diff --git a/src/database/datasets.py b/src/database/datasets.py
index 664e7bdb..5f9c0e5c 100644
--- a/src/database/datasets.py
+++ b/src/database/datasets.py
@@ -2,7 +2,7 @@
 
 import datetime
 from collections import defaultdict
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Literal
 
 from sqlalchemy import text
 from sqlalchemy.exc import IntegrityError
@@ -13,6 +13,7 @@
     DuplicatePrimaryKeyError,
     ForeignKeyConstraintError,
 )
+from routers.types import Identifier, TagString
 from schemas.datasets.openml import DatasetStatus, Feature
 
 if TYPE_CHECKING:
@@ -20,7 +21,7 @@
     from sqlalchemy.ext.asyncio import AsyncConnection
 
 
-async def get(id_: int, connection: AsyncConnection) -> Row | None:
+async def get(id_: Identifier, connection: AsyncConnection) -> Row | None:
     row = await connection.execute(
         text(
             """
@@ -34,7 +35,7 @@ async def get(id_: int, connection: AsyncConnection) -> Row | None:
     return row.one_or_none()
 
 
-async def get_file(*, file_id: int, connection: AsyncConnection) -> Row | None:
+async def get_file(*, file_id: Identifier, connection: AsyncConnection) -> Row | None:
     row = await connection.execute(
         text(
             """
@@ -48,7 +49,11 @@ async def get_file(*, file_id: int, connection: AsyncConnection) -> Row | None:
     return row.one_or_none()
 
 
-async def get_tag(dataset_id: int, tag: str, connection: AsyncConnection) -> Row | None:
+async def get_tag(
+    dataset_id: Identifier,
+    tag: TagString,
+    connection: AsyncConnection,
+) -> Row | None:
     return (
         await connection.execute(
             text(
@@ -63,7 +68,7 @@ async def get_tag(dataset_id: int, tag: str, connection: AsyncConnection) -> Row
     ).first()
 
 
-async def delete_tag(dataset_id: int, tag: str, connection: AsyncConnection) -> None:
+async def delete_tag(dataset_id: Identifier, tag: TagString, connection: AsyncConnection) -> None:
     await connection.execute(
         text(
             """
@@ -75,7 +80,7 @@ async def delete_tag(dataset_id: int, tag: str, connection: AsyncConnection) ->
     )
 
 
-async def get_tags_for(id_: int, connection: AsyncConnection) -> list[str]:
+async def get_tags_for(id_: Identifier, connection: AsyncConnection) -> list[str]:
     row = await connection.execute(
         text(
             """
@@ -115,7 +120,7 @@ async def tag(id_: int, tag_: str, *, user_id: int, connection: AsyncConnection)
 
 
 async def get_description(
-    id_: int,
+    id_: Identifier,
     connection: AsyncConnection,
 ) -> Row | None:
     """Get the most recent description for the dataset."""
@@ -133,7 +138,7 @@ async def get_description(
     return row.first()
 
 
-async def get_status(id_: int, connection: AsyncConnection) -> DatasetStatus:
+async def get_status(id_: Identifier, connection: AsyncConnection) -> DatasetStatus:
     """Get most recent status for the dataset."""
     row = (
         await connection.execute(
@@ -152,7 +157,10 @@ async def get_status(id_: int, connection: AsyncConnection) -> DatasetStatus:
     return DatasetStatus(row.status) if row else DatasetStatus.IN_PREPARATION
 
 
-async def get_latest_processing_update(dataset_id: int, connection: AsyncConnection) -> Row | None:
+async def get_latest_processing_update(
+    dataset_id: Identifier,
+    connection: AsyncConnection,
+) -> Row | None:
     row = await connection.execute(
         text(
             """
@@ -167,7 +175,7 @@ async def get_latest_processing_update(dataset_id: int, connection: AsyncConnect
     return row.first()
 
