diff --git a/src/paperscout/config.py b/src/paperscout/config.py
index eb45f61..3b825df 100644
--- a/src/paperscout/config.py
+++ b/src/paperscout/config.py
@@ -1,10 +1,25 @@
-"""Environment-backed runtime configuration (see ``settings`` singleton)."""
+"""Environment-backed runtime configuration (see ``settings`` singleton).
+
+The module-level :data:`settings` object is the process-wide singleton loaded at
+import time.  For temporary field overrides in tests or integration scenarios,
+use :func:`override_settings` — it mutates that instance in place so all
+``from paperscout.config import settings`` importers observe the change.
+
+``_PAPERSCOUT_TESTING=1`` is an import-time bypass for Slack credential
+validation during pytest collection; it is not a substitute for per-test field
+overrides (use :func:`override_settings` for those).
+"""
 
 from __future__ import annotations
 
+import asyncio
 import os
+import threading
+from collections.abc import Iterator
+from contextlib import contextmanager
+from dataclasses import dataclass
 from pathlib import Path
-from typing import NamedTuple
+from typing import Any, NamedTuple
 
 from pydantic import Field, model_validator
 from pydantic_settings import BaseSettings, PydanticBaseSettingsSource, SettingsConfigDict
@@ -209,4 +224,78 @@ def _require_slack_credentials_unless_testing(self) -> Settings:
 }
 
 
+@dataclass(frozen=True, slots=True)
+class _OverrideFrame:
+    snapshot: dict[str, Any]
+    owner: tuple[int, int | None]  # (thread_id, asyncio task id or None)
+
+
+_override_stack: list[_OverrideFrame] = []
+
+
+def _execution_context() -> tuple[int, int | None]:
+    """Identify the current thread and asyncio task (if any)."""
+    thread_id = threading.get_ident()
+    try:
+        task = asyncio.current_task()
+    except RuntimeError:
+        task_id = None
+    else:
+        task_id = id(task) if task is not None else None
+    return (thread_id, task_id)
+
+
+def _restore_snapshot(snapshot: dict[str, Any]) -> None:
+    for key, value in snapshot.items():
+        setattr(settings, key, value)
+
+
+def _apply_validated_overrides(snapshot: dict[str, Any], **kwargs: Any) -> None:
+    """Merge *kwargs* into *snapshot*, validate, and apply to ``settings`` in place."""
+    validated = Settings.model_validate({**snapshot, **kwargs})
+    for key in kwargs:
+        setattr(settings, key, getattr(validated, key))
+
+
+@contextmanager
+def override_settings(**kwargs: Any) -> Iterator[Settings]:
+    """Temporarily override fields on the module ``settings`` singleton.
+
+    Supported mechanism for tests and integration scenarios.  Mutates the
+    existing instance in place so ``from paperscout.config import settings``
+    importers observe changes.  Nested calls on the **same thread/task** are
+    supported (LIFO); concurrent use from other threads or asyncio tasks raises
+    :exc:`RuntimeError` on exit rather than restoring out of order.
+
+    Objects that copied setting values at construction time (e.g. queue size
+    captured in ``__init__``) are not updated — only live reads of ``settings``
+    or references to the singleton itself see the override.
+    """
+    unknown = set(kwargs) - set(Settings.model_fields)
+    if unknown:
+        raise TypeError(f"Unknown settings field(s): {sorted(unknown)}")
+
+    frame = _OverrideFrame(snapshot=settings.model_dump(), owner=_execution_context())
+    _override_stack.append(frame)
+    try:
+        _apply_validated_overrides(frame.snapshot, **kwargs)
+        yield settings
+    finally:
+        current = _execution_context()
+        if frame.owner != current:
+            raise RuntimeError(
+                "override_settings: owner context changed during override "
+                f"(entered as {frame.owner!r}, exiting as {current!r})"
+            )
+        if not _override_stack or _override_stack[-1] is not frame:
+            if frame in _override_stack:
+                _override_stack.remove(frame)
+            raise RuntimeError(
+                "override_settings: non-LIFO or concurrent override detected; "
+                "nested overrides must exit in reverse order on the same thread/task"
+            )
+        _override_stack.pop()
+        _restore_snapshot(frame.snapshot)
+
+
 settings = Settings()
diff --git a/tests/conftest.py b/tests/conftest.py
index dca2179..13249fb 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -4,7 +4,10 @@
 
 import os
 
-# Tests import ``paperscout.config`` which instantiates ``Settings()`` at module load; keep Slack placeholders.
+# Import-time only: ``Settings()`` loads at module import and skips Slack validation
+# when ``_PAPERSCOUT_TESTING=1``.  Per-test field overrides use
+# ``paperscout.config.override_settings``; ``make_test_settings()`` is for explicit
+# ``cfg=`` injection into constructors.
 os.environ.setdefault("_PAPERSCOUT_TESTING", "1")
 os.environ.setdefault("SLACK_BOT_TOKEN", "xoxb-test")
 os.environ.setdefault("SLACK_SIGNING_SECRET", "test-secret")
diff --git a/tests/test_config.py b/tests/test_config.py
index 7d5e277..67ceacd 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -3,8 +3,16 @@
 from __future__ import annotations
 
 import pytest
-
-from paperscout.config import ENV_VAR_MAP, Settings, legacy_env_name, prefixed_env_name
+from pydantic import ValidationError
+
+from paperscout.config import (
+    ENV_VAR_MAP,
+    Settings,
+    legacy_env_name,
+    override_settings,
+    prefixed_env_name,
+    settings,
+)
 from paperscout.errors import ConfigurationError
 
