diff --git a/.github/workflows/gate.yml b/.github/workflows/gate.yml
new file mode 100644
index 0000000..7366bd8
--- /dev/null
+++ b/.github/workflows/gate.yml
@@ -0,0 +1,64 @@
+# Emits the `gate` status context required by the paiml org ruleset
+# "Green Main" (id 13878864). Without a check literally named `gate`, every PR
+# in this repo reports mergeStateStatus: BLOCKED while every visible check is
+# green, and no contributor can fix it.
+#
+# The job id AND name must stay `gate` - the ruleset matches the CONTEXT, not a
+# display name. Renaming it silently re-blocks every PR in this repository.
+#
+# This repo is a Jupyter Book / Jekyll course and has no test suite to
+# aggregate, so the gate measures what the repo actually is: notebooks, YAML,
+# JSON, helper scripts, and the table of contents. Every step prints its
+# denominator and fails when it is zero.
+name: gate
+
+on:
+  pull_request:
+  push:
+    branches: [master]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: gate-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  gate:
+    name: gate
+    # ubuntu-latest, not the self-hosted fleet: this repo is PUBLIC, and a
+    # `pull_request` job runs the head ref's own code, so a fork PR would
+    # execute untrusted content on the clean-room runners. Never bare
+    # `self-hosted` either - that matches the aarch64 gx10 node (paiml/infra#342).
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install YAML parser
+        run: python3 -m pip install --quiet --disable-pip-version-check pyyaml
+
+      # Runs every checker against deliberately broken fixtures and asserts it
+      # rejects them. A guard never shown to fail is not evidence of anything.
+      - name: Checkers can fail (selftest)
+        run: python3 scripts/ci_gate_check.py selftest
+
+      - name: Notebooks parse
+        run: python3 scripts/ci_gate_check.py notebooks
+
+      - name: YAML parses
+        run: python3 scripts/ci_gate_check.py yaml
+
+      - name: JSON parses
+        run: python3 scripts/ci_gate_check.py json
+
+      - name: Python compiles
+        run: python3 scripts/ci_gate_check.py python
+
+      - name: Table of contents resolves
+        run: python3 scripts/ci_gate_check.py toc
+
+      - name: Lessons published to content/lessons
+        run: python3 scripts/ci_gate_check.py lessons
diff --git a/scripts/ci_gate_check.py b/scripts/ci_gate_check.py
new file mode 100755
index 0000000..e450ac4
--- /dev/null
+++ b/scripts/ci_gate_check.py
@@ -0,0 +1,318 @@
+#!/usr/bin/env python3
+"""Structural checks for the `gate` status context.
+
+This repository is a Jupyter Book / Jekyll course: notebooks, YAML config, a
+table of contents and two helper scripts. There is no test suite to aggregate,
+so the gate measures the things this repo actually IS.
+
+Every check prints its DENOMINATOR ("checked N ...") and FAILS when N is zero.
+A check that silently inspected nothing must never read as a pass.
+
+`selftest` proves each checker can actually fail: it builds deliberately broken
+fixtures in a temp directory and asserts every checker rejects them. A guard
+that has never been shown to fail is not evidence of anything.
+
+Usage:
+    ci_gate_check.py notebooks | yaml | json | python | toc | lessons | selftest
+"""
+
+from __future__ import annotations
+
+import json
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+
+try:
+    import yaml
+except ImportError:  # pragma: no cover - surfaced as a hard failure, never a skip
+    print("FAIL: PyYAML is not importable; the YAML checks cannot run.", file=sys.stderr)
+    print("An absent verifier is a NO-GO, not a pass.", file=sys.stderr)
+    sys.exit(2)
+
+REPO = Path(__file__).resolve().parent.parent
+
+
+def discover(root: Path, suffixes: tuple[str, ...]) -> list[Path]:
+    """Tracked files under `root` matching `suffixes`.
+
+    Uses `git ls-files` inside a checkout so untracked scratch files can never
+    inflate or deflate a denominator; falls back to a walk for selftest
+    fixtures, which are not git repositories.
+    """
+    try:
+        out = subprocess.run(
+            ["git", "-C", str(root), "ls-files", "-z"],
+            check=True,
+            capture_output=True,
+            text=True,
+        ).stdout
+        names = [n for n in out.split("\0") if n]
+    except (subprocess.CalledProcessError, FileNotFoundError):
+        names = [
+            str(p.relative_to(root))
+            for p in root.rglob("*")
+            if p.is_file()
+        ]
+    return sorted(root / n for n in names if n.endswith(suffixes))
+
+
+def report(kind: str, n: int, errors: list[str]) -> int:
+    for e in errors:
+        print(f"  ERROR: {e}")
+    print(f"checked {n} {kind}")
+    if n == 0:
+        print(f"FAIL: found 0 {kind} to check - the check inspected nothing.")
