diff --git a/.github/workflows/make-pdf-gate.yml b/.github/workflows/make-pdf-gate.yml
index ddbd184f1..eca26beab 100644
--- a/.github/workflows/make-pdf-gate.yml
+++ b/.github/workflows/make-pdf-gate.yml
@@ -52,7 +52,7 @@ jobs:
 
       - uses: oven-sh/setup-bun@v2
         with:
-          bun-version: latest
+          bun-version: 1.3.13
 
       - name: Install dependencies
         run: bun install --frozen-lockfile
diff --git a/.github/workflows/quality-gate.yml b/.github/workflows/quality-gate.yml
index 5760d1a6b..531a23cbb 100644
--- a/.github/workflows/quality-gate.yml
+++ b/.github/workflows/quality-gate.yml
@@ -36,7 +36,7 @@ jobs:
           fetch-depth: 0
       - uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2
         with:
-          bun-version: latest
+          bun-version: 1.3.13
 
       - name: Install frozen dependencies
         run: bun install --frozen-lockfile --ignore-scripts
diff --git a/.github/workflows/skill-docs.yml b/.github/workflows/skill-docs.yml
index 173f0467f..721a5afdb 100644
--- a/.github/workflows/skill-docs.yml
+++ b/.github/workflows/skill-docs.yml
@@ -26,7 +26,9 @@ jobs:
     steps:
       - uses: actions/checkout@v7
       - uses: oven-sh/setup-bun@v2
-      - run: bun install
+        with:
+          bun-version: 1.3.13
+      - run: bun install --frozen-lockfile
       # One generation pass for ALL 10 hosts. gen-skill-docs --host all
       # hard-fails on any per-host generation error (scripts/gen-skill-docs.ts
       # aggregates failures and exits non-zero), so every host is gated on
diff --git a/.github/workflows/version-gate.yml b/.github/workflows/version-gate.yml
index 00a2e25ec..96f9185bd 100644
--- a/.github/workflows/version-gate.yml
+++ b/.github/workflows/version-gate.yml
@@ -27,6 +27,8 @@ jobs:
 
       - name: Setup Bun
         uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: 1.3.13
 
       - name: Read versions
         id: versions
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 7e5e1fa31..583d5d78d 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -6,7 +6,7 @@ stages:
   - check
 
 variables:
-  BUN_VERSION: "1.3.10"
+  BUN_VERSION: "1.3.13"
 
 .setup-bun: &setup-bun
   - apt-get update -qq && apt-get install -qq -y curl jq git
diff --git a/test/bun-version-drift.test.ts b/test/bun-version-drift.test.ts
new file mode 100644
index 000000000..ce3dd4dd5
--- /dev/null
+++ b/test/bun-version-drift.test.ts
@@ -0,0 +1,76 @@
+/**
+ * One Bun version across every CI surface.
+ *
+ * The drift class this pins: Dockerfile.ci's comment records that the old
+ * `| BUN_VERSION=x.y.z bash` form silently installed latest on every image
+ * rebuild (observed 1.3.13/1.3.14 drift vs the 1.3.10 devs ran locally),
+ * and before 2026-08-29 the lanes disagreed four ways (1.3.13 / latest /
+ * unpinned / 1.3.10). Different Bun versions change test-runner OUTPUT
+ * SHAPES the strict classifiers regex-match, spawn semantics, and shell
+ * parsing — a lane on a different Bun is testing a different product.
+ *
+ * Bumping Bun: change every surface in one commit; this test names each one.
+ */
+import { describe, expect, test } from 'bun:test';
+import * as fs from 'node:fs';
+import * as path from 'node:path';
+
+const ROOT = path.resolve(__dirname, '..');
+const WORKFLOWS_DIR = path.join(ROOT, '.github', 'workflows');
+
+interface Pin {
+  surface: string;
+  version: string;
+}
+
+function collectPins(): Pin[] {
+  const pins: Pin[] = [];
+
+  for (const name of fs.readdirSync(WORKFLOWS_DIR).sort()) {
+    if (!/\.ya?ml$/.test(name)) continue;
+    const source = fs.readFileSync(path.join(WORKFLOWS_DIR, name), 'utf-8');
+    const lines = source.split('\n');
+    for (let i = 0; i < lines.length; i++) {
+      if (!/uses:\s*oven-sh\/setup-bun@/.test(lines[i])) continue;
+      // A pinned stanza is `with:` + `bun-version: <v>` within the next few
+      // lines; an unpinned setup-bun is itself drift (installs latest).
+      const window = lines.slice(i + 1, i + 4).join('\n');
+      const m = window.match(/bun-version:\s*["']?([\w.]+)["']?/);
+      pins.push({
+        surface: `${name}:${i + 1}`,
+        version: m ? m[1] : '<unpinned setup-bun — installs latest>',
+      });
+    }
+  }
+
+  const dockerfile = fs.readFileSync(
+    path.join(ROOT, '.github', 'docker', 'Dockerfile.ci'), 'utf-8');
+  const dockerPin = dockerfile.match(/bash -s ["']?bun-v([\w.]+)["']?/);
+  pins.push({
+    surface: 'Dockerfile.ci',
+    version: dockerPin ? dockerPin[1] : '<no bun-vX.Y.Z positional arg>',
+  });
+
+  const gitlab = fs.readFileSync(path.join(ROOT, '.gitlab-ci.yml'), 'utf-8');
+  const gitlabPin = gitlab.match(/BUN_VERSION:\s*["']?([\w.]+)["']?/);
+  pins.push({
+    surface: '.gitlab-ci.yml',
+    version: gitlabPin ? gitlabPin[1] : '<no BUN_VERSION>',
+  });
+
+  return pins;
+}
+
+describe('bun version pins', () => {
+  test('every CI surface pins the same bun version', () => {
+    const pins = collectPins();
+    // Sanity: the scan found the known surfaces (a regex rot that finds
+    // nothing must fail loudly, not vacuously pass).
+    expect(pins.length).toBeGreaterThanOrEqual(6);
+
+    const versions = [...new Set(pins.map((p) => p.version))];
+    const detail = pins.map((p) => `${p.surface} → ${p.version}`).join('\n');
+    expect(versions, `bun version drift across CI surfaces:\n${detail}`).toHaveLength(1);
+    expect(versions[0]).toMatch(/^\d+\.\d+\.\d+$/);
+  });
+});