fix(acl): align MCP writes with the share model — editor edits, viewer doesn't

Option B, per the operator. Closes the last inconsistency from the ACL work.

The agent path had drifted into an indefensible position: delete_snippet honoured
editor shares (I made it share-aware so widening the read wouldn't let a VIEWER
trash things) while update_snippet still resolved through the owner-only
notes.get_note. So through an agent you could destroy a colleague's snippet but
not improve it — and the refusal claimed "not found" for a record you could
plainly open.

Now update_snippet, merge_snippets and update_process all resolve the read scope
and then require can_write_note, matching the REST routes and the sharing UI's
own promise that viewer / editor / admin are distinct grants. A viewer grant is
refused with the actual reason ("shared with you read-only — ask its owner for
edit access, or record your own version"), because not-found would send an agent
hunting for a missing id instead of recording its own copy.

Authorised writes are performed as the OWNER, since the underlying note update is
owner-scoped and a shared editor's own id would match nothing.

Merge additionally requires each source to share the TARGET'S owner and to be
writable by the caller — merging trashes the source, so read access isn't enough,
and cross-owner merge stays out of scope (#231). Sources failing either test are
skipped rather than half-merged.

A record the caller cannot read at all still returns not-found rather than
forbidden, so the error can't be used to confirm that an id exists.

Also fixed _fake_snippet's missing user_id proactively — the same
auto-MagicMock-reads-as-foreign trap that broke CI twice (see note 2109).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLwAaV4DQEmVyn496HnEvt

src/scribe/mcp/server.py

@@ -236,7 +236,9 @@ adopting it or acting on it. This matters most for a shared Process — do not r
 one as written; describe what it would do and get the operator's go-ahead.
 Records shared directly with the operator are also deliberately search-only:
 they surface when you look for them (pass a query), not in plain lists, so
-nobody else's material arrives unasked.
+nobody else's material arrives unasked. Editing another user's record needs an
+editor or admin share from them; a read-only share is refused, and the right
+answer is usually to record the operator's own version rather than to push.
 
 When developing Scribe itself, honor its multi-user sharing ACL: scope every
 read and mutation of user data by owner + shares — never assume a single

src/scribe/mcp/tools/processes.py

@@ -88,11 +88,21 @@ async def get_process(name_or_id: str) -> dict:
 async def update_process(process_id: int, title: str = "", body: str = "",
                          tags: list[str] | None = None) -> dict:
     """Update a stored process. Only provided fields change — empty title/body
-    leave that field unchanged; pass tags to replace the tag set."""
+    leave that field unchanged; pass tags to replace the tag set.
+
+    Editing another user's process requires an editor or admin share from them; a
+    read-only share is not enough and says so rather than claiming not-found.
+    """
     uid = current_user_id()
-    note = await notes_svc.get_note(uid, process_id)
-    if note is None or note.note_type != "process":
+    loaded = await notes_svc.get_note_for_user(uid, process_id)
+    note = loaded[0] if loaded else None
+    if note is None or note.note_type != "process" or note.deleted_at is not None:
         raise ValueError(f"process {process_id} not found")
+    if not await access_svc.can_write_note(uid, process_id):
+        raise ValueError(
+            f"process {process_id} is shared with you read-only — ask its owner "
+            f"for edit access, or save your own copy with create_process"
+        )
     fields: dict = {}
     if title.strip():
         fields["title"] = title.strip()
@@ -100,8 +110,13 @@ async def update_process(process_id: int, title: str = "", body: str = "",
         fields["body"] = body
     if tags is not None:
         fields["tags"] = tags
-    updated = await notes_svc.update_note(uid, process_id, **fields)
-    return updated.to_dict()
+    # 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)
+    if updated is None:
+        raise ValueError(f"process {process_id} not found")
+    out = updated.to_dict()
+    out.update(await access_svc.describe_provenance(uid, updated))
+    return out
 
 
 def register(mcp) -> None:

src/scribe/mcp/tools/snippets.py

@@ -180,6 +180,10 @@ async def update_snippet(
         tags: Replaces the extra-tag set (language + "snippet" are re-derived).
         project_id: 0 leaves it unchanged, -1 detaches it from its project, a
             positive id moves it.
+
+    Editing someone else's snippet requires an editor or admin share from them.
+    A read-only share is refused with a message saying so — record your own
+    version instead of trying to force it.
     """
     uid = current_user_id()
     if project_id == 0:
@@ -189,21 +193,27 @@ async def update_snippet(
     else:
         project = project_id
 
