From f53d377766cdf4f61b0d02c74070c2bc170a74e8 Mon Sep 17 00:00:00 2001
From: Bryan Van Deusen <bryan.vandeusen@fabledsword.com>
Date: Wed, 7 Oct 2026 15:01:53 -0400
Subject: [PATCH] sharing: share a note from the web, at view or edit, with
 anyone on the instance
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

The ACL has gated every read since M0, but nothing could write a share.

Server:
- shares_api: GET /api/users/directory (everyone but you, signed-in only),
  GET/POST /api/notes/<id>/shares and DELETE …/shares/<share_id>, owner
  only. Sharing again with the same person changes the permission
  (ON CONFLICT on the new unique index).
- acl.visible_to_user takes permission=; granted_to and shared_ids feed
  the serializer.
- Edit covers body and checklist (_get_editable). Everything else stays
  _get_owned. A view share's write is a 404 like a stranger's (#1984). An
  editor's PATCH naming anything but body is a 403.
- Serialized notes carry permission, shared and shared_by. A recipient
  never gets the owner's labels, and a #tag an editor types files under
  the owner's (it always went to note.owner_id).
- ?shared=with_me, also allowed in saved views. Trash and reminders are the
  owner's. purge_note drops the note's shares.
- Migration 0034: one share per note and person (and per group), permission
  limited to view and edit, an index for "shared with me".

Web:
- ShareDialog (one, mounted by the shell): pick a member, Can view or Can
  edit, change or remove existing shares, with loading, error and empty
  states.
- Card: "Shared by X" or "Shared" chip; owner-only actions and reminder
  buttons hidden for recipients; checkboxes inert at view.
- Editor: read-only at view; text and checklist only at edit; Share button
  for the owner.
- FilterBar: Shared with me. Repo seam gains `shares`; the offline desktop
  shows none of it (#5175 brings sharing there).

Tests: owner, recipient and stranger across reads, every write at view and
edit, tag filing, unshare, trash, delete and validation; web unit tests for
the facet and permission helpers. #5174.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 alembic/versions/0034_share_constraints.py |  50 +++++++
 frontend/src/adapters/local.ts             |  11 +-
 frontend/src/adapters/repo.ts              |  28 ++++
 frontend/src/adapters/rest.ts              |  12 ++
 frontend/src/components/AppShell.vue       |   2 +
 frontend/src/components/FilterBar.vue      |  15 ++
 frontend/src/components/Icon.vue           |   1 +
 frontend/src/components/NoteCard.vue       |  54 ++++++-
 frontend/src/components/NoteEditor.vue     |  60 ++++++--
 frontend/src/components/ShareDialog.vue    | 163 +++++++++++++++++++++
 frontend/src/notes/facets.ts               |   3 +
 frontend/src/notes/sharing.test.ts         |  36 +++++
 frontend/src/notes/sharing.ts              |  14 ++
 frontend/src/stores/notes.ts               |  21 +++
 frontend/src/stores/ui.ts                  |  13 ++
 src/inkwell/acl.py                         |  50 ++++++-
 src/inkwell/app.py                         |   2 +
 src/inkwell/notes/__init__.py              |  56 ++++---
 src/inkwell/notes/helpers.py               |  25 +++-
 src/inkwell/notes/serialize.py             |  54 +++++--
 src/inkwell/retention.py                   |   3 +
 src/inkwell/saved_filters.py               |   1 +
 src/inkwell/shares_api.py                  | 127 ++++++++++++++++
 tests/test_integration.py                  | 160 +++++++++++++++++++-
 24 files changed, 911 insertions(+), 50 deletions(-)
 create mode 100644 alembic/versions/0034_share_constraints.py
 create mode 100644 frontend/src/components/ShareDialog.vue
 create mode 100644 frontend/src/notes/sharing.test.ts
 create mode 100644 frontend/src/notes/sharing.ts
 create mode 100644 src/inkwell/shares_api.py

diff --git a/alembic/versions/0034_share_constraints.py b/alembic/versions/0034_share_constraints.py
new file mode 100644
index 0000000..10181f2
--- /dev/null
+++ b/alembic/versions/0034_share_constraints.py
@@ -0,0 +1,50 @@
+"""shares: one grant per note and person, and only the permissions the app knows
+
+Revision ID: 0034
+Revises: 0033
+Create Date: 2026-10-07
+
+Nothing wrote a share until #5174, so the table never needed these. Now the Share
+dialog does:
+- A unique index on (resource_type, resource_id, shared_with_user_id) where a user is
+  the target, so sharing a note with someone twice updates the one grant rather than
+  stacking two (the same for a group, ahead of step 15).
+- A check that `permission` is `view` or `edit`.
+- An index on `shared_with_user_id` for "Shared with me".
+
+## Downgrade
+
+Drops the indexes and the check. The rows stay.
+"""
+from alembic import op
+
+revision = "0034"
+down_revision = "0033"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_index(
+        "uq_shares_resource_user",
+        "shares",
+        ["resource_type", "resource_id", "shared_with_user_id"],
+        unique=True,
+        postgresql_where="shared_with_user_id IS NOT NULL",
+    )
+    op.create_index(
+        "uq_shares_resource_group",
+        "shares",
+        ["resource_type", "resource_id", "shared_with_group_id"],
+        unique=True,
+        postgresql_where="shared_with_group_id IS NOT NULL",
+    )
+    op.create_index("ix_shares_user", "shares", ["shared_with_user_id"])
+    op.create_check_constraint("ck_shares_permission", "shares", "permission IN ('view', 'edit')")
+
+
+def downgrade() -> None:
+    op.drop_constraint("ck_shares_permission", "shares", type_="check")
+    op.drop_index("ix_shares_user", table_name="shares")
+    op.drop_index("uq_shares_resource_group", table_name="shares")
+    op.drop_index("uq_shares_resource_user", table_name="shares")
diff --git a/frontend/src/adapters/local.ts b/frontend/src/adapters/local.ts
index d2a54fa..e5ccee6 100644
--- a/frontend/src/adapters/local.ts
+++ b/frontend/src/adapters/local.ts
@@ -16,7 +16,7 @@ import type { Device } from "../stores/devices";
 import type { TitleEntry } from "../stores/titles";
 import type { User } from "../stores/session";
 import type { PublicConfig } from "../stores/config";
-import type { DeviceToken, ImportResult, Repo } from "./repo";
+import type { DeviceToken, ImportResult, Member, NoteShare, Repo } from "./repo";
 
 const NEEDS_SERVER = "That's not available offline — connect a server to use it.";
 
@@ -91,4 +91,13 @@ export const local: Repo = {
     remove: (id) => invoke<void>("saved_filters_remove", { id }),
     rename: (id, name) => invoke<SavedFilter>("saved_filters_rename", { id, name }),
   },
+
+  // Sharing is between accounts on a server; the desktop gets it with sync (#5175).
+  // The Share controls are not shown here, so these only answer a stray call.
+  shares: {
+    directory: () => Promise.resolve<Member[]>([]),
+    list: () => Promise.resolve<NoteShare[]>([]),
+    share: () => Promise.reject<NoteShare[]>(new Error(NEEDS_SERVER)),
+    unshare: () => Promise.reject<NoteShare[]>(new Error(NEEDS_SERVER)),
+  },
 };
diff --git a/frontend/src/adapters/repo.ts b/frontend/src/adapters/repo.ts
index 367ad1e..0705674 100644
--- a/frontend/src/adapters/repo.ts
+++ b/frontend/src/adapters/repo.ts
@@ -46,6 +46,24 @@ export interface ChecklistItemChanges {
 }
 
 
+// ---- sharing (#5174) ---------------------------------------------------------
+
+/** A person on this instance, as the member directory shows them. */
+export interface Member {
+  id: string;
+  display_name: string;
+  email: string;
+}
+
+export type SharePermission = "view" | "edit";
+
+export interface NoteShare {
+  id: string;
+  member: Member;
+  permission: SharePermission;
+  created_at: string | null;
+}
+
 export interface ImportResult {
   source: string;
   imported: number;
@@ -116,6 +134,15 @@ export interface NotesRepo {
   titles(): Promise<TitleEntry[]>;
 }
 
