diff --git a/.github/scripts/schema_guard.py b/.github/scripts/schema_guard.py
new file mode 100755
index 0000000..4051259
--- /dev/null
+++ b/.github/scripts/schema_guard.py
@@ -0,0 +1,143 @@
+#!/usr/bin/env python3
+"""Guard the store schema against changes an older lerd cannot read.
+
+A store definition reaches every install within a day, whatever version of lerd
+it runs, so the schema may only grow. This compares the YAML in a pull request
+against what is already published and refuses the two changes that break a
+binary older than the definition: a key that disappeared, and a key whose type
+moved under it. A value never before seen for a key that has always held a small
+closed set is reported as a warning, since widening an enum breaks an old
+binary's switch exactly as retyping does, but only the definition's author can
+say whether the binary already knows the new value.
+
+Usage: schema_guard.py <published-dir> <candidate-dir> [glob]
+"""
+
+import sys
+import pathlib
+import yaml
+
+# A key whose published values number no more than this across the whole store
+# is treated as a closed set, so a value outside it is worth a second look.
+CLOSED_SET_MAX = 8
+
+
+def type_name(value):
+    if isinstance(value, bool):
+        return "bool"
+    if isinstance(value, dict):
+        return "map"
+    if isinstance(value, list):
+        return "list"
+    if isinstance(value, (int, float)):
+        return "number"
+    if value is None:
+        return "null"
+    return "string"
+
+
+def walk(node, prefix, types, values):
+    """Flatten a document into path -> type and path -> observed scalar values.
+
+    List elements collapse onto one `[]` path: the store cares that a list holds
+    maps or strings, not what sits at index three.
+    """
+    kind = type_name(node)
+    if prefix:
+        types.setdefault(prefix, set()).add(kind)
+    if isinstance(node, dict):
+        for key, child in node.items():
+            walk(child, f"{prefix}.{key}" if prefix else str(key), types, values)
+    elif isinstance(node, list):
+        for child in node:
+            walk(child, f"{prefix}[]", types, values)
+    elif prefix and kind in ("string", "bool", "number"):
+        values.setdefault(prefix, set()).add(str(node))
+
+
+def load(path):
+    with open(path, "r", encoding="utf-8") as handle:
+        return yaml.safe_load(handle) or {}
+
+
+def profile(root, pattern):
+    """Map every YAML file under root to its flattened type and value profile."""
+    out = {}
+    for path in sorted(pathlib.Path(root).rglob(pattern)):
+        rel = str(path.relative_to(root))
+        types, values = {}, {}
+        try:
+            walk(load(path), "", types, values)
+        except yaml.YAMLError as err:
+            out[rel] = ("unparseable", err)
+            continue
+        out[rel] = (types, values)
+    return out
+
+
+def closed_sets(profiles):
+    """Union every file's observed values per path, keeping the small ones."""
+    merged = {}
+    for entry in profiles.values():
+        if entry[0] == "unparseable":
+            continue
+        for path, vals in entry[1].items():
+            merged.setdefault(path, set()).update(vals)
+    return {p: v for p, v in merged.items() if len(v) <= CLOSED_SET_MAX}
+
+
+def main():
+    if len(sys.argv) < 3:
+        print(__doc__)
+        return 2
+    published_dir, candidate_dir = sys.argv[1], sys.argv[2]
+    pattern = sys.argv[3] if len(sys.argv) > 3 else "*.yaml"
+
+    published = profile(published_dir, pattern)
+    candidate = profile(candidate_dir, pattern)
+    enums = closed_sets(published)
+
+    failures, warnings = [], []
+
+    for rel, entry in published.items():
+        if rel not in candidate:
+            warnings.append(f"{rel}: no longer published, an install that refetches it gets a 404")
+            continue
+        if entry[0] == "unparseable" or candidate[rel][0] == "unparseable":
+            continue
+        old_types, _ = entry
+        new_types, new_values = candidate[rel]
+
+        for path, kinds in sorted(old_types.items()):
+            if path not in new_types:
+                failures.append(f"{rel}: key `{path}` was removed, an older lerd still reads it")
+                continue
+            if kinds != new_types[path] and not kinds & new_types[path]:
+                failures.append(
+                    f"{rel}: key `{path}` changed type from {'/'.join(sorted(kinds))} "
+                    f"to {'/'.join(sorted(new_types[path]))}"
+                )
+
+        for path, vals in sorted(new_values.items()):
+            if path not in enums:
+                continue
+            for value in sorted(vals - enums[path]):
+                warnings.append(
+                    f"{rel}: key `{path}` takes the new value `{value}`; published values are "
+                    f"{', '.join(sorted(enums[path]))}. Confirm every supported lerd understands it"
+                )
+
+    for line in warnings:
+        print(f"warning: {line}")
+    for line in failures:
+        print(f"error: {line}")
+
+    if failures:
+        print(f"\n{len(failures)} change(s) an older lerd cannot read.")
+        return 1
+    print(f"\nschema guard passed ({len(published)} published file(s) checked, {len(warnings)} warning(s)).")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/.github/workflows/schema-guard.yml b/.github/workflows/schema-guard.yml
new file mode 100644
index 0000000..c8bcd6d
--- /dev/null
+++ b/.github/workflows/schema-guard.yml
@@ -0,0 +1,30 @@
+name: Schema guard
+
+on:
+  pull_request:
+
+# A store definition reaches every install within a day, whatever version of
+# lerd it runs, so a pull request may only grow the schema. This refuses a key
+# that disappeared or changed type, and reports a value no published definition
+# has used before.
+jobs:
+  schema-guard:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - run: pip install pyyaml
+
+      - name: Extract the published tree
+        run: |
+          mkdir -p "$RUNNER_TEMP/published"
+          git archive "origin/${{ github.base_ref }}" | tar -x -C "$RUNNER_TEMP/published"
+
+      - name: Compare this pull request against it
+        run: python3 .github/scripts/schema_guard.py "$RUNNER_TEMP/published" .