diff --git a/ava.setup.mjs b/ava.setup.mjs
index e6c370e01..b40599112 100644
--- a/ava.setup.mjs
+++ b/ava.setup.mjs
@@ -1,3 +1,4 @@
 import pkg from "./package.json" with { type: "json" };
 
 globalThis.__CODEQL_ACTION_VERSION__ = pkg.version;
+globalThis.__CODEQL_ACTION_TEST_ENV__ = "unit-test";
diff --git a/build.mjs b/build.mjs
index a0c3c0618..ecbe5be84 100644
--- a/build.mjs
+++ b/build.mjs
@@ -78,7 +78,7 @@ const UPLOAD_LIB_SRC = "./src/upload-lib";
  *
  * The virtual module additionally re-exports `upload-lib` under the `uploadLib` namespace so that
  * external consumers can access it via the small `lib/upload-lib.js` stub emitted below.
- * 
+ *
  * A tiny stub file is emitted for each Action entrypoint, and one for `upload-lib`. Each stub
  * imports the shared bundle and calls/re-exports from the respective entry point.
  *
@@ -212,6 +212,7 @@ const context = await esbuild.context({
   target: ["node20"],
   define: {
     __CODEQL_ACTION_VERSION__: JSON.stringify(pkg.version),
+    __CODEQL_ACTION_TEST_ENV__: JSON.stringify(""),
   },
   metafile: true,
 });
diff --git a/lib/entry-points.js b/lib/entry-points.js
index 6bf927591..d537ba343 100644
--- a/lib/entry-points.js
+++ b/lib/entry-points.js
@@ -166161,7 +166161,7 @@ var ReadOnlyEnv = class {
    * or if this instance was constructed by `getTestEnv`.
    */
   isTestingEnv() {
-    return this.getOptional("NODE_ENV" /* NODE_ENV */) === "test";
+    return false;
   }
 };
 var Env = class extends ReadOnlyEnv {
diff --git a/src/environment.test.ts b/src/environment.test.ts
new file mode 100644
index 000000000..2c8a8a8f1
--- /dev/null
+++ b/src/environment.test.ts
@@ -0,0 +1,12 @@
+import test from "ava";
+
+import { Env, ReadOnlyEnv } from "./environment";
+import { getTestEnv, setupTests } from "./testing-utils";
+
+setupTests(test);
+
+test("isTestingEnv() is true", (t) => {
+  t.true(new ReadOnlyEnv(process.env).isTestingEnv());
+  t.true(new Env(process.env).isTestingEnv());
+  t.true(getTestEnv().isTestingEnv());
+});
diff --git a/src/environment.ts b/src/environment.ts
index c5ef6c579..c3aad9086 100644
--- a/src/environment.ts
+++ b/src/environment.ts
@@ -1,5 +1,10 @@
 import * as core from "@actions/core";
 
+/**
+ * This constant is set in `ava.setup.mjs` for tests.
+ */
+declare const __CODEQL_ACTION_TEST_ENV__: string | undefined;
+
 /**
  * Environment variables used by Default Setup to communicate the private registry proxy configuration.
  */
@@ -321,7 +326,7 @@ export class ReadOnlyEnv<T extends string | undefined = string | undefined> {
    * or if this instance was constructed by `getTestEnv`.
    */
   public isTestingEnv(): boolean {
-    return this.getOptional(SystemEnvVar.NODE_ENV) === "test";
+    return __CODEQL_ACTION_TEST_ENV__ === "unit-test";
   }
 }
 
diff --git a/src/testing-utils.ts b/src/testing-utils.ts
index 2011b56e0..cd6031894 100644
--- a/src/testing-utils.ts
+++ b/src/testing-utils.ts
@@ -184,12 +184,6 @@ export function makeMacro<Args extends unknown[]>(
 }
 
 export function getTestEnv(testEnv: NodeJS.ProcessEnv = {}): Env {
-  // In unit tests run by `ava`, this environment variable is set for the process,
-  // but we default to an empty environment here for `testEnv`, so we must set it
-  // explicitly to ensure that we get test-specific behaviour in functions when
-  // we need it.
-  testEnv["NODE_ENV"] = "test";
-
   return getEnv(testEnv);
 }