diff --git a/docs/plans/2026-08-05-inter-app-comms-implementation.md b/docs/plans/2026-08-05-inter-app-comms-implementation.md
index 6f12901c..921deeb0 100644
--- a/docs/plans/2026-08-05-inter-app-comms-implementation.md
+++ b/docs/plans/2026-08-05-inter-app-comms-implementation.md
@@ -561,7 +561,14 @@ export class ProcedureBuilder<Input, Output> {
   }
 
   input<S extends ObjectSchema<object>>(schema: S): ProcedureBuilder<InferInput<S>, Output> {
-    return new ProcedureBuilder<InferInput<S>, Output>({ ...this.def, input: schema });
+    // The cast is required for the same reason .output<T>() needs one: the
+    // phantom __input/__output markers make ProcedureDef invariant, so
+    // spreading a ProcedureDef<Input, Output> into a ProcedureDef<InferInput<S>,
+    // Output> is not assignable without it. No runtime effect.
+    return new ProcedureBuilder<InferInput<S>, Output>({
+      ...this.def,
+      input: schema,
+    } as ProcedureDef<InferInput<S>, Output>);
   }
 
   output<T>(): ProcedureBuilder<Input, T> {