diff --git a/plugins/hermes-achievements/dashboard/plugin_api.py b/plugins/hermes-achievements/dashboard/plugin_api.py
index c2f69a2299..643c306464 100644
--- a/plugins/hermes-achievements/dashboard/plugin_api.py
+++ b/plugins/hermes-achievements/dashboard/plugin_api.py
@@ -142,16 +142,47 @@ ACHIEVEMENTS: List[Dict[str, Any]] = [
 ]
 
 
+def _data_dir() -> Path:
+    """Durable data root (``<hermes home>/plugin-data/hermes-achievements/``).
+
+    Was the install tree (``plugins/hermes-achievements/``) before the
+    plugin-data convention existed — state parked there died on
+    ``hermes plugins remove``/``update``. Legacy files migrate on first read.
+    """
+    try:
+        from plugins.plugin_storage import plugin_data_dir
+
+        return plugin_data_dir("hermes-achievements")
+    except Exception:
+        # Standalone dashboard import (no plugins package on sys.path):
+        # keep the plugin working with the same layout, computed locally.
+        root = get_hermes_home() / "plugin-data" / "hermes-achievements"
+        root.mkdir(parents=True, exist_ok=True)
+        return root
+
+
+def _data_file(name: str) -> Path:
+    path = _data_dir() / name
+    if not path.exists():
+        legacy = get_hermes_home() / "plugins" / "hermes-achievements" / name
+        if legacy.exists():
+            try:
+                path.write_text(legacy.read_text(encoding="utf-8"), encoding="utf-8")
+            except Exception:
+                pass
+    return path
+
+
 def state_path() -> Path:
-    return get_hermes_home() / "plugins" / "hermes-achievements" / "state.json"
+    return _data_file("state.json")
 
 
 def snapshot_path() -> Path:
-    return get_hermes_home() / "plugins" / "hermes-achievements" / "scan_snapshot.json"
+    return _data_file("scan_snapshot.json")
 
 
 def checkpoint_path() -> Path:
-    return get_hermes_home() / "plugins" / "hermes-achievements" / "scan_checkpoint.json"
+    return _data_file("scan_checkpoint.json")
 
