diff --git a/scripts/bootstrap-seed.mjs b/scripts/bootstrap-seed.mjs
index fc9a297d8..6bce075f5 100644
--- a/scripts/bootstrap-seed.mjs
+++ b/scripts/bootstrap-seed.mjs
@@ -25,6 +25,19 @@ export function verifyBootstrapSeed(directory, revision, sanitize) {
   }
 }
 
+/** The environment a bootstrap seed builds with, over the inherited one. The
+ * bootstrap compares the seed's LLVM with the native compiler's own emission,
+ * which has no function layout. The layout only orders the functions, so the
+ * seed is built without it. */
+export function seedBuildEnvironment(sanitize) {
+  return {
+    SCRIPTC_SAN: sanitize ? "1" : "",
+    SCRIPTC_NATIVE_EMIT_IR: "1",
+    SCRIPTC_NATIVE_OPTIMIZATION: sanitize ? "dev" : "release",
+    SCRIPTC_NATIVE_LAYOUT: "",
+  };
+}
+
 async function main() {
   const directory = process.env.SCRIPTC_BOOTSTRAP_SEED_DIRECTORY;
   const revision = process.env.GITHUB_SHA;
@@ -47,15 +60,7 @@ async function main() {
     {
       cwd: root,
       stdio: "inherit",
-      env: {
-        ...process.env,
-        SCRIPTC_NATIVE_EMIT_IR: "1",
-        SCRIPTC_NATIVE_OPTIMIZATION: sanitize ? "dev" : "release",
-        // The bootstrap compares the seed's LLVM with the native compiler's
-        // own emission, which has no function layout. The layout only orders
-        // the functions, so the seed is built without it.
-        SCRIPTC_NATIVE_LAYOUT: "",
-      },
+      env: { ...process.env, ...seedBuildEnvironment(sanitize) },
     },
   );
   writeFileSync(
diff --git a/tests/harness/bootstrap-seed.test.ts b/tests/harness/bootstrap-seed.test.ts
index 76b1f6de5..2da27baea 100644
--- a/tests/harness/bootstrap-seed.test.ts
+++ b/tests/harness/bootstrap-seed.test.ts
@@ -3,7 +3,7 @@ import { tmpdir } from "node:os";
 import { dirname, join } from "node:path";
 import { load } from "js-yaml";
 import { expect, test } from "vitest";
-import { verifyBootstrapSeed } from "../../scripts/bootstrap-seed.mjs";
+import { seedBuildEnvironment, verifyBootstrapSeed } from "../../scripts/bootstrap-seed.mjs";
 
 test("bootstrap artifacts require a complete seed from the same revision, lane, and path", () => {
   const directory = mkdtempSync(join(tmpdir(), "scriptc-seed-contract-"));
@@ -36,6 +36,22 @@ test("bootstrap artifacts require a complete seed from the same revision, lane,
   }
 });
 
+test("seeds build without the function layout in both lanes, shared or not", () => {
+  for (const sanitize of [false, true])
+    expect(seedBuildEnvironment(sanitize)).toEqual({
+      SCRIPTC_SAN: sanitize ? "1" : "",
+      SCRIPTC_NATIVE_EMIT_IR: "1",
+      SCRIPTC_NATIVE_OPTIMIZATION: sanitize ? "dev" : "release",
+      SCRIPTC_NATIVE_LAYOUT: "",
+    });
+  // The bootstrap test builds its own seed when CI shares none.
+  const driver = readFileSync(
+    new URL("./self-hosting-native-driver.test.ts", import.meta.url),
+    "utf8",
+  );
+  expect(driver).toContain("...seedBuildEnvironment(sanitize)");
+});
+
 test("CI restores each seed only into its matching instrumented bootstrap lane", () => {
   const workflow = load(
     readFileSync(new URL("../../.github/workflows/ci.yml", import.meta.url), "utf8"),
diff --git a/tests/harness/self-hosting-native-driver.test.ts b/tests/harness/self-hosting-native-driver.test.ts
index 1cc920bd2..8cfd16eea 100644
--- a/tests/harness/self-hosting-native-driver.test.ts
+++ b/tests/harness/self-hosting-native-driver.test.ts
@@ -1,4 +1,4 @@
-import { verifyBootstrapSeed } from "../../scripts/bootstrap-seed.mjs";
+import { seedBuildEnvironment, verifyBootstrapSeed } from "../../scripts/bootstrap-seed.mjs";
 import { execFile, spawnSync } from "node:child_process";
 import {
   chmodSync,
@@ -89,12 +89,7 @@ test(
               // seed avoids optimizing the huge compiler module before ASan can
               // exercise its complete traversal, serialization and LLVM emission.
               ...options,
-              env: {
-                ...process.env,
-                SCRIPTC_SAN: sanitize ? "1" : "",
-                SCRIPTC_NATIVE_EMIT_IR: "1",
-                SCRIPTC_NATIVE_OPTIMIZATION: sanitize ? "dev" : "release",
-              },
+              env: { ...process.env, ...seedBuildEnvironment(sanitize) },
             },
           ),
         );