diff --git a/README.md b/README.md
index d4e2ca1..24c8240 100644
--- a/README.md
+++ b/README.md
@@ -42,9 +42,17 @@ Inside the plugin, reference bundled files via `${CLAUDE_PLUGIN_ROOT}/...` — i
 
 ## Verify before done
 
+`ruff` and `pytest` are dev tooling only — they never ship inside `plugin/`. If `python3 -m pip`
+is unavailable, install them isolated with [`uv`](https://docs.astral.sh/uv/)
+(`uv tool install ruff pytest`, then `export PATH="$HOME/.local/bin:$PATH"`).
+
 ```sh
-ruff check . && ruff format --check .
+ruff check . && ruff format --check .   # apply with `ruff format .` if it wants changes
 python3 -m pytest -q
+
+# Stdlib-only guard (mirrored in tests/test_stdlib_only.py): the shipped payload must import
+# only the standard library. This must find nothing.
+grep -rnE '^\s*(import|from)\s+(msal|azure|requests|urllib3|httpx|aiohttp|msgraph|pydantic|yaml|dotenv)\b' plugin/src/
 ```
 
 ## Non-negotiable conventions
diff --git a/tests/test_stdlib_only.py b/tests/test_stdlib_only.py
new file mode 100644
index 0000000..1ab356d
--- /dev/null
+++ b/tests/test_stdlib_only.py
@@ -0,0 +1,80 @@
+"""Enforce — not just assert — the stdlib-only guarantee for the shipped payload.
+
+The plugin under plugin/src/ must import only the Python standard library (plus its own package)
+at runtime: that is the whole portability/auditability promise (no install friction, no third-party
+attack surface). ruff/pytest are dev tooling and live OUTSIDE plugin/, so they never count here.
+
+Two layers of defence:
+  - test_no_forbidden_imports — a fast denylist grep, mirrored verbatim in README's verify steps,
+    so a human and CI fail on the same obvious offenders (msal, azure, requests, ...).
+  - test_every_import_is_stdlib_or_first_party — the real guarantee: AST-walk every import and assert
+    each top-level module is in the stdlib or is the plugin's own package. Catches anything the
+    denylist forgot.
+"""
+
+from __future__ import annotations
+
+import ast
+import re
+import sys
+from pathlib import Path
+
+PLUGIN_SRC = Path(__file__).resolve().parent.parent / "plugin" / "src"
+
+# First-party package(s) shipped in the plugin. Rename alongside src/example when instantiating.
+FIRST_PARTY = {"example"}
+
+# Mirrored in README "Verify before done". Keep the two in sync.
+FORBIDDEN = (
+    "msal",
+    "azure",
+    "requests",
+    "urllib3",
+    "httpx",
+    "aiohttp",
+    "msgraph",
+    "pydantic",
+    "yaml",
+    "dotenv",
+)
+
+
+def _py_files() -> list[Path]:
+    return sorted(PLUGIN_SRC.rglob("*.py"))
+
+
+def test_plugin_src_has_python_files():
+    # Guard against the test silently passing because the path moved.
+    assert _py_files(), f"no .py files under {PLUGIN_SRC} — did the layout change?"
+
+
+def test_no_forbidden_imports():
+    """The denylist grep, applied as code. Same intent as the README one-liner."""
+    pattern = re.compile(r"^\s*(?:import|from)\s+(" + "|".join(FORBIDDEN) + r")\b", re.MULTILINE)
+    offenders = []
+    for path in _py_files():
+        for m in pattern.finditer(path.read_text(encoding="utf-8")):
+            offenders.append(f"{path.relative_to(PLUGIN_SRC.parent.parent)}: {m.group(1)}")
+    assert not offenders, "forbidden third-party imports in shipped plugin:\n  " + "\n  ".join(offenders)
+
+
+def test_every_import_is_stdlib_or_first_party():
+    """The real guarantee: every imported top-level module is stdlib or the plugin's own package."""
+    stdlib = sys.stdlib_module_names  # Python 3.10+
+    bad: list[str] = []
+    for path in _py_files():
+        tree = ast.parse(path.read_text(encoding="utf-8"), filename=str(path))
+        for node in ast.walk(tree):
+            if isinstance(node, ast.Import):
+                mods = [a.name.split(".")[0] for a in node.names]
+            elif isinstance(node, ast.ImportFrom) and node.level == 0 and node.module:
+                mods = [node.module.split(".")[0]]
+            else:
+                continue
+            for mod in mods:
+                if mod not in stdlib and mod not in FIRST_PARTY:
+                    bad.append(f"{path.relative_to(PLUGIN_SRC.parent.parent)}: {mod}")
+    assert not bad, (
+        "non-stdlib / non-first-party imports in shipped plugin (stdlib-only is non-negotiable):\n  "
+        + "\n  ".join(bad)
+    )