 
-async def get_features(dataset_id: int, connection: AsyncConnection) -> list[Feature]:
+async def get_features(dataset_id: Identifier, connection: AsyncConnection) -> list[Feature]:
     row = await connection.execute(
         text(
             """
@@ -184,7 +192,7 @@ async def get_features(dataset_id: int, connection: AsyncConnection) -> list[Fea
 
 
 async def get_feature_ontologies(
-    dataset_id: int,
+    dataset_id: Identifier,
     connection: AsyncConnection,
 ) -> dict[int, list[str]]:
     rows = await connection.execute(
@@ -204,7 +212,7 @@ async def get_feature_ontologies(
 
 
 async def get_feature_values(
-    dataset_id: int,
+    dataset_id: Identifier,
     *,
     feature_index: int,
     connection: AsyncConnection,
@@ -224,10 +232,10 @@ async def get_feature_values(
 
 
 async def update_status(
-    dataset_id: int,
-    status: str,
+    dataset_id: Identifier,
+    status: Literal[DatasetStatus.ACTIVE, DatasetStatus.DEACTIVATED],
     *,
-    user_id: int,
+    user_id: Identifier,
     connection: AsyncConnection,
 ) -> None:
     await connection.execute(
@@ -246,7 +254,7 @@ async def update_status(
     )
 
 
-async def remove_deactivated_status(dataset_id: int, connection: AsyncConnection) -> None:
+async def remove_deactivated_status(dataset_id: Identifier, connection: AsyncConnection) -> None:
     await connection.execute(
         text(
             """
diff --git a/src/database/flows.py b/src/database/flows.py
index ed022c40..504e7821 100644
--- a/src/database/flows.py
+++ b/src/database/flows.py
@@ -3,11 +3,13 @@
 
 from sqlalchemy import Row, text
 
+from routers.types import Identifier
+
 if TYPE_CHECKING:
     from sqlalchemy.ext.asyncio import AsyncConnection
 
 
-async def get_subflows(for_flow: int, expdb: AsyncConnection) -> Sequence[Row]:
+async def get_subflows(for_flow: Identifier, expdb: AsyncConnection) -> Sequence[Row]:
     rows = await expdb.execute(
         text(
             """
@@ -24,7 +26,7 @@ async def get_subflows(for_flow: int, expdb: AsyncConnection) -> Sequence[Row]:
     )
 
 
-async def get_tags(flow_id: int, expdb: AsyncConnection) -> list[str]:
+async def get_tags(flow_id: Identifier, expdb: AsyncConnection) -> list[str]:
     rows = await expdb.execute(
         text(
             """
@@ -39,7 +41,7 @@ async def get_tags(flow_id: int, expdb: AsyncConnection) -> list[str]:
     return [tag.tag for tag in tag_rows]
 
 
-async def get_parameters(flow_id: int, expdb: AsyncConnection) -> Sequence[Row]:
+async def get_parameters(flow_id: Identifier, expdb: AsyncConnection) -> Sequence[Row]:
     rows = await expdb.execute(
         text(
             """
@@ -71,7 +73,7 @@ async def get_by_name(name: str, external_version: str, expdb: AsyncConnection)
     return row.one_or_none()
 
 
-async def get(id_: int, expdb: AsyncConnection) -> Row | None:
+async def get(id_: Identifier, expdb: AsyncConnection) -> Row | None:
     row = await expdb.execute(
         text(
             """
diff --git a/src/database/qualities.py b/src/database/qualities.py
index 9180b137..4899f028 100644
--- a/src/database/qualities.py
+++ b/src/database/qualities.py
@@ -4,13 +4,14 @@
 
 from sqlalchemy import text
 
+from routers.types import Identifier
 from schemas.datasets.openml import Quality
 
 if TYPE_CHECKING:
     from sqlalchemy.ext.asyncio import AsyncConnection
 
 
-async def get_for_dataset(dataset_id: int, connection: AsyncConnection) -> list[Quality]:
+async def get_for_dataset(dataset_id: Identifier, connection: AsyncConnection) -> list[Quality]:
     row = await connection.execute(
         text(
             """
@@ -26,7 +27,7 @@ async def get_for_dataset(dataset_id: int, connection: AsyncConnection) -> list[
 
 
 async def get_for_datasets(
-    dataset_ids: Iterable[int],
+    dataset_ids: Iterable[Identifier],
     quality_names: Iterable[str],
     connection: AsyncConnection,
 ) -> dict[int, list[Quality]]:
diff --git a/src/database/runs.py b/src/database/runs.py
index 6eef0b3b..22b0678d 100644
--- a/src/database/runs.py
+++ b/src/database/runs.py
@@ -5,11 +5,13 @@
 
 from sqlalchemy import Row, text
 
+from routers.types import Identifier
+
 if TYPE_CHECKING:
     from sqlalchemy.ext.asyncio import AsyncConnection
 
 
-async def exist(id_: int, expdb: AsyncConnection) -> bool:
+async def exist(id_: Identifier, expdb: AsyncConnection) -> bool:
     """Check if a run exists by ID."""
     row = await expdb.execute(
         text(
@@ -24,7 +26,7 @@ async def exist(id_: int, expdb: AsyncConnection) -> bool:
     return bool(row.one_or_none())
 
 
-async def get_trace(run_id: int, expdb: AsyncConnection) -> Sequence[Row]:
+async def get_trace(run_id: Identifier, expdb: AsyncConnection) -> Sequence[Row]:
     """Get trace rows for a run from the trace table."""
     rows = await expdb.execute(
         text(
diff --git a/src/database/setups.py b/src/database/setups.py
index c8651d93..1c959b34 100644
--- a/src/database/setups.py
+++ b/src/database/setups.py
@@ -4,12 +4,14 @@
 
 from sqlalchemy import text
 
+from routers.types import Identifier, TagString
+
 if TYPE_CHECKING:
     from sqlalchemy.engine import Row, RowMapping
     from sqlalchemy.ext.asyncio import AsyncConnection
 
 
-async def get(setup_id: int, connection: AsyncConnection) -> Row | None:
+async def get(setup_id: Identifier, connection: AsyncConnection) -> Row | None:
     """Get the setup with id `setup_id` from the database."""
     row = await connection.execute(
         text(
@@ -24,7 +26,7 @@ async def get(setup_id: int, connection: AsyncConnection) -> Row | None:
     return row.first()
 
 
-async def get_parameters(setup_id: int, connection: AsyncConnection) -> list[RowMapping]:
+async def get_parameters(setup_id: Identifier, connection: AsyncConnection) -> list[RowMapping]:
     """Get all parameters for setup with `setup_id` from the database."""
     rows = await connection.execute(
         text(
@@ -51,7 +53,7 @@ async def get_parameters(setup_id: int, connection: AsyncConnection) -> list[Row
     return list(rows.mappings().all())
 
 
-async def get_tags(setup_id: int, connection: AsyncConnection) -> list[Row]:
+async def get_tags(setup_id: Identifier, connection: AsyncConnection) -> list[Row]:
     """Get all tags for setup with `setup_id` from the database."""
     rows = await connection.execute(
         text(
@@ -66,7 +68,7 @@ async def get_tags(setup_id: int, connection: AsyncConnection) -> list[Row]:
     return list(rows.all())
 
 
-async def untag(setup_id: int, tag: str, connection: AsyncConnection) -> None:
+async def untag(setup_id: Identifier, tag: TagString, connection: AsyncConnection) -> None:
     """Remove tag `tag` from setup with id `setup_id`."""
     await connection.execute(
         text(
@@ -79,7 +81,12 @@ async def untag(setup_id: int, tag: str, connection: AsyncConnection) -> None:
     )
 
 
-async def tag(setup_id: int, tag: str, user_id: int, connection: AsyncConnection) -> None:
+async def tag(
+    setup_id: Identifier,
+    tag: TagString,
+    user_id: Identifier,
+    connection: AsyncConnection,
+) -> None:
     """Add tag `tag` to setup with id `setup_id`."""
     await connection.execute(
         text(
diff --git a/src/database/studies.py b/src/database/studies.py
index 3c7c8e96..19394526 100644
--- a/src/database/studies.py
+++ b/src/database/studies.py
@@ -6,13 +6,14 @@
 from sqlalchemy import Row, text
 
 from database.users import User
+from routers.types import Identifier
 from schemas.study import CreateStudy, StudyType
 
 if TYPE_CHECKING:
     from sqlalchemy.ext.asyncio import AsyncConnection
 
 
-async def get_by_id(id_: int, connection: AsyncConnection) -> Row | None:
+async def get_by_id(id_: Identifier, connection: AsyncConnection) -> Row | None:
     row = await connection.execute(
         text(
             """
@@ -114,7 +115,12 @@ async def create(study: CreateStudy, user: User, expdb: AsyncConnection) -> int:
     return cast("int", study_id)
 
 
-async def attach_task(task_id: int, study_id: int, user: User, expdb: AsyncConnection) -> None:
+async def attach_task(
+    task_id: Identifier,
+    study_id: Identifier,
+    user: User,
+    expdb: AsyncConnection,
+) -> None:
     await expdb.execute(
         text(
             """
@@ -126,7 +132,13 @@ async def attach_task(task_id: int, study_id: int, user: User, expdb: AsyncConne
     )
 
 
-async def attach_run(*, run_id: int, study_id: int, user: User, expdb: AsyncConnection) -> None:
+async def attach_run(
+    *,
+    run_id: Identifier,
+    study_id: Identifier,
+    user: User,
+    expdb: AsyncConnection,
+) -> None:
     await expdb.execute(
         text(
             """
@@ -171,8 +183,8 @@ async def attach_tasks(
 
 
 async def attach_runs(
-    study_id: int,
-    run_ids: list[int],
+    study_id: Identifier,
+    run_ids: list[Identifier],
     user: User,
     connection: AsyncConnection,
 ) -> None:
diff --git a/src/database/tasks.py b/src/database/tasks.py
index ba4eeccb..39fadb01 100644
--- a/src/database/tasks.py
+++ b/src/database/tasks.py
@@ -3,11 +3,13 @@
 
 from sqlalchemy import Row, text
 
+from routers.types import Identifier
+
 if TYPE_CHECKING:
     from sqlalchemy.ext.asyncio import AsyncConnection
 
 
-async def get(id_: int, expdb: AsyncConnection) -> Row | None:
+async def get(id_: Identifier, expdb: AsyncConnection) -> Row | None:
     row = await expdb.execute(
         text(
             """
@@ -36,7 +38,7 @@ async def get_task_types(expdb: AsyncConnection) -> Sequence[Row]:
     )
 
 
-async def get_task_type(task_type_id: int, expdb: AsyncConnection) -> Row | None:
+async def get_task_type(task_type_id: Identifier, expdb: AsyncConnection) -> Row | None:
     row = await expdb.execute(
         text(
             """
@@ -67,7 +69,7 @@ async def get_input_for_task_type(task_type_id: int, expdb: AsyncConnection) ->
     )
 
 
-async def get_input_for_task(id_: int, expdb: AsyncConnection) -> Sequence[Row]:
+async def get_input_for_task(id_: Identifier, expdb: AsyncConnection) -> Sequence[Row]:
     rows = await expdb.execute(
         text(
             """
@@ -85,7 +87,7 @@ async def get_input_for_task(id_: int, expdb: AsyncConnection) -> Sequence[Row]:
 
 
 async def get_task_type_inout_with_template(
-    task_type: int,
+    task_type: Identifier,
     expdb: AsyncConnection,
 ) -> Sequence[Row]:
     rows = await expdb.execute(
@@ -104,7 +106,7 @@ async def get_task_type_inout_with_template(
     )
 
 
-async def get_tags(id_: int, expdb: AsyncConnection) -> list[str]:
+async def get_tags(id_: Identifier, expdb: AsyncConnection) -> list[str]:
     rows = await expdb.execute(
         text(
             """
diff --git a/src/database/users.py b/src/database/users.py
index bc2d645b..beeb14e6 100644
--- a/src/database/users.py
+++ b/src/database/users.py
@@ -8,6 +8,7 @@
 from sqlalchemy import text
 
 from config import get_config
+from routers.types import Identifier
 
 if TYPE_CHECKING:
     from sqlalchemy.ext.asyncio import AsyncConnection
@@ -57,7 +58,11 @@ async def get_user_id_for(*, api_key: APIKey, connection: AsyncConnection) -> in
     return user.id if user else None
 
 
-async def get_user_groups_for(*, user_id: int, connection: AsyncConnection) -> list[int]:
+async def get_user_groups_for(
+    *,
+    user_id: Identifier,
+    connection: AsyncConnection,
+) -> list[UserGroup]:
     row = await connection.execute(
         text(
             """
@@ -69,12 +74,12 @@ async def get_user_groups_for(*, user_id: int, connection: AsyncConnection) -> l
         parameters={"user_id": user_id},
     )
     rows = row.all()
-    return [group for (group,) in rows]
+    return [UserGroup(group) for (group,) in rows]
 
 
 @dataclasses.dataclass
 class User:
-    user_id: int
+    user_id: Identifier
     _database: AsyncConnection
     _groups: list[UserGroup] | None = None
 
@@ -86,8 +91,10 @@ async def fetch(cls, api_key: APIKey, user_db: AsyncConnection) -> Self | None:
 
     async def get_groups(self) -> list[UserGroup]:
         if self._groups is None:
-            group_ids = await get_user_groups_for(user_id=self.user_id, connection=self._database)
-            self._groups = [UserGroup(group_id) for group_id in group_ids]
+            self._groups = await get_user_groups_for(
+                user_id=self.user_id,
+                connection=self._database,
+            )
         return self._groups
 
     async def is_admin(self) -> bool:
diff --git a/src/routers/openml/datasets.py b/src/routers/openml/datasets.py
index 045f4e50..270c71b5 100644
--- a/src/routers/openml/datasets.py
+++ b/src/routers/openml/datasets.py
@@ -46,7 +46,7 @@
     CasualString128,
     Identifier,
     IntegerRange,
-    SystemString64,
+    TagString,
     integer_range_regex,
 )
 from schemas.datasets.openml import DatasetMetadata, DatasetStatus, Feature, FeatureType
@@ -63,7 +63,7 @@
 )
 async def tag_dataset(
     data_id: Annotated[Identifier, Body()],
-    tag: Annotated[SystemString64, Body()],
+    tag: Annotated[TagString, Body()],
     user: Annotated[User, Depends(fetch_user_or_raise)],
     expdb_db: Annotated[AsyncConnection, Depends(expdb_connection)],
 ) -> dict[str, dict[str, Any]]:
@@ -87,13 +87,13 @@ async def tag_dataset(
 
 class TagInfo(TypedDict):
     id: str
-    tag: NotRequired[SystemString64 | list[SystemString64]]
+    tag: NotRequired[TagString | list[TagString]]
 
 
 @router.post(path="/untag", deprecated=True)
 async def untag_dataset_like_php(
     data_id: Annotated[Identifier, Body()],
-    tag: Annotated[SystemString64, Body()],
+    tag: Annotated[TagString, Body()],
     user: Annotated[User, Depends(fetch_user_or_raise)],
     expdb_db: Annotated[AsyncConnection, Depends(expdb_connection)],
 ) -> dict[Literal["data_untag"], TagInfo]:
@@ -110,7 +110,7 @@ async def untag_dataset_like_php(
 @router.delete(path="/{identifier}/tag", status_code=HTTPStatus.NO_CONTENT)
 async def untag_dataset(
     identifier: Identifier,
-    tag: Annotated[SystemString64, Query()],
+    tag: Annotated[TagString, Query()],
     user: Annotated[User, Depends(fetch_user_or_raise)],
     expdb_db: Annotated[AsyncConnection, Depends(expdb_connection)],
 ) -> None:
@@ -158,7 +158,7 @@ def _quality_clause(quality: str, range_: str | None) -> str:
 async def list_datasets(  # noqa: PLR0913, C901
     pagination: Annotated[Pagination, Body(default_factory=Pagination)],
     data_name: Annotated[CasualString128 | None, Body()] = None,
-    tag: Annotated[SystemString64 | None, Body()] = None,
+    tag: Annotated[TagString | None, Body()] = None,
     data_version: Annotated[
         Identifier | None,
         Body(description="The dataset version to include in the search."),
diff --git a/src/routers/openml/setups.py b/src/routers/openml/setups.py
index dbca9038..ef71ce61 100644
--- a/src/routers/openml/setups.py
+++ b/src/routers/openml/setups.py
@@ -15,7 +15,7 @@
 )
 from database.users import User
 from routers.dependencies import expdb_connection, fetch_user_or_raise
-from routers.types import Identifier, SystemString64
+from routers.types import Identifier, TagString
 from schemas.setups import SetupParameters, SetupResponse
 
 if TYPE_CHECKING:
@@ -49,7 +49,7 @@ async def get_setup(
 @router.post(path="/tag")
 async def tag_setup(
     setup_id: Annotated[Identifier, Body()],
-    tag: Annotated[SystemString64, Body()],
+    tag: Annotated[TagString, Body()],
     user: Annotated[User, Depends(fetch_user_or_raise)],
     expdb_db: Annotated[AsyncConnection, Depends(expdb_connection)],
 ) -> dict[str, dict[str, str | list[str]]]:
@@ -76,7 +76,7 @@ async def tag_setup(
 @router.post(path="/untag")
 async def untag_setup(
     setup_id: Annotated[Identifier, Body()],
-    tag: Annotated[SystemString64, Body()],
+    tag: Annotated[TagString, Body()],
     user: Annotated[User, Depends(fetch_user_or_raise)],
     expdb_db: Annotated[AsyncConnection, Depends(expdb_connection)],
 ) -> dict[str, dict[str, str | list[str]]]:
diff --git a/src/routers/openml/tasks.py b/src/routers/openml/tasks.py
index 411ea5b5..8faa898c 100644
--- a/src/routers/openml/tasks.py
+++ b/src/routers/openml/tasks.py
@@ -17,7 +17,7 @@
     CasualString128,
     Identifier,
     IntegerRange,
-    SystemString64,
+    TagString,
     integer_range_regex,
 )
 from schemas.datasets.openml import Task
@@ -231,8 +231,8 @@ def _quality_clause(quality: str, range_: str | None) -> str:
 async def list_tasks(  # noqa: PLR0913, PLR0912, C901, PLR0915
     pagination: Annotated[Pagination, Body(default_factory=Pagination)],
     task_type_id: Annotated[Identifier | None, Body(description="Filter by task type id.")] = None,
-    tag: Annotated[SystemString64 | None, Body()] = None,
-    data_tag: Annotated[SystemString64 | None, Body()] = None,
+    tag: Annotated[TagString | None, Body()] = None,
+    data_tag: Annotated[TagString | None, Body()] = None,
     status: Annotated[TaskStatusFilter, Body()] = TaskStatusFilter.ACTIVE,
     task_id: Annotated[
         list[Identifier] | None,
diff --git a/src/routers/types.py b/src/routers/types.py
index fcdb876a..fe945778 100644
--- a/src/routers/types.py
+++ b/src/routers/types.py
@@ -2,8 +2,13 @@
 
 from pydantic import Field
 
-SystemString64 = Annotated[str, Field(pattern=r"^[\w\-\.]+$", min_length=1, max_length=64)]
-CasualString128 = Annotated[str, Field(pattern=r"^[\w\-\.\(\),]+$", min_length=1, max_length=128)]
+# Known as SystemString64 in the XSD
+TagString = Annotated[str, Field(pattern=r"^[\w\-\.]+$", min_length=1, max_length=64)]
+
+# Currently used for a variety of fields, like `name` or `feature`.
+CasualString = Annotated[str, Field(pattern=r"^[\w\-\.\(\),]+$", min_length=1)]
+CasualString128 = Annotated[CasualString, Field(max_length=128)]
+
 Identifier = Annotated[int, Field(gt=0)]
 
 integer_range_regex = r"^(\d+)(\.\.\d+)?$"
diff --git a/tests/routers/openml/dataset_tag_test.py b/tests/routers/openml/dataset_tag_test.py
index 5bcfb4a3..bb12a74b 100644
--- a/tests/routers/openml/dataset_tag_test.py
+++ b/tests/routers/openml/dataset_tag_test.py
@@ -31,25 +31,6 @@ async def test_dataset_tag_rejects_unauthorized(key: ApiKey, py_api: httpx.Async
     assert response.status_code == HTTPStatus.UNAUTHORIZED
 