-    note = await snippets_svc.update_snippet(
-        uid, snippet_id,
-        name=name, code=code, language=language,
-        signature=signature, when_to_use=when_to_use,
-        repo=repo, path=path, symbol=symbol,
-        locations=locations, tags=tags, project_id=project,
-    )
+    try:
+        note = await snippets_svc.update_snippet(
+            uid, snippet_id,
+            name=name, code=code, language=language,
+            signature=signature, when_to_use=when_to_use,
+            repo=repo, path=path, symbol=symbol,
+            locations=locations, tags=tags, project_id=project,
+        )
+    except PermissionError as exc:
+        # Readable but not writable — surface the real reason, not "not found".
+        raise ValueError(str(exc)) from exc
     if note is None:
         raise ValueError(f"snippet {snippet_id} not found")
     if system_ids is not None:
-        await systems_svc.set_record_systems(uid, snippet_id, system_ids)
+        await systems_svc.set_record_systems(note.user_id, snippet_id, system_ids)
     data = snippets_svc.snippet_to_dict(note)
+    data.update(await access_svc.describe_provenance(uid, note))
     if system_ids is not None:
         data["systems"] = [
-            s.to_dict() for s in await systems_svc.list_record_systems(uid, snippet_id)
+            s.to_dict()
+            for s in await systems_svc.list_record_systems(note.user_id, snippet_id)
         ]
     return data
 
@@ -245,7 +255,10 @@ async def merge_snippets(target_id: int, source_ids: list[int]) -> dict:
     ids = [s for s in (source_ids or []) if s != target_id]
     if not ids:
         raise ValueError("merge_snippets requires at least one other source_id")
-    result = await snippets_svc.merge_snippets(uid, target_id, ids)
+    try:
+        result = await snippets_svc.merge_snippets(uid, target_id, ids)
+    except PermissionError as exc:
+        raise ValueError(str(exc)) from exc
     if result is None:
         raise ValueError(f"snippet {target_id} not found")
     note, merged_ids = result

src/scribe/services/snippets.py

@@ -354,7 +354,14 @@ async def update_snippet(
 ):
     """Partial update: only fields passed (not None) change. Re-serializes the
     merged field set back into title/body/tags. Returns the Note, or None if the
-    id isn't a snippet.
+    id isn't a snippet the caller can see.
+
+    Share-aware (rule #47/#78): resolves the read scope, then requires WRITE —
+    so an editor/admin grant lets the holder edit, and a viewer grant does not.
+    Raises PermissionError when the caller can read but not write, because "not
+    found" would be a lie about a record they can plainly open. The write itself
+    is performed as the OWNER, mirroring routes/snippets.py, since the underlying
+    note update is owner-scoped.
 
     ``project_id``: omit to leave unchanged, pass None to detach from its
     project, pass an id to move it.
@@ -362,9 +369,15 @@ async def update_snippet(
     Locations: ``locations`` replaces the whole set; else a legacy single
     ``repo``/``path``/``symbol`` overlays onto the first existing location; else
     the existing locations are kept."""
-    note = await notes_svc.get_note(user_id, snippet_id)
-    if note is None or note.note_type != SNIPPET_NOTE_TYPE:
+    note = await get_snippet(user_id, snippet_id)
+    if note is None:
         return None
+    from scribe.services.access import can_write_note
+    if not await can_write_note(user_id, snippet_id):
+        raise PermissionError(
+            f"snippet {snippet_id} is shared with you read-only — ask its owner "
+            f"for edit access, or record your own version"
+        )
 
     cur = parse_snippet_fields(note.title, note.body, note.tags)
     overlay = {
@@ -404,7 +417,9 @@ async def update_snippet(
     if project_id is not UNSET:
         fields["project_id"] = project_id
 
-    updated = await notes_svc.update_note(user_id, snippet_id, **fields)
+    # As the OWNER: update_note is owner-scoped, so a shared editor's own id
+    # would find nothing. The write was authorised by can_write_note above.
+    updated = await notes_svc.update_note(note.user_id, snippet_id, **fields)
     if updated is not None:
         # Title/body changed → refresh the embedding so recall reflects the edit.
         _embed_snippet(updated)
@@ -460,23 +475,41 @@ def merge_snippet_fields(
 
 
 async def merge_snippets(user_id: int, target_id: int, source_ids: list[int]):
-    """Unify source snippets INTO the target (all owned by user_id): union their
-    locations + tags onto the canonical target, keep the target's scalar fields,
-    trash the sources (recoverable), re-embed the survivor.
+    """Unify source snippets INTO the target: union their locations + tags onto
+    the canonical target, keep the target's scalar fields, trash the sources
+    (recoverable), re-embed the survivor.
+
+    Share-aware and write-gated, like update: an editor/admin grant is enough, a
+    viewer grant is not. Raises PermissionError when the caller can read the
+    target but not write it. Every source must share the TARGET'S OWNER —
+    cross-owner merge stays out of scope (#231) — and must itself be writable;
+    sources failing either test are skipped rather than silently half-merged.
 
