diff --git a/tests/harness/differential.test.ts b/tests/harness/differential.test.ts
index fbf9cb1ec..a2206fd97 100644
--- a/tests/harness/differential.test.ts
+++ b/tests/harness/differential.test.ts
@@ -22,6 +22,7 @@ import { describe, expect, test } from "vitest";
 import ts5 from "typescript";
 import { compile } from "@scriptc/compiler";
 import { corpusSelect, shardSuffix } from "./shard.js";
+import { oracleEnvKey } from "./oracle-env.js";
 
 const execFileAsync = promisify(execFile);
 const repoRoot = join(import.meta.dirname, "../..");
@@ -238,8 +239,8 @@ function programInputs(file: string): string[] {
 // Node's verdict for a corpus program is a pure function of the program bytes,
 // the shims, and the Node build (corpus stdout is deterministic by
 // construction — it must be identical to a non-Node native binary's output). So
-// cache it, keyed by all of those plus the invocation shape (SCRIPTC_TEST_ENV
-// and the cwd). Only the SPAWN is skipped: the native side always runs live
+// cache it, keyed by all of those plus the invocation shape (the inherited
+// environment, see oracle-env.ts, and the cwd). Only the SPAWN is skipped: the native side always runs live
 // and the comparison itself never changes. SCRIPTC_NO_CACHE=1 (or an unset
 // SCRIPTC_CACHE_DIR) disables the cache in both directions — no reads, no writes.
 // Storage shares the compile cache's root and its LRU sweep (see native-toolchain.ts).
@@ -265,7 +266,7 @@ function oracleKeyBase(): Promise<string> {
       .update(readFileSync(fileURLToPath(comptimeShim)))
       .update("\0")
       .update("\0")
-      .update(process.env["SCRIPTC_TEST_ENV"] ?? "")
+      .update(oracleEnvKey(process.env))
       .update("\0")
       .update(process.cwd())
       .update("\0")
diff --git a/tests/harness/oracle-env.test.ts b/tests/harness/oracle-env.test.ts
new file mode 100644
index 000000000..da09dde25
--- /dev/null
+++ b/tests/harness/oracle-env.test.ts
@@ -0,0 +1,25 @@
+import { expect, test } from "vitest";
+import { oracleEnvKey } from "./oracle-env.js";
+
+test("the oracle key changes with any inherited variable", () => {
+  const base = { PATH: "/bin", SCRIPTC_TEST_ENV: "from-harness" };
+  expect(oracleEnvKey({ ...base, NODE_ENV: "production" })).not.toBe(
+    oracleEnvKey({ ...base, NODE_ENV: "test" }),
+  );
+  expect(oracleEnvKey({ ...base, NODE_ENV: "test" })).not.toBe(oracleEnvKey(base));
+  expect(oracleEnvKey({ ...base, FOO: "a=b" })).not.toBe(
+    oracleEnvKey({ ...base, FOO: "a", B: "" }),
+  );
+});
+
+test("the oracle key ignores the runner's worker numbers and variable order", () => {
+  expect(oracleEnvKey({ A: "1", B: "2", VITEST_WORKER_ID: "1", VITEST_POOL_ID: "1" })).toBe(
+    oracleEnvKey({
+      VITEST_POOL_ID: "7",
+      B: "2",
+      TINYPOOL_WORKER_ID: "3",
+      A: "1",
+      VITEST_WORKER_ID: "9",
+    }),
+  );
+});
diff --git a/tests/harness/oracle-env.ts b/tests/harness/oracle-env.ts
new file mode 100644
index 000000000..5f66f26bb
--- /dev/null
+++ b/tests/harness/oracle-env.ts
@@ -0,0 +1,15 @@
+/* The environment part of the differential oracle cache key. The Node
+ * oracle runs with the harness's inherited environment, so a program whose
+ * output reads a variable (NODE_ENV, SCRIPTC_TEST_ENV, ...) must not reuse a
+ * verdict recorded under a different value. The variables that number the
+ * test runner's own workers differ between workers of one run and are left
+ * out, so every worker shares the cache. */
+const WORKER_VARIABLES = new Set(["VITEST_POOL_ID", "VITEST_WORKER_ID", "TINYPOOL_WORKER_ID"]);
+
+export function oracleEnvKey(env: NodeJS.ProcessEnv): string {
+  return Object.keys(env)
+    .filter((name) => !WORKER_VARIABLES.has(name) && env[name] !== undefined)
+    .sort()
+    .map((name) => `${name}=${env[name]}\0`)
+    .join("");
+}