diff --git a/.github/scripts/check-runner-labels.py b/.github/scripts/check-runner-labels.py
new file mode 100644
index 0000000000000..15ca898e08423
--- /dev/null
+++ b/.github/scripts/check-runner-labels.py
@@ -0,0 +1,143 @@
+"""Keep workflow runner selection on hosted pools.
+
+This repository has no self-hosted runners. Pull requests from forks can propose
+workflow changes, and a workflow that selects a private runner would put
+untrusted code on hardware that holds credentials and reaches the internal
+network. Every `runs-on` value must therefore name a Blacksmith pool
+(`blacksmith-*`) or a GitHub-hosted image (`ubuntu-*`, `macos-*`, `windows-*`).
+
+Two modes:
+
+    check-runner-labels.py
+        Scan `.github/workflows/*.yml` and fail on `self-hosted` or a
+        private-runner label (`linux-runner`, `windows-runner`,
+        `macos-runner`, `bench-*`). Comments are ignored.
+
+    check-runner-labels.py --runner LABEL [--verify-runner LABEL]
+        Validate labels passed to a reusable workflow, where the static scan
+        cannot see through `${{ inputs.runner }}`.
+"""
+
+import argparse
+import os
+import re
+import sys
+from pathlib import Path
+
+WORKFLOWS_DIR = Path(__file__).resolve().parent.parent / "workflows"
+
+# A label a hosted pool provides: Blacksmith's per-capacity labels and GitHub's
+# own images. `[a-z0-9.-]*` covers suffixes such as `ubuntu-24.04-arm`.
+HOSTED_LABEL = re.compile(r"^(?:blacksmith|ubuntu|macos|windows)-[a-z0-9][a-z0-9.-]*$")
+
+# Runner labels that name hardware this account owns. `-runner` is a stronger
+# signal than `-hosted`, so a label such as `linux-runner` is private by
+# construction; `self-hosted` is GitHub's own selector.
+PRIVATE_LABEL_HINT = ("self-hosted", "-runner")
+
+# What the static scan looks for in workflow text: GitHub's self-hosted
+# selector, the private labels this repository used to have, and the `bench-*`
+# capacity pool. `android-emulator-runner` and other action names do not match.
+FORBIDDEN_REFERENCE = re.compile(
+    r"self-hosted|\b(?:linux|windows|macos)-runner\b|\bbench-[a-z0-9][a-z0-9.-]*"
+)
+
+
+def strip_comment(line):
+    """Drop a trailing YAML comment without touching quoted `#` characters."""
+    quote = None
+    for index, char in enumerate(line):
+        if quote:
+            if char == quote:
+                quote = None
+        elif char in ("'", '"'):
+            quote = char
+        elif char == "#" and (index == 0 or line[index - 1] in " \t"):
+            return line[:index]
+    return line
+
+
+def scan_workflows(workflows_dir=WORKFLOWS_DIR):
+    """Return one message per workflow line that references a private runner."""
+    problems = []
+    paths = sorted(workflows_dir.glob("*.yml")) + sorted(workflows_dir.glob("*.yaml"))
+    for path in paths:
+        for number, line in enumerate(path.read_text(encoding="utf-8").splitlines(), 1):
+            code = strip_comment(line)
+            match = FORBIDDEN_REFERENCE.search(code)
+            if match:
+                problems.append(
+                    f"{path.name}:{number}: references private runner {match.group(0)!r}: "
+                    f"{code.strip()}"
+                )
+    return problems
+
+
+def check_label(name, label, required):
+    """Return a message when `label` is empty-but-required or not a hosted pool."""
+    if not label:
+        if required:
+            return f"inputs.{name} is empty; this workflow needs a hosted runner label"
+        return None
+    if any(hint in label for hint in PRIVATE_LABEL_HINT) or not HOSTED_LABEL.match(label):
+        return (
+            f"inputs.{name}={label!r} is not a hosted runner label; "
+            "use blacksmith-*, ubuntu-*, macos-* or windows-*"
+        )
+    return None
+
+
+def check_labels(runner, verify_runner):
+    """Validate the runner inputs of a reusable workflow."""