+        return 1
+    if errors:
+        print(f"FAIL: {len(errors)} of {n} {kind} are broken.")
+        return 1
+    print(f"OK: {n} {kind} pass.")
+    return 0
+
+
+# --------------------------------------------------------------------------
+# checkers: each returns (denominator, errors)
+# --------------------------------------------------------------------------
+
+def _cell_defects(cells: list, rel) -> list[str]:
+    return [
+        f"{rel}: cell {i} lacks 'cell_type'/'source'"
+        for i, cell in enumerate(cells)
+        if not isinstance(cell, dict)
+        or "cell_type" not in cell
+        or "source" not in cell
+    ]
+
+
+def _notebook_defects(nb, rel) -> list[str]:
+    if not isinstance(nb, dict):
+        return [f"{rel}: top level is {type(nb).__name__}, expected object"]
+    errors: list[str] = []
+    if not isinstance(nb.get("nbformat"), int):
+        errors.append(f"{rel}: missing or non-integer 'nbformat'")
+    cells = nb.get("cells")
+    if not isinstance(cells, list):
+        errors.append(f"{rel}: missing or non-list 'cells'")
+        return errors
+    return errors + _cell_defects(cells, rel)
+
+
+def check_notebooks(root: Path) -> tuple[int, list[str]]:
+    files = discover(root, (".ipynb",))
+    errors: list[str] = []
+    for f in files:
+        rel = f.relative_to(root)
+        try:
+            nb = json.loads(f.read_text(encoding="utf-8"))
+        except (json.JSONDecodeError, UnicodeDecodeError) as exc:
+            errors.append(f"{rel}: not valid JSON: {exc}")
+            continue
+        errors.extend(_notebook_defects(nb, rel))
+    return len(files), errors
+
+
+def check_yaml(root: Path) -> tuple[int, list[str]]:
+    files = discover(root, (".yml", ".yaml"))
+    errors: list[str] = []
+    for f in files:
+        try:
+            yaml.safe_load(f.read_text(encoding="utf-8"))
+        except (yaml.YAMLError, UnicodeDecodeError) as exc:
+            errors.append(f"{f.relative_to(root)}: invalid YAML: {exc}")
+    return len(files), errors
+
+
+def check_json(root: Path) -> tuple[int, list[str]]:
+    files = discover(root, (".json",))
+    errors: list[str] = []
+    for f in files:
+        try:
+            json.loads(f.read_text(encoding="utf-8"))
+        except (json.JSONDecodeError, UnicodeDecodeError) as exc:
+            errors.append(f"{f.relative_to(root)}: invalid JSON: {exc}")
+    return len(files), errors
+
+
+def check_python(root: Path) -> tuple[int, list[str]]:
+    files = discover(root, (".py",))
+    errors: list[str] = []
+    for f in files:
+        try:
+            compile(f.read_text(encoding="utf-8"), str(f), "exec")
+        except (SyntaxError, ValueError, UnicodeDecodeError) as exc:
+            errors.append(f"{f.relative_to(root)}: does not compile: {exc}")
+    return len(files), errors
+
+
+def _toc_urls(node, out: list[str]) -> None:
+    """Collect internal (non-external) urls from a jupyter-book toc tree."""
+    if isinstance(node, list):
+        for item in node:
+            _toc_urls(item, out)
+    elif isinstance(node, dict):
+        url = node.get("url")
+        if isinstance(url, str) and not node.get("external") and url.startswith("/"):
+            out.append(url)
+        _toc_urls(node.get("sections"), out)
+
+
+def check_toc(root: Path) -> tuple[int, list[str]]:
+    """Every internal toc entry must resolve to a real file under content/.
+
+    This is what catches a renamed or deleted lesson: the book still builds a
+    sidebar entry, but the page behind it is gone.
+    """
+    tocs = [p for p in discover(root, (".yml",)) if p.name == "toc.yml"]
+    errors: list[str] = []
+    checked = 0
+    for toc in tocs:
+        book_root = toc.parent.parent  # <book>/_data/toc.yml
+        content = book_root / "content"
+        if not content.is_dir():
+            errors.append(f"{toc.relative_to(root)}: no content/ dir beside it")
+            continue
+        urls: list[str] = []
+        _toc_urls(yaml.safe_load(toc.read_text(encoding="utf-8")), urls)
+        for url in urls:
+            checked += 1
+            stem = content / url.lstrip("/")
+            if not any(
+                stem.with_suffix(ext).is_file() for ext in (".md", ".ipynb", ".markdown")
+            ):
+                errors.append(
+                    f"{toc.relative_to(root)}: url {url} resolves to no "
+                    f"{stem.relative_to(book_root)}.{{md,ipynb}}"
+                )
+    return checked, errors
+
+
+def check_lessons(root: Path) -> tuple[int, list[str]]:
+    """`make book` copies the top-level Colab notebooks into content/lessons/.
