diff --git a/.github/workflows/free-tests.yml b/.github/workflows/free-tests.yml
index 9cfb037ab..7b65cf9d9 100644
--- a/.github/workflows/free-tests.yml
+++ b/.github/workflows/free-tests.yml
@@ -227,6 +227,57 @@ jobs:
       - name: CSO source formatting
         run: bun run format:cso:check
 
+  # Capy machines install GSTACK_BUN_DEFAULT (bin/gstack-bun-version.sh) when
+  # no project pin decides, above the 1.4.2 minimum every other lane pins, so
+  # the quick tier also runs on it. Its pin equals GSTACK_BUN_DEFAULT;
+  # test/bun-version-drift.test.ts checks this job exists and agrees.
+  quick-bun-default:
+    runs-on: ubuntu-24.04
+    timeout-minutes: 30
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
+        with:
+          persist-credentials: false
+      - uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2
+        with:
+          bun-version: 1.4.3
+      - name: Install dependencies
+        run: bun install --frozen-lockfile
+      - name: Install Playwright Chromium
+        run: npx playwright install --with-deps chromium
+      - name: Configure the bundled Chromium sandbox helper
+        run: |
+          set -euo pipefail
+          chrome=$(bun -e 'import { chromium } from "playwright"; import { realpathSync } from "node:fs"; console.log(realpathSync(chromium.executablePath()))')
+          case "$chrome" in
+            "$HOME"/.cache/ms-playwright/chromium-*/chrome-linux*/chrome) ;;
+            *) echo "Unexpected Chromium installation path" >&2; exit 1 ;;
+          esac
+          helper="${chrome%/*}/chrome_sandbox"
+          installed="${chrome%/*}/chrome-sandbox"
+          test -f "$helper" && test ! -L "$helper"
+          sudo install -T -o root -g root -m 4755 "$helper" "$installed"
+      - name: Install Xvfb + X11 utilities + gate tools
+        run: sudo apt-get install -y --no-install-recommends xvfb x11-utils poppler-utils fonts-noto-color-emoji zsh
+      - name: Configure git identity (tests init temp repos)
+        run: |
+          git config --global user.email "free-tests-ci@gstack.test"
+          git config --global user.name "Free Tests CI"
+          git config --global init.defaultBranch main
+          git config --global --add safe.directory '*'
+      - name: Generate host SKILL.md outputs (.agents, .factory)
+        run: bun run gen:skill-docs --host all
+      - name: Vendor xterm assets into the extension
+        run: bun run vendor:xterm
+      - name: Build server-node bundle (loaded by browse cli imports)
+        run: bash browse/scripts/build-node-server.sh
+      - name: Build gate binaries (make-pdf e2e gates)
+        run: bun run build:gates
+      - name: Build trusted CSO helper used by free contract tests
+        run: bun run build:cso
+      - name: Run the quick free tier on the install-default Bun
+        run: xvfb-run -a bun run test:quick
+
   free-suite:
     needs: free-plan
     runs-on: ubicloud-standard-8
@@ -396,14 +447,15 @@ jobs:
   # gate is merge-blocking without a separate branch-protection migration.
   free-tests:
     if: always()
-    needs: [free-suite, typecheck, cso-macos-launcher, macos-named-regressions, cso-windows-launcher, cso-docker-integration]
+    needs: [free-suite, typecheck, quick-bun-default, cso-macos-launcher, macos-named-regressions, cso-windows-launcher, cso-docker-integration]
     runs-on: ubuntu-24.04
     timeout-minutes: 5
     steps:
-      - name: Require the free suite, typecheck, macOS regressions, and every CSO platform gate
+      - name: Require the free suite, typecheck, the quick tier on the default Bun, macOS regressions, and every CSO platform gate
         env:
           FREE_SUITE_RESULT: ${{ needs.free-suite.result }}
           TYPECHECK_RESULT: ${{ needs.typecheck.result }}
+          QUICK_BUN_DEFAULT_RESULT: ${{ needs.quick-bun-default.result }}
           CSO_MACOS_RESULT: ${{ needs.cso-macos-launcher.result }}
           MACOS_REGRESSIONS_RESULT: ${{ needs.macos-named-regressions.result }}
           CSO_WINDOWS_RESULT: ${{ needs.cso-windows-launcher.result }}
@@ -412,6 +464,7 @@ jobs:
           set -eu
           test "$FREE_SUITE_RESULT" = success
           test "$TYPECHECK_RESULT" = success
+          test "$QUICK_BUN_DEFAULT_RESULT" = success
           test "$CSO_MACOS_RESULT" = success
           test "$MACOS_REGRESSIONS_RESULT" = success
           test "$CSO_WINDOWS_RESULT" = success
diff --git a/test/bun-version-drift.test.ts b/test/bun-version-drift.test.ts
index 55a83348f..724bc337e 100644
--- a/test/bun-version-drift.test.ts
+++ b/test/bun-version-drift.test.ts
@@ -28,8 +28,13 @@ const WORKFLOWS_DIR = path.join(ROOT, '.github', 'workflows');
 interface Pin {
   surface: string;
   version: string;
+  /** The workflow job the pin sits in (empty outside a workflow). */
+  job?: string;
 }
 
+/** The one job that runs on GSTACK_BUN_DEFAULT instead of the minimum (plan A-g). */
+const DEFAULT_BUN_JOB = 'quick-bun-default';
+
 function collectPins(): Pin[] {
   const pins: Pin[] = [];
 
