diff --git a/packages/compiler/src/node-islands/probe-recorder.test.ts b/packages/compiler/src/node-islands/probe-recorder.test.ts
new file mode 100644
index 000000000..2e82be3e9
--- /dev/null
+++ b/packages/compiler/src/node-islands/probe-recorder.test.ts
@@ -0,0 +1,74 @@
+import { spawnSync } from "node:child_process";
+import { existsSync, mkdirSync, mkdtempSync, realpathSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { dirname, join } from "node:path";
+import { afterEach, expect, test } from "vitest";
+import { PROBE_RECORDER } from "./probe-recorder.js";
+
+const made: string[] = [];
+afterEach(() => {
+  for (const dir of made.splice(0)) rmSync(dir, { recursive: true, force: true });
+});
+
+function project(files: Record<string, string>): string {
+  const root = realpathSync(mkdtempSync(join(tmpdir(), "scriptc-probe-recorder-")));
+  made.push(root);
+  for (const [path, text] of Object.entries(files)) {
+    mkdirSync(dirname(join(root, path)), { recursive: true });
+    writeFileSync(join(root, path), text);
+  }
+  return root;
+}
+
+interface ProbeResult {
+  path: string;
+  throws?: string;
+  root?: number;
+}
+
+/** Runs the recorder in a Node process of its own, as the probe host runs
+ * it in libnode, over `modules`. */
+function probe(root: string, modules: string[]): ProbeResult[] {
+  writeFileSync(join(root, "recorder.cjs"), PROBE_RECORDER);
+  writeFileSync(
+    join(root, "run.cjs"),
+    `const out = require("./recorder.cjs").probe({ modules: ${JSON.stringify(modules)} });\n` +
+      `process.stdout.write(JSON.stringify(out.results));\n`,
+  );
+  const run = spawnSync(process.execPath, [join(root, "run.cjs")], { cwd: root, encoding: "utf8" });
+  expect(run.status, run.stderr).toBe(0);
+  return JSON.parse(run.stdout) as ProbeResult[];
+}
+
+// A source file that, when it evaluates, writes `evaluated-<name>` beside itself.
+const marker = (name: string): string =>
+  `require("fs").writeFileSync(require("path").join(__dirname, "evaluated-${name}"), "");\nmodule.exports = { n: 1 };\n`;
+
+test("the probe evaluates an installed package", () => {
+  const root = project({
+    "node_modules/pkg/package.json": '{ "name": "pkg" }',
+    "node_modules/pkg/index.js": "module.exports = { answer: 42 };\n",
+  });
+  const [result] = probe(root, [join(root, "node_modules/pkg/index.js")]);
+  expect(result!.throws).toBeUndefined();
+  expect(result!.root).toBeDefined();
+});
+
+test("the probe never evaluates the program's own source", () => {
+  const root = project({
+    "src/program.js": marker("program"),
+    "src/shared.js": marker("shared"),
+    "node_modules/reaches/package.json": '{ "name": "reaches" }',
+    "node_modules/reaches/index.js": 'module.exports = require("../../src/shared.js");\n',
+  });
+  const results = probe(root, [
+    join(root, "src/program.js"),
+    join(root, "node_modules/reaches/index.js"),
+  ]);
+  expect(results.map((r) => r.throws)).toEqual([
+    `the build does not evaluate ${join(root, "src/program.js")}, a file of the program`,
+    `the build does not evaluate ${join(root, "src/shared.js")}, a file of the program`,
+  ]);
+  expect(existsSync(join(root, "src/evaluated-program"))).toBe(false);
+  expect(existsSync(join(root, "src/evaluated-shared"))).toBe(false);
+});
diff --git a/packages/compiler/src/node-islands/probe-recorder.ts b/packages/compiler/src/node-islands/probe-recorder.ts
index 499de3593..af29be084 100644
--- a/packages/compiler/src/node-islands/probe-recorder.ts
+++ b/packages/compiler/src/node-islands/probe-recorder.ts
@@ -20,6 +20,8 @@
  *     (island-local, since native code has its own builtins, but
  *     observable by other island modules evaluated out of order);
  *   - every file the load required (the module's closure), and a throw;
+ *     a load that resolves a file outside every node_modules directory (the
+ *     program's own source) throws before that file evaluates;
  *
  * and takes the interface snapshot of the module's value (snapshot.ts).
  * Calls made by Node's own internals (the module loader reading files,
@@ -429,11 +431,19 @@ exports.probe = function probe(input) {
   undo.push(() => hook.disable());
   // The closure: a resolve hook (no frame of ours stays on the stack while
   // modules evaluate, so Node's node_modules checks see the real callers).
+  // The probe evaluates installed packages only: a file outside every
+  // node_modules directory is the program's own source (a program module,
+  // or a project file a package requires), and its load throws before the
+  // file evaluates, so the module that asked for it stays eager.
   const hooks = Module.registerHooks({
     resolve(specifier, context, nextResolve) {
       const result = nextResolve(specifier, context);
-      if (recording && typeof result.url === "string" && result.url.startsWith("file:"))
-        files.add(require("node:url").fileURLToPath(result.url));
+      if (recording && typeof result.url === "string" && result.url.startsWith("file:")) {
+        const file = require("node:url").fileURLToPath(result.url);
+        if (!/[\\/]node_modules[\\/]/.test(file))
+          throw new Error("the build does not evaluate " + file + ", a file of the program");
+        files.add(file);
+      }
       return result;
     },
   });