diff --git a/.github/actions/setup-workspace/action.yml b/.github/actions/setup-workspace/action.yml
index e9b025aeb6..e363407887 100644
--- a/.github/actions/setup-workspace/action.yml
+++ b/.github/actions/setup-workspace/action.yml
@@ -6,7 +6,7 @@ inputs:
     description: Node version to install
     required: true
   pnpm-version:
-    description: pnpm version to install. Needs a matching pnpm-<version>.sha256 file beside this action.
+    description: pnpm version to install. Must match the root packageManager and have a pnpm-<version>.sha256 file beside this action.
     required: true
   cache-prefix:
     description: Names this job's Turbo cache entry (e.g. checks, test-server). Every job reads every prefix; this only decides which entry the job writes back.
diff --git a/.github/actions/setup-workspace/install-pnpm.sh b/.github/actions/setup-workspace/install-pnpm.sh
index e255d097eb..63bf47b0e2 100755
--- a/.github/actions/setup-workspace/install-pnpm.sh
+++ b/.github/actions/setup-workspace/install-pnpm.sh
@@ -13,6 +13,12 @@ set -euo pipefail
 version="${PNPM_VERSION:?PNPM_VERSION is required}"
 action_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 checksums="${action_dir}/pnpm-${version}.sha256"
+package_manager="$(node --print 'require(process.argv[1]).packageManager' "${GITHUB_WORKSPACE:?GITHUB_WORKSPACE is required}/package.json")"
+
+if [[ "${package_manager}" != "pnpm@${version}" ]]; then
+  echo "::error::pnpm version mismatch: package.json declares ${package_manager#pnpm@}, but the action requested ${version}."
+  exit 1
+fi
 
 if [[ ! -f "${checksums}" ]]; then
   echo "::error::No checksums for pnpm ${version}. Download the release assets from https://github.com/pnpm/pnpm/releases/tag/v${version}, run sha256sum on them, and commit the output as ${checksums}."
diff --git a/packages/scripts/test/ci-workflow.test.ts b/packages/scripts/test/ci-workflow.test.ts
index 113dfa1847..05be52a2b0 100644
--- a/packages/scripts/test/ci-workflow.test.ts
+++ b/packages/scripts/test/ci-workflow.test.ts
@@ -1,7 +1,16 @@
-import { readFileSync } from "node:fs";
-import { dirname, resolve } from "node:path";
+import { spawnSync } from "node:child_process";
+import {
+  chmodSync,
+  mkdirSync,
+  mkdtempSync,
+  readFileSync,
+  rmSync,
+  writeFileSync,
+} from "node:fs";
+import { tmpdir } from "node:os";
+import { dirname, join, resolve } from "node:path";
 import { fileURLToPath } from "node:url";
-import { expect, it } from "vitest";
+import { expect, it, onTestFinished } from "vitest";
 
 const testDir = dirname(fileURLToPath(import.meta.url));
 const repoRoot = resolve(testDir, "..", "..", "..");
@@ -15,3 +24,36 @@ it("limits concurrent Turbo test tasks to the CI runner CPU count", () => {
 
   expect(testStep).toContain("--concurrency=4");
 });
+
+it("rejects a pnpm version that disagrees with the root manifest", () => {
+  const fixture = mkdtempSync(join(tmpdir(), "bb-pnpm-version-"));
+  onTestFinished(() => rmSync(fixture, { force: true, recursive: true }));
+  const fakeBin = resolve(fixture, "bin");
+  mkdirSync(fakeBin);
+  writeFileSync(
+    resolve(fixture, "package.json"),
+    '{"packageManager":"pnpm@9.15.1"}\n',
+  );
+  writeFileSync(resolve(fakeBin, "curl"), "#!/bin/sh\nexit 23\n");
+  chmodSync(resolve(fakeBin, "curl"), 0o755);
+
+  const result = spawnSync(
+    "bash",
+    [resolve(repoRoot, ".github/actions/setup-workspace/install-pnpm.sh")],
+    {
+      cwd: fixture,
+      encoding: "utf8",
+      env: {
+        ...process.env,
+        GITHUB_WORKSPACE: fixture,
+        PATH: `${fakeBin}:${process.env.PATH}`,
+        PNPM_VERSION: "9.15.0",
+      },
+    },
+  );
+
+  expect(result.status).toBe(1);
+  expect(result.stdout).toContain(
+    "pnpm version mismatch: package.json declares 9.15.1, but the action requested 9.15.0",
+  );
+});
diff --git a/turbo.json b/turbo.json
index 3dcf8a84d7..df9371f8d6 100644
--- a/turbo.json
+++ b/turbo.json
@@ -454,6 +454,7 @@
       "dependsOn": ["//#ensure-native-modules", "@bb/cli#build", "topo"],
       "inputs": [
         "$TURBO_DEFAULT$",
+        "$TURBO_ROOT$/.github/actions/setup-workspace/**",
         "$TURBO_ROOT$/.github/APPROVED_CONTRIBUTORS",
         "$TURBO_ROOT$/.github/workflows/approve-contributor.yml",
         "$TURBO_ROOT$/.github/workflows/pr-gate.yml",