+export interface SharesRepo {
+  /** Everyone on the instance but you. */
+  directory(): Promise<Member[]>;
+  list(noteId: string): Promise<NoteShare[]>;
+  /** Share with one member, or change their permission. Answers the note's shares. */
+  share(noteId: string, userId: string, permission: SharePermission): Promise<NoteShare[]>;
+  unshare(noteId: string, shareId: string): Promise<NoteShare[]>;
+}
+
 export interface SavedFiltersRepo {
   list(): Promise<SavedFilter[]>;
   create(name: string, params: NoteFacets): Promise<SavedFilter>;
@@ -130,4 +157,5 @@ export interface Repo {
   labels: LabelsRepo;
   notes: NotesRepo;
   savedFilters: SavedFiltersRepo;
+  shares: SharesRepo;
 }
diff --git a/frontend/src/adapters/rest.ts b/frontend/src/adapters/rest.ts
index 015b91b..d3fd680 100644
--- a/frontend/src/adapters/rest.ts
+++ b/frontend/src/adapters/rest.ts
@@ -15,7 +15,9 @@ import type { PublicConfig } from "../stores/config";
 import type {
   DeviceToken,
   ImportResult,
+  Member,
   NoteChanges,
+  NoteShare,
   NoteCreateInput,
   NoteListQuery,
   ChecklistItemChanges,
@@ -35,6 +37,7 @@ function notesQuery(q: NoteListQuery): string {
   if (q.facets?.has_attachment) params.set("has_attachment", "true");
   if (q.facets?.created_after) params.set("created_after", q.facets.created_after);
   if (q.facets?.created_before) params.set("created_before", q.facets.created_before);
+  if (q.facets?.shared) params.set("shared", q.facets.shared);
   if (q.sort) params.set("sort", q.sort);
   return params.toString();
 }
@@ -115,4 +118,13 @@ export const rest: Repo = {
     remove: (id) => api.del<void>(`/api/saved-filters/${id}`),
     rename: (id, name) => api.patch<SavedFilter>(`/api/saved-filters/${id}`, { name }),
   },
+
+  shares: {
+    directory: async () => (await api.get<{ members: Member[] }>("/api/users/directory")).members,
+    list: async (noteId) => (await api.get<{ shares: NoteShare[] }>(`/api/notes/${noteId}/shares`)).shares,
+    share: async (noteId, userId, permission) =>
+      (await api.post<{ shares: NoteShare[] }>(`/api/notes/${noteId}/shares`, { user_id: userId, permission })).shares,
+    unshare: async (noteId, shareId) =>
+      (await api.del<{ shares: NoteShare[] }>(`/api/notes/${noteId}/shares/${shareId}`)).shares,
+  },
 };
diff --git a/frontend/src/components/AppShell.vue b/frontend/src/components/AppShell.vue
index c8480bd..28f2507 100644
--- a/frontend/src/components/AppShell.vue
+++ b/frontend/src/components/AppShell.vue
@@ -13,6 +13,7 @@ import Icon from "./Icon.vue";
 import ExportNotes from "./ExportNotes.vue";
 import ImportNotes from "./ImportNotes.vue";
 import LabelsModal from "./LabelsModal.vue";
+import ShareDialog from "./ShareDialog.vue";
 import { isDesktop } from "../desktop/bridge";
 import { useLensName } from "../composables/useLensName";
 import { facetsToQuery } from "../notes/facets";
@@ -552,6 +553,7 @@ async function signOut() {
     <LabelsModal v-if="managing" @close="managing = false" />
 
     <CommandPalette v-if="paletteOpen" @close="paletteOpen = false" />
+    <ShareDialog v-if="ui.shareNoteId" :note-id="ui.shareNoteId" @close="ui.closeShare()" />
 
     <BaseModal
       v-if="showShortcuts"
diff --git a/frontend/src/components/FilterBar.vue b/frontend/src/components/FilterBar.vue
index 0cf3359..f23d7f5 100644
--- a/frontend/src/components/FilterBar.vue
+++ b/frontend/src/components/FilterBar.vue
@@ -9,6 +9,7 @@ import { facetCount, facetsFromQuery, facetsToQuery } from "../notes/facets";
 import { addLocalDays, formatLocalDay, parseLocalDate } from "../notes/datetime";
 import Icon from "./Icon.vue";
 import { errorMessage } from "../api/errors";
+import { isDesktop } from "../desktop/bridge";
 
 // A dead-simple facet bar over the board: color + labels + has-reminder
 // + has-attachment + created-date range. The URL query IS the state, so a
@@ -43,6 +44,12 @@ function toggleReminder() {
 function toggleAttachment() {
   patch({ has_attachment: facets.value.has_attachment ? undefined : true });
 }
+// Sharing is between accounts on a server, so the offline desktop has no shared notes
+// to narrow to (#5174).
+const sharingAvailable = !isDesktop();
+function toggleShared() {
+  patch({ shared: facets.value.shared ? undefined : "with_me" });
+}
 
 function onFrom(e: Event) {
   const v = (e.target as HTMLInputElement).value;
@@ -129,6 +136,14 @@ const chipOff = "border-neutral-300 text-neutral-600 hover:bg-neutral-100 dark:b
         <button type="button" :class="[chipBase, facets.has_attachment ? chipOn : chipOff]" @click="toggleAttachment">
           Has attachment
         </button>
+        <button
+          v-if="sharingAvailable"
+          type="button"
+          :class="[chipBase, facets.shared ? chipOn : chipOff]"
+          @click="toggleShared"
+        >
+          Shared with me
+        </button>
       </div>
 
       <div class="flex flex-wrap items-center gap-2">
diff --git a/frontend/src/components/Icon.vue b/frontend/src/components/Icon.vue
index 478ec00..8ba0586 100644
--- a/frontend/src/components/Icon.vue
+++ b/frontend/src/components/Icon.vue
@@ -29,6 +29,7 @@ const paths: Record<string, string> = {
   paperclip: '<path d="m21.44 11.05-9.19 9.19a6 6 0 0 1-8.49-8.49l8.57-8.57A4 4 0 1 1 18 8.84l-8.59 8.57a2 2 0 0 1-2.83-2.83l8.49-8.48"/>',
   link: '<path d="M10 13a5 5 0 0 0 7.54.54l3-3a5 5 0 0 0-7.07-7.07l-1.72 1.71"/><path d="M14 11a5 5 0 0 0-7.54-.54l-3 3a5 5 0 0 0 7.07 7.07l1.71-1.71"/>',
   filter: '<polygon points="22 3 2 3 10 12.46 10 19 14 21 14 12.46 22 3"/>',
+  users: '<path d="M16 21v-2a4 4 0 0 0-4-4H6a4 4 0 0 0-4 4v2"/><circle cx="9" cy="7" r="4"/><path d="M22 21v-2a4 4 0 0 0-3-3.87"/><path d="M16 3.13a4 4 0 0 1 0 7.75"/>',
   copy: '<rect width="14" height="14" x="8" y="8" rx="2" ry="2"/><path d="M4 16c-1.1 0-2-.9-2-2V4c0-1.1.9-2 2-2h10c1.1 0 2 .9 2 2"/>',
 };
 </script>
diff --git a/frontend/src/components/NoteCard.vue b/frontend/src/components/NoteCard.vue
index 5836313..251cb37 100644
--- a/frontend/src/components/NoteCard.vue
+++ b/frontend/src/components/NoteCard.vue
@@ -16,6 +16,9 @@ import {
 } from "../composables/useCardDrag";
 import { formatReminder, formatTrashCountdown, isOverdue, trashDaysLeft } from "../notes/datetime";
 import { useConfigStore } from "../stores/config";
+import { useUiStore } from "../stores/ui";
+import { canEditText, isOwnNote } from "../notes/sharing";
+import { isDesktop } from "../desktop/bridge";
 
 const props = defineProps<{ note: Note; reorderable?: boolean; active?: boolean }>();
 const emit = defineEmits<{
@@ -32,6 +35,21 @@ const emit = defineEmits<{
 }>();
 const notes = useNotesStore();
 const config = useConfigStore();
+const ui = useUiStore();
+
+// --- Sharing (#5174). Someone else's note shows who shared it and offers none of
+// the owner's controls; at `view` its checkboxes are inert too. Sharing needs a
+// server, so the offline desktop has no Share button (its notes are all its own). ---
+const own = computed(() => isOwnNote(props.note));
+const editable = computed(() => canEditText(props.note));
+const canShare = computed(() => own.value && !isDesktop());
+const sharedLine = computed(() => {
+  if (!own.value) {
+    const who = props.note.shared_by?.display_name || "someone";
+    return props.note.permission === "view" ? `Shared by ${who} · view only` : `Shared by ${who}`;
+  }
+  return props.note.shared ? "Shared" : "";
+});
 
 // --- Retention countdown. A note in Trash is on a clock, and the card is the only
 // place someone browsing Trash would ever find that out in time to restore it.
@@ -105,7 +123,8 @@ const root = ref<HTMLElement | null>(null);
 // Pointer rather than native HTML5 drag-and-drop because that API never fires from
 // touch — on a phone this did nothing at all. One code path now covers mouse, touch
 // and stylus. See composables/useCardDrag.ts for why the state is shared. ---
-const canDrag = () => !!props.reorderable && !props.note.trashed;
+// Board order is the owner's; a shared note's place is the recipient's from #5176.
+const canDrag = () => !!props.reorderable && !props.note.trashed && own.value;
 const dragging = computed(() => draggingId.value === props.note.id);
 const dragOver = computed(() => overId.value === props.note.id);
 
@@ -308,7 +327,12 @@ const tagColors = computed<Record<string, string>>(() => {
            blank and the link is never unreachable. -->
       <LinkPreview v-if="loneUrlPreview" :preview="loneUrlPreview" />
       <div v-else-if="note.body" class="text-sm text-neutral-700 dark:text-neutral-300">
-        <MarkdownText :text="bodyPreview" :tag-colors="tagColors" toggleable @toggle="toggleTask" />
+        <MarkdownText
+          :text="bodyPreview"
+          :tag-colors="tagColors"
+          :toggleable="editable"
+          @toggle="toggleTask"
+        />
       </div>
       <p
         v-if="!note.body && !note.items.length && !note.attachments.length"
@@ -329,7 +353,17 @@ const tagColors = computed<Record<string, string>>(() => {
          two paragraphs instead of always after them. Rendering both would have shown
          every list twice. -->
 