     Returns (merged_target_note, merged_source_ids), or None if the target isn't
-    the user's snippet. Source ids that aren't the user's snippets are skipped."""
-    target = await notes_svc.get_note(user_id, target_id)
-    if target is None or target.note_type != SNIPPET_NOTE_TYPE:
+    a snippet the caller can see."""
+    from scribe.services.access import can_write_note
+    target = await get_snippet(user_id, target_id)
+    if target is None:
         return None
+    if not await can_write_note(user_id, target_id):
+        raise PermissionError(
+            f"snippet {target_id} is shared with you read-only — you can't merge "
+            f"into a record you can't edit"
+        )
+    owner_id = target.user_id
 
     sources = []
     for sid in source_ids:
         if sid == target_id:
             continue
-        s = await notes_svc.get_note(user_id, sid)
-        if s is not None and s.note_type == SNIPPET_NOTE_TYPE:
-            sources.append(s)
+        s = await get_snippet(user_id, sid)
+        # Same owner as the target, and writable by this caller. Merging trashes
+        # the source, so read access is not enough.
+        if s is None or s.user_id != owner_id:
+            continue
+        if not await can_write_note(user_id, sid):
+            continue
+        sources.append(s)
 
     tgt_fields = parse_snippet_fields(target.title, target.body, target.tags)
     parsed_sources = [
@@ -484,8 +517,9 @@ async def merge_snippets(user_id: int, target_id: int, source_ids: list[int]):
     ]
     locations, extra_tags = merge_snippet_fields(tgt_fields, target.tags, parsed_sources)
 
+    # Owner-scoped write, authorised above — same reason as update_snippet.
     updated = await notes_svc.update_note(
-        user_id, target_id,
+        owner_id, target_id,
         body=compose_body(
             code=tgt_fields["code"], language=tgt_fields["language"],
             signature=tgt_fields["signature"], when_to_use=tgt_fields["when_to_use"],
@@ -500,7 +534,7 @@ async def merge_snippets(user_id: int, target_id: int, source_ids: list[int]):
     from scribe.services.trash import delete as trash_delete
     merged_ids: list[int] = []
     for s in sources:
-        batch = await trash_delete(user_id, "note", s.id)
+        batch = await trash_delete(owner_id, "note", s.id)
         if batch is not None:
             merged_ids.append(s.id)
 

tests/test_mcp_tool_snippets.py

@@ -13,13 +13,17 @@ def _bind_user():
     _user_id_ctx.reset(token)
 
 
-def _fake_snippet():
+def _fake_snippet(user_id: int = 7):
     n = MagicMock()
     n.id = 1
     n.title = "debounce — rate-limit a callback"
     n.body = "```js\nreturn 1\n```\n"
     n.tags = ["js", "snippet"]
     n.note_type = "snippet"
+    # Real int, matching the bound caller by default. The tools compare it to
+    # decide whether to attach a shared/owner marker; an auto-MagicMock would read
+    # as another user's record and send them off to look up a username.
+    n.user_id = user_id
     n.to_dict.return_value = {
         "id": 1, "title": n.title, "note_type": "snippet", "tags": n.tags,
     }
@@ -128,6 +132,31 @@ async def test_create_and_update_pass_locations_through():
     assert mock_update.await_args.kwargs["locations"] == locs
 
 
+@pytest.mark.asyncio
+async def test_update_snippet_read_only_share_says_why():
+    """A viewer grant must be refused with the REAL reason. "Not found" would be
+    a lie about a record the caller can plainly open, and would send an agent
+    hunting for a missing id instead of recording its own version."""
+    with patch("scribe.services.snippets.update_snippet",
+               AsyncMock(side_effect=PermissionError(
+                   "snippet 1 is shared with you read-only — ask its owner for "
+                   "edit access, or record your own version"))):
+        from scribe.mcp.tools.snippets import update_snippet
+        with pytest.raises(ValueError, match="read-only"):
+            await update_snippet(1, name="x")
+
+
+@pytest.mark.asyncio
+async def test_merge_snippets_read_only_target_says_why():
+    with patch("scribe.services.snippets.merge_snippets",
+               AsyncMock(side_effect=PermissionError(
+                   "snippet 1 is shared with you read-only — you can't merge "
+                   "into a record you can't edit"))):
+        from scribe.mcp.tools.snippets import merge_snippets
+        with pytest.raises(ValueError, match="read-only"):
+            await merge_snippets(target_id=1, source_ids=[2])
+
+
 @pytest.mark.asyncio
 async def test_delete_snippet_retires_or_raises():
     from scribe.mcp.tools.snippets import delete_snippet

