diff --git a/lib/ship-preflight.ts b/lib/ship-preflight.ts
index 6ccb08011..b9f5c06fb 100644
--- a/lib/ship-preflight.ts
+++ b/lib/ship-preflight.ts
@@ -154,7 +154,7 @@ export async function preflight(argv: readonly string[], cwd: string): Promise<{
   if (!base || typeof list === 'string') return { text: `gstack-ship-preflight: ${typeof list === 'string' ? list : '--base <base> is required'}\n`, exit: 2 };
   if (has('--tests-only') && !list.length) return { text: 'gstack-ship-preflight: --tests-only needs at least one --lane <label>=<command>\n', exit: 2 };
   const mode = flag('--lanes');
-  const concurrency = laneConcurrency(mode);
+  const concurrency = has('--lanes') && mode === undefined ? '--lanes needs serial or parallel' : laneConcurrency(mode);
   if (typeof concurrency === 'string') return { text: `gstack-ship-preflight: ${concurrency}\n`, exit: 2 };
   const r = new Report();
   if (has('--tests-only') || integrate(cwd, base, r)) {
diff --git a/test/ship-preflight.test.ts b/test/ship-preflight.test.ts
index cbe478189..a376a4116 100644
--- a/test/ship-preflight.test.ts
+++ b/test/ship-preflight.test.ts
@@ -124,6 +124,20 @@ describe('gstack-ship-preflight', () => {
     expect(r.stdout.split('\n').filter(l => l.startsWith('LANE: ')).map(l => l.replace(/ log=\S+$/, ''))).toEqual(['LANE: a pass exit=0 tests_ran=2', 'LANE: b pass exit=0 tests_ran=2']);
     expect(lineOf(r.stdout, 'LANES')).toBe('LANES: serial (--lanes)');
     expect(f.run(PREFLIGHT, ['--tests-only', '--base', 'main', '--lanes', 'fast', '--lane', LANE]).status).toBe(2);
+    expect(f.run(PREFLIGHT, ['--tests-only', '--base', 'main', '--lane', LANE, '--lanes']).status).toBe(2);
+  });
+
+  // Value: protects=parallel lanes report in the order given even when a later lane finishes first, and an unflagged run names the core count
+  //   behind its default (GST-851); fails_when=results come back in finish order or the LANES line drops its reason; why_new=runLanes became a
+  //   worker pool; seam=runLanes, LANES line; no_claim=which default any given machine picks
+  test('--lanes parallel keeps lane order when a later lane finishes first; unflagged LANES names the core count', () => {
+    const f = fx();
+    const lane = (n: string, wait: number) => `${n}=sleep ${wait}; printf " 1 pass\\n 0 fail\\nRan 1 tests across 1 file. [5.00ms]\\n"`;
+    const r = f.run(PREFLIGHT, ['--tests-only', '--base', 'main', '--lanes', 'parallel', '--lane', lane('slow', 1), '--lane', lane('fast', 0)]);
+    expect(r.status).toBe(0);
+    expect(lineOf(r.stdout, 'LANES')).toBe('LANES: parallel (--lanes)');
+    expect(r.stdout.split('\n').filter(l => l.startsWith('LANE: ')).map(l => l.split(' ')[1])).toEqual(['slow', 'fast']);
+    expect(lineOf(f.run(PREFLIGHT, ['--tests-only', '--base', 'main', '--lane', LANE]).stdout, 'LANES')).toMatch(/^LANES: (serial|parallel) \(\d+ cores\)$/);
   });
 
   // Value: protects=a small machine runs lanes one at a time without a flag, a large one keeps running them together, and the flag overrides