-    <div v-if="note.remind_at" class="mt-2 flex flex-wrap items-center gap-1.5">
+    <div v-if="sharedLine" class="mt-2">
+      <span
+        class="inline-flex items-center gap-1 rounded-full bg-black/5 px-2 py-0.5 text-xs text-neutral-600 dark:bg-white/10 dark:text-neutral-300"
+      >
+        <Icon name="users" class="!h-3 !w-3" />
+        {{ sharedLine }}
+      </span>
+    </div>
+
+    <!-- A reminder is its owner's: someone the note is shared with isn't alerted by it. -->
+    <div v-if="note.remind_at && own" class="mt-2 flex flex-wrap items-center gap-1.5">
       <span
         class="inline-flex items-center gap-1 rounded-full px-2 py-0.5 text-xs"
         :class="
@@ -424,7 +458,9 @@ const tagColors = computed<Record<string, string>>(() => {
         <Icon name="grip" />
       </button>
 
+      <!-- Every action here is the owner's, so a shared note has no action set. -->
       <div
+        v-if="own"
         class="note-actions-set hover-reveal pointer-events-none flex items-center gap-0.5 rounded-full bg-white/85 p-0.5 opacity-0 shadow-sm ring-1 ring-black/5 backdrop-blur-sm transition focus-within:pointer-events-auto focus-within:opacity-100 group-hover:pointer-events-auto group-hover:opacity-100 dark:bg-neutral-900/85 dark:ring-white/10"
       >
         <template v-if="note.trashed">
@@ -441,7 +477,17 @@ const tagColors = computed<Record<string, string>>(() => {
             <Icon name="trash" />
           </button>
         </template>
-        <template v-else>
+        <template v-else-if="own">
+          <button
+            v-if="canShare"
+            type="button"
+            class="icon-btn"
+            title="Share"
+            aria-label="Share"
+            @click="ui.openShare(note.id)"
+          >
+            <Icon name="users" />
+          </button>
           <button
             type="button"
             class="icon-btn"
diff --git a/frontend/src/components/NoteEditor.vue b/frontend/src/components/NoteEditor.vue
index 7341fc0..5406d6e 100644
--- a/frontend/src/components/NoteEditor.vue
+++ b/frontend/src/components/NoteEditor.vue
@@ -11,6 +11,9 @@ import { takeMorphOrigin } from "../composables/useEditorMorph";
 import { prefersReducedMotion } from "../composables/useReducedMotion";
 import type { Note, NoteLabel, NoteRevision } from "../stores/notes";
 import { labelChipClasses } from "../notes/colors";
+import { canEditText, isOwnNote } from "../notes/sharing";
+import { useUiStore } from "../stores/ui";
+import { isDesktop } from "../desktop/bridge";
 import {
   afterEnter,
   type EditorBlock,
@@ -114,6 +117,19 @@ const liveNote = computed<Note>(() =>
 );
 const bodyPlaceholder = "Take a note…";
 
+// Sharing (#5174). Someone else's note: at `edit` its text and checklist are theirs to
+// change and nothing else is; at `view` it is read-only. The server enforces both —
+// this only stops the editor offering what would be refused.
+const ui = useUiStore();
+const own = computed(() => isOwnNote(liveNote.value));
+const editable = computed(() => canEditText(liveNote.value));
+const canShare = computed(() => own.value && !isCreate.value && !isDesktop());
+const sharedLine = computed(() => {
+  if (own.value) return "";
+  const who = liveNote.value.shared_by?.display_name || "someone";
+  return editable.value ? `Shared by ${who} · you can edit the text` : `Shared by ${who} · view only`;
+});
+
 // Keep local state in sync when the edited note changes (modal reused for another note).
 watch(
   () => props.note,
@@ -353,6 +369,7 @@ function onProseInput(index: number, e: Event): void {
  * however many lines became checkboxes and would otherwise keep its old height.
  */
 function onProseBlur(index: number): void {
+  if (!editable.value) return;
   const promoted = promoteTasks(blocks.value, index);
   if (promoted === blocks.value) return;
   blocks.value = promoted;
@@ -369,6 +386,7 @@ function onProseKeydown(e: KeyboardEvent): void {
 
 /** Enter on an item makes the next one; on an EMPTY item it ends the list. */
 function onTaskEnter(index: number): void {
+  if (!editable.value) return;
   const next = afterEnter(blocks.value, index);
   blocks.value = next.blocks;
   void focusBlock(next.focus);
@@ -383,6 +401,7 @@ function onTaskEnter(index: number): void {
  * only one of them can offer.
  */
 function onTaskBackspace(index: number, e: KeyboardEvent): void {
+  if (!editable.value) return;
   const el = e.target as HTMLInputElement;
   if (el.value !== "" || el.selectionStart !== 0) return;
   e.preventDefault();
@@ -487,6 +506,7 @@ async function onFileChange(e: Event) {
   input.value = "";
 }
 async function onPaste(e: ClipboardEvent) {
+  if (!own.value) return;
   const item = Array.from(e.clipboardData?.items ?? []).find((i) => i.type.startsWith("image/"));
   const file = item?.getAsFile();
   if (file) {
@@ -563,12 +583,17 @@ function revPreview(rev: NoteRevision): string {
       @paste="onPaste"
     >
       <div class="flex flex-col gap-2 p-4">
+        <p v-if="sharedLine" class="flex items-center gap-1.5 text-xs text-neutral-500 dark:text-neutral-400">
+          <Icon name="users" class="!h-3.5 !w-3.5" />
+          {{ sharedLine }}
+        </p>
         <div v-if="liveNote.attachments.length" class="flex flex-wrap items-center gap-2">
           <template v-for="att in liveNote.attachments" :key="att.id">
             <!-- Image → inline thumbnail -->
             <div v-if="attKind(att.mime) === 'image'" class="group/att relative">
               <img :src="att.url" alt="" loading="lazy" decoding="async" class="h-24 w-24 rounded-lg object-cover" />
               <button
+                v-if="own"
                 type="button"
                 class="hover-reveal absolute right-1 top-1 rounded-full bg-black/50 px-1.5 text-white opacity-0 transition group-hover/att:opacity-100"
                 aria-label="Remove attachment"
@@ -584,6 +609,7 @@ function revPreview(rev: NoteRevision): string {
             >
               <audio controls :src="att.url" class="h-8 max-w-[220px]"></audio>
               <button
+                v-if="own"
                 type="button"
                 class="text-neutral-400 hover:text-red-500"
                 aria-label="Remove attachment"
@@ -603,6 +629,7 @@ function revPreview(rev: NoteRevision): string {
               <span class="max-w-[160px] truncate">{{ att.filename || "file" }}</span>
               <span v-if="att.size" class="text-neutral-400">{{ fmtSize(att.size) }}</span>
               <button
+                v-if="own"
                 type="button"
                 class="ml-1 text-neutral-400 hover:text-red-500"
                 aria-label="Remove attachment"
@@ -631,7 +658,7 @@ function revPreview(rev: NoteRevision): string {
             v-for="p in liveNote.previews"
             :key="p.id"
             :preview="p"
-            :removable="!liveNote.trashed"
+            :removable="!liveNote.trashed && own"
             @remove="notes.deletePreview(liveNote.id, p.id)"
           />
         </div>
@@ -646,6 +673,7 @@ function revPreview(rev: NoteRevision): string {
                 type="checkbox"
                 class="h-4 w-4 shrink-0 accent-brand"
                 :checked="block.checked"
+                :disabled="!editable"
                 :aria-label="block.text || 'Checklist item'"
                 @change="setChecked(i, ($event.target as HTMLInputElement).checked)"
               />
@@ -653,6 +681,7 @@ function revPreview(rev: NoteRevision): string {
                 :ref="(el) => setBlockEl(block.id, el)"
                 :value="block.text"
                 type="text"
+                :readonly="!editable"
                 class="min-w-0 flex-1 bg-transparent text-sm leading-relaxed outline-none"
                 :class="block.checked ? 'text-neutral-400 line-through' : ''"
                 @input="setText(i, ($event.target as HTMLInputElement).value)"
@@ -660,6 +689,7 @@ function revPreview(rev: NoteRevision): string {
                 @keydown.backspace="onTaskBackspace(i, $event)"
               />
               <button
+                v-if="editable"
                 type="button"
                 class="hover-reveal shrink-0 text-neutral-300 opacity-0 hover:text-neutral-600 focus:opacity-100 group-hover/item:opacity-100 dark:hover:text-neutral-200"
                 aria-label="Delete item"
@@ -673,7 +703,8 @@ function revPreview(rev: NoteRevision): string {
               :ref="(el) => setBlockEl(block.id, el)"
               :value="block.text"
               rows="1"
-              :placeholder="i === 0 ? bodyPlaceholder : ''"
+              :readonly="!editable"
+              :placeholder="i === 0 && editable ? bodyPlaceholder : ''"
               class="w-full resize-none overflow-hidden bg-transparent text-sm leading-relaxed outline-none placeholder:text-neutral-400"
               @input="onProseInput(i, $event)"
               @keydown="onProseKeydown"
@@ -707,7 +738,7 @@ function revPreview(rev: NoteRevision): string {
           </span>
         </div>
 
-        <div v-if="richEnabled" class="flex items-center gap-2 pt-1">
+        <div v-if="richEnabled && own" class="flex items-center gap-2 pt-1">
           <Icon name="bell" class="text-neutral-400" />
           <input
             type="datetime-local"
@@ -725,7 +756,7 @@ function revPreview(rev: NoteRevision): string {
           </button>
         </div>
 