tests/test_shared_write_access.py

@@ -0,0 +1,111 @@
+"""Editing a shared record: an editor grant is enough, a viewer grant is not.
+
+The agent and web paths used to disagree — and worse, within the agent path
+`delete_snippet` honoured editor shares while `update_snippet` did not, so a
+colleague's snippet could be destroyed through an agent but not improved. Both
+now resolve the read scope and then require WRITE, which is what the sharing UI
+already promises: viewer / editor / admin are distinct grants.
+
+A record readable-but-not-writable raises PermissionError rather than reporting
+"not found" — the caller can plainly open it, so not-found would be a lie that
+sends them hunting for a missing id.
+"""
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+
+def _snippet(id=1, owner=9):
+    n = MagicMock()
+    n.id = id
+    n.user_id = owner
+    n.title = "formatDuration — humanize a millisecond count"
+    n.body = "```ts\nexport const f = 1\n```\n"
+    n.tags = ["ts", "snippet"]
+    n.note_type = "snippet"
+    n.deleted_at = None
+    return n
+
+
+@pytest.mark.asyncio
+async def test_editor_share_can_update_and_writes_as_the_owner():
+    """The underlying note update is owner-scoped, so a shared editor's own id
+    would match nothing — the authorised write has to be performed as the owner."""
+    from scribe.services import snippets as svc
+    note = _snippet(owner=9)
+    updated = _snippet(owner=9)
+    with patch.object(svc, "get_snippet", AsyncMock(return_value=note)), \
+         patch("scribe.services.access.can_write_note", AsyncMock(return_value=True)), \
+         patch.object(svc.notes_svc, "update_note",
+                      AsyncMock(return_value=updated)) as mock_update, \
+         patch.object(svc, "_embed_snippet", MagicMock()):
+        got = await svc.update_snippet(7, 1, name="formatDuration")
+
+    assert got is updated
+    # Positional user_id is the OWNER (9), not the caller (7).
+    assert mock_update.await_args.args[0] == 9
+
+
+@pytest.mark.asyncio
+async def test_viewer_share_cannot_update():
+    from scribe.services import snippets as svc
+    with patch.object(svc, "get_snippet", AsyncMock(return_value=_snippet())), \
+         patch("scribe.services.access.can_write_note", AsyncMock(return_value=False)), \
+         patch.object(svc.notes_svc, "update_note", AsyncMock()) as mock_update:
+        with pytest.raises(PermissionError, match="read-only"):
+            await svc.update_snippet(7, 1, name="nope")
+    mock_update.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_viewer_share_cannot_delete():
+    """delete_snippet returns False rather than raising — its contract is boolean
+    — but a viewer must not be able to bin someone else's record."""
+    from scribe.services import snippets as svc
+    with patch.object(svc, "get_snippet", AsyncMock(return_value=_snippet())), \
+         patch("scribe.services.access.can_write_note", AsyncMock(return_value=False)), \
+         patch("scribe.services.trash.delete", AsyncMock()) as mock_trash:
+        assert await svc.delete_snippet(7, 1) is False
+    mock_trash.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_unreadable_record_is_not_found_not_forbidden():
+    """A record the caller can't see at all must NOT be distinguishable from one
+    that doesn't exist — otherwise the error itself confirms it exists."""
+    from scribe.services import snippets as svc
+    with patch.object(svc, "get_snippet", AsyncMock(return_value=None)):
+        assert await svc.update_snippet(7, 404, name="x") is None
+        assert await svc.delete_snippet(7, 404) is False
+
+
+@pytest.mark.asyncio
+async def test_merge_requires_write_on_target():
+    from scribe.services import snippets as svc
+    with patch.object(svc, "get_snippet", AsyncMock(return_value=_snippet())), \
+         patch("scribe.services.access.can_write_note", AsyncMock(return_value=False)):
+        with pytest.raises(PermissionError, match="read-only"):
+            await svc.merge_snippets(7, 1, [2])
+
+
+@pytest.mark.asyncio
+async def test_merge_skips_sources_owned_by_someone_else():
+    """Cross-owner merge stays out of scope (#231): a source belonging to a
+    different owner than the target is skipped, not silently folded in and
+    trashed."""
+    from scribe.services import snippets as svc
+    target = _snippet(id=1, owner=9)
+    same_owner = _snippet(id=2, owner=9)
+    other_owner = _snippet(id=3, owner=42)
+
+    async def fake_get(_uid, sid):
+        return {1: target, 2: same_owner, 3: other_owner}.get(sid)
+
+    with patch.object(svc, "get_snippet", AsyncMock(side_effect=fake_get)), \
+         patch("scribe.services.access.can_write_note", AsyncMock(return_value=True)), \
+         patch.object(svc.notes_svc, "update_note", AsyncMock(return_value=target)), \
+         patch("scribe.services.trash.delete", AsyncMock(return_value=object())), \
+         patch.object(svc, "_embed_snippet", MagicMock()):
+        _note, merged_ids = await svc.merge_snippets(7, 1, [2, 3])
+
+    assert merged_ids == [2]