diff --git a/packages/cursorless-engine/src/core/commandRunner/CommandRunnerImpl.ts b/packages/cursorless-engine/src/core/commandRunner/CommandRunnerImpl.ts
index c1230aaf33..77bfe82bc3 100644
--- a/packages/cursorless-engine/src/core/commandRunner/CommandRunnerImpl.ts
+++ b/packages/cursorless-engine/src/core/commandRunner/CommandRunnerImpl.ts
@@ -211,6 +211,13 @@ export class CommandRunnerImpl implements CommandRunner {
 
       default: {
         const action = this.actions[actionDescriptor.name];
+
+        // Ensure we don't miss any new actions. Needed because we don't have input validation.
+        // FIXME: remove once we have schema validation (#983)
+        if (action == null) {
+          throw new Error(`Unknown action: ${actionDescriptor.name}`);
+        }
+
         this.finalStages = action.getFinalStages?.() ?? [];
         this.noAutomaticTokenExpansion =
           action.noAutomaticTokenExpansion ?? false;
diff --git a/packages/cursorless-engine/src/processTargets/MarkStageFactoryImpl.ts b/packages/cursorless-engine/src/processTargets/MarkStageFactoryImpl.ts
index 97b884dea0..b88b030393 100644
--- a/packages/cursorless-engine/src/processTargets/MarkStageFactoryImpl.ts
+++ b/packages/cursorless-engine/src/processTargets/MarkStageFactoryImpl.ts
@@ -47,6 +47,13 @@ export class MarkStageFactoryImpl implements MarkStageFactory {
         return new TargetMarkStage(this.targetPipelineRunner, mark);
       case "explicit":
         return new ExplicitMarkStage(mark);
+      default: {
+        // Ensure we don't miss any new marks. Needed because we don't have input validation.
+        // FIXME: remove once we have schema validation (#983)
+        const _exhaustiveCheck: never = mark;
+        const { type } = mark;
+        throw new Error(`Unknown mark: ${type}`);
+      }
     }
   }
 }
diff --git a/packages/cursorless-engine/src/processTargets/ModifierStageFactoryImpl.ts b/packages/cursorless-engine/src/processTargets/ModifierStageFactoryImpl.ts
index 5f7b6ec009..e414e99d1c 100644
--- a/packages/cursorless-engine/src/processTargets/ModifierStageFactoryImpl.ts
+++ b/packages/cursorless-engine/src/processTargets/ModifierStageFactoryImpl.ts
@@ -109,6 +109,13 @@ export class ModifierStageFactoryImpl implements ModifierStageFactory {
         throw Error(
           `Unexpected modifier '${modifier.type}'; it should have been removed during inference`,
         );
+      default: {
+        // Ensure we don't miss any new modifiers. Needed because we don't have input validation.
+        // FIXME: remove once we have schema validation (#983)
+        const _exhaustiveCheck: never = modifier;
+        const { type } = modifier;
+        throw new Error(`Unknown modifier: ${type}`);
+      }
     }
   }