diff --git a/SEAMS.md b/SEAMS.md
index 6d76436..696e3fb 100644
--- a/SEAMS.md
+++ b/SEAMS.md
@@ -15,6 +15,7 @@ surface: `graph/plugins/registry.py` in the protoAgent repo.
 | `register_surface` | `nudge.py` | Lifecycle-managed due-card checker; inert at `nudge_interval_hours: 0` — the pull path (`review_next`) always works. |
 | `register_skill_dir` | `skills/learning-tutor/` | The tutor *policy* lives in a skill, not a pasted prompt — skill-level placement survives the long chats where mega-prompts decay. |
 | `register_subagent` ×2 | `subagents.py` | `review-coach` (runs one session, grades honestly) and `wiki-lint` (**read-only by tool allowlist** — curation can't edit). Trust boundaries expressed as tool lists. |
+| `graph.sdk.plugin_store` | `__init__._data_dir` | Resolves the instance-scoped durable store. A missing or failed host seam raises instead of silently crossing into a global home-directory fallback. |
 | `graph.sdk.schedule_recurring` | `__init__._arm_crons`, `/learn` | Plugin-owned crons (`plugin:learning_wiki:*`, swept on disable — #1642). Scheduled review + weekly lint = the dream/distill pattern (ADR 0054): a cron fires a normal agent turn; no new scheduling machinery. |
 | `register_goal_verifier` ×2 | `goals.py` | Learning goals become ground-truthed conditions: `learning_wiki:strength` and `learning_wiki:reviews_clear` read the ledger, never the conversation. Usable from `/goal` and from watches. |
 | `register_watch_hook` | `goals.py` | When a `/learn` watch trips (target strength reached), the hook retires the loop — `stop_goal_loop` for sugar-armed loops, a hand `cancel_scheduled` for composed ones — and emits `goal_achieved`. Both watch-id schemes recognized. |
diff --git a/__init__.py b/__init__.py
index fbdbb49..f89837a 100644
--- a/__init__.py
+++ b/__init__.py
@@ -29,7 +29,13 @@ def _data_dir(cfg: dict) -> Path:
         # manifest's v0.148 floor guarantees this seam in a real host.
         from graph import sdk
 
-        p = sdk.plugin_store(plugin_id="learning_wiki")
+        try:
+            p = sdk.plugin_store(plugin_id="learning_wiki")
+        except Exception as exc:
+            raise RuntimeError("learning_wiki requires an available instance-scoped plugin store") from exc
+        if p is None:
+            raise RuntimeError("learning_wiki received no instance-scoped plugin store")
+        p = Path(p)
     p.mkdir(parents=True, exist_ok=True)
     return p
 
diff --git a/tests/conftest.py b/tests/conftest.py
index a5d2d2a..9506189 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -116,9 +116,17 @@ def registry(tmp_path, monkeypatch):
 
 
 @pytest.fixture
-def host_stub(monkeypatch):
+def host_stub(monkeypatch, tmp_path):
     """Fake graph.* modules; returns a call-capture dict."""
-    calls = {"scheduled": [], "cancelled": [], "watches": [], "metrics": [], "loops": [], "stopped_loops": []}
+    calls = {
+        "scheduled": [],
+        "cancelled": [],
+        "watches": [],
+        "metrics": [],
+        "loops": [],
+        "stopped_loops": [],
+        "plugin_stores": [],
+    }
 
     g = types.ModuleType("graph")
     goals_pkg = types.ModuleType("graph.goals")
@@ -157,6 +165,10 @@ def record_metric(name, value, *, ts=None, plugin_id):
         calls["metrics"].append((name, value, plugin_id))
         return {}
 
+    def plugin_store(*, plugin_id):
+        calls["plugin_stores"].append(plugin_id)
+        return tmp_path / "plugin-store" / plugin_id
+
     class Knobs:
         def __init__(self, **kw):
             self.values_map: dict = {}
@@ -197,6 +209,7 @@ def stop_goal_loop(*, plugin_id, loop_id):
     sdk.cancel_scheduled = cancel_scheduled
     sdk.create_watch = create_watch
     sdk.record_metric = record_metric
+    sdk.plugin_store = plugin_store
     sdk.Knobs = Knobs
     sdk.make_knob_tools = make_knob_tools
     sdk.start_goal_loop = start_goal_loop
diff --git a/tests/test_store_path.py b/tests/test_store_path.py
index 15467dd..8691c1d 100644
--- a/tests/test_store_path.py
+++ b/tests/test_store_path.py
@@ -3,6 +3,8 @@
 import sys
 import types
 
+import pytest
+
 import learning_wiki
 
 
@@ -26,6 +28,26 @@ def plugin_store(*, plugin_id):
     assert calls == ["learning_wiki"]
 
 
+@pytest.mark.parametrize("result", [None, RuntimeError("store unavailable")])
+def test_default_store_refuses_an_unscoped_fallback(monkeypatch, result):
+    monkeypatch.delenv("LEARNING_WIKI_DIR", raising=False)
+    sdk = types.ModuleType("graph.sdk")
+
+    def plugin_store(*, plugin_id):
+        if isinstance(result, Exception):
+            raise result
+        return result
+
+    sdk.plugin_store = plugin_store
+    graph = types.ModuleType("graph")
+    graph.sdk = sdk
+    monkeypatch.setitem(sys.modules, "graph", graph)
+    monkeypatch.setitem(sys.modules, "graph.sdk", sdk)
+
+    with pytest.raises(RuntimeError, match="instance-scoped plugin store"):
+        learning_wiki._data_dir({})
+
+
 def test_config_and_env_overrides_stay_literal_and_skip_the_sdk(monkeypatch, tmp_path):
     environment = tmp_path / "environment"
     configured = tmp_path / "configured"