+    problems = [
+        message
+        for message in (
+            check_label("runner", runner, required=True),
+            check_label("verify_runner", verify_runner, required=False),
+        )
+        if message
+    ]
+    return problems
+
+
+def report(message):
+    prefix = "::error::" if os.environ.get("GITHUB_ACTIONS") else "error: "
+    print(f"{prefix}{message}", file=sys.stderr)
+
+
+def main(argv=None):
+    parser = argparse.ArgumentParser(description=__doc__.splitlines()[0])
+    parser.add_argument(
+        "--runner",
+        help="runner input of a reusable workflow; validates labels instead of scanning",
+    )
+    parser.add_argument(
+        "--verify-runner",
+        help="optional verify_runner input; empty means the workflow has no verify job",
+    )
+    parser.add_argument(
+        "--workflows-dir",
+        type=Path,
+        default=WORKFLOWS_DIR,
+        help=f"directory to scan (default: {WORKFLOWS_DIR})",
+    )
+    args = parser.parse_args(argv)
+
+    if args.runner is None and args.verify_runner is None:
+        problems = scan_workflows(args.workflows_dir)
+        checked = "workflow runner references"
+    else:
+        # A caller may pass neither flag through the shell, so both are checked.
+        problems = check_labels(args.runner or "", args.verify_runner or "")
+        checked = "runner inputs"
+
+    for problem in problems:
+        report(problem)
+    if problems:
+        return 1
+    print(f"All {checked} name hosted runner pools.")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/.github/scripts/check-runner-labels.test.py b/.github/scripts/check-runner-labels.test.py
new file mode 100644
index 0000000000000..dc6186a9dae50
--- /dev/null
+++ b/.github/scripts/check-runner-labels.test.py
@@ -0,0 +1,109 @@
+import importlib.util
+import tempfile
+import unittest
+from pathlib import Path
+
+spec = importlib.util.spec_from_file_location(
+    "check_runner_labels", Path(__file__).with_name("check-runner-labels.py")
+)
+labels = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(labels)
+
+
+class ScanWorkflowsTests(unittest.TestCase):
+    def setUp(self):
+        self.temp = tempfile.TemporaryDirectory()
+        self.addCleanup(self.temp.cleanup)
+        self.workflows = Path(self.temp.name)
+
+    def scan(self, filename, text):
+        (self.workflows / filename).write_text(text, encoding="utf-8")
+        return labels.scan_workflows(self.workflows)
+
+    def test_accepts_hosted_pools(self):
+        self.assertEqual(self.scan("build.yml", """
+            jobs:
+              build:
+                runs-on: blacksmith-8vcpu-ubuntu-2404
+              verify:
+                runs-on: windows-11-arm
+              lint:
+                runs-on: ubuntu-24.04-arm
+              mac:
+                runs-on: macos-26
+        """), [])
+
+    def test_rejects_self_hosted(self):
+        problems = self.scan("build.yml", "        runs-on: [self-hosted, linux]\n")
+        self.assertEqual(len(problems), 1)
+        self.assertIn("build.yml:1", problems[0])
+        self.assertIn("self-hosted", problems[0])
+
+    def test_rejects_private_runner_labels(self):
+        for label in ("linux-runner", "windows-runner", "macos-runner", "bench-32vcpu"):
+            with self.subTest(label=label):
+                self.workflows.joinpath("build.yml").unlink(missing_ok=True)
+                problems = self.scan("build.yml", f"        runs-on: {label}\n")
+                self.assertEqual(len(problems), 1, problems)
+
+    def test_rejects_matrix_values(self):
+        problems = self.scan("release.yml", """
+            strategy:
+              matrix:
+                runner: [blacksmith-8vcpu-ubuntu-2404, bench-arm64]
+            runs-on: ${{ matrix.runner }}
+        """)
+        self.assertEqual(len(problems), 1)
+        self.assertIn("bench-arm64", problems[0])
+
+    def test_ignores_comments_and_action_names(self):
+        self.assertEqual(self.scan("notes.yml", """
+            # A private runner would be self-hosted or linux-runner.
