diff --git a/tests/conftest.py b/tests/conftest.py
index 649f4f0..af2f5a1 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -24,6 +24,32 @@ from transcription.services.documents import DocumentService
 from transcription.services.jobs import JobService
 
 
+@pytest.fixture(autouse=True, scope="session")
+def isolate_settings_from_local_env_files(tmp_path_factory):
+    """Point `Settings` at a controlled stub env file instead of a developer one.
+
+    `Settings.model_config` declares `env_file=".env.production"`, resolved against the
+    current working directory, so a repository-root pytest run would otherwise read real
+    deployment values into tests that assert declared defaults.
+
+    The stub mirrors what `.github/workflows/quality-gate.yml` writes in CI: only
+    `OPENROUTER_API_KEY`, which is required and which many tests need `get_settings()` to
+    find. Everything else falls back to declared defaults, so local and CI runs agree. This
+    stays a file rather than a process environment variable because
+    `test_config.py::test_requires_api_key` asserts the missing-key failure via
+    `_env_file=None`. Guarded by `tests/test_config_isolation.py`.
+    """
+    stub = tmp_path_factory.mktemp("settings-env") / ".env.test"
+    stub.write_text("OPENROUTER_API_KEY=test-placeholder-not-a-real-key\n", encoding="utf-8")
+
+    original = Settings.model_config.get("env_file")
+    Settings.model_config["env_file"] = str(stub)
+    try:
+        yield
+    finally:
+        Settings.model_config["env_file"] = original
+
+
 @pytest.fixture
 def session():
     """Provide a clean synchronous database session for sync tests."""
diff --git a/tests/test_config_isolation.py b/tests/test_config_isolation.py
new file mode 100644
index 0000000..6886e81
--- /dev/null
+++ b/tests/test_config_isolation.py
@@ -0,0 +1,39 @@
+"""Suite-wide isolation of `Settings` from developer environment files.
+
+`Settings.model_config` declares `env_file=".env.production"`, resolved relative to the
+current working directory. Running pytest from the repository root therefore loads a real
+developer env file into tests that construct `Settings(...)` directly, and the declared
+field defaults stop being what the suite actually exercises.
+
+That breaks verification in both directions: tests asserting default behavior fail locally
+for environmental reasons, and tests asserting configured behavior can pass locally against
+values that do not exist in CI. The autouse fixture in `conftest.py` neutralizes the class
+level `env_file` so defaults are authoritative; tests that need file loading still pass
+`_env_file=` explicitly, which takes precedence over the class config.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from transcription.config import Settings
+
+
+def test_settings_defaults_are_not_overridden_by_a_local_env_file():
+    """A bare `Settings(...)` must observe declared defaults, not developer machine state."""
+    settings = Settings(openrouter_api_key="test-key")
+
+    assert settings.environment == "development"
+    assert settings.log_dir == Path("./data/logs")
+    assert settings.host == "0.0.0.0"
+    assert settings.port == 8000
+
+
+def test_explicit_env_file_still_loads(tmp_path):
+    """Isolation must not disable env-file loading for tests that opt into it."""
+    env_path = tmp_path / ".env"
+    env_path.write_text("PORT=9123\n", encoding="utf-8")
+
+    settings = Settings(openrouter_api_key="test-key", _env_file=env_path, _cli_parse_args=False)
+
+    assert settings.port == 9123