diff --git a/tests/architecture/test_top_level_module_budget.py b/tests/architecture/test_top_level_module_budget.py
new file mode 100644
index 000000000..7209e6fe8
--- /dev/null
+++ b/tests/architecture/test_top_level_module_budget.py
@@ -0,0 +1,106 @@
+"""`loopx/` keeps the top-level module count it pinned; moves lower it.
+
+Anchored on `docs/architecture/rfcs/monorepo-distribution-split-v0.md` milestone
+M0 and Section 9: the import-boundary tests protect inward edges of
+`loopx/control_plane`, and nothing at all bounded how many modules sit directly
+under `loopx/`. The budget may only be lowered, in the PR that moves files.
+"""
+
+from __future__ import annotations
+
+import json
+import re
+from collections.abc import Sequence
+from pathlib import Path
+from typing import Any
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+BUDGET_FIXTURE = Path(__file__).with_name("top_level_module_budget.json")
+BUDGET_SCHEMA = "loopx_top_level_module_budget_v0"
+_COMMIT_SHA = re.compile(r"^[0-9a-f]{40}$")
+
+
+def _top_level_modules(package_root: Path | None = None) -> list[str]:
+    """Name the modules that sit directly under ``loopx/``."""
+    root = package_root or (REPO_ROOT / "loopx")
+    return sorted(path.name for path in root.glob("*.py"))
+
+
+def _budget() -> dict[str, Any]:
+    payload = json.loads(BUDGET_FIXTURE.read_text(encoding="utf-8"))
+    assert isinstance(payload, dict), f"{BUDGET_FIXTURE.name} must hold one object"
+    assert payload["schema"] == BUDGET_SCHEMA, (
+        f"{BUDGET_FIXTURE.name} schema is {payload['schema']!r}, expected {BUDGET_SCHEMA!r}"
+    )
+    assert _COMMIT_SHA.match(str(payload["baseline_commit"])), (
+        "baseline_commit must be the full 40-character commit the count was measured at"
+    )
+    maximum = payload["max_top_level_modules"]
+    assert isinstance(maximum, int) and maximum > 0, (
+        f"max_top_level_modules must be a positive integer, got {maximum!r}"
+    )
+    allowlist = payload["allowlist"]
+    assert isinstance(allowlist, list) and all(
+        isinstance(name, str) for name in allowlist
+    ), f"allowlist must be a list of module file names, got {allowlist!r}"
+    assert len(allowlist) == len(set(allowlist)), (
+        f"allowlist repeats a name: {allowlist}"
+    )
+    return payload
+
+
+def _count_offenders(modules: Sequence[str], maximum: int) -> list[str]:
+    """Return the modules that push ``modules`` past ``maximum``, newest path first."""
+    if len(modules) <= maximum:
+        return []
+    return sorted(modules, reverse=True)[: len(modules) - maximum]
+
+
+def test_top_level_module_count_stays_at_the_pinned_budget() -> None:
+    budget = _budget()
+    modules = _top_level_modules()
+    maximum = int(str(budget["max_top_level_modules"]))
+    assert not _count_offenders(modules, maximum), (
+        f"loopx/ grew from {maximum} to {len(modules)} top-level modules; the RFC only "
+        "allows a move into a subpackage, and new top-level modules need a recorded "
+        "design decision first."
+    )
+    assert len(modules) == maximum, (
+        f"loopx/ now holds {len(modules)} top-level modules but the budget pins {maximum}; "
+        "a PR that moves files must lower max_top_level_modules by the number moved."
+    )
+
+
+def test_allowlisted_entry_points_stay_at_the_top_level() -> None:
+    budget = _budget()
+    allowlist = [str(name) for name in budget["allowlist"]]
+    modules = set(_top_level_modules())
+    missing = [name for name in allowlist if name not in modules]
+    assert not missing, (
+        f"allowlisted entry points left the top level: {sorted(missing)}"
+    )
+
+
+def test_guard_flags_a_tree_that_grew_past_its_budget(tmp_path: Path) -> None:
+    for index in range(3):
+        (tmp_path / f"module_{index}.py").write_text("", encoding="utf-8")
+    assert _count_offenders(_top_level_modules(tmp_path), 2) == ["module_2.py"]
+
+
+def test_guard_accepts_a_tree_exactly_at_its_budget(tmp_path: Path) -> None:
+    for index in range(3):
+        (tmp_path / f"module_{index}.py").write_text("", encoding="utf-8")
+    assert _count_offenders(_top_level_modules(tmp_path), 3) == []
+
+
+def test_guard_accepts_a_tree_under_its_budget(tmp_path: Path) -> None:
+    for index in range(3):
+        (tmp_path / f"module_{index}.py").write_text("", encoding="utf-8")
+    assert _count_offenders(_top_level_modules(tmp_path), 5) == []
+
+
+def test_guard_ignores_modules_below_the_top_level(tmp_path: Path) -> None:
+    (tmp_path / "subpackage").mkdir()
+    (tmp_path / "subpackage" / "deep.py").write_text("", encoding="utf-8")
+    (tmp_path / "only.py").write_text("", encoding="utf-8")
+    assert _top_level_modules(tmp_path) == ["only.py"]
diff --git a/tests/architecture/top_level_module_budget.json b/tests/architecture/top_level_module_budget.json
new file mode 100644
index 000000000..7a87f560b
--- /dev/null
+++ b/tests/architecture/top_level_module_budget.json
@@ -0,0 +1,6 @@
+{
+  "schema": "loopx_top_level_module_budget_v0",
+  "baseline_commit": "9eaacfbf2ff93d5386cee82ddf0847d9c739e78a",
+  "max_top_level_modules": 147,
+  "allowlist": ["__init__.py", "entrypoint.py", "cli.py"]
+}