diff --git a/lib/ship-bin-common.ts b/lib/ship-bin-common.ts
index 2702f251f..0bb6e6b05 100644
--- a/lib/ship-bin-common.ts
+++ b/lib/ship-bin-common.ts
@@ -7,6 +7,7 @@
  */
 import { spawn, spawnSync } from 'node:child_process';
 import * as fs from 'node:fs';
+import * as os from 'node:os';
 import * as path from 'node:path';
 import { fragmentSlug } from './changelog-fragments';
 import { parseUnifiedDiff, stampOnlyLine, versionAtRef } from './restamp';
@@ -150,13 +151,32 @@ export class Report {
   }
 }
 
-/** Run each lane through gstack-evidence run in parallel and read its recorded line. */
-export async function runLanes(cwd: string, list: { label: string; command: string }[]): Promise<{ label: string; exit: number | null; testsRan: string; log: string }[]> {
-  return Promise.all(list.map(async l => {
-    const r = await binAsync(cwd, 'gstack-evidence', ['run', '--label', l.label, '--', l.command]);
-    const m = /gstack-evidence: recorded label=\S+ exit=(\S+) tests_ran=(\S+) log=(\S+)/.exec(r.stderr);
-    return { label: l.label, exit: r.status, testsRan: m?.[2] ?? 'unknown', log: m?.[3] ?? '-' };
-  }));
+/**
+ * How many lanes run at once (GST-851): `--lanes serial` one, `--lanes parallel` all; unset, one on a machine with 4 or fewer
+ * cores (six lanes at once took 4-core machines down on gstack-evals) and all above that. Any other value is a usage error.
+ */
+export function laneConcurrency(mode: string | undefined, cpus = os.availableParallelism()): number | string {
+  if (mode === 'serial') return 1;
+  if (mode === 'parallel') return Infinity;
+  if (mode !== undefined) return `--lanes ${mode} is not serial or parallel`;
+  return cpus <= 4 ? 1 : Infinity;
+}
+
+/** Run each lane through gstack-evidence run, at most `concurrency` at a time, and read its recorded line; results keep the lanes' order. */
+export async function runLanes(cwd: string, list: { label: string; command: string }[], concurrency = Infinity): Promise<{ label: string; exit: number | null; testsRan: string; log: string }[]> {
+  const out: { label: string; exit: number | null; testsRan: string; log: string }[] = [];
+  let next = 0;
+  const worker = async () => {
+    while (next < list.length) {
+      const i = next++;
+      const l = list[i]!;
+      const r = await binAsync(cwd, 'gstack-evidence', ['run', '--label', l.label, '--', l.command]);
+      const m = /gstack-evidence: recorded label=\S+ exit=(\S+) tests_ran=(\S+) log=(\S+)/.exec(r.stderr);
+      out[i] = { label: l.label, exit: r.status, testsRan: m?.[2] ?? 'unknown', log: m?.[3] ?? '-' };
+    }
+  };
+  await Promise.all(Array.from({ length: Math.max(1, Math.min(concurrency, list.length)) }, worker));
+  return out;
 }
 
 /** Step 5's lane verdict: a lane passes on its runner count, never exit 0 alone. */
diff --git a/lib/ship-preflight.ts b/lib/ship-preflight.ts
index dd1f51d5e..6ccb08011 100644
--- a/lib/ship-preflight.ts
+++ b/lib/ship-preflight.ts
@@ -6,22 +6,24 @@
  * agent (a merge conflict, a failing or zero-run lane, the carry route, the Apple target).
  */
 import * as fs from 'node:fs';
+import * as os from 'node:os';
 import * as path from 'node:path';
 import { classifyDiff } from './review-depth';
 import { audit } from './ship-audit';
-import { bin, firstLine, flagReader, gh, git, hostedOrigin, laneLine, lanes, Report, runLanes, shipPolicy, short } from './ship-bin-common';
+import { bin, firstLine, flagReader, gh, git, hostedOrigin, laneConcurrency, laneLine, lanes, Report, runLanes, shipPolicy, short } from './ship-bin-common';
 
