"id": 0, "vram_mb": 24576, "compute_cap": 6.0,
+                           "services": [], "role": "p", "card": "GTX 1080",
+                           "always_on": False}],
+                "agent_url": "http://10.1.10.71:7701",
+            }
+        }
+    }
+    _setup_profile(tmp_path, low_cap_profile)
+
+    r = client.post(
+        "/api/nodes-mgmt/nodes/heimdall/gpu/0/services",
+        json={"services": ["cf-text"]},
+    )
+    assert r.status_code == 422
+    assert "compute_cap" in r.json()["detail"]
+
+
+def test_update_services_insufficient_vram_returns_422(client, tmp_path):
+    tiny_vram_profile = {
+        **_BASE_PROFILE,
+        "nodes": {
+            "heimdall": {
+                "gpus": [{"id": 0, "vram_mb": 512, "compute_cap": 8.6,
+                           "services": [], "role": "p", "card": "old",
+                           "always_on": False}],
+                "agent_url": "http://10.1.10.71:7701",
+            }
+        }
+    }
+    _setup_profile(tmp_path, tiny_vram_profile)
+
+    r = client.post(
+        "/api/nodes-mgmt/nodes/heimdall/gpu/0/services",
+        json={"services": ["cf-text"]},
+    )
+    assert r.status_code == 422
+    assert "VRAM" in r.json()["detail"]
+
+
+def test_update_services_unknown_service_returns_422(client, tmp_path):
+    _setup_profile(tmp_path)
+    r = client.post(
+        "/api/nodes-mgmt/nodes/heimdall/gpu/0/services",
+        json={"services": ["not-a-real-service"]},
+    )
+    assert r.status_code == 422
+
+
+def test_update_services_reload_failure_returns_reloaded_false(client, tmp_path):
+    """YAML saved but coordinator reload fails — ok: true, reloaded: false."""
+    _setup_profile(tmp_path)
+
+    mock_reload = MagicMock()
+    mock_reload.status_code = 500
+
+    with patch("httpx.post", return_value=mock_reload):
+        r = client.post(
+            "/api/nodes-mgmt/nodes/heimdall/gpu/0/services",
+            json={"services": ["ollama"]},
+        )
+
+    assert r.status_code == 200
+    data = r.json()
+    assert data["ok"] is True
+    assert data["reloaded"] is False