-        <div v-if="richEnabled && liveNote.remind_at" class="flex flex-wrap items-center gap-2 pl-6 text-xs">
+        <div v-if="richEnabled && own && liveNote.remind_at" class="flex flex-wrap items-center gap-2 pl-6 text-xs">
           <label class="text-neutral-400">Repeat</label>
           <select
             :value="liveNote.recurrence ?? ''"
@@ -785,7 +816,7 @@ function revPreview(rev: NoteRevision): string {
       <div class="flex items-center justify-end gap-2 border-t border-neutral-100 px-3 py-2 dark:border-neutral-800">
         <div class="flex items-center gap-0.5">
           <button
-            v-if="richEnabled && !liveNote.trashed"
+            v-if="richEnabled && !liveNote.trashed && own"
             type="button"
             class="icon-btn"
             title="Attach a file"
@@ -796,7 +827,7 @@ function revPreview(rev: NoteRevision): string {
           </button>
           <input ref="fileInput" type="file" class="hidden" @change="onFileChange" />
           <button
-            v-if="!liveNote.trashed"
+            v-if="!liveNote.trashed && editable"
             type="button"
             class="icon-btn"
             title="Add a checklist"
@@ -806,12 +837,23 @@ function revPreview(rev: NoteRevision): string {
             <Icon name="checkbox" />
           </button>
           <LabelPicker
-            v-if="richEnabled && !liveNote.trashed"
+            v-if="richEnabled && !liveNote.trashed && own"
             :model-value="labelList"
             @update:model-value="onLabelsChange"
           />
           <button
-            v-if="!isCreate"
+            v-if="canShare && !liveNote.trashed"
+            type="button"
+            class="icon-btn"
+            :class="liveNote.shared ? 'text-brand-700 dark:text-brand' : ''"
+            title="Share"
+            aria-label="Share"
+            @click="ui.openShare(liveNote.id)"
+          >
+            <Icon name="users" />
+          </button>
+          <button
+            v-if="!isCreate && own"
             type="button"
             class="icon-btn"
             :class="showHistory ? 'text-brand-700 dark:text-brand' : ''"
@@ -822,7 +864,7 @@ function revPreview(rev: NoteRevision): string {
           >
             <Icon name="history" />
           </button>
-          <template v-if="!isCreate && !liveNote.trashed">
+          <template v-if="!isCreate && !liveNote.trashed && own">
             <button
               type="button"
               class="icon-btn"
diff --git a/frontend/src/components/ShareDialog.vue b/frontend/src/components/ShareDialog.vue
new file mode 100644
index 0000000..62119d8
--- /dev/null
+++ b/frontend/src/components/ShareDialog.vue
@@ -0,0 +1,163 @@
+<script setup lang="ts">
+import { computed, onMounted, ref } from "vue";
+import { repo } from "../adapters";
+import type { Member, NoteShare, SharePermission } from "../adapters/repo";
+import { errorMessage } from "../api/errors";
+import { useNotesStore } from "../stores/notes";
+import BaseModal from "./BaseModal.vue";
+import Icon from "./Icon.vue";
+
+// Share a note with other people on this instance (#5174). The owner picks a member
+// from the directory and says whether they may only read it or also change its text.
+// Only the owner opens this; the server refuses everyone else regardless.
+
+const props = defineProps<{ noteId: string }>();
+const emit = defineEmits<{ (e: "close"): void }>();
+const notes = useNotesStore();
+
+const PERMISSIONS: { value: SharePermission; label: string }[] = [
+  { value: "view", label: "Can view" },
+  { value: "edit", label: "Can edit" },
+];
+
+const members = ref<Member[]>([]);
+const shares = ref<NoteShare[]>([]);
+const loading = ref(true);
+const loadError = ref("");
+const error = ref("");
+const busy = ref(false);
+const pick = ref("");
+const permission = ref<SharePermission>("view");
+
+/** The people it isn't shared with yet. */
+const available = computed(() => {
+  const taken = new Set(shares.value.map((s) => s.member.id));
+  return members.value.filter((m) => !taken.has(m.id));
+});
+
+async function load() {
+  loading.value = true;
+  loadError.value = "";
+  try {
+    [members.value, shares.value] = await Promise.all([repo.shares.directory(), repo.shares.list(props.noteId)]);
+  } catch (e) {
+    loadError.value = errorMessage(e, "Couldn't load who this is shared with.");
+  } finally {
+    loading.value = false;
+  }
+}
+
+async function run(fn: () => Promise<NoteShare[]>, fallback: string) {
+  busy.value = true;
+  error.value = "";
+  try {
+    shares.value = await fn();
+    notes.setShared(props.noteId, shares.value.length > 0);
+  } catch (e) {
+    error.value = errorMessage(e, fallback);
+  } finally {
+    busy.value = false;
+  }
+}
+
+async function add() {
+  if (!pick.value) return;
+  const userId = pick.value;
+  await run(() => repo.shares.share(props.noteId, userId, permission.value), "Couldn't share the note.");
+  if (!error.value) pick.value = "";
+}
+
+function change(share: NoteShare, next: SharePermission) {
+  void run(() => repo.shares.share(props.noteId, share.member.id, next), "Couldn't change that.");
+}
+
+function remove(share: NoteShare) {
+  void run(() => repo.shares.unshare(props.noteId, share.id), "Couldn't stop sharing.");
+}
+
+const selectClass =
+  "rounded-md border border-neutral-300 bg-white px-2 py-1.5 text-sm outline-none focus-visible:ring-2 focus-visible:ring-brand disabled:opacity-60 dark:border-neutral-700 dark:bg-neutral-800";
+
+onMounted(load);
+</script>
+
+<template>
+  <BaseModal panel-class="w-full max-w-md shadow-xl" @close="emit('close')">
+    <div class="flex items-center justify-between border-b border-neutral-100 px-4 py-3 dark:border-neutral-800">
+      <h2 class="text-sm font-semibold">Share note</h2>
+      <button type="button" class="icon-btn" aria-label="Close" @click="emit('close')"><Icon name="close" /></button>
+    </div>
+
+    <div class="flex flex-col gap-4 p-4">
+      <p v-if="loading" class="py-4 text-center text-sm text-neutral-400">Loading…</p>
+
+      <div v-else-if="loadError" class="py-4 text-center">
+        <p class="text-sm text-red-600 dark:text-red-400">{{ loadError }}</p>
+        <button
+          type="button"
+          class="mt-2 rounded-md border border-neutral-300 px-3 py-1 text-sm hover:bg-neutral-100 dark:border-neutral-700 dark:hover:bg-neutral-800"
+          @click="load"
+        >
+          Retry
+        </button>
+      </div>
+
+      <template v-else>
+        <p v-if="!members.length" class="text-sm text-neutral-500 dark:text-neutral-400">
+          No one else has an account here yet. An admin can invite people from Settings.
+        </p>
+
+        <form v-else-if="available.length" class="flex flex-wrap items-center gap-2" @submit.prevent="add">
+          <select v-model="pick" :class="[selectClass, 'min-w-0 flex-1']" aria-label="Person" :disabled="busy">
+            <option value="" disabled>Choose someone…</option>
+            <option v-for="m in available" :key="m.id" :value="m.id">{{ m.display_name }} ({{ m.email }})</option>
+          </select>
+          <select v-model="permission" :class="selectClass" aria-label="Permission" :disabled="busy">
+            <option v-for="p in PERMISSIONS" :key="p.value" :value="p.value">{{ p.label }}</option>
+          </select>
+          <button
+            type="submit"
+            class="rounded-md bg-brand px-3 py-1.5 text-sm font-semibold text-neutral-900 hover:brightness-95 focus:outline-none focus-visible:ring-2 focus-visible:ring-brand disabled:opacity-60"
+            :disabled="busy || !pick"
+          >
+            Share
+          </button>
+        </form>
+
+        <p v-if="error" role="alert" class="text-sm text-red-600 dark:text-red-400">{{ error }}</p>
+
+        <div class="flex flex-col gap-1">
+          <p class="text-xs font-semibold uppercase tracking-wide text-neutral-400">Shared with</p>
+          <p v-if="!shares.length" class="text-sm text-neutral-400">Only you.</p>
+          <ul v-else class="flex flex-col gap-1">
+            <li v-for="s in shares" :key="s.id" class="flex items-center gap-2 rounded-lg px-1 py-1">
+              <div class="min-w-0 flex-1">
+                <p class="truncate text-sm text-neutral-800 dark:text-neutral-100">{{ s.member.display_name }}</p>
+                <p class="truncate text-xs text-neutral-400">{{ s.member.email }}</p>
+              </div>
+              <select
+                :value="s.permission"
+                :class="selectClass"
+                :aria-label="`Permission for ${s.member.display_name}`"
+                :disabled="busy"
+                @change="change(s, ($event.target as HTMLSelectElement).value as SharePermission)"
+              >
+                <option v-for="p in PERMISSIONS" :key="p.value" :value="p.value">{{ p.label }}</option>
+              </select>
+              <button
+                type="button"
+                class="icon-btn"
+                :title="`Stop sharing with ${s.member.display_name}`"
+                :aria-label="`Stop sharing with ${s.member.display_name}`"
+                :disabled="busy"
+                @click="remove(s)"
+              >
+                <Icon name="close" />
+              </button>
+            </li>
+          </ul>
+        </div>
+      </template>
+    </div>
+  </BaseModal>
+</template>
diff --git a/frontend/src/notes/facets.ts b/frontend/src/notes/facets.ts
index c918dd0..138c2dd 100644
--- a/frontend/src/notes/facets.ts
+++ b/frontend/src/notes/facets.ts
@@ -22,6 +22,7 @@ export function facetsFromQuery(q: LocationQuery): NoteFacets {
   if (after) f.created_after = after;
   const before = one(q.created_before);
   if (before) f.created_before = before;
+  if (one(q.shared) === "with_me") f.shared = "with_me";
   return f;
 }
 
