diff --git a/nanobot/cli/commands.py b/nanobot/cli/commands.py
index eae7eefb..0edc2e70 100644
--- a/nanobot/cli/commands.py
+++ b/nanobot/cli/commands.py
@@ -1149,13 +1149,13 @@ def serve(
     console.print("  [cyan]Session[/cyan]  : api:default")
     console.print(f"  [cyan]Timeout[/cyan]  : {timeout}s")
     api_key = api_cfg.api_key.strip() if api_cfg.api_key else ""
+    if not api_key:
+        console.print(
+            "[red]Error: api_key is not set. "
+            "Set api.api_key in config to prevent unauthenticated API access.[/red]"
+        )
+        raise typer.Exit(1)
     if host in {"0.0.0.0", "::"}:
-        if not api_key:
-            console.print(
-                "[red]Error: host is 0.0.0.0 (all interfaces) but api_key is not set. "
-                "Set api.api_key in config to prevent unauthenticated access.[/red]"
-            )
-            raise typer.Exit(1)
         console.print(
             "[yellow]API is bound to all interfaces "
             "(authentication required).[/yellow]"
diff --git a/tests/cli/test_commands.py b/tests/cli/test_commands.py
index 3369f8bb..29c5813d 100644
--- a/tests/cli/test_commands.py
+++ b/tests/cli/test_commands.py
@@ -2806,6 +2806,7 @@ def test_serve_uses_api_config_defaults_and_workspace_override(
     config.api.host = "127.0.0.2"
     config.api.port = 18900
     config.api.timeout = 45.0
+    config.api.api_key = "secret"
     override_workspace = tmp_path / "override-workspace"
     seen: dict[str, object] = {}
 
@@ -2821,7 +2822,7 @@ def test_serve_uses_api_config_defaults_and_workspace_override(
     assert seen["host"] == "127.0.0.2"
     assert seen["port"] == 18900
     assert seen["request_timeout"] == 45.0
-    assert seen["api_key"] == ""
+    assert seen["api_key"] == "secret"
 
 
 def test_trigger_cli_queues_message_in_workspace(
@@ -2862,6 +2863,7 @@ def test_serve_cli_options_override_api_config(monkeypatch, tmp_path: Path) -> N
     config.api.host = "127.0.0.2"
     config.api.port = 18900
     config.api.timeout = 45.0
+    config.api.api_key = "secret"
     seen: dict[str, object] = {}
 
     _patch_serve_runtime(monkeypatch, config, seen)
@@ -2885,7 +2887,21 @@ def test_serve_cli_options_override_api_config(monkeypatch, tmp_path: Path) -> N
     assert seen["host"] == "127.0.0.1"
     assert seen["port"] == 18901
     assert seen["request_timeout"] == 46.0
-    assert seen["api_key"] == ""
+    assert seen["api_key"] == "secret"
+
+
+def test_serve_rejects_loopback_without_api_key(monkeypatch, tmp_path: Path) -> None:
+    config_file = _write_instance_config(tmp_path)
+    config = Config()
+    seen: dict[str, object] = {}
+
+    _patch_serve_runtime(monkeypatch, config, seen)
+
+    result = runner.invoke(app, ["serve", "--config", str(config_file)])
+
+    assert result.exit_code == 1
+    assert "api_key is not set" in result.stdout
+    assert "api_app" not in seen
 
 
 def test_serve_passes_configured_api_key(monkeypatch, tmp_path: Path) -> None: