diff --git a/src/domains/auth/__init__.py b/src/domains/auth/__init__.py
index 9b1afa6..f0ec8f0 100644
--- a/src/domains/auth/__init__.py
+++ b/src/domains/auth/__init__.py
@@ -20,6 +20,7 @@ from src.domains.auth.oidc import (
 )
 from src.domains.auth.service import AuthService, get_auth_service
 from src.domains.auth.controller import auth_controller
+from src.domains.auth.callers import Caller, require_caller
 from src.domains.auth.models import (
     User,
     Role,
@@ -50,6 +51,9 @@ __all__ = [
     "get_auth_service",
     # Controller
     "auth_controller",
+    # Callers (T-1)
+    "Caller",
+    "require_caller",
     # Models
     "User",
     "Role",
diff --git a/src/domains/auth/callers.py b/src/domains/auth/callers.py
new file mode 100644
index 0000000..75e5514
--- /dev/null
+++ b/src/domains/auth/callers.py
@@ -0,0 +1,128 @@
+"""
+Caller Authentication
+
+core-api has no service-key auth today: AuthService.validate_api_key is
+never called in any route, api_keys stays empty, and `CORE_API_KEY` is read
+by nothing. This module is the first thing that reads it.
+
+Resolution order, static keys before any JWT parsing:
+1. Bearer == office_service_key -> kind "office"
+2. Bearer == tatlock_service_key -> kind "tatlock"
+3. Bearer == scheduler_service_key -> kind "scheduler"
+4. Otherwise -> get_current_user() -> kind "user"
+
+Each static key is matched with hmac.compare_digest, and an unconfigured
+(empty-string) key never matches — checked explicitly rather than trusting
+compare_digest("", "") to be the right thing to compare, which would make an
+empty configured key match an empty bearer.
+"""
+import hmac
+import re
+from dataclasses import dataclass
+from typing import Callable, Literal, Optional
+
+from fastapi import Depends, HTTPException, Request, Security
+from fastapi.security import HTTPAuthorizationCredentials
+
+from src.domains.auth.oidc import get_current_user, security
+from src.shared.config import get_settings
+from src.shared.logging import get_logger
+
+logger = get_logger(__name__)
+
+CallerKind = Literal["office", "tatlock", "scheduler", "user"]
+
+# Reserved identities: the system nouns, never a caller's own identity.
+RESERVED_IDENTITIES = {"book", "office", "scheduler", "tatlock"}
+
+# ^[a-z0-9][a-z0-9-]{0,62}$, and additionally not a "group-*" shape (checked
+# separately below, since the character class alone can't exclude a prefix).
+_IDENTITY_RE = re.compile(r"^[a-z0-9][a-z0-9-]{0,62}$")
+
+
+@dataclass(frozen=True)
+class Caller:
+    """Who is calling, resolved once per request."""
+
+    kind: CallerKind
+    identity: str          # stamped on every write and event
+    client: Optional[str]  # X-Office-Client, informational
+
+
+def _identity_error(code: str) -> HTTPException:
+    return HTTPException(status_code=400, detail={"code": code})
+
+
+def _validate_identity(identity: str) -> None:
+    """Raise 400 identity_invalid for a malformed or reserved identity."""
+    if not _IDENTITY_RE.match(identity):
+        raise _identity_error("identity_invalid")
+    if identity.startswith("group-") or identity in RESERVED_IDENTITIES:
+        raise _identity_error("identity_invalid")
+
+
+def require_caller(*allowed: CallerKind, identity_required: bool = False) -> Callable:
+    """
+    Build a FastAPI dependency that resolves the caller and enforces it is
+    one of `allowed`.
+
+    Args:
+        allowed: caller kinds permitted on this route.
+        identity_required: for an "office" caller, whether X-Office-Identity
+            must be present (true on writes). Other kinds carry a fixed
+            identity and ignore this.
+    """
+
+    async def dependency(
+        request: Request,
+        credentials: Optional[HTTPAuthorizationCredentials] = Security(security),
+    ) -> Caller:
+        settings = get_settings()
+        token = credentials.credentials if credentials else None
+        client = request.headers.get("X-Office-Client")
+
+        caller: Optional[Caller] = None
+
+        if (
+            token
+            and settings.office_service_key
+            and hmac.compare_digest(token, settings.office_service_key)
+        ):
+            identity_header = request.headers.get("X-Office-Identity")
+            if not identity_header:
+                if identity_required:
+                    raise _identity_error("identity_required")
+                identity = ""
+            else:
+                _validate_identity(identity_header)
+                identity = identity_header
+            caller = Caller(kind="office", identity=identity, client=client)
+
+        elif (
+            token
+            and settings.tatlock_service_key
+            and hmac.compare_digest(token, settings.tatlock_service_key)
+        ):
+            caller = Caller(kind="tatlock", identity="tatlock", client=client)
+
+        elif (
+            token
+            and settings.scheduler_service_key
+            and hmac.compare_digest(token, settings.scheduler_service_key)
+        ):
+            caller = Caller(kind="scheduler", identity="scheduler", client=client)
+
+        else:
+            # No static key matched (or none configured) — fall through to
+            # the existing OIDC/JWT path. A static key never reaches here.
+            user = await get_current_user(credentials)
+            identity = (user or {}).get("preferred_username") or "user"
+            caller = Caller(kind="user", identity=identity, client=client)
+
+        if caller.kind not in allowed:
+            logger.warning(f"Caller kind '{caller.kind}' forbidden on this route (allowed: {allowed})")
+            raise HTTPException(status_code=403, detail={"code": "forbidden"})
+
+        return caller
+
+    return dependency
diff --git a/src/shared/config.py b/src/shared/config.py
index 8b60d06..1af96ae 100644
--- a/src/shared/config.py
+++ b/src/shared/config.py
@@ -104,6 +104,15 @@ class Settings(BaseSettings):
     authentik_username: str = ""
     authentik_password: str = ""
 
+    # Caller auth — static service keys (T-1). Empty by default so an unset
+    # key can never match an empty Bearer token (see src.domains.auth.callers).
+    office_service_key: str = ""
+    tatlock_service_key: str = ""
+    scheduler_service_key: str = ""
+
+    # The Book (T-1) — local time for due/remind/recurrence handling.
+    book_timezone: str = "Europe/Amsterdam"
+
     class Config:
         env_file = ".env"
         case_sensitive = False
diff --git a/tests/test_callers.py b/tests/test_callers.py
new file mode 100644
index 0000000..22c4bf2
--- /dev/null
+++ b/tests/test_callers.py
@@ -0,0 +1,189 @@
+"""
+Tests for require_caller (T-1).
+
+Builds a tiny standalone FastAPI app rather than reusing src.main's real
+routes, so these are pure caller-resolution tests with no database and no
+coupling to a particular domain's routing.
+"""
+import pytest
+from fastapi import Depends, FastAPI
+from fastapi.testclient import TestClient
+from unittest.mock import patch
+
+from src.domains.auth.callers import Caller, require_caller
+from src.domains.auth.oidc import oidc_config
+from src.shared.config import get_settings
+
+settings = get_settings()
+
+
+def _reset_service_keys():
+    settings.office_service_key = ""
+    settings.tatlock_service_key = ""
+    settings.scheduler_service_key = ""
+
+
+@pytest.fixture(autouse=True)
+def clean_caller_state():
+    """Service keys and OIDC state are process-global; isolate every test."""
+    original_keys = (
+        settings.office_service_key,
+        settings.tatlock_service_key,
+        settings.scheduler_service_key,
+    )
+    original_oidc = (oidc_config.enabled, oidc_config.issuers, oidc_config.audiences)
+    _reset_service_keys()
+    yield
+    (
+        settings.office_service_key,
+        settings.tatlock_service_key,
+        settings.scheduler_service_key,
+    ) = original_keys
+    oidc_config.enabled, oidc_config.issuers, oidc_config.audiences = original_oidc
+
+
+# Named with a leading underscore so pytest does not try to collect it as
+# a test module attribute (it previously warned: "cannot collect 'test_app'
+# because it is not a function").
+_caller_app = FastAPI()
+
+
+@_caller_app.get("/only-office")
+async def only_office(caller: Caller = Depends(require_caller("office"))):
+    return {"kind": caller.kind, "identity": caller.identity, "client": caller.client}
+
+
+@_caller_app.get("/office-tatlock-user")
+async def office_tatlock_user(
+    caller: Caller = Depends(require_caller("office", "tatlock", "user")),
+):
+    return {"kind": caller.kind, "identity": caller.identity}
+
+
+@_caller_app.post("/office-user-write")
+async def office_user_write(
+    caller: Caller = Depends(require_caller("office", "user", identity_required=True)),
+):
+    return {"kind": caller.kind, "identity": caller.identity}
+
+
+@pytest.fixture
+def client():
+    return TestClient(_caller_app)
+
+
+class TestNoAuth:
+    def test_no_token_defaults_to_local_user_when_oidc_disabled(self, client):
+        """Matches existing housekeeping/dashboard tests: OIDC off -> local user."""
