diff --git a/Makefile b/Makefile
index da78f87..c7fc1ad 100644
--- a/Makefile
+++ b/Makefile
@@ -14,7 +14,7 @@ install: ## Sync project-local dependencies from both lockfiles
 	cd apps/web && bun install --frozen-lockfile
 
 dev: ## Start PostgreSQL and run the native backend and web together
-	$(MAKE) --jobs=2 backend-dev web-dev
+	$(MAKE) --jobs=2 backend-dev web-dev BACKEND_PORT=$(BACKEND_PORT)
 
 db-up: ## Start local PostgreSQL and wait until ready
 	docker compose up -d --wait postgres
diff --git a/tests/conftest.py b/tests/conftest.py
index 2f63992..82c98e7 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,4 +1,5 @@
 import asyncio
+import os
 import time
 from collections.abc import Callable, Iterator
 from typing import Any
@@ -19,6 +20,15 @@ from ai_platform.infrastructure.persistence.models import AIProfile, AppCredenti
 from ai_platform.platform.security import digest
 
 
+@pytest.fixture
+def isolated_settings(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Keep pure configuration tests independent of operator credentials and local URLs."""
+    monkeypatch.setitem(Settings.model_config, "env_file", None)
+    for name in tuple(os.environ):
+        if name.startswith("AI_PLATFORM_"):
+            monkeypatch.delenv(name)
+
+
 @pytest.fixture
 def valid_run_payload():
     return {
diff --git a/tests/test_aihub_settings.py b/tests/test_aihub_settings.py
index 3d27011..f2b7265 100644
--- a/tests/test_aihub_settings.py
+++ b/tests/test_aihub_settings.py
@@ -4,6 +4,7 @@ from pydantic import SecretStr, ValidationError
 from ai_platform.infrastructure.config.settings import Settings
 
 _TEST_DATABASE_URL = "postgresql+asyncpg://test:test@localhost/ai_platform_test"
+pytestmark = pytest.mark.usefixtures("isolated_settings")
 
 
 @pytest.mark.parametrize(
diff --git a/tests/test_dev_commands.py b/tests/test_dev_commands.py
index 75d75f5..0708dab 100644
--- a/tests/test_dev_commands.py
+++ b/tests/test_dev_commands.py
@@ -35,7 +35,9 @@ def test_dev_uses_the_same_backend_port_for_both_servers(
     (tmp_path / ".env").write_text(config)
     # Only the port lookup executes during dry-run; bypass uv project discovery in this fixture.
     uv = tmp_path / "uv"
-    uv.write_text(f'#!/bin/sh\nshift 3\nexec {shlex.quote(sys.executable)} "$@"\n')
+    uv.write_text(
+        f'#!/bin/sh\necho port-lookup >&2\nshift 3\nexec {shlex.quote(sys.executable)} "$@"\n'
+    )
     uv.chmod(0o700)
     environment = os.environ.copy()
     for name in ("BACKEND_PORT", "MAKEFLAGS", "MFLAGS", "MAKEOVERRIDES"):
@@ -62,3 +64,5 @@ def test_dev_uses_the_same_backend_port_for_both_servers(
     assert "alembic" not in result.stdout
     assert secret not in result.stdout + result.stderr
     assert "AI_PLATFORM_OPENAI_API_KEY" not in result.stdout + result.stderr
+    expected_lookups = 0 if environment_port is not None or argument_port is not None else 1
+    assert result.stderr.count("port-lookup") == expected_lookups
diff --git a/tests/test_notifications.py b/tests/test_notifications.py
index 4eb6c41..89c0a8a 100644
--- a/tests/test_notifications.py
+++ b/tests/test_notifications.py
@@ -27,6 +27,7 @@ NOW = datetime(2026, 9, 28, tzinfo=UTC)
 SECRET = "test-only-webhook-secret-32-characters-long"
 
 
+@pytest.mark.usefixtures("isolated_settings")
 def test_webhook_configuration_requires_pair_and_safe_url():
     with pytest.raises(ValidationError):
         AppWebhook.model_validate({"url": "https://omni.example.invalid/events"})