+
+    If a lesson exists at the top level but not under content/lessons/, the
+    published book is silently missing it until someone re-runs `make book`.
+    """
+    lessons_dir = root / "content" / "lessons"
+    top = [p for p in discover(root, (".ipynb",)) if p.parent == root]
+    errors: list[str] = []
+    if not lessons_dir.is_dir():
+        errors.append("content/lessons/ does not exist")
+        return len(top), errors
+    for nb in top:
+        if not (lessons_dir / nb.name).is_file():
+            errors.append(
+                f"{nb.name} is not published to content/lessons/ (run `make book`)"
+            )
+    return len(top), errors
+
+
+CHECKS = {
+    "notebooks": ("notebook(s)", check_notebooks),
+    "yaml": ("YAML file(s)", check_yaml),
+    "json": ("JSON file(s)", check_json),
+    "python": ("Python file(s)", check_python),
+    "toc": ("table-of-contents entr(ies)", check_toc),
+    "lessons": ("top-level lesson notebook(s)", check_lessons),
+}
+
+
+# --------------------------------------------------------------------------
+# selftest: prove every checker can fail
+# --------------------------------------------------------------------------
+
+GOOD_NB = json.dumps(
+    {"nbformat": 4, "nbformat_minor": 0, "metadata": {}, "cells": [
+        {"cell_type": "markdown", "source": ["# hi"], "metadata": {}}]}
+)
+
+
+def _fixture(tmp: Path, name: str, files: dict[str, str]) -> Path:
+    root = tmp / name
+    for rel, body in files.items():
+        p = root / rel
+        p.parent.mkdir(parents=True, exist_ok=True)
+        p.write_text(body, encoding="utf-8")
+    return root
+
+
+def _selftest_cases() -> list:
+    """(label, checker, broken fixture, empty fixture-or-None)."""
+    good_toc = (
+        "- title: Home\n  url: /intro\n"
+        "- title: Ext\n  url: https://example.com\n  external: true\n"
+    )
+    no_source = {"README.md": "x"}
+    return [
+        ("notebooks", check_notebooks, {"a.ipynb": "{not json"}, no_source),
+        ("notebooks-shape", check_notebooks,
+         {"a.ipynb": json.dumps({"nbformat": 4, "cells": [{"source": []}]})}, None),
+        ("yaml", check_yaml, {"a.yml": "a:\n  - b\n c: bad indent\n"}, no_source),
+        ("json", check_json, {"a.json": "{,}"}, no_source),
+        ("python", check_python, {"a.py": "def broken(:\n"}, no_source),
+        ("toc", check_toc,
+         {"_data/toc.yml": good_toc, "content/other.md": "x"},
+         {"_data/toc.yml": "- title: Ext\n  url: https://e.com\n  external: true\n",
+          "content/intro.md": "x"}),
+        ("lessons", check_lessons,
+         {"L1.ipynb": GOOD_NB, "content/lessons/keep.md": "x"}, no_source),
+    ]
+
+
+def _assert_rejects(label: str, fn, root: Path) -> int:
+    """A checker must report at least one error on a deliberately broken tree."""
+    n, errors = fn(root)
+    if not errors:
+        print(f"  SELFTEST FAIL: {label} accepted a broken fixture")
+        return 1
+    print(f"  selftest ok: {label} rejects broken input (n={n})")
+    return 0
+
+
+def _assert_zero_denominator(label: str, fn, root: Path) -> int:
+    """A checker must report denominator 0 when there is nothing to inspect."""
+    n, _ = fn(root)
+    if n != 0:
+        print(f"  SELFTEST FAIL: {label} counted {n} on an empty fixture")
+        return 1
+    print(f"  selftest ok: {label} reports denominator 0 on empty input")
+    return 0
+
+
+def selftest() -> int:
+    cases = _selftest_cases()
+    failures = 0
+    with tempfile.TemporaryDirectory() as td:
+        tmp = Path(td)
+        for i, (label, fn, broken, empty) in enumerate(cases):
+            failures += _assert_rejects(label, fn, _fixture(tmp, f"broken{i}", broken))
+            if empty is not None:
+                failures += _assert_zero_denominator(
+                    label, fn, _fixture(tmp, f"empty{i}", empty)
+                )
+    print(f"checked {len(cases)} checker(s) for falsifiability")
+    if failures:
+        print(f"FAIL: {failures} selftest assertion(s) failed.")
+        return 1
+    print("OK: every checker demonstrably fails on broken input.")
+    return 0
+
+
+def main(argv: list[str]) -> int:
+    if len(argv) != 2:
+        print(__doc__)
+        return 2
+    name = argv[1]
+    if name == "selftest":
+        return selftest()
+    if name not in CHECKS:
+        print(f"unknown check: {name}", file=sys.stderr)
+        return 2
+    kind, fn = CHECKS[name]
+    n, errors = fn(REPO)
+    return report(kind, n, errors)
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv))