diff --git a/src/content/__tests__/schema.test.ts b/src/content/__tests__/schema.test.ts
index fe5b756..b15c946 100644
--- a/src/content/__tests__/schema.test.ts
+++ b/src/content/__tests__/schema.test.ts
@@ -208,4 +208,21 @@ describe('action kind schema', () => {
     });
     expect(content.actionsById.rest.kind).toBe('loop');
   });
+
+  it('rejects loop action without durationMs', () => {
+    expect(() =>
+      buildContent({
+        resources: [{ id: 'supplies', name: 'Supplies' }],
+        actions: [
+          {
+            id: 'broken_loop',
+            name: 'Broken loop',
+            kind: 'loop',
+            group: { id: 'camp', label: 'Camp' },
+            yields: [],
+          },
+        ],
+      }),
+    ).toThrow(/durationMs/);
+  });
 });
diff --git a/src/content/schema.ts b/src/content/schema.ts
index d42ad9d..527d274 100644
--- a/src/content/schema.ts
+++ b/src/content/schema.ts
@@ -61,7 +61,7 @@ export const actionDefSchema = z
         path: ['durationMs'],
       });
     }
-    if (action.kind === 'story' && !action.storyChoiceId) {
+    if (action.kind === 'story' && action.storyChoiceId === undefined) {
       ctx.addIssue({
         code: 'custom',
         message: 'story actions require storyChoiceId',
diff --git a/src/engine/game.ts b/src/engine/game.ts
index b83b52e..4b7a040 100644
--- a/src/engine/game.ts
+++ b/src/engine/game.ts
@@ -169,6 +169,7 @@ export function tickGame(state: GameState, content: Content, tickMs: number): Ti
     const actionId = state.activeActionId;
     const action = content.actionsById[actionId];
     if (!action) return { completedActionIds };
+    if (action.durationMs === undefined) return { completedActionIds };
     if (state.actionElapsedMs < action.durationMs) return { completedActionIds };
 
     state.actionElapsedMs -= action.durationMs;
diff --git a/src/state/viewModel.ts b/src/state/viewModel.ts
index 64b9957..168df55 100644
--- a/src/state/viewModel.ts
+++ b/src/state/viewModel.ts
@@ -80,7 +80,7 @@ export function toView(state: GameState, content: GameContent): GameView {
   }));
 
   const action = state.activeActionId ? content.actionsById[state.activeActionId] : undefined;
-  const actionProgress = action ? Math.min(1, state.actionElapsedMs / action.durationMs) : 0;
+  const actionProgress = action && action.durationMs ? Math.min(1, state.actionElapsedMs / action.durationMs) : 0;
   const queuedActionIds = [...state.actionQueue];
   const queuedActionNames = queuedActionIds.map((id) => content.actionsById[id]?.name ?? id);