packages/mosaic/src/commands/lease-activation-probe.spec.ts

@@ -1,5 +1,8 @@
-import { describe, it, expect } from 'vitest';
+import { describe, it, expect, afterEach } from 'vitest';
 import { Command } from 'commander';
+import { existsSync, mkdirSync, rmSync, writeFileSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
 import {
   LEASE_ACTIVATION_CAPABILITY,
   LEASE_CAPABILITY_PROBE_COMMAND,
@@ -115,6 +118,54 @@ describe('defaultCapabilityProbe', () => {
     // artifact advertising the capability.
     expect(defaultCapabilityProbe()).toBeNull();
   });
+
+  describe('positive path — a real built dist/cli.js advertises the capability', () => {
+    // Resolve the ACTUAL @mosaicstack/mosaic package root on disk (two levels
+    // up from src/commands/), so this test exercises the exact same
+    // require.resolve('@mosaicstack/mosaic') self-reference codepath
+    // defaultCapabilityProbe() itself uses — no mocking of the resolver.
+    // This is the regression proof for the reviewer-found bug: the previous
+    // implementation resolved via the NOT-exported './package.json' subpath
+    // (ERR_PACKAGE_PATH_NOT_EXPORTED on every real install), which the
+    // catch-all silently turned into an always-null probe. That bug returns
+    // null here regardless of a built dist/cli.js being present — so this
+    // test fails red against it and only passes once resolution goes through
+    // the package's already-exported "." entry.
+    const mosaicRoot = join(dirname(fileURLToPath(import.meta.url)), '..', '..');
+    const distDir = join(mosaicRoot, 'dist');
+    const distIndexPath = join(distDir, 'index.js');
+    const distCliPath = join(distDir, 'cli.js');
+
+    // dist/ is gitignored and unbuilt in a fresh checkout; only remove what
+    // THIS test created, never a real build that predates it.
+    const distDirPreexisted = existsSync(distDir);
+    const indexPreexisted = existsSync(distIndexPath);
+    const cliPreexisted = existsSync(distCliPath);
+
+    afterEach(() => {
+      if (!cliPreexisted) rmSync(distCliPath, { force: true });
+      if (!indexPreexisted) rmSync(distIndexPath, { force: true });
+      if (!distDirPreexisted) rmSync(distDir, { recursive: true, force: true });
+    });
+
+    it('returns the real {name, version} capability object', () => {
+      mkdirSync(distDir, { recursive: true });
+      // Minimal stand-in for the built "." export target — only needs to
+      // exist for require.resolve('@mosaicstack/mosaic') to succeed; its
+      // content is never loaded by defaultCapabilityProbe().
+      writeFileSync(distIndexPath, 'export {};\n');
+      // Minimal stand-in for the built CLI's hidden __lease-capability
+      // subcommand — prints exactly what registerLeaseCapabilityProbe()
+      // wires the real `mosaic __lease-capability` command to print.
+      writeFileSync(
+        distCliPath,
+        `process.stdout.write(JSON.stringify(${JSON.stringify(LEASE_ACTIVATION_CAPABILITY)}));\n`,
+      );
+
+      const result = defaultCapabilityProbe();
+      expect(result).toEqual(LEASE_ACTIVATION_CAPABILITY);
+    });
+  });
 });
 
 describe('defaultSupervisorProbe', () => {

packages/mosaic/src/commands/lease-activation-probe.ts

@@ -94,8 +94,15 @@ export function registerLeaseCapabilityProbe(program: Command): void {
 export function defaultCapabilityProbe(): LeaseActivationCapability | null {
   try {
     const req = createRequire(import.meta.url);
-    const pkgJsonPath = req.resolve('@mosaicstack/mosaic/package.json');
-    const cliEntry = join(dirname(pkgJsonPath), 'dist', 'cli.js');
+    // Resolve via the package's "." export (already present in package.json's
+    // `exports` map) rather than a "./package.json" subpath — the latter is
+    // NOT exported, so `require.resolve('@mosaicstack/mosaic/package.json')`
+    // throws ERR_PACKAGE_PATH_NOT_EXPORTED on every real install, which the
+    // catch below would silently turn into an always-null probe. The "."
+    // export resolves to `dist/index.js`; `cli.js` is its sibling in the same
+    // built `dist/` directory (see package.json's `bin.mosaic`).
+    const mainEntry = req.resolve('@mosaicstack/mosaic');
+    const cliEntry = join(dirname(mainEntry), 'cli.js');
     if (!existsSync(cliEntry)) return null;
 
     const output = execFileSync(process.execPath, [cliEntry, LEASE_CAPABILITY_PROBE_COMMAND], {