diff --git a/src/action-common.ts b/src/action-common.ts
index eb120d357..5291e31b8 100644
--- a/src/action-common.ts
+++ b/src/action-common.ts
@@ -6,6 +6,7 @@ import { ActionsEnv, getActionsEnv } from "./actions-util";
 import type { ApiClient } from "./api-client";
 import { Env, ReadOnlyEnv } from "./environment";
 import type { FeatureEnablement } from "./feature-flags";
+import type { FileSystem } from "./fs";
 import { getActionsLogger, Logger } from "./logging";
 import {
   ActionName,
@@ -57,6 +58,10 @@ export interface FeatureState {
     /** Information about enabled feature flags. */
     features: FeatureEnablement;
   };
+  FS: {
+    /** The file system operations to use. */
+    fs: FileSystem;
+  };
 }
 
 /** Identifies a type of state an Action may have. */
diff --git a/src/testing-utils.ts b/src/testing-utils.ts
index e8d3db706..d8c149329 100644
--- a/src/testing-utils.ts
+++ b/src/testing-utils.ts
@@ -1,3 +1,4 @@
+import * as fs from "fs";
 import { TextDecoder } from "node:util";
 import * as os from "os";
 import path from "path";
@@ -226,6 +227,7 @@ type AllState = [
   "Actions",
   "Api",
   "FeatureFlags",
+  "FS",
 ];
 
 /** Initialise a fresh `ActionState<AllState>` value. */
@@ -244,6 +246,7 @@ export function initAllState(
     actions: getTestActionsEnv(env),
     apiClient: github.getOctokit("123"),
     features: createFeatures([]),
+    fs,
     ...overrides,
   };
 }