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diff --git a/mailrules.py b/mailrules.py
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+#!/usr/bin/env python3
+#
+# Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+"""Shared notmuch tagging-rule store.
+
+The rules live in ~/.config/mailrules/rules.json and are read by both this
+tool and qtmaildir, so the format belongs to neither: a field one tool does
+not understand is preserved verbatim across a save by the other.
+
+A rule carries NO scope. The post-new hook supplies `tag:new`, a dry run
+supplies nothing and counts against the whole corpus. This is what lets one
+rule serve arrivals, a dry run, and (later) a backfill over history.
+
+Stdlib only, deliberately: this module is imported by a notmuch hook that
+runs on every sync, and mailctl has no dependencies to inherit.
+"""
+
+import json
+import os
+from dataclasses import dataclass, field
+from pathlib import Path
+
+FORMAT_VERSION = 1
+DEFAULT_STAGE = 50
+
+# Fields this version understands. Anything else in a rule object is kept in
+# `unknown` and written back untouched, which is what makes the file neutral
+# rather than this tool's file that another program may read.
+KNOWN_KEYS = {"id", "stage", "enabled", "add", "remove", "query", "note"}
+
+
+@dataclass
+class Rule:
+    id: str
+    query: str
+    add: list = field(default_factory=list)
+    remove: list = field(default_factory=list)
+    stage: int = DEFAULT_STAGE
+    enabled: bool = True
+    note: str = ""
+    unknown: dict = field(default_factory=dict)
+
+
+@dataclass
+class Store:
+    rules: list = field(default_factory=list)
+    warnings: list = field(default_factory=list)
+    unknown: dict = field(default_factory=dict)
+
+
+def default_path():
+    """$XDG_CONFIG_HOME/mailrules/rules.json, or ~/.config/... as fallback.
+
+    No hardcoded home directory: both tools must resolve the same path, and
+    a user with XDG_CONFIG_HOME set expects it honoured.
+    """
+    base = os.environ.get("XDG_CONFIG_HOME") or Path.home() / ".config"
+    return Path(base) / "mailrules" / "rules.json"
+
+
+def load(path=None):
+    """Read the store. Never raises for a bad file: problems land in
+    Store.warnings and the offending rule is dropped, so one malformed rule
+    cannot stop the other nineteen from running."""
+    path = Path(path) if path else default_path()
+    store = Store()
+
+    raw = json.loads(path.read_text())
+
+    for obj in raw.get("rules", []):
+        store.rules.append(Rule(
+            id=obj["id"],
+            query=obj["query"],
+            add=list(obj.get("add", [])),
+            remove=list(obj.get("remove", [])),
+            stage=int(obj.get("stage", DEFAULT_STAGE)),
+            enabled=bool(obj.get("enabled", True)),
+            note=obj.get("note", ""),
+            unknown={k: v for k, v in obj.items() if k not in KNOWN_KEYS},
+        ))
+
+    return store