diff --git a/src/database/setups.py b/src/database/setups.py
index 1c959b3..ce0f0cc 100644
--- a/src/database/setups.py
+++ b/src/database/setups.py
@@ -3,7 +3,14 @@
 from typing import TYPE_CHECKING
 
 from sqlalchemy import text
+from sqlalchemy.exc import IntegrityError
 
+from database.exceptions import (
+    _DUPLICATE_ENTRY,
+    _FOREIGN_KEY_CONSTRAINT_FAILED,
+    DuplicatePrimaryKeyError,
+    ForeignKeyConstraintError,
+)
 from routers.types import Identifier, TagString
 
 if TYPE_CHECKING:
@@ -88,12 +95,20 @@ async def tag(
     connection: AsyncConnection,
 ) -> None:
     """Add tag `tag` to setup with id `setup_id`."""
-    await connection.execute(
-        text(
-            """
-            INSERT INTO setup_tag (id, tag, uploader)
-            VALUES (:setup_id, :tag, :user_id)
-            """,
-        ),
-        parameters={"setup_id": setup_id, "tag": tag, "user_id": user_id},
-    )
+    try:
+        await connection.execute(
+            text(
+                """
+                INSERT INTO setup_tag (id, tag, uploader)
+                VALUES (:setup_id, :tag, :user_id)
+                """,
+            ),
+            parameters={"setup_id": setup_id, "tag": tag, "user_id": user_id},
+        )
+    except IntegrityError as e:
+        code, msg = e.orig.args
+        if code == _FOREIGN_KEY_CONSTRAINT_FAILED:
+            raise ForeignKeyConstraintError(msg) from e
+        if code == _DUPLICATE_ENTRY:
+            raise DuplicatePrimaryKeyError(msg) from e
+        raise
diff --git a/src/routers/openml/setups.py b/src/routers/openml/setups.py
index ef71ce6..34b28e0 100644
--- a/src/routers/openml/setups.py
+++ b/src/routers/openml/setups.py
@@ -13,6 +13,7 @@
     TagNotFoundError,
     TagNotOwnedError,
 )
+from database.exceptions import DuplicatePrimaryKeyError, ForeignKeyConstraintError
 from database.users import User
 from routers.dependencies import expdb_connection, fetch_user_or_raise
 from routers.types import Identifier, TagString
@@ -54,22 +55,19 @@ async def tag_setup(
     expdb_db: Annotated[AsyncConnection, Depends(expdb_connection)],
 ) -> dict[str, dict[str, str | list[str]]]:
     """Add tag `tag` to setup with id `setup_id`."""
-    setup, setup_tags = await asyncio.gather(
-        database.setups.get(setup_id, expdb_db),
-        database.setups.get_tags(setup_id, expdb_db),
-    )
-    if not setup:
+    try:
+        await database.setups.tag(setup_id, tag, user.user_id, expdb_db)
+    except ForeignKeyConstraintError:
         msg = f"Setup {setup_id} not found."
-        raise SetupNotFoundError(msg)
-    matched_tag_row = next((t for t in setup_tags if t.tag.casefold() == tag.casefold()), None)
+        raise SetupNotFoundError(msg, code=472) from None
+    except DuplicatePrimaryKeyError:
+        msg = f"Setup {setup_id} already tagged with {tag!r}."
+        raise TagAlreadyExistsError(msg) from None
 
-    if matched_tag_row:
-        msg = f"Setup {setup_id} already has tag {tag!r}."
-        raise TagAlreadyExistsError(msg)
-
-    await database.setups.tag(setup_id, tag, user.user_id, expdb_db)
     logger.info("Setup {setup_id} tagged '{tag}'.", setup_id=setup_id, tag=tag)
-    all_tags = [t.tag for t in setup_tags] + [tag]
+    all_tag_rows = await database.setups.get_tags(setup_id, expdb_db)
+    all_tags = [t.tag for t in all_tag_rows]
+
     return {"setup_tag": {"id": str(setup_id), "tag": all_tags}}
 
 
diff --git a/tests/routers/openml/setups_tag_test.py b/tests/routers/openml/setups_tag_test.py
index c674ca2..32c85ef 100644
--- a/tests/routers/openml/setups_tag_test.py
+++ b/tests/routers/openml/setups_tag_test.py
@@ -65,7 +65,7 @@ async def test_setup_tag_already_exists(expdb_test: AsyncConnection) -> None:
         text("INSERT INTO setup_tag (id, tag, uploader) VALUES (1, :tag, 2);"),
         parameters={"tag": tag},
     )
-    with pytest.raises(TagAlreadyExistsError, match=rf"Setup 1 already has tag '{tag}'\."):
+    with pytest.raises(TagAlreadyExistsError, match=rf"Setup 1 already tagged with '{tag}'\."):
         await tag_setup(
             setup_id=1,
             tag=tag,
@@ -203,4 +203,4 @@ async def test_setup_tag_response_is_identical_tag_already_exists(
     assert php_response.status_code == HTTPStatus.INTERNAL_SERVER_ERROR
     assert py_response.status_code == HTTPStatus.CONFLICT
     assert php_response.json()["error"]["message"] == "Entity already tagged by this tag."
-    assert py_response.json()["detail"] == f"Setup {setup_id} already has tag {tag!r}."
+    assert py_response.json()["detail"] == f"Setup {setup_id} already tagged with {tag!r}."