 
 def load_state() -> Dict[str, Any]:
diff --git a/plugins/plugin_storage.py b/plugins/plugin_storage.py
new file mode 100644
index 0000000000..ca3e363a05
--- /dev/null
+++ b/plugins/plugin_storage.py
@@ -0,0 +1,76 @@
+"""Per-plugin persistent storage convention.
+
+Plugins that want durable state today invent their own paths, and most of
+them invent the same wrong one: a scratch directory inside
+``<hermes home>/plugins/<name>/``. That tree is the plugin *install* dir —
+``hermes plugins remove`` deletes it and ``hermes plugins update`` git-pulls
+into it — so user data parked there dies with the code that wrote it.
+
+This module is the sanctioned alternative: one data root per plugin under
+``<hermes home>/plugin-data/<name>/``, owned by the user, untouched by
+install/update/remove. Agent-built plugins get durable state without
+inventing a storage story, and every plugin's data is inspectable in one
+predictable place.
+
+Secrets are deliberately NOT part of this convention — credential reads go
+through ``agent.secret_scope`` / ``.env`` like everywhere else in Hermes.
+
+Usage::
+
+    from plugins.plugin_storage import plugin_data_dir, plugin_db
+
+    state_file = plugin_data_dir("my-plugin") / "state.json"
+
+    with plugin_db("my-plugin") as conn:      # <data dir>/data.db
+        conn.execute("CREATE TABLE IF NOT EXISTS ...")
+"""
+
+from __future__ import annotations
+
+import re
+import sqlite3
+from pathlib import Path
+
+__all__ = ["plugin_data_dir", "plugin_db"]
+
+# Mirrors the plugin-name shape `hermes plugins install` accepts. Anything
+# else could escape the data root via separators or traversal.
+_NAME_RE = re.compile(r"^[a-zA-Z0-9][a-zA-Z0-9._-]{0,63}$")
+
+
+def _validate_name(name: str) -> str:
+    if not _NAME_RE.fullmatch(name) or ".." in name:
+        raise ValueError(f"invalid plugin name for storage: {name!r}")
+    return name
+
+
+def plugin_data_dir(name: str) -> Path:
+    """Return (and create) this plugin's durable data directory.
+
+    ``<hermes home>/plugin-data/<name>/`` — survives plugin update and
+    removal, and follows the active profile because it resolves through
+    :func:`hermes_constants.get_hermes_home` on every call. Don't cache the
+    result across profile switches.
+    """
+    from hermes_constants import get_hermes_home
+
+    root = get_hermes_home() / "plugin-data" / _validate_name(name)
+    root.mkdir(parents=True, exist_ok=True)
+    return root
+
+
+def plugin_db(name: str, filename: str = "data.db") -> sqlite3.Connection:
+    """Open this plugin's SQLite database (created on first use).
+
+    Lives at ``<data dir>/<filename>``. WAL mode so a dashboard reader and an
+    agent-tool writer can coexist; ``check_same_thread=False`` matches the
+    multi-threaded FastAPI/tool environment plugins actually run in — the
+    caller still owns transaction discipline.
+    """
+    if Path(filename).name != filename or not filename:
+        raise ValueError(f"invalid plugin db filename: {filename!r}")
+
+    conn = sqlite3.connect(plugin_data_dir(name) / filename, check_same_thread=False)
+    conn.execute("PRAGMA journal_mode=WAL")
+    conn.execute("PRAGMA foreign_keys=ON")
+    return conn
diff --git a/tests/test_plugin_storage.py b/tests/test_plugin_storage.py
new file mode 100644
index 0000000000..28f2ded8cf
--- /dev/null
+++ b/tests/test_plugin_storage.py
@@ -0,0 +1,65 @@
+"""Tests for the per-plugin durable storage convention (plugins/plugin_storage).
+
+The contract under test: data lives under ``<hermes home>/plugin-data/<name>/``
+(NOT the ``plugins/<name>/`` install tree), names that could escape the root
+are rejected, and the sqlite helper opens a WAL-mode connection inside the
+data dir.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from hermes_constants import reset_hermes_home_override, set_hermes_home_override
+from plugins.plugin_storage import plugin_data_dir, plugin_db
+
+
+@pytest.fixture
+def hermes_home(tmp_path):
+    token = set_hermes_home_override(str(tmp_path))
+    try:
+        yield tmp_path
+    finally:
+        reset_hermes_home_override(token)
+
+
+def test_data_dir_lives_outside_the_install_tree(hermes_home):
+    root = plugin_data_dir("my-plugin")
+
+    assert root == hermes_home / "plugin-data" / "my-plugin"
+    assert root.is_dir()
+    # The invariant that motivated the module: data must not live under the
+    # install tree that `hermes plugins remove` deletes.
+    assert (hermes_home / "plugins") not in root.parents
+
+
+def test_data_dir_is_stable_across_calls(hermes_home):
+    assert plugin_data_dir("p") == plugin_data_dir("p")
+
+
+@pytest.mark.parametrize("bad", ["", ".", "..", "../escape", "a/b", "a\\b", "x" * 65])
+def test_hostile_names_are_rejected(hermes_home, bad):
+    with pytest.raises(ValueError):
+        plugin_data_dir(bad)
+
+
+def test_plugin_db_opens_wal_sqlite_in_the_data_dir(hermes_home):
+    conn = plugin_db("board")
+    try:
+        conn.execute("CREATE TABLE t (x)")
+        conn.execute("INSERT INTO t VALUES (1)")
+        conn.commit()
+
+        mode = conn.execute("PRAGMA journal_mode").fetchone()[0]
+        assert mode == "wal"
+    finally:
+        conn.close()
+
+    assert (hermes_home / "plugin-data" / "board" / "data.db").exists()
+
+
+def test_plugin_db_rejects_path_shaped_filenames(hermes_home):
+    with pytest.raises(ValueError):
+        plugin_db("board", filename="../outside.db")
+    with pytest.raises(ValueError):
+        plugin_db("board", filename="")
diff --git a/website/docs/developer-guide/plugins/index.md b/website/docs/developer-guide/plugins/index.md
index 3ba389c4e0..8688d181a3 100644
--- a/website/docs/developer-guide/plugins/index.md
+++ b/website/docs/developer-guide/plugins/index.md
@@ -659,6 +659,31 @@ with open(_DATA_FILE) as f:
     _DATA = yaml.safe_load(f)
 ```
 
+That's for files you *ship*. State you *write* is different — see the next
+section.
+
+### Store durable state
+
+Never write runtime state into your plugin directory: that's the install
+tree, and `hermes plugins update` / `remove` git-pull or delete it — your
+users' data dies with it. The sanctioned home is the per-plugin data root,
+which survives both and follows the active profile:
+
+```python
+from plugins.plugin_storage import plugin_data_dir, plugin_db
+
+# <hermes home>/plugin-data/<name>/ — created on first use
+state_file = plugin_data_dir("my-plugin") / "state.json"
+
+# Or a SQLite database at <data dir>/data.db (WAL mode, thread-friendly)
+conn = plugin_db("my-plugin")
+conn.execute("CREATE TABLE IF NOT EXISTS runs (id TEXT PRIMARY KEY)")
+```
+
+One directory per plugin means every plugin's data is inspectable in one
+predictable place. Secrets don't belong here — credential reads go through
+the standard `.env` / secret-scope path like everywhere else.
+
 ### Bundle skills
 
 Plugins can ship skill files that the agent loads via `skill_view("plugin:skill")`. Register them in your `__init__.py`: