diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 9da40d6..8e5a945 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -15,3 +15,12 @@ repos:
       - id: ruff
         args: [--fix]
       - id: ruff-format
+
+  - repo: local
+    hooks:
+      - id: mypy-protocols
+        name: mypy (protocols + monitor)
+        language: system
+        entry: uv run mypy
+        args: [src/paperscout/protocols.py, src/paperscout/monitor.py, src/paperscout/__main__.py]
+        pass_filenames: false
diff --git a/pyproject.toml b/pyproject.toml
index 1db4818..82c229b 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -34,6 +34,7 @@ packages = ["src/paperscout"]
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
 testpaths = ["tests"]
+norecursedirs = [".git", ".tox", "dist", "build", "*.egg", "typing"]
 markers = [
     "benchmark: ISO probe cycle performance regression (run: pytest benchmarks/ -m benchmark)",
 ]
@@ -93,3 +94,11 @@ module = [
     "fitz.*",
 ]
 ignore_missing_imports = true
+
+[[tool.mypy.overrides]]
+module = [
+    "paperscout.protocols",
+    "paperscout.monitor",
+    "paperscout.__main__",
+]
+strict = true
diff --git a/src/paperscout/__main__.py b/src/paperscout/__main__.py
index dca6078..c0a42a6 100644
--- a/src/paperscout/__main__.py
+++ b/src/paperscout/__main__.py
@@ -8,13 +8,16 @@
 import signal
 import sys
 import threading
+from collections.abc import Callable
 from datetime import datetime, timezone
 from pathlib import Path
+from typing import Any, cast
 
 from .config import settings
 from .db import init_db, init_pool
 from .health import start_health_server
-from .monitor import Scheduler
+from .monitor import PollResult, Scheduler
+from .protocols import DataSource, OpsAlertFn
 from .scout import (
     MessageQueue,
     create_app,
@@ -40,7 +43,7 @@
 )
 
 
-def _mq_health_fields(mq: MessageQueue) -> dict:
+def _mq_health_fields(mq: MessageQueue) -> dict[str, Any]:
     """MQ metrics for /health; from health_fields() when present, else depth only."""
     if hasattr(mq, "health_fields"):
         try:
@@ -72,13 +75,13 @@ def _mq_health_fields(mq: MessageQueue) -> dict:
 
 
 def _merge_extra_health_fields(
-    scheduler_snap: dict,
-    mq_extra: dict,
-    db_pool: dict,
-) -> dict:
+    scheduler_snap: dict[str, Any],
+    mq_extra: dict[str, Any],
+    db_pool: dict[str, Any],
+) -> dict[str, Any]:
     """Merge health JSON with scheduler winning on key conflicts."""
     scheduler_keys = set(scheduler_snap)
-    mq_filtered: dict = {}
+    mq_filtered: dict[str, Any] = {}
     for key, value in mq_extra.items():
         if key in _MQ_HEALTH_FIELD_NAMES:
             if key in scheduler_keys:
@@ -137,17 +140,27 @@ def _register_shutdown_signals(
 ) -> None:
     """Register SIGTERM/SIGINT handlers that set *shutdown_event*."""
 
-    def _on_signal(signame: str) -> None:
-        """Record the first shutdown signal and wake the scheduler."""
-        if shutdown_reason[0] is None:
-            shutdown_reason[0] = signame
-        shutdown_event.set()
+    def _bind_shutdown_handler(signame: str) -> Callable[[], None]:
+        def handler() -> None:
+            if shutdown_reason[0] is None:
+                shutdown_reason[0] = signame
+            shutdown_event.set()
+
+        return handler
+
+    def _bind_sys_signal_handler(signame: str) -> Callable[..., None]:
+        def handler(*_a: object) -> None:
+            if shutdown_reason[0] is None:
+                shutdown_reason[0] = signame
+            shutdown_event.set()
+
+        return handler
 
     for sig, name in ((signal.SIGTERM, "SIGTERM"), (signal.SIGINT, "SIGINT")):
         try:
-            loop.add_signal_handler(sig, lambda n=name: _on_signal(n))
+            loop.add_signal_handler(sig, _bind_shutdown_handler(name))
         except NotImplementedError:
-            signal.signal(sig, lambda *_a, n=name: _on_signal(n))
+            signal.signal(sig, _bind_sys_signal_handler(name))
 
 
 async def _async_main() -> None:
@@ -222,7 +235,7 @@ async def _async_main() -> None:
     user_watchlist = UserWatchlist(pool)
     index = WG21Index(pool, cfg=settings)
     prober = ISOProber(index, state, user_watchlist)
-    sources: list = [index, prober]
+    sources: list[DataSource] = [index, prober]
     if settings.enable_open_std:
         sources.append(OpenStdSource())
     app = create_app()
@@ -232,7 +245,7 @@ async def _async_main() -> None:
     def paper_count_fn() -> int:
         return len(index.papers)
 
