diff --git a/src/extensions/endpoints/EndpointsExtension.ts b/src/extensions/endpoints/EndpointsExtension.ts
index 80e6434c2..408613f10 100644
--- a/src/extensions/endpoints/EndpointsExtension.ts
+++ b/src/extensions/endpoints/EndpointsExtension.ts
@@ -26,7 +26,7 @@ export type GraphQLConfigEnpointsMapData = {
 }
 
 export type GraphQLConfigEnpointsMap = {
-  [env: string]: GraphQLConfigEnpointConfig
+  [env: string]: GraphQLConfigEnpointConfig | GraphQLEndpoint
 }
 
 export type GraphQLConfigEnpointsData = GraphQLConfigEnpointsMapData
@@ -68,7 +68,15 @@ export class GraphQLEndpointsExtension {
   ): GraphQLEndpoint {
     const endpoint = this.getRawEndpoint(endpointName)
     try {
-      return new GraphQLEndpoint(resolveEnvsInValues(endpoint, env))
+      const resolved = resolveEnvsInValues(endpoint, env)
+
+      // graphql-config extensions might have already instantiated a GraphQLEndpoint
+      // or derived class from the GraphQLConfigEndpointConfig data. In that case,
+      // getRawEndpoint will already return a GraphQLEndpoint and it should not be overwritten.
+      if (!(resolved instanceof GraphQLEndpoint)) {
+        return new GraphQLEndpoint(resolved)
+      }
+      return resolved
     } catch (e) {
       e.message = `${this.configPath}: ${e.message}`
       throw e
@@ -99,7 +107,7 @@ export class GraphQLEndpointsExtension {
       )
     }
 
-    if (!endpoint.url) {
+    if (!endpoint.url && !(endpoint instanceof GraphQLEndpoint)) {
       throw new Error(
         `${this
           .configPath}: "url" is required but is not specified for "${endpointName}" endpoint`,
diff --git a/src/extensions/endpoints/resolveRefString.ts b/src/extensions/endpoints/resolveRefString.ts
index 533a0acf9..992a4d334 100644
--- a/src/extensions/endpoints/resolveRefString.ts
+++ b/src/extensions/endpoints/resolveRefString.ts
@@ -15,7 +15,6 @@ export function resolveEnvsInValues<T extends any>(
   config: T,
   env: { [name: string]: string | undefined },
 ): T {
-  config = Object.assign({}, config)
   for (let key in config) {
     const value = config[key]
     if (typeof value === 'string') {