diff --git a/.changeset/witty-boxes-teach.md b/.changeset/witty-boxes-teach.md
new file mode 100644
index 00000000000..80d8af4bab6
--- /dev/null
+++ b/.changeset/witty-boxes-teach.md
@@ -0,0 +1,5 @@
+---
+"@effect/platform": patch
+---
+
+fix HttpApiGroup.addError signature
diff --git a/packages/platform/src/HttpApiGroup.ts b/packages/platform/src/HttpApiGroup.ts
index 36705537a3b..9225361ce35 100644
--- a/packages/platform/src/HttpApiGroup.ts
+++ b/packages/platform/src/HttpApiGroup.ts
@@ -66,12 +66,12 @@ export interface HttpApiGroup<
    * Add an error schema to an `HttpApiGroup`, which is shared by all endpoints in the
    * group.
    */
-  addError<A, I, R>(
-    schema: Schema.Schema<A, I, R>,
+  addError<A, I, RX>(
+    schema: Schema.Schema<A, I, RX>,
     annotations?: {
       readonly status?: number | undefined
     }
-  ): HttpApiGroup<Id, Endpoints, Error | A, R | R, TopLevel>
+  ): HttpApiGroup<Id, Endpoints, Error | A, R | RX, TopLevel>
 
   /**
    * Add a path prefix to all endpoints in an `HttpApiGroup`. Note that this will only