diff --git a/.devcontainer/Dockerfile b/.devcontainer/Dockerfile
index e8d01d7..283786a 100644
--- a/.devcontainer/Dockerfile
+++ b/.devcontainer/Dockerfile
@@ -1,7 +1,12 @@
 FROM mcr.microsoft.com/devcontainers/python:1-3.13-bookworm
 
 # `mysql` CLI for poking at the database during development.
-RUN apt-get update \
+#
+# The base image ships a yarn apt source whose signing key has since rotated,
+# which makes `apt-get update` fail outright. We don't use yarn, so drop that
+# source list rather than carrying its key.
+RUN rm -f /etc/apt/sources.list.d/yarn.list \
+    && apt-get update \
     && apt-get install -y --no-install-recommends default-mysql-client \
     && rm -rf /var/lib/apt/lists/*
 
diff --git a/.devcontainer/compose.override.yaml b/.devcontainer/compose.override.yaml
index f17afdc..bf977c7 100644
--- a/.devcontainer/compose.override.yaml
+++ b/.devcontainer/compose.override.yaml
@@ -1,16 +1,20 @@
 # Adds the development container to the stack defined in ../compose.yaml.
 # Merged by devcontainer.json; not useful on its own.
+#
+# NOTE: relative paths below resolve against the *project directory* -- the
+# directory of the first compose file (the repo root), not this file's own
+# directory. Hence "." rather than "..".
 
 services:
   app:
     build:
-      context: ..
+      context: .
       dockerfile: .devcontainer/Dockerfile
     command: sleep infinity
     ports:
       - "8000:8000"
     volumes:
-      - ..:/workspaces/v2x-server:cached
+      - .:/workspaces/v2x-server:cached
       # Keep the Linux venv out of the macOS/Windows bind mount. Without this,
       # host-side tooling and platform-specific wheels collide.
       - venv:/workspaces/v2x-server/.venv
diff --git a/pyproject.toml b/pyproject.toml
index 83f4b10..9098a63 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -81,7 +81,10 @@ ignore_missing_imports = true
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
-asyncio_default_fixture_loop_scope = "session"
+# Must match the test loop scope (function). If fixtures run on a session loop
+# while tests run on a per-function loop, asyncmy's loop-bound futures raise
+# "attached to a different loop". aiosqlite hides this; MySQL does not.
+asyncio_default_fixture_loop_scope = "function"
 testpaths = ["tests"]
 addopts = "-ra --strict-markers"
 filterwarnings = ["error"]
diff --git a/src/v2x_server/schemas/device.py b/src/v2x_server/schemas/device.py
index 6e4cec4..fcaaf8f 100644
--- a/src/v2x_server/schemas/device.py
+++ b/src/v2x_server/schemas/device.py
@@ -4,7 +4,7 @@ from __future__ import annotations
 
 import datetime as dt
 
-from pydantic import BaseModel, ConfigDict, Field
+from pydantic import BaseModel, ConfigDict, Field, field_validator
 
 from v2x_server.models.device import DeviceStatus
 
@@ -40,6 +40,18 @@ class DeviceRead(DeviceBase):
     created_at: dt.datetime
     updated_at: dt.datetime
 
+    @field_validator("created_at", "updated_at")
+    @classmethod
+    def _mark_as_utc(cls, value: dt.datetime) -> dt.datetime:
+        """Attach the offset MySQL cannot store.
+
+        DATETIME columns carry no timezone, so these come back naive. Every
+        connection is pinned to UTC (see db/session.py), so a naive value read
+        from the database *is* UTC -- say so explicitly rather than emitting a
+        bare timestamp the client has to guess about.
+        """
+        return value.replace(tzinfo=dt.UTC) if value.tzinfo is None else value
+
 
 class DevicePage(BaseModel):
     items: list[DeviceRead]
diff --git a/tests/test_devices_api.py b/tests/test_devices_api.py
index f31ba59..138becd 100644
--- a/tests/test_devices_api.py
+++ b/tests/test_devices_api.py
@@ -101,6 +101,14 @@ class TestCrud:
         assert patched.json()["description"] == "original"
         assert patched.json()["name"] == created["name"]
 
+    async def test_timestamps_carry_an_explicit_utc_offset(
+        self, client: AsyncClient, as_user: AsUser
+    ) -> None:
+        """MySQL DATETIME has no offset; the API must not emit a bare timestamp."""
+        as_user("operator")
+        created = (await client.post(PREFIX, json=_payload())).json()
+        assert created["created_at"].endswith(("Z", "+00:00"))
+
     async def test_missing_device_is_404(self, client: AsyncClient, as_user: AsUser) -> None:
         as_user("viewer")
         response = await client.get(f"{PREFIX}/999999")