-    def _on_poll_result(result):
+    def _on_poll_result(result: PollResult) -> None:
         notify_channel(app, result, mq)
         notify_users(app, result, mq)
 
@@ -243,9 +256,9 @@ def _ops_alert(msg: str) -> None:
                 f":rotating_light: PaperScout alert: {msg}",
             )
 
-    def _pool_status(p) -> dict:
+    def _pool_status(p: Any) -> dict[str, Any]:
         """Best-effort pool stats (psycopg2 ThreadedConnectionPool uses private attrs)."""
-        status: dict = {"max": getattr(p, "maxconn", None)}
+        status: dict[str, Any] = {"max": getattr(p, "maxconn", None)}
         try:
             status["in_use"] = len(p._used)
             status["available"] = len(p._pool)
@@ -260,10 +273,10 @@ def _pool_status(p) -> dict:
         state=state,
         cfg=settings,
         notify_callback=_on_poll_result,
-        ops_alert_fn=_ops_alert,
+        ops_alert_fn=cast(OpsAlertFn, _ops_alert),
     )
 
-    def _extra_health_fields() -> dict:
+    def _extra_health_fields() -> dict[str, Any]:
         return _merge_extra_health_fields(
             scheduler.health_snapshot(),
             _mq_health_fields(mq),
diff --git a/src/paperscout/monitor.py b/src/paperscout/monitor.py
index 4c6ccec..643bd0b 100644
--- a/src/paperscout/monitor.py
+++ b/src/paperscout/monitor.py
@@ -8,7 +8,7 @@
 import logging
 import threading
 import time
-from collections.abc import Callable, Mapping, Sequence
+from collections.abc import Mapping, Sequence
 from dataclasses import dataclass
 from datetime import datetime, timezone
 from types import MappingProxyType
@@ -20,7 +20,14 @@
 from .config import Settings, settings
 from .errors import ConfigurationError, FailureCategory
 from .models import CycleResult, CycleStatus, Paper, PerUserMatches, ProbeHit
-from .protocols import SOURCE_ISO_PROBE, SOURCE_OPEN_STD, SOURCE_WG21_INDEX, DataSource
+from .protocols import (
+    SOURCE_ISO_PROBE,
+    SOURCE_OPEN_STD,
+    SOURCE_WG21_INDEX,
+    DataSource,
+    NotifyCallback,
+    OpsAlertFn,
+)
 from .sources import ISOProber, OpenStdEntry, WG21Index
 from .storage import ProbeState, UserWatchlist
 
@@ -163,15 +170,15 @@ def __init__(
         user_watchlist: UserWatchlist,
         state: ProbeState,
         cfg: Settings | None = None,
-        notify_callback=None,
-        ops_alert_fn: Callable[[str], None] | None = None,
+        notify_callback: NotifyCallback | None = None,
+        ops_alert_fn: OpsAlertFn | None = None,
     ):
         self.sources = list(sources)
         self.user_watchlist = user_watchlist
         self.state = state
         self.cfg = cfg or settings
-        self.notify_callback = notify_callback
-        self.ops_alert_fn = ops_alert_fn
+        self.notify_callback: NotifyCallback | None = notify_callback
+        self.ops_alert_fn: OpsAlertFn | None = ops_alert_fn
         self._snapshots: dict[str, Any] = {}
         self._seeded = False
         self._poll_count = 0
diff --git a/src/paperscout/protocols.py b/src/paperscout/protocols.py
index 601b077..5c4a30c 100644
--- a/src/paperscout/protocols.py
+++ b/src/paperscout/protocols.py
@@ -1,4 +1,4 @@
-"""Structural typing contracts for pluggable data sources.
+"""Structural typing contracts for pluggable data sources and callbacks.
 
 Known ``source_id`` values:
 
@@ -9,7 +9,10 @@
 
 from __future__ import annotations
 
-from typing import Any, Protocol, runtime_checkable
+from typing import TYPE_CHECKING, Any, Protocol, runtime_checkable
+
+if TYPE_CHECKING:
+    from .monitor import PollResult
 
