diff --git a/packages/babel-core/src/config/caching.js b/packages/babel-core/src/config/caching.js
index b9703560b5..7c3a28bda2 100644
--- a/packages/babel-core/src/config/caching.js
+++ b/packages/babel-core/src/config/caching.js
@@ -1,6 +1,6 @@
 // @flow
 
-type SimpleCacheConfigurator = SimpleCacheConfiguratorFn &
+export type SimpleCacheConfigurator = SimpleCacheConfiguratorFn &
   SimpleCacheConfiguratorObj;
 
 type SimpleCacheConfiguratorFn = {
diff --git a/packages/babel-core/src/config/files/configuration.js b/packages/babel-core/src/config/files/configuration.js
index 73786b89fb..c1a55cc598 100644
--- a/packages/babel-core/src/config/files/configuration.js
+++ b/packages/babel-core/src/config/files/configuration.js
@@ -6,6 +6,7 @@ import fs from "fs";
 import json5 from "json5";
 import resolve from "resolve";
 import { makeStrongCache, type CacheConfigurator } from "../caching";
+import makeAPI from "../helpers/config-api";
 
 const debug = buildDebug("babel:config:loading:files:configuration");
 
@@ -150,12 +151,7 @@ const readConfigJS = makeStrongCache(
     }
 
     if (typeof options === "function") {
-      options = options({
-        cache: cache.simple(),
-        // Expose ".env()" so people can easily get the same env that we expose using the "env" key.
-        env: () => cache.using(data => data.envName),
-        async: () => false,
-      });
+      options = options(makeAPI(cache));
 
       if (!cache.configured()) throwConfigError();
     }
diff --git a/packages/babel-core/src/config/helpers/config-api.js b/packages/babel-core/src/config/helpers/config-api.js
new file mode 100644
index 0000000000..933c605016
--- /dev/null
+++ b/packages/babel-core/src/config/helpers/config-api.js
@@ -0,0 +1,40 @@
+// @flow
+import type { CacheConfigurator, SimpleCacheConfigurator } from "../caching";
+
+type EnvFunction = {
+  (): string,
+  <T>((string) => T): T,
+  (string): boolean,
+  (Array<string>): boolean,
+};
+
+export type PluginAPI = {
+  cache: SimpleCacheConfigurator,
+  env: EnvFunction,
+  async: () => boolean,
+};
+
+export default function makeAPI(
+  cache: CacheConfigurator<{ envName: string }>,
+): PluginAPI {
+  const env: any = value =>
+    cache.using(data => {
+      if (typeof value === "undefined") return data.envName;
+      if (typeof value === "function") return value(data.envName);
+      if (!Array.isArray(value)) value = [value];
+
+      return value.some(entry => {
+        if (typeof entry !== "string") {
+          throw new Error("Unexpected non-string value");
+        }
+        return entry === data.envName;
+      });
+    });
+
+  return {
+    cache: cache.simple(),
+    // Expose ".env()" so people can easily get the same env that we expose using the "env" key.
+    env,
+    async: () => false,
+  };
+}
diff --git a/packages/babel-core/src/config/index.js b/packages/babel-core/src/config/index.js
index 05ac8e7ce9..5ee1c04818 100644
--- a/packages/babel-core/src/config/index.js
+++ b/packages/babel-core/src/config/index.js
@@ -16,6 +16,7 @@ import { makeWeakCache, type CacheConfigurator } from "./caching";
 import { getEnv } from "./helpers/environment";
 import { validate, type ValidatedOptions } from "./validation/options";
 import { validatePluginObject } from "./validation/plugins";
+import makeAPI from "./helpers/config-api";
 
 type LoadedDescriptor = {
   value: {},
@@ -202,11 +203,7 @@ const loadDescriptor = makeWeakCache(
 
     let item = value;
     if (typeof value === "function") {
-      const api = Object.assign(Object.create(context), {
-        cache: cache.simple(),
-        env: () => cache.using(data => data.envName),
-        async: () => false,
-      });
+      const api = Object.assign(Object.create(context), makeAPI(cache));
 
       try {
         item = value(api, options, dirname);