@@ -33,6 +34,7 @@ export function facetsToQuery(f: NoteFacets): LocationQueryRaw {
   if (f.has_attachment) q.has_attachment = "true";
   if (f.created_after) q.created_after = f.created_after;
   if (f.created_before) q.created_before = f.created_before;
+  if (f.shared) q.shared = f.shared;
   return q;
 }
 
@@ -44,5 +46,6 @@ export function facetCount(f: NoteFacets): number {
   if (f.has_reminder) n++;
   if (f.has_attachment) n++;
   if (f.created_after || f.created_before) n++;
+  if (f.shared) n++;
   return n;
 }
diff --git a/frontend/src/notes/sharing.test.ts b/frontend/src/notes/sharing.test.ts
new file mode 100644
index 0000000..20b1c81
--- /dev/null
+++ b/frontend/src/notes/sharing.test.ts
@@ -0,0 +1,36 @@
+import { describe, expect, it } from "vitest";
+
+import { canEditText, isOwnNote } from "./sharing";
+import { facetCount, facetsFromQuery, facetsToQuery } from "./facets";
+import type { Note } from "../stores/notes";
+
+const note = (extra: Partial<Note>): Note => ({ ...({} as Note), ...extra });
+
+describe("how a note is held", () => {
+  it("is your own when the source says nothing of sharing", () => {
+    expect(isOwnNote(note({}))).toBe(true);
+    expect(canEditText(note({}))).toBe(true);
+  });
+
+  it("lets an edit share change the text but not own the note", () => {
+    expect(isOwnNote(note({ permission: "edit" }))).toBe(false);
+    expect(canEditText(note({ permission: "edit" }))).toBe(true);
+  });
+
+  it("leaves a view share read-only", () => {
+    expect(canEditText(note({ permission: "view" }))).toBe(false);
+  });
+});
+
+describe("the Shared with me facet", () => {
+  it("round-trips through the URL and counts as one filter", () => {
+    const facets = facetsFromQuery({ shared: "with_me" });
+    expect(facets).toEqual({ shared: "with_me" });
+    expect(facetsToQuery(facets)).toEqual({ shared: "with_me" });
+    expect(facetCount(facets)).toBe(1);
+  });
+
+  it("ignores a value it doesn't know", () => {
+    expect(facetsFromQuery({ shared: "by_me" })).toEqual({});
+  });
+});
diff --git a/frontend/src/notes/sharing.ts b/frontend/src/notes/sharing.ts
new file mode 100644
index 0000000..3213345
--- /dev/null
+++ b/frontend/src/notes/sharing.ts
@@ -0,0 +1,14 @@
+import type { Note } from "../stores/notes";
+
+// How the signed-in person holds a note (#5174). The owner does everything; someone it
+// is shared with at `edit` changes the body and checklist; at `view`, nothing. A note
+// with no permission came from a source without sharing (the offline desktop): it is
+// the person's own.
+
+export function isOwnNote(note: Note): boolean {
+  return (note.permission ?? "owner") === "owner";
+}
+
+export function canEditText(note: Note): boolean {
+  return (note.permission ?? "owner") !== "view";
+}
diff --git a/frontend/src/stores/notes.ts b/frontend/src/stores/notes.ts
index 67daa4b..a1b4c74 100644
--- a/frontend/src/stores/notes.ts
+++ b/frontend/src/stores/notes.ts
@@ -13,8 +13,14 @@ export interface NoteFacets {
   has_attachment?: boolean;
   created_after?: string;
   created_before?: string;
+  /** "with_me": only notes other people have shared with you (#5174). */
+  shared?: "with_me";
 }
 
+/** How the signed-in person holds a note. A note from a source that knows nothing of
+ *  sharing (the offline desktop) has none, and is theirs. */
+export type NotePermission = "owner" | "edit" | "view";
+
 export interface NoteLabel {
   id: string;
   name: string;
@@ -86,6 +92,13 @@ export interface Note {
   previews: LinkPreview[];
   created_at: string | null;
   updated_at: string | null;
+  // Sharing (#5174). `edit` may change the body and checklist; everything else is the
+  // owner's. A recipient's copy arrives without the owner's labels.
+  permission?: NotePermission;
+  /** The owner's own note: whether it is shared with anyone. */
+  shared?: boolean;
+  /** Someone else's note: who shared it. */
+  shared_by?: { id: string; display_name: string } | null;
 }
 
 export const useNotesStore = defineStore("notes", () => {
@@ -108,6 +121,7 @@ export const useNotesStore = defineStore("notes", () => {
     const inView =
       v === "trash" ? n.trashed : v === "archived" ? !n.trashed && n.archived : !n.trashed && !n.archived;
     if (!inView) return false;
+    if (activeFacets.value.shared === "with_me" && (n.permission ?? "owner") === "owner") return false;
     if (activeLabel.value) return n.labels.some((lb) => lb.id === activeLabel.value);
     return true;
   }
@@ -262,9 +276,16 @@ export const useNotesStore = defineStore("notes", () => {
     return note;
   }
 
+  /** The Share dialog changed who a note is shared with; the card's indicator follows. */
+  function setShared(id: string, shared: boolean): void {
+    const note = items.value.find((n) => n.id === id);
+    if (note) note.shared = shared;
+  }
+
   return {
     items,
     loading,
+    setShared,
     view,
     activeLabel,
     activeFacets,
diff --git a/frontend/src/stores/ui.ts b/frontend/src/stores/ui.ts
index 7494c4d..7d58db1 100644
--- a/frontend/src/stores/ui.ts
+++ b/frontend/src/stores/ui.ts
@@ -53,7 +53,20 @@ export const useUiStore = defineStore("ui", () => {
     run?.();
   }
 
+  // The note whose Share dialog is open (#5174), or null. One dialog for the app,
+  // mounted by the shell, so the card and the editor open the same one.
+  const shareNoteId = ref<string | null>(null);
+  function openShare(id: string) {
+    shareNoteId.value = id;
+  }
+  function closeShare() {
+    shareNoteId.value = null;
+  }
+
   return {
+    shareNoteId,
+    openShare,
+    closeShare,
     composeTick,
     composeSeed,
     requestCompose,
diff --git a/src/inkwell/acl.py b/src/inkwell/acl.py
index 6e078bc..04baca4 100644
--- a/src/inkwell/acl.py
+++ b/src/inkwell/acl.py
@@ -7,12 +7,21 @@ from sqlalchemy import ColumnElement, exists, or_, select
 from .models.group import GroupMember
 from .models.share import Share
 
+# What a share grants (#5174). `edit` is the note's body and checklist; everything
+# else about a note stays its owner's. Ordered weakest first, so the strongest of a
+# person's grants is the one with the highest index.
+VIEW = "view"
+EDIT = "edit"
+PERMISSIONS = (VIEW, EDIT)
+
 
 def visible_to_user(
     resource_type: str,
     owner_column,
     resource_id_column,
     user_id: uuid.UUID,
+    *,
+    permission: str | None = None,
 ) -> ColumnElement[bool]:
     """Boolean SQL predicate: is this resource visible to ``user_id``?
 
@@ -21,7 +30,10 @@ def visible_to_user(
     mutation) of shareable user data through this — never assume a single operator
     (family rule 47).
 
-    Usage (M1+), e.g. for notes::
+    With ``permission``, only shares at that level count: ``permission=EDIT`` is
+    "may this user change it", which the owner always may.
+
+    Usage, e.g. for notes::
 
         stmt = select(Note).where(visible_to_user("note", Note.owner_id, Note.id, uid))
 