 
@@ -91,3 +99,96 @@ def test_legacy_keys_in_dotenv_file_load(tmp_path, monkeypatch):
     env_file.write_text("POLL_INTERVAL_MINUTES=66\n")
     s = Settings(_env_file=env_file)
     assert s.poll_interval_minutes == 66
+
+
+class TestOverrideSettings:
+    def test_override_settings_applies_within_block(self):
+        original = settings.poll_interval_minutes
+        with override_settings(poll_interval_minutes=99):
+            assert settings.poll_interval_minutes == 99
+        assert settings.poll_interval_minutes == original
+
+    def test_override_settings_restores_after_block(self):
+        original = settings.wg21_index_timeout_s
+        with override_settings(wg21_index_timeout_s=42.0):
+            assert settings.wg21_index_timeout_s == 42.0
+        assert settings.wg21_index_timeout_s == original
+
+    def test_override_settings_restores_on_exception(self):
+        original = settings.poll_interval_minutes
+        with pytest.raises(RuntimeError, match="boom"):
+            with override_settings(poll_interval_minutes=77):
+                assert settings.poll_interval_minutes == 77
+                raise RuntimeError("boom")
+        assert settings.poll_interval_minutes == original
+
+    def test_override_settings_nested(self):
+        original = settings.poll_interval_minutes
+        with override_settings(poll_interval_minutes=10):
+            assert settings.poll_interval_minutes == 10
+            with override_settings(poll_interval_minutes=20):
+                assert settings.poll_interval_minutes == 20
+            assert settings.poll_interval_minutes == 10
+        assert settings.poll_interval_minutes == original
+
+    def test_override_settings_rejects_unknown_field(self):
+        with pytest.raises(TypeError, match="Unknown settings field"):
+            with override_settings(not_a_real_field=1):
+                pass
+
+    def test_override_settings_rejects_invalid_value(self):
+        with pytest.raises(ValidationError):
+            with override_settings(wg21_index_timeout_s=-1):
+                pass
+
+    def test_importers_see_override(self):
+        from paperscout.config import settings as imported_settings
+
+        assert imported_settings is settings
+        original = settings.enable_iso_probe
+        with override_settings(enable_iso_probe=not original):
+            assert imported_settings.enable_iso_probe is not original
+        assert imported_settings.enable_iso_probe == original
+
+    def test_override_settings_rejects_concurrent_threads(self):
+        import threading
+
+        import paperscout.config as config_module
+
+        a_inside = threading.Event()
+        b_inside = threading.Event()
+        a_may_exit = threading.Event()
+        errors: list[RuntimeError] = []
+        original = settings.poll_interval_minutes
+
+        def thread_a() -> None:
+            try:
+                with override_settings(poll_interval_minutes=111):
+                    a_inside.set()
+                    b_inside.wait(timeout=5)
+            except RuntimeError as exc:
+                errors.append(exc)
+            finally:
+                a_may_exit.set()
+
+        def thread_b() -> None:
+            with override_settings(poll_interval_minutes=222):
+                b_inside.set()
+                a_may_exit.wait(timeout=5)
+
+        try:
+            t_a = threading.Thread(target=thread_a)
+            t_b = threading.Thread(target=thread_b)
+            t_a.start()
+            assert a_inside.wait(timeout=5)
+            t_b.start()
+            assert b_inside.wait(timeout=5)
+            t_a.join(timeout=5)
+            t_b.join(timeout=5)
+            assert not t_a.is_alive()
+            assert not t_b.is_alive()
+            assert errors, "expected concurrent override exit to raise RuntimeError"
+            assert any("non-LIFO or concurrent" in str(exc) for exc in errors)
+        finally:
+            config_module._override_stack.clear()
+            settings.poll_interval_minutes = original
diff --git a/tests/test_sources.py b/tests/test_sources.py
index 6c1fe09..b8a714e 100644
--- a/tests/test_sources.py
+++ b/tests/test_sources.py
@@ -11,6 +11,7 @@
 import httpx
 import pytest
 
+from paperscout.config import override_settings
 from paperscout.models import CycleStatus, Paper
 from paperscout.sources import (
     ISOProber,
@@ -176,16 +177,16 @@ async def test_download_http_status_500_emits_network(self, fake_pool, caplog):
         assert "failure_category=NETWORK" in caplog.text
 
     async def test_download_uses_wg21_index_timeout_from_settings(self, fake_pool):
-        cfg = make_test_settings(wg21_index_timeout_s=42.0)
-        index = WG21Index(fake_pool, cfg=cfg)
+        index = WG21Index(fake_pool)
         mock_resp = _make_response(200, json_data=SAMPLE_INDEX_DATA)
         mock_client = _make_async_client(get_resp=mock_resp)
-        with patch("paperscout.sources.httpx.AsyncClient") as mock_cls:
-            mock_cls.return_value.__aenter__ = AsyncMock(return_value=mock_client)
-            mock_cls.return_value.__aexit__ = AsyncMock(return_value=False)
-            await index._download()
-        mock_cls.assert_called_once()
-        arg_timeout = mock_cls.call_args.kwargs["timeout"]
+        with override_settings(wg21_index_timeout_s=42.0):
+            with patch("paperscout.sources.httpx.AsyncClient") as mock_cls:
+                mock_cls.return_value.__aenter__ = AsyncMock(return_value=mock_client)
+                mock_cls.return_value.__aexit__ = AsyncMock(return_value=False)
+                await index._download()
+            mock_cls.assert_called_once()
+            arg_timeout = mock_cls.call_args.kwargs["timeout"]
         assert isinstance(arg_timeout, httpx.Timeout)
         assert arg_timeout == httpx.Timeout(42.0)