diff --git a/.github/workflows/workflow-lint.yml b/.github/workflows/workflow-lint.yml
index f5c4efe..28388a7 100644
--- a/.github/workflows/workflow-lint.yml
+++ b/.github/workflows/workflow-lint.yml
@@ -26,6 +26,10 @@ on:
         description: sha256 of that release's linux.x86_64 tarball; travels with shellcheck-version, so overriding one without the other fails the install
         type: string
         default: "8c3be12b05d5c177a04c29e3c78ce89ac86f1595681cab149b65b97c4e227198"
+      tooling-isolation:
+        description: Fail when a job that holds a secret also installs third-party tooling
+        type: boolean
+        default: true
 
 permissions:
   contents: read
@@ -83,3 +87,127 @@ jobs:
         env:
           SHELLCHECK_BIN: ${{ env.SHELLCHECK_BIN }}
         run: actionlint -color "-shellcheck=${SHELLCHECK_BIN}"
+
+      # A job that installs third-party tooling is running code we did not
+      # write: `cargo install` (and the other installers) executes build
+      # scripts over the whole dependency tree, so the secrets a job holds
+      # are reachable from code outside this organisation. Keeping the
+      # install out of any job that holds a secret bounds that reach to
+      # nothing. `pip install --require-hashes` is the exception because a
+      # hash-pinned install resolves nothing — the artifacts are fixed by
+      # the lockfile, so no surprise tree appears.
+      - name: A job holding a secret must not install third-party tooling
+        if: ${{ inputs.tooling-isolation }}
+        run: |
+          python3 - <<'PY'
+          import glob
+          import re
+          import sys
+
+          import yaml
+
+          # `secrets.<id>` is how every GitHub Actions expression reaches a
+          # secret. `secrets: inherit` and `secrets:` declarations on
+          # reusable calls don't match — those are declaration blocks, not
+          # references, and `secrets:` does not contain a `.`.
+          SECRET_RE = re.compile(r"(?<!\w)secrets\.[A-Za-z_]\w*")
+
+          # (`pattern`, `opt-out marker on the same line`). The marker, when
+          # present on the line, exempts that install: the only case where
+          # the resolved tree is fixed is pip with --require-hashes, which
+          # reads the lockfile verbatim and installs exactly what it lists.
+          INSTALL_PATTERNS = (
+              ("cargo install",   None),
+              ("go install",      None),
+              ("gem install",     None),
+              ("npm install -g",  None),
+              ("npm i -g",        None),
+              ("pip install",     "--require-hashes"),
+          )
+
+
+          def normalize_run(value):
+              # `run:` is either a single string or a list of strings. Split
+              # each into lines so the install scan is per-line: a
+              # `--require-hashes` marker on one line only exempts that line.
+              if isinstance(value, list):
+                  out = []
+                  for v in value:
+                      if isinstance(v, str):
+                          out.extend(v.splitlines())
+                  return out
+              if isinstance(value, str):
+                  return value.splitlines()
+              return []
+
+
+          def find_violations(spec_path):
+              violations = []
+              try:
+                  with open(spec_path) as fh:
+                      spec = yaml.safe_load(fh)
+              except yaml.YAMLError as e:
+                  # Silent skip would hide a misconfigured workflow that the
+                  # caller is going to learn about elsewhere in CI for the
+                  # wrong reason. Warning annotation instead.
+                  print(f"::warning file={spec_path}::skipped: YAML parse error: {e}")
+                  return violations
+              if not isinstance(spec, dict):
+                  return violations
+              jobs = spec.get("jobs") or {}
+              for job_id, job in jobs.items():
+                  if not isinstance(job, dict):
+                      continue
+                  # Re-serialize the job so `with:`, `env:`, `if:` and
+                  # `run:` strings are all scanned for a secret reference.
+                  # Comments are stripped by safe_load, so they cannot
+                  # produce a false positive.
+                  job_text = yaml.safe_dump(job)
+                  if not SECRET_RE.search(job_text):
+                      continue
+                  for step in (job.get("steps") or []):
+                      if not isinstance(step, dict):
+                          continue
+                      run = step.get("run")
+                      if run is None:
+                          continue
+                      for line in normalize_run(run):
+                          stripped = line.lstrip()
+                          if not stripped or stripped.startswith("#"):
+                              continue
+                          for pattern, except_marker in INSTALL_PATTERNS:
+                              if pattern in line and not (
+                                  except_marker and except_marker in line
+                              ):
+                                  violations.append((spec_path, job_id, line))
+                                  break
+              return violations
+
+
+          def main():
+              files = sorted(set(
+                  glob.glob(".github/workflows/*.yml")
+                  + glob.glob(".github/workflows/*.yaml")
+              ))
+              violations = []
+              for path in files:
+                  violations.extend(find_violations(path))
+              if violations:
+                  for path, job_id, line in violations:
+                      msg = (
+                          f"job `{job_id}` installs third-party tooling while holding a secret: "
+                          f"`{line.strip()}` — see Glyndor/apt#121. Fix by moving the install "
+                          f"into a job that holds no secrets, or pin it by hash "
+                          f"(e.g. `pip install --require-hashes`)."
+                      )
+                      print(f"::error file={path}::{msg}")
+                  sys.exit(1)
+              print(
+                  "tooling-isolation: no job holding a secret installs third-party tooling."
+              )
+              sys.exit(0)
+
+
+          if __name__ == "__main__":
+              sys.exit(main())
+          PY
diff --git a/docs/reusables/README.md b/docs/reusables/README.md
index 0be0751..856b1e0 100644
--- a/docs/reusables/README.md
+++ b/docs/reusables/README.md
@@ -30,7 +30,7 @@ which blocks every pull request until someone works out why.
 | [`rust-toolchain-watch`](rust-toolchain-watch.md) | 1 | 0 | 1 |
 | [`schedule-freshness`](schedule-freshness.md) | 1 | 0 | 2 |
 | [`shell-ci`](shell-ci.md) | 2 | 1 | 7 |
-| [`workflow-lint`](workflow-lint.md) | 1 | 0 | 2 |
+| [`workflow-lint`](workflow-lint.md) | 1 | 0 | 3 |
 
 *Conditional* checks only run when an input turns them on, so they emit no
 check name at all when it is left at its default.
diff --git a/docs/reusables/workflow-lint.md b/docs/reusables/workflow-lint.md
index d6c7e85..d1f9be0 100644
--- a/docs/reusables/workflow-lint.md
+++ b/docs/reusables/workflow-lint.md
@@ -32,6 +32,7 @@ a required check whose name nothing emits blocks every pull request.
 |---|---|---|---|---|
 | `shellcheck-version` | string | `v0.11.0` | no | Exact shellcheck release tag to install from koalaman/shellcheck for actionlint's embedded shellcheck pass; bumped via .github releases |
 | `shellcheck-sha256` | string | `8c3be12b05d5c177a04c29e3c78ce89ac86f1595681cab149b65b97c4e227198` | no | sha256 of that release's linux.x86_64 tarball; travels with shellcheck-version, so overriding one without the other fails the install |
+| `tooling-isolation` | boolean | `true` | no | Fail when a job that holds a secret also installs third-party tooling |
 
 ---