@@ -30,17 +42,53 @@ def visible_to_user(
         owner_column: the resource's owner column (e.g. ``Note.owner_id``).
         resource_id_column: the resource's primary-key column (e.g. ``Note.id``).
         user_id: the viewer.
+        permission: the level a share must grant to count, or None for any.
     """
     user_group_ids = select(GroupMember.group_id).where(GroupMember.user_id == user_id)
+    granted = [Share.permission == permission] if permission is not None else []
 
     direct_share = exists().where(
         Share.resource_type == resource_type,
         Share.resource_id == resource_id_column,
         Share.shared_with_user_id == user_id,
+        *granted,
     )
     group_share = exists().where(
         Share.resource_type == resource_type,
         Share.resource_id == resource_id_column,
         Share.shared_with_group_id.in_(user_group_ids),
+        *granted,
     )
     return or_(owner_column == user_id, direct_share, group_share)
+
+
+async def granted_to(db, resource_type: str, resource_ids: list, user_id: uuid.UUID) -> dict:
+    """resource_id -> the strongest permission shared with ``user_id``, directly or
+    through a group. Resources with no share to them are absent."""
+    if not resource_ids:
+        return {}
+    user_group_ids = select(GroupMember.group_id).where(GroupMember.user_id == user_id)
+    rows = await db.execute(
+        select(Share.resource_id, Share.permission).where(
+            Share.resource_type == resource_type,
+            Share.resource_id.in_(resource_ids),
+            or_(Share.shared_with_user_id == user_id, Share.shared_with_group_id.in_(user_group_ids)),
+        )
+    )
+    best: dict = {}
+    for rid, perm in rows.all():
+        if PERMISSIONS.index(perm) >= PERMISSIONS.index(best.get(rid, VIEW)):
+            best[rid] = perm
+    return best
+
+
+async def shared_ids(db, resource_type: str, resource_ids: list) -> set:
+    """Which of these resources have been shared with anyone."""
+    if not resource_ids:
+        return set()
+    rows = await db.scalars(
+        select(Share.resource_id)
+        .where(Share.resource_type == resource_type, Share.resource_id.in_(resource_ids))
+        .distinct()
+    )
+    return set(rows.all())
diff --git a/src/inkwell/app.py b/src/inkwell/app.py
index 148de07..214ea23 100644
--- a/src/inkwell/app.py
+++ b/src/inkwell/app.py
@@ -24,6 +24,7 @@ from .retention import run_sweeper
 from .saved_filters import bp as saved_filters_bp
 from .settings import get_public_config, get_setting, load_or_create_secret_key, refresh_live
 from .settings_api import bp as settings_bp
+from .shares_api import bp as shares_bp
 from .sync import bp as sync_bp, protocol_advertisement
 
 # Without this, `logger.info` from this package goes nowhere: hypercorn configures its
@@ -95,6 +96,7 @@ def create_app() -> Quart:
     app.register_blueprint(settings_bp)
     app.register_blueprint(invites_bp)
     app.register_blueprint(accounts_bp)
+    app.register_blueprint(shares_bp)
     app.register_blueprint(sync_bp)
     app.register_blueprint(saved_filters_bp)
     app.register_blueprint(client_bp)
diff --git a/src/inkwell/notes/__init__.py b/src/inkwell/notes/__init__.py
index 13e04be..1331a12 100644
--- a/src/inkwell/notes/__init__.py
+++ b/src/inkwell/notes/__init__.py
@@ -49,6 +49,7 @@ from .helpers import (
     ALLOWED_IMAGE_MIMES,
     VALID_FILTERS,
     _attachment_ext,
+    _get_editable,
     _get_owned,
     _header_filename,
     _safe_filename,
@@ -119,9 +120,18 @@ async def list_notes():
     after_param = request.args.get("created_after")
     before_param = request.args.get("created_before")
     sort = request.args.get("sort")
+    shared = request.args.get("shared")
+    if shared not in (None, "", "with_me"):
+        return json_error("invalid shared", 400)
     async with session_scope() as db:
         stmt = select(Note).where(visible_to_user("note", Note.owner_id, Note.id, g.user_id))
         stmt = apply_filter(stmt, filter_name)
+        # Trash is the owner's: a note its owner trashed leaves a recipient's board
+        # rather than turning up in their Trash, where they could do nothing with it.
+        if filter_name == "trash":
+            stmt = stmt.where(Note.owner_id == g.user_id)
+        if shared == "with_me":
+            stmt = stmt.where(Note.owner_id != g.user_id)
         for raw_label in label_params:
             lid = parse_uuid(raw_label)
             if lid is None:
@@ -156,7 +166,7 @@ async def list_notes():
         else:
             stmt = stmt.order_by(Note.pinned.desc(), Note.position.desc(), Note.updated_at.desc())
         notes = (await db.scalars(stmt)).all()
-        return jsonify({"notes": await _serialize_notes(db, notes)})
+        return jsonify({"notes": await _serialize_notes(db, notes, g.user_id)})
 
 
 @bp.get("/reminders")
@@ -166,14 +176,16 @@ async def list_reminders():
         stmt = (
             select(Note)
             .where(
-                visible_to_user("note", Note.owner_id, Note.id, g.user_id),
+                # A reminder is its owner's, like everything but a note's text: someone
+                # a note is shared with isn't alerted by it and can't clear it.
+                Note.owner_id == g.user_id,
                 Note.deleted_at.is_(None),
                 Note.remind_at.is_not(None),
             )
             .order_by(Note.remind_at.asc())
         )
         notes = (await db.scalars(stmt)).all()
-        return jsonify({"notes": await _serialize_notes(db, notes)})
+        return jsonify({"notes": await _serialize_notes(db, notes, g.user_id)})
 
 
 @bp.post("/<note_id>/reminder/complete")
@@ -191,7 +203,7 @@ async def complete_reminder(note_id: str):
             note.remind_at = None
             note.recurrence = None
         await db.commit()
-        return jsonify(await _serialize_note(db, note))
+        return jsonify(await _serialize_note(db, note, g.user_id))
 
 
 @bp.post("/<note_id>/reminder/snooze")
@@ -210,7 +222,7 @@ async def snooze_reminder(note_id: str):
             return not_found()
         note.remind_at = datetime.now(timezone.utc) + timedelta(minutes=minutes)
         await db.commit()
-        return jsonify(await _serialize_note(db, note))
+        return jsonify(await _serialize_note(db, note, g.user_id))
 
 
 @bp.get("/export")
@@ -421,7 +433,7 @@ async def get_note(note_id: str):
         )
         if note is None:
             return not_found()
-        return jsonify(await _serialize_note(db, note))
+        return jsonify(await _serialize_note(db, note, g.user_id))
 
 
 @bp.patch("/<note_id>")
@@ -429,9 +441,13 @@ async def get_note(note_id: str):
 async def update_note(note_id: str):
     data = await request.get_json(silent=True) or {}
     async with session_scope() as db:
-        note = await _get_owned(db, note_id)
+        note = await _get_editable(db, note_id)
         if note is None:
             return not_found()
+        # Someone the note is shared with at `edit` may change its text and nothing
+        # else. They can already read it, so saying so is no oracle.
+        if note.owner_id != g.user_id and set(data) - {"body"}:
+            return json_error("only the owner can change that", 403)
         changed = False
         if isinstance(data.get("body"), str):
             # Version history: the first write of an editing session snapshots, not
@@ -468,7 +484,7 @@ async def _commit_note(db, note: Note, text_changed: bool):
     await db.refresh(note)
     if text_changed:
         schedule_unfurls(note.id, note.body)
-    return jsonify(await _serialize_note(db, note))
+    return jsonify(await _serialize_note(db, note, g.user_id))
 
 
 def _serialize_revision(rev: NoteRevision) -> dict:
@@ -511,7 +527,7 @@ async def restore_revision(note_id: str, rev_id: str):
         if rev is None:
             return not_found()
         if note.body == rev.body:
-            return jsonify(await _serialize_note(db, note))  # already at this version — no-op
+            return jsonify(await _serialize_note(db, note, g.user_id))  # already at this version — no-op
         # Snapshot the CURRENT text unconditionally, so restoring is itself undoable,
         # then apply the revision through the same path as any edit — which is what
         # gives a restored link its preview back.
@@ -541,7 +557,7 @@ async def set_note_labels(note_id: str):
         owned = await resolve_owned_label_ids(db, label_ids, g.user_id)
         await reconcile_manual_labels(db, note, owned)
         await db.commit()
-        return jsonify(await _serialize_note(db, note))
+        return jsonify(await _serialize_note(db, note, g.user_id))
 
 
 def _item_index(item_id: str) -> int | None:
@@ -565,7 +581,7 @@ async def add_item(note_id: str):
     data = await request.get_json(silent=True) or {}
     text = data["text"].strip() if isinstance(data.get("text"), str) else ""
     async with session_scope() as db:
-        note = await _get_owned(db, note_id)
+        note = await _get_editable(db, note_id)
         if note is None:
             return not_found()
         response = await _rewrite_body(db, note, append_item(note.body, text))
@@ -580,7 +596,7 @@ async def update_item(note_id: str, item_id: str):
     if index is None:
         return not_found()
     async with session_scope() as db:
-        note = await _get_owned(db, note_id)
+        note = await _get_editable(db, note_id)
         if note is None:
             return not_found()
         if index >= len(parse_items(note.body)):
@@ -600,7 +616,7 @@ async def delete_item(note_id: str, item_id: str):
     if index is None:
         return not_found()
     async with session_scope() as db:
-        note = await _get_owned(db, note_id)
+        note = await _get_editable(db, note_id)
         if note is None:
             return not_found()
         if index >= len(parse_items(note.body)):
@@ -634,7 +650,7 @@ async def upload_attachment(note_id: str):
                 return json_error("invalid attachment id", 400)
             existing = await db.scalar(select(NoteAttachment.note_id).where(NoteAttachment.id == att_id))
             if existing == note.id:
-                return jsonify(await _serialize_note(db, note))  # already have this blob
+                return jsonify(await _serialize_note(db, note, g.user_id))  # already have this blob
             if existing is not None:
                 return json_error("attachment id already in use", 409)
         raw = upload.stream.read()
@@ -644,7 +660,7 @@ async def upload_attachment(note_id: str):
         # Any file type is allowed — images render inline, everything else downloads.
         store_attachment(db, note, raw, upload.filename, upload.content_type, att_id)
         await db.commit()
-        return jsonify(await _serialize_note(db, note)), 201
+        return jsonify(await _serialize_note(db, note, g.user_id)), 201
 