 
-@pytest.mark.parametrize(
-    "tag",
-    ["", "h@", " a", "a" * 65],
-    ids=["too short", "@", "space", "too long"],
-)
-async def test_dataset_tag_invalid_tag_is_rejected(
-    # Constraints for the tag are handled by FastAPI
-    tag: str,
-    py_api: httpx.AsyncClient,
-) -> None:
-    response = await py_api.post(
-        f"/datasets/tag?api_key={ApiKey.ADMIN}",
-        json={"data_id": 1, "tag": tag},
-    )
-
-    assert response.status_code == HTTPStatus.UNPROCESSABLE_ENTITY
-    assert response.json()["errors"][0]["loc"] == ["body", "tag"]
-
-
 # ── Direct call tests: tag_dataset ──
 
 
diff --git a/tests/types_test.py b/tests/types_test.py
new file mode 100644
index 00000000..7a4df612
--- /dev/null
+++ b/tests/types_test.py
@@ -0,0 +1,84 @@
+"""Tests validation of custom types.
+
+Note that for parametrized tests, it is important that the value order and
+amount must be consistent to allow distribution with pytest-xdist:
+https://pytest-xdist.readthedocs.io/en/latest/known-limitations.html#order-and-amount-of-test-must-be-consistent
+
+"""
+
+import string
+
+import pytest
+from pydantic import TypeAdapter, ValidationError
+
+from routers.types import CasualString, Identifier, TagString
+
+_identifier = TypeAdapter(Identifier)
+
+
+def test_identifier_accepts_positive_integer() -> None:
+    assert _identifier.validate_strings("1") == 1
+
+
+def test_identifier_rejects_non_integer() -> None:
+    with pytest.raises(ValidationError):
+        _identifier.validate_strings("foo")
+
+    with pytest.raises(ValidationError):
+        _identifier.validate_strings("1.2")
+
+
+def test_identifier_rejects_negative() -> None:
+    with pytest.raises(ValidationError):
+        _identifier.validate_strings("-1")
+
+
+def test_identifier_rejects_zero() -> None:
+    with pytest.raises(ValidationError):
+        _identifier.validate_strings("0")
+
+
+_tag_string = TypeAdapter(TagString)
+_valid_punctuation_tag = list("_-.")
+_invalid_punctuation_tag = sorted(set(string.punctuation) - set(_valid_punctuation_tag))
+
+
+def test_tag_string_pattern() -> None:
+    assert _tag_string.json_schema()["pattern"] == r"^[\w\-\.]+$"
+
+
+@pytest.mark.parametrize("tag", ["a", "c" * 64, "version2.0", "study-14", "study_15"])
+def test_tag_string_accepts_valid(tag: str) -> None:
+    assert _tag_string.validate_strings(tag) == tag
+
+
+@pytest.mark.parametrize(
+    "tag", ["", " ", "a ", " a", "a b", "a\t", "c" * 65, *_invalid_punctuation_tag]
+)
+def test_tag_string_rejects_invalid(tag: str) -> None:
+    with pytest.raises(ValidationError):
+        _tag_string.validate_strings(tag)
+
+
+_casual_string = TypeAdapter(CasualString)
+_valid_punctuation_casual_string = list(set("_-.(),"))
+_invalid_punctuation_casual_string = sorted(
+    set(string.punctuation) - set(_valid_punctuation_casual_string)
+)
+
+
+def test_casual_string_pattern() -> None:
+    assert _casual_string.json_schema()["pattern"] == r"^[\w\-\.\(\),]+$"
+
+
+@pytest.mark.parametrize("string", ["a", "a" * 1000, "_-.(),"])
+def test_casual_string_accepts_valid(string: str) -> None:
+    assert _casual_string.validate_strings(string)
+
+
+@pytest.mark.parametrize(
+    "string", ["", " ", "a ", " a", "a b", "a\t", *_invalid_punctuation_casual_string]
+)
+def test_casual_string_rejects_invalid(string: str) -> None:
+    with pytest.raises(ValidationError):
+        _casual_string.validate_strings(string)