 # Well-known source identifiers (stable across releases).
 SOURCE_WG21_INDEX = "wg21_index"
@@ -31,3 +34,17 @@ async def fetch(self) -> Any:
     def diff(self, previous: Any, current: Any) -> Any:
         """Compare *previous* and *current* snapshots; return source-specific diff."""
         ...
+
+
+@runtime_checkable
+class NotifyCallback(Protocol):
+    """Deliver a completed poll result (channel + per-user notifications)."""
+
+    def __call__(self, result: PollResult) -> None: ...
+
+
+@runtime_checkable
+class OpsAlertFn(Protocol):
+    """Surface operational errors to operators. Caller catches exceptions."""
+
+    def __call__(self, message: str) -> None: ...
diff --git a/tests/test_callback_protocols.py b/tests/test_callback_protocols.py
new file mode 100644
index 0000000..bca724e
--- /dev/null
+++ b/tests/test_callback_protocols.py
@@ -0,0 +1,85 @@
+"""Tests for NotifyCallback and OpsAlertFn protocol conformance."""
+
+from __future__ import annotations
+
+import os
+import subprocess
+import sys
+from pathlib import Path
+from unittest.mock import AsyncMock, MagicMock
+
+from paperscout.models import CycleResult, CycleStatus
+from paperscout.monitor import DiffResult, PollResult, Scheduler
+from paperscout.protocols import SOURCE_ISO_PROBE, SOURCE_WG21_INDEX, NotifyCallback, OpsAlertFn
+from paperscout.sources import ISOProber, WG21Index
+from paperscout.storage import ProbeState, UserWatchlist
+from tests.conftest import make_test_settings
+
+_REPO_ROOT = Path(__file__).resolve().parents[1]
+_INVALID_CALLBACKS = _REPO_ROOT / "tests" / "typing" / "invalid_callbacks.py"
+
+
+def test_notify_callback_protocol_isinstance() -> None:
+    def ok(result: PollResult) -> None:
+        del result
+
+    assert isinstance(ok, NotifyCallback)
+
+
+def test_ops_alert_fn_protocol_isinstance() -> None:
+    def ok(message: str) -> None:
+        del message
+
+    assert isinstance(ok, OpsAlertFn)
+
+
+async def test_scheduler_accepts_protocol_callbacks(fake_pool) -> None:
+    notified: list[PollResult] = []
+    alerts: list[str] = []
+
+    def on_notify(result: PollResult) -> None:
+        notified.append(result)
+
+    def on_ops_alert(message: str) -> None:
+        alerts.append(message)
+
+    wg21 = MagicMock(spec=WG21Index)
+    wg21.source_id = SOURCE_WG21_INDEX
+    wg21.fetch = AsyncMock(return_value={})
+    wg21.diff = MagicMock(return_value=DiffResult([], []))
+
+    iso = MagicMock(spec=ISOProber)
+    iso.source_id = SOURCE_ISO_PROBE
+    iso.fetch = AsyncMock(return_value=CycleResult(CycleStatus.EMPTY))
+    iso.diff = MagicMock(return_value=[])
+    iso.snapshot_stats = MagicMock(return_value={})
+
+    scheduler = Scheduler(
+        sources=[wg21, iso],
+        user_watchlist=MagicMock(spec=UserWatchlist),
+        state=ProbeState(fake_pool),
+        cfg=make_test_settings(),
+        notify_callback=on_notify,
+        ops_alert_fn=on_ops_alert,
+    )
+    assert isinstance(scheduler.notify_callback, NotifyCallback)
+    assert isinstance(scheduler.ops_alert_fn, OpsAlertFn)
+
+    wg21.papers = {}
+    await scheduler.poll_once()
+    assert notified == []
+
+
+def test_mypy_rejects_invalid_callbacks() -> None:
+    env = {**os.environ, "MYPYPATH": str(_REPO_ROOT / "src")}
+    result = subprocess.run(
+        [sys.executable, "-m", "mypy", str(_INVALID_CALLBACKS)],
+        cwd=_REPO_ROOT,
+        capture_output=True,
+        text=True,
+        check=False,
+        env=env,
+    )
+    assert result.returncode != 0
+    combined = result.stdout + result.stderr
+    assert "Incompatible types" in combined
diff --git a/tests/typing/invalid_callbacks.py b/tests/typing/invalid_callbacks.py
new file mode 100644
index 0000000..74326eb
--- /dev/null
+++ b/tests/typing/invalid_callbacks.py
@@ -0,0 +1,21 @@
+"""Deliberate mypy failures for callback protocol conformance.
+
+Not collected by pytest (see pyproject.toml norecursedirs). Verified by
+``test_mypy_rejects_invalid_callbacks`` in tests/test_callback_protocols.py.
+"""
+
+from __future__ import annotations
+
+from paperscout.protocols import NotifyCallback, OpsAlertFn
+
+
+def bad_notify(x: str) -> None:
+    del x
+
+
+def bad_ops_alert(n: int) -> None:
+    del n
+
+
+_reject_notify: NotifyCallback = bad_notify
+_reject_ops: OpsAlertFn = bad_ops_alert