+            jobs:
+              emulator:
+                runs-on: blacksmith-4vcpu-ubuntu-2404
+                steps:
+                  - uses: reactivecircus/android-emulator-runner@v2
+        """), [])
+
+    def test_ignores_trailing_comments(self):
+        self.assertEqual(self.scan("build.yml", "runs-on: ubuntu-24.04 # not self-hosted\n"), [])
+
+    def test_strip_comment_keeps_hash_inside_quotes(self):
+        self.assertEqual(
+            labels.strip_comment('run: echo "a # b" # mentions linux-runner'),
+            'run: echo "a # b" ',
+        )
+
+
+class CheckLabelsTests(unittest.TestCase):
+    def test_accepts_hosted_labels(self):
+        self.assertEqual(
+            labels.check_labels("blacksmith-16vcpu-ubuntu-2404", "windows-2025"), []
+        )
+        self.assertEqual(labels.check_labels("ubuntu-24.04-arm", ""), [])
+
+    def test_rejects_private_labels(self):
+        for label in ("self-hosted", "linux-runner", "bench-32vcpu", "macos-26-runner"):
+            with self.subTest(label=label):
+                problems = labels.check_labels(label, "")
+                self.assertEqual(len(problems), 1, problems)
+                self.assertIn("inputs.runner", problems[0])
+
+    def test_rejects_verify_runner_on_a_private_pool(self):
+        problems = labels.check_labels("ubuntu-24.04", "linux-runner")
+        self.assertEqual(len(problems), 1)
+        self.assertIn("inputs.verify_runner", problems[0])
+
+    def test_requires_runner_but_not_verify_runner(self):
+        self.assertEqual(len(labels.check_labels("", "")), 1)
+        self.assertIn("inputs.runner is empty", labels.check_labels("", "")[0])
+
+
+class MainTests(unittest.TestCase):
+    def test_scan_mode_passes_on_the_repository_workflows(self):
+        self.assertEqual(labels.scan_workflows(), [])
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c4777f032705a..b7b74e3049625 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -45,11 +45,23 @@ jobs:
             /pnpm-workspace.yaml
             /.github/scripts/ci-changes.cjs
             /.github/actions/ci-changes/
+            /.github/workflows/
+            /.github/scripts/check-runner-labels.py
+            /.github/scripts/check-runner-labels.test.py
             /*/package.json
             /*/*/package.json
             !/.repos/
           sparse-checkout-cone-mode: false
 
+      # The repository has no self-hosted runners, so a workflow that selects
+      # one is a security bug rather than a configuration choice. This is the
+      # cheapest job and it runs before everything else, so a pull request that
+      # introduces a private runner label fails before any other job starts.
+      - name: Check workflow runner labels
+        run: |
+          python3 -B .github/scripts/check-runner-labels.test.py
+          python3 -B .github/scripts/check-runner-labels.py
+
       - name: Detect affected jobs
         id: detect
         uses: ./.github/actions/ci-changes
diff --git a/.github/workflows/fork-ci.yml b/.github/workflows/fork-ci.yml
index efff2a86fdcaf..4b6ef83ed4820 100644
--- a/.github/workflows/fork-ci.yml
+++ b/.github/workflows/fork-ci.yml
@@ -39,6 +39,14 @@ jobs:
           git fetch --no-tags --depth=2 origin "$GITHUB_SHA"
           git sparse-checkout set --no-cone '/*' '!/.repos/' '!/apps/mobile/modules/t3-terminal/Vendor/**/*.a'
           git checkout --detach FETCH_HEAD
+
+      # Mirrors ci.yml: the repository has no self-hosted runners, so a workflow
+      # that selects one fails here, before any other job starts.