 
 @bp.get("/<note_id>/attachments/<att_id>")
@@ -696,7 +712,7 @@ async def delete_attachment(note_id: str, att_id: str):
         rel = att.path
         await db.delete(att)
         await db.commit()
-        result = await _serialize_note(db, note)
+        result = await _serialize_note(db, note, g.user_id)
     unlink_media(rel)
     return jsonify(result)
 
@@ -738,7 +754,7 @@ async def unfurl_link(note_id: str):
         row.image_url = preview["image_url"]
         row.site_name = preview["site_name"]
         await db.commit()
-        return jsonify(await _serialize_note(db, note)), 201
+        return jsonify(await _serialize_note(db, note, g.user_id)), 201
 
 
 @bp.delete("/<note_id>/previews/<preview_id>")
@@ -758,7 +774,7 @@ async def delete_preview(note_id: str, preview_id: str):
             return not_found()
         await db.delete(row)
         await db.commit()
-        return jsonify(await _serialize_note(db, note))
+        return jsonify(await _serialize_note(db, note, g.user_id))
 
 
 @bp.post("/<note_id>/trash")
@@ -771,7 +787,7 @@ async def trash_note(note_id: str):
         note.deleted_at = datetime.now(timezone.utc)
         await db.commit()
         await db.refresh(note)
-        return jsonify(await _serialize_note(db, note))
+        return jsonify(await _serialize_note(db, note, g.user_id))
 
 
 @bp.post("/<note_id>/restore")
@@ -784,7 +800,7 @@ async def restore_note(note_id: str):
         note.deleted_at = None
         await db.commit()
         await db.refresh(note)
-        return jsonify(await _serialize_note(db, note))
+        return jsonify(await _serialize_note(db, note, g.user_id))
 
 
 @bp.delete("/<note_id>")
diff --git a/src/inkwell/notes/helpers.py b/src/inkwell/notes/helpers.py
index 9254de4..0329c31 100644
--- a/src/inkwell/notes/helpers.py
+++ b/src/inkwell/notes/helpers.py
@@ -15,6 +15,7 @@ import uuid
 from quart import g
 from sqlalchemy import select
 
+from ..acl import EDIT, visible_to_user
 from ..config import Config
 from ..models.note import Note
 from ..models.note_attachment import NoteAttachment
@@ -85,7 +86,13 @@ def apply_filter(stmt, filter_name: str):
 
 
 async def _get_owned(db, note_id: str) -> Note | None:
-    """Fetch a note the current user OWNS (mutations are owner-only in M1/M2).
+    """Fetch a note the current user OWNS, for every change only an owner may make:
+    trash, delete, labels, reminders, pin, archive, attachments, previews, history and
+    sharing (#5174).
+
+    Someone the note is shared with gets None here, and so a 404, even when they can
+    read the note: a 403 would confirm the note exists to a caller who only has its
+    id (#1984).
 
     A purged note reads as absent: the REST API must treat it as gone, so opening,
     editing or restoring one 404s. The sync push path looks rows up directly rather
@@ -99,6 +106,22 @@ async def _get_owned(db, note_id: str) -> Note | None:
     )
 
 
+async def _get_editable(db, note_id: str) -> Note | None:
+    """Fetch a note the current user may change the TEXT of: its body and its
+    checklist. The owner, or someone it is shared with at `edit`; a view share gets
+    None, and so a 404, like a stranger (#5174)."""
+    nid = parse_uuid(note_id)
+    if nid is None:
+        return None
+    return await db.scalar(
+        select(Note).where(
+            Note.id == nid,
+            visible_to_user("note", Note.owner_id, Note.id, g.user_id, permission=EDIT),
+            Note.purged_at.is_(None),
+        )
+    )
+
+
 def _slugify(text: str) -> str:
     """A filesystem-safe slug from a note's display name (for the .md filename)."""
     s = re.sub(r"[^\w\s-]", "", (text or "").strip().lower())
diff --git a/src/inkwell/notes/serialize.py b/src/inkwell/notes/serialize.py
index 67587ed..c5cbfbe 100644
--- a/src/inkwell/notes/serialize.py
+++ b/src/inkwell/notes/serialize.py
@@ -5,10 +5,12 @@ from __future__ import annotations
 
 from sqlalchemy import select
 
+from ..acl import granted_to, shared_ids
 from ..models.label import Label, NoteLabel
 from ..models.note import Note
 from ..models.note_attachment import NoteAttachment
 from ..models.note_link_preview import NoteLinkPreview
+from ..models.user import User
 from .checklist import parse_items
 
 
@@ -104,30 +106,56 @@ async def _previews_for_notes(db, note_ids: list) -> dict:
     return result
 
 
-async def _serialize_note(db, note: Note) -> dict:
-    data = note.serialize()
-    labels = await _labels_for_notes(db, [note.id])
-    data["labels"] = labels.get(note.id, [])
-    data["items"] = items_of(note.body)
-    attachments = await _attachments_for_notes(db, [note.id])
-    data["attachments"] = attachments.get(note.id, [])
-    previews = await _previews_for_notes(db, [note.id])
-    data["previews"] = previews.get(note.id, [])
-    return data
+async def _sharing_for_notes(db, notes: list, viewer) -> dict:
+    """note_id -> how `viewer` holds the note (#5174): `permission` (owner, edit or
+    view), `shared` (whether it is shared at all, as far as the viewer may know) and
+    `shared_by` (the owner, for a note someone shared with them)."""
+    foreign = [n for n in notes if n.owner_id != viewer]
+    own_ids = [n.id for n in notes if n.owner_id == viewer]
+    granted = await granted_to(db, "note", [n.id for n in foreign], viewer)
+    sharing = await shared_ids(db, "note", own_ids)
+    owner_ids = {n.owner_id for n in foreign}
+    names = (
+        dict((await db.execute(select(User.id, User.display_name).where(User.id.in_(owner_ids)))).all())
+        if owner_ids
+        else {}
+    )
+    out: dict = {}
+    for n in notes:
+        if n.owner_id == viewer:
+            out[n.id] = {"permission": "owner", "shared": n.id in sharing, "shared_by": None}
+        else:
+            out[n.id] = {
+                "permission": granted.get(n.id, "view"),
+                "shared": True,
+                "shared_by": {"id": str(n.owner_id), "display_name": names.get(n.owner_id, "")},
+            }
+    return out
 
 
-async def _serialize_notes(db, notes: list) -> list:
+async def _serialize_note(db, note: Note, viewer=None) -> dict:
+    return (await _serialize_notes(db, [note], viewer))[0]
+
+
+async def _serialize_notes(db, notes: list, viewer=None) -> list:
+    """The API shape of each note. With `viewer`, each also says how the viewer holds
+    it, and a note someone shared with them comes without the owner's labels: labels
+    are personal, and a recipient's board files nothing under someone else's tags.
+    Sync passes no viewer, since its feed is the caller's own notes."""
     ids = [n.id for n in notes]
     labels_map = await _labels_for_notes(db, ids)
-
     attach_map = await _attachments_for_notes(db, ids)
     preview_map = await _previews_for_notes(db, ids)
+    sharing = await _sharing_for_notes(db, notes, viewer) if viewer is not None else {}
     out = []
     for n in notes:
         data = n.serialize()
-        data["labels"] = labels_map.get(n.id, [])
+        mine = viewer is None or n.owner_id == viewer
+        data["labels"] = labels_map.get(n.id, []) if mine else []
         data["items"] = items_of(n.body)
         data["attachments"] = attach_map.get(n.id, [])
         data["previews"] = preview_map.get(n.id, [])
+        if viewer is not None:
+            data.update(sharing[n.id])
         out.append(data)
     return out
diff --git a/src/inkwell/retention.py b/src/inkwell/retention.py
index ea4166a..70c13a5 100644
--- a/src/inkwell/retention.py
+++ b/src/inkwell/retention.py
@@ -34,6 +34,7 @@ from .models.note import Note
 from .models.note_attachment import NoteAttachment
 from .models.note_link_preview import NoteLinkPreview
 from .models.note_revision import NoteRevision
+from .models.share import Share
 from .settings import get_setting
 
 logger = logging.getLogger(__name__)
@@ -87,6 +88,8 @@ async def purge_note(db, note: Note, edited_at: datetime | None = None) -> None:
     await db.execute(sa_delete(NoteLabel).where(NoteLabel.note_id == note.id))
     await db.execute(sa_delete(NoteLinkPreview).where(NoteLinkPreview.note_id == note.id))
     await db.execute(sa_delete(NoteRevision).where(NoteRevision.note_id == note.id))
+    # Its shares too: a deleted note is shared with nobody.
+    await db.execute(sa_delete(Share).where(Share.resource_type == "note", Share.resource_id == note.id))
     note.body = ""
     note.display_title = ""
     # `deleted_at` deliberately SURVIVES. It's still true — that is when the note was
diff --git a/src/inkwell/saved_filters.py b/src/inkwell/saved_filters.py
index 3a4e127..8fc0085 100644
--- a/src/inkwell/saved_filters.py
+++ b/src/inkwell/saved_filters.py
@@ -26,6 +26,7 @@ _ALLOWED_PARAM_KEYS = {
     "has_attachment",
     "created_after",
     "created_before",
+    "shared",  # "with_me": notes other people shared with you (#5174)
     "filter",
 }
 