@@ -37,7 +42,9 @@ function collectPins(): Pin[] {
     if (!/\.ya?ml$/.test(name)) continue;
     const source = fs.readFileSync(path.join(WORKFLOWS_DIR, name), 'utf-8');
     const lines = source.split('\n');
+    let job = '';
     for (let i = 0; i < lines.length; i++) {
+      job = lines[i].match(/^ {2}([A-Za-z0-9_-]+):\s*$/)?.[1] ?? job;
       if (!/uses:\s*oven-sh\/setup-bun@/.test(lines[i])) continue;
       // A pinned stanza is `with:` + `bun-version: <v>` within the next few
       // lines; an unpinned setup-bun is itself drift (installs latest).
@@ -46,6 +53,7 @@ function collectPins(): Pin[] {
       pins.push({
         surface: `${name}:${i + 1}`,
         version: m ? m[1] : '<unpinned setup-bun — installs latest>',
+        job,
       });
     }
   }
@@ -69,8 +77,8 @@ function collectPins(): Pin[] {
 }
 
 describe('bun version pins', () => {
-  test('every CI surface pins the same bun version', () => {
-    const pins = collectPins();
+  test('every CI surface pins the same bun version (the install-default job aside)', () => {
+    const pins = collectPins().filter(p => p.job !== DEFAULT_BUN_JOB);
     // Sanity: the scan found the known surfaces (a regex rot that finds
     // nothing must fail loudly, not vacuously pass).
     expect(pins.length).toBeGreaterThanOrEqual(6);
@@ -96,19 +104,40 @@ describe('bun version pins', () => {
   });
 });
 
-function bunVersionsFile(): { floor: string; tested: string } {
+function bunVersionsFile(): { floor: string; tested: string; installDefault: string; verified: string[] } {
   const src = fs.readFileSync(path.join(ROOT, 'bin', 'gstack-bun-version.sh'), 'utf-8');
   return {
     floor: src.match(/^GSTACK_BUN_FLOOR="([^"]+)"$/m)?.[1] ?? '<no GSTACK_BUN_FLOOR>',
     tested: src.match(/^GSTACK_BUN_TESTED="([^"]+)"$/m)?.[1] ?? '<no GSTACK_BUN_TESTED>',
+    installDefault: src.match(/^GSTACK_BUN_DEFAULT="([^"]+)"$/m)?.[1] ?? '<no GSTACK_BUN_DEFAULT>',
+    verified: (src.match(/^GSTACK_BUN_VERIFIED="([^"]+)"$/m)?.[1] ?? '').split(/\s+/).filter(Boolean),
   };
 }
 
+// Plan A2/A-g: the Capy install default is separate from the floor. It must be
+// a verified archive at or above the minimum, and exactly one CI job (the
+// quick free tier) runs on it so the default Bun is never untested.
+describe('install default Bun (A2, A-g)', () => {
+  const { tested, installDefault, verified } = bunVersionsFile();
+  test('GSTACK_BUN_DEFAULT is a verified archive and not below the supported minimum', () => {
+    expect(verified).toContain(installDefault);
+    expect(verified).toContain(tested);
+    expect(Bun.semver.order(installDefault, tested)).toBeGreaterThanOrEqual(0);
+  });
+  test('one CI job runs the quick free tier on GSTACK_BUN_DEFAULT, and the free-tests aggregate requires it', () => {
+    const onDefault = collectPins().filter(p => p.job === DEFAULT_BUN_JOB);
+    expect(onDefault.map(p => p.version)).toEqual([installDefault]);
+    const workflow = Bun.YAML.parse(fs.readFileSync(path.join(WORKFLOWS_DIR, 'free-tests.yml'), 'utf-8')) as { jobs: Record<string, { needs?: string[]; steps: { run?: string }[] }> };
+    expect(workflow.jobs[DEFAULT_BUN_JOB].steps.some(step => step.run?.includes('bun run test:quick'))).toBe(true);
+    expect(workflow.jobs['free-tests'].needs).toContain(DEFAULT_BUN_JOB);
+  });
+});
+
 describe('Bun requirement surfaces agree (E1)', () => {
   const { floor, tested } = bunVersionsFile();
 
-  test('tested version == every CI pin == engines.bun lower bound; floor sits below it', () => {
-    const ciVersions = [...new Set(collectPins().map((p) => p.version))];
+  test('tested version == every CI pin (the install-default job aside) == engines.bun lower bound; floor sits below it', () => {
+    const ciVersions = [...new Set(collectPins().filter((p) => p.job !== DEFAULT_BUN_JOB).map((p) => p.version))];
     expect(ciVersions).toEqual([tested]);
     const pkg = JSON.parse(fs.readFileSync(path.join(ROOT, 'package.json'), 'utf-8'));
     expect(pkg.engines?.bun).toBe(`>=${tested}`);
diff --git a/test/cso-distribution.test.ts b/test/cso-distribution.test.ts
index 2233d2a54..dd0417e6b 100644
--- a/test/cso-distribution.test.ts
+++ b/test/cso-distribution.test.ts
@@ -388,7 +388,7 @@ describe('CSO runtime staging gates', () => {
     expect(gate['continue-on-error']).not.toBe(true);
     const required = workflow.jobs['free-tests'];
     expect(required.if).toBe('always()');
-    expect(required.needs).toEqual(['free-suite', 'typecheck', 'cso-macos-launcher', 'macos-named-regressions', 'cso-windows-launcher', 'cso-docker-integration']);
+    expect(required.needs).toEqual(['free-suite', 'typecheck', 'quick-bun-default', 'cso-macos-launcher', 'macos-named-regressions', 'cso-windows-launcher', 'cso-docker-integration']);
     expect(required.steps[0].run).toContain('test "$CSO_DOCKER_RESULT" = success');
     for (const current of Object.values(workflow.jobs) as any[]) for (const step of current.steps) {
       if (step.uses?.startsWith('oven-sh/setup-bun')) expect(step.uses).toBe('oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6');