diff --git a/src/scribe/mcp/tools/processes.py b/src/scribe/mcp/tools/processes.py
index b4d831c..1246406 100644
--- a/src/scribe/mcp/tools/processes.py
+++ b/src/scribe/mcp/tools/processes.py
@@ -190,10 +190,12 @@ async def update_process(process_id: int, title: str = "", body: str = "",
         fields["tags"] = tags
     # As the owner — update_note is owner-scoped and the write is authorised above.
     updated = await notes_svc.update_note(note.user_id, process_id, **fields)
-    # Written as the OWNER, matching the update above: an editor-shared process
-    # keeps its owner's associations rather than sprouting a second set.
+    # The CALLER, not the owner. `update_note` above is owner-scoped because
+    # the service demands it; `set_record_systems` is not — it runs its own
+    # share-aware check and links only Systems the acting user can read, so
+    # passing the owner would bypass the check and borrow their reach (#47).
     if system_ids is not None:
-        await systems_svc.set_record_systems(note.user_id, process_id, system_ids)
+        await systems_svc.set_record_systems(uid, process_id, system_ids)
     if updated is None:
         raise ValueError(f"process {process_id} not found")
     out = updated.to_dict()
diff --git a/src/scribe/mcp/tools/snippets.py b/src/scribe/mcp/tools/snippets.py
index e4ad0ad..5824d6b 100644
--- a/src/scribe/mcp/tools/snippets.py
+++ b/src/scribe/mcp/tools/snippets.py
@@ -483,7 +483,10 @@ async def update_snippet(
     if note is None:
         raise ValueError(f"snippet {snippet_id} not found")
     if system_ids is not None:
-        await systems_svc.set_record_systems(note.user_id, snippet_id, system_ids)
+        # The CALLER, not the owner (#4249). `update_snippet` above already
+        # raised PermissionError if this user may not write, so the tagging
+        # runs with the actor's own System visibility rather than the owner's.
+        await systems_svc.set_record_systems(uid, snippet_id, system_ids)
     data = snippets_svc.snippet_to_dict(note)
     data.update(await access_svc.describe_provenance(uid, note))
     await systems_tools.attach_systems(
diff --git a/src/scribe/routes/lessons.py b/src/scribe/routes/lessons.py
index cd384d1..e7e19d6 100644
--- a/src/scribe/routes/lessons.py
+++ b/src/scribe/routes/lessons.py
@@ -240,9 +240,13 @@ async def update_lesson_route(lesson_id: int):
     if updated is None:
         return not_found("Lesson")
     if data.get("system_ids") is not None:
-        await systems_svc.set_record_systems(
-            owner_uid, lesson_id, data["system_ids"]
-        )
+        # The CALLER, not owner_uid (#4249). `set_record_systems` runs its own
+        # `can_write_note` and links only Systems the acting user can read;
+        # handing it the owner makes that check trivially pass and filters by
+        # the owner's visibility instead. The caller's write permission is
+        # already established above, so this neither loosens nor tightens who
+        # may edit — it decides WHOSE reach the tagging uses (#47).
+        await systems_svc.set_record_systems(uid, lesson_id, data["system_ids"])
     out = lessons_svc.lesson_to_dict(updated)
     out["systems"] = [
         s.to_dict()
diff --git a/src/scribe/routes/snippets.py b/src/scribe/routes/snippets.py
index 5f9978f..ff28699 100644
--- a/src/scribe/routes/snippets.py
+++ b/src/scribe/routes/snippets.py
@@ -208,7 +208,8 @@ async def update_snippet_route(snippet_id: int):
     if updated is None:
         return not_found("Snippet")
     if data.get("system_ids") is not None:
-        await systems_svc.set_record_systems(owner_uid, snippet_id, data["system_ids"])
+        # The CALLER, not owner_uid — see routes/lessons.py for the why (#4249).
+        await systems_svc.set_record_systems(uid, snippet_id, data["system_ids"])
     out = snippets_svc.snippet_to_dict(updated)
     out["systems"] = [
         s.to_dict() for s in await systems_svc.list_record_systems(owner_uid, snippet_id)
diff --git a/tests/test_system_tagging_door_parity.py b/tests/test_system_tagging_door_parity.py
index 8bb4fdd..80e9966 100644
--- a/tests/test_system_tagging_door_parity.py
+++ b/tests/test_system_tagging_door_parity.py
@@ -178,6 +178,60 @@ def test_the_registry_names_functions_that_exist():
     assert not missing, f"registry names functions that no longer exist: {missing}"
 
 
+def test_every_tagging_write_acts_as_the_caller():
+    """`set_record_systems` is handed the ACTING user, never the record's owner.
+
+    THE ARGUMENT. `set_record_systems` is not a dumb setter — it runs its own
+    `can_write_note` and then links only Systems the given user can READ.
+    Handing it `note.user_id` therefore does two things at once, both quiet:
+    the access check becomes trivially true (an owner can always write their
+    own record), and the System filter uses the owner's visibility instead of
+    the actor's. On a single-user install those are invisible. With a share
+    they are an editor acting with the owner's reach — the shape #47 exists to
+    prevent, and the same reasoning routes/notes.py already applies to
+    `set_supersedes`.
+
+    It is NOT a permission loosening either way: every call site establishes
+    the caller's write access first (a route's `can_write_note`, or a service
+    that raises PermissionError). What this decides is WHOSE reach the tagging
+    runs with, which is exactly the kind of difference that survives review
+    because each call site reads fine on its own.
+
+    Four of twenty sites passed the owner before #4249 — two routes, two MCP
+    tools, one of them written earlier in the very session that unified them.
+    That is the tell that this needs a guard rather than care.
+    """
+    offenders = []
+    for path in ROOT.rglob("*.py"):
+        src = path.read_text()
+        if "set_record_systems" not in src:
+            continue
+        tree = ast.parse(src)
+        for node in ast.walk(tree):
+            if not isinstance(node, ast.Call):
+                continue
+            fn = node.func
+            name = fn.attr if isinstance(fn, ast.Attribute) else getattr(fn, "id", None)
+            if name != "set_record_systems" or not node.args:
+                continue
+            first = node.args[0]
+            # The acting user arrives as a bare local — `uid` in the tools and
+            # routes, `user_id` in the service's own signature. An attribute
+            # access (`note.user_id`, `target.user_id`) is a record's owner,
+            # and `owner_uid` is the same thing already unpacked.
+            ok = isinstance(first, ast.Name) and first.id in {"uid", "user_id"}
+            if not ok:
+                shown = ast.unparse(first) if hasattr(ast, "unparse") else "?"
+                rel = path.relative_to(ROOT.parent.parent)
+                offenders.append(f"{rel}:{node.lineno} passes {shown!r}")
+    assert not offenders, (
+        "set_record_systems must be called with the acting user, not the "
+        "record's owner — passing the owner bypasses its own access check and "
+        "borrows the owner's System visibility (#47, #4249):\n  "
+        + "\n  ".join(offenders)
+    )
+
+
 def test_milestones_are_absent_on_purpose():
     """The one 'gap' that is not a gap, pinned so it is not re-opened.