 export const PREFLIGHT_HELP = `gstack-ship-preflight — /ship Steps 1-3 and Step 5's pregate and lanes in one call (ship/SKILL.md)
 
   gstack-ship-preflight --base <base> [--lane <label>=<command>]...   Steps 1-3; with lanes also Step 5
   gstack-ship-preflight --tests-only --base <base> --lane <label>=<command>...   Step 5 only (after a repair)
+      --lanes serial|parallel   how Step 5 runs its lanes; unset, one at a time on 4 or fewer cores, all at once above
       --policy-from head   Step 5's pregate reads the branch's .gstack/pregate.json, which may only add test lanes
                            (a branch that adds the lane for a file it touches); anything else is PREGATE_HEAD_POLICY_LOOSENS
   gstack-ship-preflight --audit --base <base> --out <dir>   the single reads and Steps 7-8's deterministic audits
       (CONTEXT, TODOS, COVERAGE_MAP, Coverage, PLAN_*, READER: coverage=needed|none plan=needed|none)
 
 Prints at most 30 lines: BRANCH, STATUS, DIFF, PR/CARRY, APPLE, SCOPE, MERGE, DISTRIBUTION, POLICY,
-RUNTIME_LINES, PREGATE (with up to three PREGATE_DETAIL lines when it does not pass), one LANE line per lane,
+RUNTIME_LINES, PREGATE (with up to three PREGATE_DETAIL lines when it does not pass), LANES, one LANE line per lane,
 then NEXT: lines. STOP: names what the agent must
 decide or repair; nothing after it ran. Exit codes: 0 ok · 2 usage · 3 STOP`;
 
@@ -118,7 +120,7 @@ function merge(cwd: string, base: string, r: Report): boolean {
   return true;
 }
 
-async function tests(cwd: string, base: string, list: { label: string; command: string }[], policyFrom: string, r: Report): Promise<void> {
+async function tests(cwd: string, base: string, list: { label: string; command: string }[], policyFrom: string, run: { concurrency: number; line: string }, r: Report): Promise<void> {
   const pg = bin(cwd, 'gstack-pregate', ['--allow-repo-commands', '--base', base, '--policy-from', policyFrom], 3_600_000);
   const refusal = pg.status === 3 ? /\((PREGATE_[A-Z_]+|POLICY_[A-Z_]+)[;)]/.exec(pg.stderr)?.[1] : undefined;
   const verdict = refusal ? `refused (${refusal})` : /^PREGATE: (.+?) tree=/m.exec(pg.stdout + pg.stderr)?.[1] ?? `unreadable (exit ${pg.status})`;
@@ -129,7 +131,8 @@ async function tests(cwd: string, base: string, list: { label: string; command:
     return;
   }
   if (/uncleared/.test(verdict)) r.next('pregate requires-remote: gstack-pregate clear, or --remote --spend-cap <usd> under a user cap, before Step 18');
-  const results = (await runLanes(cwd, list)).map(laneLine);
+  r.add(run.line);
+  const results = (await runLanes(cwd, list, run.concurrency)).map(laneLine);
   for (const l of results) r.add(l.line);
   const bad = results.filter(l => !l.ok);
   if (bad.length) { r.stop(`${bad.length} lane(s) failed or ran zero tests: Step 5 triage (read each log; a free-suite shard failure reruns its file list per measure.md); in-branch failures stop the ship`); return; }
@@ -150,9 +153,12 @@ export async function preflight(argv: readonly string[], cwd: string): Promise<{
   if (!['base', 'head'].includes(policyFrom)) return { text: 'gstack-ship-preflight: --policy-from needs base|head\n', exit: 2 };
   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);
+  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)) {
-    if (list.length) await tests(cwd, base, list, policyFrom, r);
+    if (list.length) await tests(cwd, base, list, policyFrom, { concurrency, line: `LANES: ${concurrency === 1 ? 'serial' : 'parallel'} (${mode ? '--lanes' : `${os.availableParallelism()} cores`})` }, r);
     else if (r.exit === 0) r.next('Step 4: confirm the test framework, then rerun with --tests-only --lane <label>=<command> per suite (Step 5)');
   }
   return { text: r.render(), exit: r.exit };
diff --git a/test/ship-preflight.test.ts b/test/ship-preflight.test.ts
index 919f40612..cbe478189 100644
--- a/test/ship-preflight.test.ts
+++ b/test/ship-preflight.test.ts
@@ -6,6 +6,7 @@
 import { afterEach, describe, expect, setDefaultTimeout, test } from 'bun:test';
 import * as fs from 'node:fs';
 import * as path from 'node:path';
+import { laneConcurrency } from '../lib/ship-bin-common';
 import { lineOf, PREFLIGHT, shipFixture, type ShipFx } from './helpers/ship-bin-fixture';
 
