diff --git a/agent/agentcontainers/api_test.go b/agent/agentcontainers/api_test.go
index 4e7926cf1d..f9fb89c16a 100644
--- a/agent/agentcontainers/api_test.go
+++ b/agent/agentcontainers/api_test.go
@@ -234,6 +234,8 @@ func (w *fakeWatcher) sendEventWaitNextCalled(ctx context.Context, event fsnotif
 // fakeSubAgentClient implements SubAgentClient for testing purposes.
 type fakeSubAgentClient struct {
 	logger slog.Logger
+
+	mu     sync.Mutex // Protects following.
 	agents map[uuid.UUID]agentcontainers.SubAgent
 
 	listErrC   chan error // If set, send to return error, close to return nil.
@@ -254,6 +256,8 @@ func (m *fakeSubAgentClient) List(ctx context.Context) ([]agentcontainers.SubAge
 			}
 		}
 	}
+	m.mu.Lock()
+	defer m.mu.Unlock()
 	var agents []agentcontainers.SubAgent
 	for _, agent := range m.agents {
 		agents = append(agents, agent)
@@ -283,6 +287,9 @@ func (m *fakeSubAgentClient) Create(ctx context.Context, agent agentcontainers.S
 		return agentcontainers.SubAgent{}, xerrors.New("operating system must be set")
 	}
 
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
 	for _, a := range m.agents {
 		if a.Name == agent.Name {
 			return agentcontainers.SubAgent{}, &pq.Error{
@@ -314,6 +321,8 @@ func (m *fakeSubAgentClient) Delete(ctx context.Context, id uuid.UUID) error {
 			}
 		}
 	}
+	m.mu.Lock()
+	defer m.mu.Unlock()
 	if m.agents == nil {
 		m.agents = make(map[uuid.UUID]agentcontainers.SubAgent)
 	}
@@ -2162,6 +2171,10 @@ func TestAPI(t *testing.T) {
 			nowRecreateSuccessTrap.MustWait(ctx).MustRelease(ctx)
 			nowRecreateSuccessTrap.Close()
 
+			// Advance the clock to run the devcontainer state update routine.
+			_, aw := mClock.AdvanceNext()
+			aw.MustWait(ctx)
+
 			req = httptest.NewRequest(http.MethodGet, "/", nil)
 			rec = httptest.NewRecorder()
 			r.ServeHTTP(rec, req)
@@ -2178,7 +2191,7 @@ func TestAPI(t *testing.T) {
 			// available for use.
 			require.Len(t, response.Devcontainers, 1)
 			assert.Equal(t, codersdk.WorkspaceAgentDevcontainerStatusRunning, response.Devcontainers[0].Status)
-			assert.NotNil(t, response.Devcontainers[0].Container)
+			require.NotNil(t, response.Devcontainers[0].Container)
 			assert.Equal(t, testContainer.ID, response.Devcontainers[0].Container.ID)
 		})