+      - name: Check workflow runner labels
+        run: |
+          python3 -B .github/scripts/check-runner-labels.test.py
+          python3 -B .github/scripts/check-runner-labels.py
+
       - name: Detect affected jobs
         id: detect
         uses: ./.github/actions/ci-changes
diff --git a/.github/workflows/mobile-eas-preview-fork.yml b/.github/workflows/mobile-eas-preview-fork.yml
index e7615d46011bd..d263461e44317 100644
--- a/.github/workflows/mobile-eas-preview-fork.yml
+++ b/.github/workflows/mobile-eas-preview-fork.yml
@@ -129,7 +129,10 @@ jobs:
 
       - name: Deploy with fingerprint check
         if: steps.expo-token.outputs.present == 'true'
-        uses: expo/expo-github-action/continuous-deploy-fingerprint@main
+        # Pinned to the commit v9.0.1 tags rather than `main`: this job runs the
+        # reviewed head commit of a fork PR with EXPO_PREVIEW_TOKEN, so a moving
+        # branch ref would let a new upstream commit run here unreviewed.
+        uses: expo/expo-github-action/continuous-deploy-fingerprint@9da89d3617bb88139671bad31b09c65714c2ee06 # v9.0.1
         env:
           EXPO_TOKEN: ${{ secrets.EXPO_PREVIEW_TOKEN }}
           RUNNER_DEBUG: "1"
diff --git a/.github/workflows/release-desktop.yml b/.github/workflows/release-desktop.yml
index 8fbe8af478b75..2deae61f01b1d 100644
--- a/.github/workflows/release-desktop.yml
+++ b/.github/workflows/release-desktop.yml
@@ -120,8 +120,53 @@ permissions:
   contents: read
 
 jobs:
+  # `runner` and `verify_runner` are caller-supplied strings used directly as
+  # `runs-on`, so this workflow cannot tell a hosted pool from a private one on
+  # its own. Fail closed before either job is scheduled: a caller that passes
+  # `self-hosted`, a `*-runner` label, or anything outside the pools this
+  # workflow is designed for stops here instead of running release work (with
+  # release secrets) on hardware this repository does not control. The static
+  # half of the same policy is .github/scripts/check-runner-labels.py.
+  validate:
+    name: Validate runner inputs
+    runs-on: ubuntu-24.04
+    timeout-minutes: 5
+    steps:
+      - name: Reject private runner labels
+        shell: bash
+        env:
+          RUNNER_LABEL: ${{ inputs.runner }}
+          VERIFY_RUNNER_LABEL: ${{ inputs.verify_runner }}
+        run: |
+          set -euo pipefail
+          # Blacksmith's per-capacity labels and GitHub's own images only.
+          allowed='^(blacksmith|ubuntu|macos|windows)-[a-z0-9][a-z0-9.-]*$'
+          failed=0
+          check_label() {
+            local name="$1" label="$2" required="$3"
+            if [[ -z "$label" ]]; then
+              if [[ "$required" == "true" ]]; then
+                echo "::error::inputs.$name is empty, but this workflow needs a hosted runner label."
+                failed=1
+              fi
+              return 0
+            fi
+            # A private label is either a `*-runner` pool or simply outside the
+            # hosted pools above (GitHub's own self-hosted selector included).
+            if [[ "$label" == *-runner* ]] || [[ ! "$label" =~ $allowed ]]; then
+              echo "::error::inputs.$name=$label is not a hosted runner label (blacksmith-*, ubuntu-*, macos-*, windows-*)."
+              failed=1
+            else
+              echo "inputs.$name=$label is a hosted runner label."
+            fi
+          }
+          check_label runner "$RUNNER_LABEL" true
+          check_label verify_runner "$VERIFY_RUNNER_LABEL" false
+          exit "$failed"
+
   build:
     name: Build ${{ inputs.label }}
+    needs: [validate]
     runs-on: ${{ inputs.runner }}
     # Windows waits for the Linux CLI archive inside its own budget (see
     # "Wait for Linux CLI archive"), so it gets that wait on top of the usual 30.
@@ -801,7 +846,7 @@ jobs:
 
   verify:
     name: Verify
-    needs: [build]
+    needs: [validate, build]
     if: inputs.verify_runner != ''
     runs-on: ${{ inputs.verify_runner }}
     timeout-minutes: 15