+        response = client.get("/office-tatlock-user")
+        assert response.status_code == 200
+        assert response.json() == {"kind": "user", "identity": "local"}
+
+    def test_no_token_401_when_oidc_enabled(self, client):
+        oidc_config.configure(enabled=True, issuers=[], audiences=[])
+        response = client.get("/office-tatlock-user")
+        assert response.status_code == 401
+
+
+class TestOfficeKey:
+    def test_office_key_with_identity_resolves(self, client):
+        settings.office_service_key = "office-secret"
+        response = client.get(
+            "/office-tatlock-user",
+            headers={"Authorization": "Bearer office-secret", "X-Office-Identity": "jeroen"},
+        )
+        assert response.status_code == 200
+        assert response.json() == {"kind": "office", "identity": "jeroen"}
+
+    def test_office_key_client_header_is_informational(self, client):
+        settings.office_service_key = "office-secret"
+        response = client.get(
+            "/only-office",
+            headers={
+                "Authorization": "Bearer office-secret",
+                "X-Office-Identity": "jeroen",
+                "X-Office-Client": "claude-desktop",
+            },
+        )
+        assert response.status_code == 200
+        assert response.json()["client"] == "claude-desktop"
+
+    def test_office_key_missing_identity_on_read_defaults_empty(self, client):
+        settings.office_service_key = "office-secret"
+        response = client.get(
+            "/office-tatlock-user", headers={"Authorization": "Bearer office-secret"}
+        )
+        assert response.status_code == 200
+        assert response.json() == {"kind": "office", "identity": ""}
+
+    def test_office_key_missing_identity_on_write_is_400(self, client):
+        settings.office_service_key = "office-secret"
+        response = client.post(
+            "/office-user-write", headers={"Authorization": "Bearer office-secret"}
+        )
+        assert response.status_code == 400
+        assert response.json()["detail"]["code"] == "identity_required"
+
+    @pytest.mark.parametrize("identity", ["Jeroen", "group-home", "tatlock", "office", "book", "scheduler", "-abc", "a_b"])
+    def test_office_key_malformed_or_reserved_identity_is_400(self, client, identity):
+        settings.office_service_key = "office-secret"
+        response = client.post(
+            "/office-user-write",
+            headers={"Authorization": "Bearer office-secret", "X-Office-Identity": identity},
+        )
+        assert response.status_code == 400
+        assert response.json()["detail"]["code"] == "identity_invalid"
+
+    def test_office_key_valid_identity_on_write_succeeds(self, client):
+        settings.office_service_key = "office-secret"
+        response = client.post(
+            "/office-user-write",
+            headers={"Authorization": "Bearer office-secret", "X-Office-Identity": "jeroen"},
+        )
+        assert response.status_code == 200
+        assert response.json() == {"kind": "office", "identity": "jeroen"}
+
+
+class TestTatlockAndSchedulerKeys:
+    def test_tatlock_key_resolves_and_ignores_identity_header(self, client):
+        settings.tatlock_service_key = "tatlock-secret"
+        response = client.get(
+            "/office-tatlock-user",
+            headers={"Authorization": "Bearer tatlock-secret", "X-Office-Identity": "jeroen"},
+        )
+        assert response.status_code == 200
+        assert response.json() == {"kind": "tatlock", "identity": "tatlock"}
+
+    def test_scheduler_key_forbidden_on_a_route_that_excludes_it(self, client):
+        settings.scheduler_service_key = "scheduler-secret"
+        response = client.get(
+            "/office-tatlock-user", headers={"Authorization": "Bearer scheduler-secret"}
+        )
+        assert response.status_code == 403
+        assert response.json()["detail"]["code"] == "forbidden"
+
+
+class TestServiceKeyBypassesJWT:
+    def test_matched_service_key_never_calls_get_current_user(self, client):
+        """Static keys are checked before any JWT parsing (dossier T-1 §1)."""
+        settings.office_service_key = "office-secret"
+        with patch("src.domains.auth.callers.get_current_user") as mock_get_current_user:
+            mock_get_current_user.side_effect = AssertionError(
+                "get_current_user must not run when a service key matched"
+            )
+            response = client.get(
+                "/office-tatlock-user",
+                headers={"Authorization": "Bearer office-secret", "X-Office-Identity": "jeroen"},
+            )
+            assert response.status_code == 200
+            mock_get_current_user.assert_not_called()
+
+    def test_unmatched_token_falls_through_to_get_current_user(self, client):
+        """A wrong-looking token with no matching key must still reach the JWT path."""
+        settings.office_service_key = "office-secret"
+        response = client.get(
+            "/office-tatlock-user", headers={"Authorization": "Bearer not-the-office-key"}
+        )
+        # OIDC disabled -> get_current_user ignores the bad bearer and returns the local user.
+        assert response.status_code == 200
+        assert response.json() == {"kind": "user", "identity": "local"}