 setDefaultTimeout(60_000);
@@ -107,6 +108,33 @@ describe('gstack-ship-preflight', () => {
     expect(lineOf(zero.stdout, 'LANE')).toMatch(/^LANE: tests ZERO-RUN exit=0 tests_ran=0/);
   });
 
+  // Value: protects=Step 5 can run its lanes one at a time, and each lane keeps its own tests_ran (GST-851: GSTA-2 saw six lanes start within
+  //   60 ms on a 4-core machine on gstack-evals#115); fails_when=--lanes serial lanes overlap, a lane loses its count, or a bad --lanes value runs;
+  //   why_new=runLanes started every lane at once with no serial mode; seam=runLanes; no_claim=how long a lane takes
+  test('--lanes serial runs one lane at a time and keeps each lane\'s count; a bad --lanes value is usage', () => {
+    const f = fx();
+    const log = path.join(f.root, 'lane-order.log');
+    const lane = (n: string) => `${n}=echo start ${n} >> '${log}'; sleep 1; echo end ${n} >> '${log}'; printf " 2 pass\\n 0 fail\\nRan 2 tests across 1 file. [5.00ms]\\n"`;
+    const r = f.run(PREFLIGHT, ['--tests-only', '--base', 'main', '--lanes', 'serial', '--lane', lane('a'), '--lane', lane('b')]);
+    expect(r.status).toBe(0);
+    const order = fs.readFileSync(log, 'utf8').trim().split('\n');
+    expect(order.length).toBe(4);
+    expect(order.map(l => l.split(' ')[0])).toEqual(['start', 'end', 'start', 'end']);
+    expect(order[1]!.split(' ')[1]).toBe(order[0]!.split(' ')[1]);
+    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);
+  });
+
+  // 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
+  //   either way (GST-851); fails_when=a 4-core default runs lanes at once or the flag is ignored; why_new=no default existed; seam=laneConcurrency;
+  //   no_claim=the core count of any CI runner
+  test('lane concurrency: serial on 4 cores or fewer, parallel above, --lanes overrides', () => {
+    expect([laneConcurrency(undefined, 2), laneConcurrency(undefined, 4), laneConcurrency(undefined, 8)]).toEqual([1, 1, Infinity]);
+    expect([laneConcurrency('serial', 64), laneConcurrency('parallel', 2)]).toEqual([1, Infinity]);
+    expect(laneConcurrency('fast', 8)).toBe('--lanes fast is not serial or parallel');
+  });
+
   // Value: protects=Step 2's candidates and Step 1.4's carry check are reported for the agent to judge, and an open PR's carry verdict is printed; fails_when=a new bin/ artifact goes unmentioned or the carry check is skipped when a PR is open;
   //   why_new=moved from Steps 1.4 and 2; seam=gh, gstack-ship-receipt carry; no_claim=whether a candidate is a distributable
   test('a new artifact path is a Step 2 candidate; an open PR runs the carry dry-run and prints its verdict', () => {