diff --git a/scripts/vitest-global-setup.ts b/scripts/vitest-global-setup.ts
new file mode 100644
index 000000000..02b5d552e
--- /dev/null
+++ b/scripts/vitest-global-setup.ts
@@ -0,0 +1,45 @@
+#!/usr/bin/env node
+/**
+ * Vitest `globalSetup` — runs once before the whole test run, regardless of
+ * whether it was invoked via `npm test` or a direct `npx vitest run <file>`.
+ *
+ * Issue #2439: the WASM engine always parses through the COMPILED
+ * `dist/domain/wasm-worker-entry.js`, even when a test imports `src/*.ts`
+ * directly — so editing an extractor under `src/` silently exercises stale
+ * compiled code until `dist/` is rebuilt. `pretest` only runs `npm run
+ * doctor`, which doesn't rebuild, and `npm install`'s `prepare` script
+ * builds `dist/` once, not on every subsequent edit. In PR #2432 this
+ * manifested as "the native engine is correct and WASM reproduces the old
+ * buggy behaviour" — indistinguishable from a genuine engine-parity bug.
+ *
+ * A hand-rolled staleness check (comparing dist/'s mtime against src/'s)
+ * was tried and reverted: `tsconfig.json` sets `incremental: true`, so tsc
+ * skips re-emitting an output file whose compiled content wouldn't change —
+ * `dist/domain/wasm-worker-entry.js`'s own mtime reflects when THAT file
+ * was last actually recompiled, not when the project was last built, and is
+ * routinely older than unrelated src/ files even in a fully up-to-date
+ * build. That produced false positives on every run. Just running the
+ * build is simpler and correct, because it defers to tsc's own incremental
+ * engine — the only thing that actually knows what's stale — instead of
+ * reimplementing it.
+ *
+ * A no-op incremental rebuild costs about a second, dominated by process
+ * startup rather than compilation, and runs ONCE per test invocation here
+ * (not once per test file), so the added cost is negligible against a
+ * multi-minute full suite run.
+ */
+
+import { execFileSync } from 'node:child_process';
+import os from 'node:os';
+import { fileURLToPath } from 'node:url';
+
+const repoRoot = fileURLToPath(new URL('..', import.meta.url));
+
+// npm on Windows is npm.cmd; Node refuses to spawn .cmd/.bat without a shell.
+// Safe with shell: true here since the argv is a fixed literal, never
+// user-controlled input (matches scripts/doctor.ts's NPM_SHELL convention).
+const NPM_SHELL = os.platform() === 'win32';
+
+export default function setup(): void {
+  execFileSync('npm', ['run', 'build'], { cwd: repoRoot, stdio: 'inherit', shell: NPM_SHELL });
+}
diff --git a/tests/unit/vitest-global-setup.test.ts b/tests/unit/vitest-global-setup.test.ts
new file mode 100644
index 000000000..f9247f78d
--- /dev/null
+++ b/tests/unit/vitest-global-setup.test.ts
@@ -0,0 +1,43 @@
+/**
+ * Unit test for scripts/vitest-global-setup.ts (issue #2439).
+ *
+ * The WASM engine's worker always loads COMPILED dist/, even when a test
+ * imports src/*.ts directly, so an edit to an extractor silently exercises
+ * stale compiled code until dist/ is rebuilt. This globalSetup hook rebuilds
+ * dist/ once before the whole vitest run (regardless of `npm test` vs. a
+ * direct `npx vitest run <file>`) rather than reimplementing tsc's own
+ * incremental staleness detection — see that file's doc comment for why a
+ * hand-rolled mtime comparison was tried and reverted.
+ */
+
+import { fileURLToPath } from 'node:url';
+import { describe, expect, it, vi } from 'vitest';
+
+vi.mock('node:child_process', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('node:child_process')>();
+  return {
+    ...actual,
+    execFileSync: vi.fn(),
+  };
+});
+
+// Mirrors exactly how scripts/vitest-global-setup.ts derives its own cwd —
+// fileURLToPath(new URL(...)) is not guaranteed trailing-slash-equivalent to
+// path.resolve() across platforms, and this test only cares that setup()
+// passes ITS OWN computed root through unchanged.
+const repoRoot = fileURLToPath(new URL('../..', import.meta.url));
+
+describe('vitest global setup (#2439)', () => {
+  it('runs npm run build in the repo root before the test run starts', async () => {
+    const { execFileSync } = await import('node:child_process');
+    const setup = (await import('../../scripts/vitest-global-setup.js')).default;
+
+    setup();
+
+    expect(execFileSync).toHaveBeenCalledWith(
+      'npm',
+      ['run', 'build'],
+      expect.objectContaining({ cwd: repoRoot, stdio: 'inherit' }),
+    );
+  });
+});
diff --git a/vitest.config.ts b/vitest.config.ts
index c78f12ae3..075ee84b7 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -18,6 +18,13 @@ export default defineConfig({
     testTimeout: 30000,
     hookTimeout: 30000,
     exclude: ['**/node_modules/**', '**/.git/**', '**/.claude/**'],
+    // Issue #2439: rebuild dist/ once before the whole run (regardless of
+    // `npm test` vs. a direct `npx vitest run <file>`), so the WASM engine's
+    // worker — which always loads compiled dist/, even when tests import
+    // src/*.ts directly — never silently runs stale extraction logic. See
+    // scripts/vitest-global-setup.ts for why this isn't a hand-rolled
+    // staleness check.
+    globalSetup: './scripts/vitest-global-setup.ts',
     // Ensure child processes spawned by tests (e.g. CLI integration tests)
     // can load .ts files via Node's built-in type stripping.
     env: {