diff --git a/src/inkwell/shares_api.py b/src/inkwell/shares_api.py
new file mode 100644
index 0000000..a1c12f8
--- /dev/null
+++ b/src/inkwell/shares_api.py
@@ -0,0 +1,127 @@
+"""Sharing a note with another member of this instance (#5174).
+
+`acl.visible_to_user` has gated every read since M0, but nothing wrote a share. This
+is the writing half: the note's owner lists, grants and revokes access, and anyone
+signed in can read the member directory the Share dialog picks people from.
+
+What a share grants is in `acl` (view, or edit: the body and checklist) and is
+enforced by the note routes (`notes.helpers._get_owned` / `_get_editable`).
+"""
+from __future__ import annotations
+
+import uuid
+
+from quart import Blueprint, g, jsonify, request
+from sqlalchemy import select
+from sqlalchemy.dialects.postgresql import insert
+
+from .acl import PERMISSIONS
+from .auth import login_required
+from .common import iso
+from .db import session_scope
+from .models.share import Share
+from .models.user import User
+from .notes.helpers import _get_owned
+from .responses import json_error, not_found, parse_uuid
+
+bp = Blueprint("shares", __name__, url_prefix="/api")
+
+
+def _member(user: User) -> dict:
+    return {"id": str(user.id), "display_name": user.display_name, "email": user.email}
+
+
+@bp.get("/users/directory")
+@login_required
+async def directory():
+    """The people on this instance a note can be shared with: everyone but you.
+
+    Signed-in only. Each entry is a name and an email and nothing more, which is what
+    choosing someone takes and all a fellow member needs to know."""
+    async with session_scope() as db:
+        users = (
+            await db.scalars(select(User).where(User.id != g.user_id).order_by(User.display_name, User.email))
+        ).all()
+    return jsonify({"members": [_member(u) for u in users]})
+
+
+async def _shares_of(db, note_id: uuid.UUID) -> list[dict]:
+    rows = (
+        await db.execute(
+            select(Share, User)
+            .join(User, User.id == Share.shared_with_user_id)
+            .where(Share.resource_type == "note", Share.resource_id == note_id)
+            .order_by(Share.created_at)
+        )
+    ).all()
+    return [
+        {"id": str(share.id), "member": _member(user), "permission": share.permission, "created_at": iso(share.created_at)}
+        for share, user in rows
+    ]
+
+
+@bp.get("/notes/<note_id>/shares")
+@login_required
+async def list_shares(note_id: str):
+    async with session_scope() as db:
+        note = await _get_owned(db, note_id)
+        if note is None:
+            return not_found()
+        return jsonify({"shares": await _shares_of(db, note.id)})
+
+
+@bp.post("/notes/<note_id>/shares")
+@login_required
+async def share_note(note_id: str):
+    """Share the note with one member at `view` or `edit`. Sharing again with the same
+    person changes their permission rather than adding a second grant."""
+    data = await request.get_json(silent=True) or {}
+    permission = data.get("permission") or "view"
+    if permission not in PERMISSIONS:
+        return json_error("permission must be view or edit", 400)
+    target = parse_uuid(str(data.get("user_id") or ""))
+    if target is None:
+        return json_error("choose someone to share with", 400)
+    async with session_scope() as db:
+        note = await _get_owned(db, note_id)
+        if note is None:
+            return not_found()
+        if target == g.user_id:
+            return json_error("this note is already yours", 400)
+        if await db.get(User, target) is None:
+            return json_error("that person isn't on this instance", 400)
+        await db.execute(
+            insert(Share)
+            .values(
+                id=uuid.uuid4(),
+                resource_type="note",
+                resource_id=note.id,
+                shared_with_user_id=target,
+                permission=permission,
+            )
+            .on_conflict_do_update(
+                index_elements=[Share.resource_type, Share.resource_id, Share.shared_with_user_id],
+                index_where=Share.shared_with_user_id.is_not(None),
+                set_={"permission": permission},
+            )
+        )
+        await db.commit()
+        return jsonify({"shares": await _shares_of(db, note.id)}), 201
+
+
+@bp.delete("/notes/<note_id>/shares/<share_id>")
+@login_required
+async def unshare_note(note_id: str, share_id: str):
+    sid = parse_uuid(share_id)
+    async with session_scope() as db:
+        note = await _get_owned(db, note_id)
+        if note is None or sid is None:
+            return not_found()
+        share = await db.scalar(
+            select(Share).where(Share.id == sid, Share.resource_type == "note", Share.resource_id == note.id)
+        )
+        if share is None:
+            return not_found()
+        await db.delete(share)
+        await db.commit()
+        return jsonify({"shares": await _shares_of(db, note.id)})
diff --git a/tests/test_integration.py b/tests/test_integration.py
index 780c0bc..65802c7 100644
--- a/tests/test_integration.py
+++ b/tests/test_integration.py
@@ -30,6 +30,7 @@ from inkwell.config import Config
 from inkwell.db import dispose_engine, session_scope
 from inkwell.models.invite import Invite
 from inkwell.models.password_reset import PasswordReset
+from inkwell.models.share import Share
 from inkwell.models.label import NoteLabel
 from inkwell.models.note import Note
 from inkwell.models.note_attachment import NoteAttachment
@@ -47,7 +48,7 @@ pytestmark = pytest.mark.integration
 
 # Every table the tests touch, child-first so FKs never block the truncate.
 # RESTART IDENTITY + CASCADE keeps this honest if a table gains children later.
-_TABLES = "notes, note_revisions, note_labels, note_link_previews, labels, invites, password_resets, users"
+_TABLES = "notes, note_revisions, note_labels, note_link_previews, labels, shares, invites, password_resets, users"
 
 
 @pytest_asyncio.fixture
@@ -1179,3 +1180,160 @@ async def test_revoking_this_device_from_a_web_session_is_a_bad_request(app_clie
     await _signed_in(app_client, "web")
     resp = await app_client.delete("/api/auth/devices/self")
     assert resp.status_code == 400
+
+
+# --- Sharing a note (#5174) ---------------------------------------------------
+#
+# Owner, a recipient, and a stranger who is on the instance but not shared with.
+
+
+async def _three_people(app_client):
+    """The owner (admin, signed in on `app_client`), a recipient and a stranger, each
+    signed in on a client of their own. Returns (recipient, stranger, ids by name)."""
+    first = await _admin_with_invite(app_client)
+    second = (await (await app_client.post("/api/invites", json={})).get_json())["token"]
+    people = {}
+    clients = []
+    for name, token in (("recipient", first), ("stranger", second)):
+        client = create_app().test_client()
+        joined = await client.post(
+            "/api/auth/register",
+            json={"email": f"{name}@example.test", "password": _PASSWORD, "display_name": name.title(), "invite": token},
+        )
+        assert joined.status_code == 201
+        people[name] = (await joined.get_json())["id"]
+        clients.append(client)
+    return clients[0], clients[1], people
+
+
+async def _owners_note(app_client, body: str = "a shared thought #idea") -> str:
+    resp = await app_client.post("/api/notes", json={"body": body})
+    assert resp.status_code == 201, await resp.get_data(as_text=True)
+    return (await resp.get_json())["id"]
+
+
+async def _share(app_client, nid: str, user_id: str, permission: str = "view"):
+    return await app_client.post(f"/api/notes/{nid}/shares", json={"user_id": user_id, "permission": permission})
+
+
+async def _board_ids(client, query: str = "") -> list[str]:
+    return [n["id"] for n in (await (await client.get(f"/api/notes?{query}")).get_json())["notes"]]
+
+
+async def test_a_shared_note_reaches_the_recipient_and_no_one_else(app_client, db):
+    recipient, stranger, people = await _three_people(app_client)
+    nid = await _owners_note(app_client)
+
+    # The directory is everyone but you.
+    members = (await (await app_client.get("/api/users/directory")).get_json())["members"]
+    assert sorted(m["email"] for m in members) == ["recipient@example.test", "stranger@example.test"]
+
+    assert (await recipient.get(f"/api/notes/{nid}")).status_code == 404
+    shared = await _share(app_client, nid, people["recipient"])
+    assert shared.status_code == 201, await shared.get_data(as_text=True)
+    assert [(s["member"]["email"], s["permission"]) for s in (await shared.get_json())["shares"]] == [
+        ("recipient@example.test", "view")
+    ]
+
+    seen = await (await recipient.get(f"/api/notes/{nid}")).get_json()
+    assert seen["permission"] == "view"
+    assert seen["shared_by"]["display_name"] == "owner"
+    # Labels are personal: the owner's #idea doesn't come with the note.
+    assert seen["labels"] == []
+    assert nid in await _board_ids(recipient)
+    assert await _board_ids(recipient, "shared=with_me") == [nid]
+
+    mine = await (await app_client.get(f"/api/notes/{nid}")).get_json()
+    assert (mine["permission"], mine["shared"], mine["shared_by"]) == ("owner", True, None)
+    assert [lb["name"] for lb in mine["labels"]] == ["idea"]
+    assert await _board_ids(app_client, "shared=with_me") == []
+
+    # The stranger, on the same instance, sees nothing of it.
+    assert (await stranger.get(f"/api/notes/{nid}")).status_code == 404
+    assert nid not in await _board_ids(stranger)
+
+
+async def test_a_view_share_writes_nothing_and_an_edit_share_writes_only_text(app_client, db):
+    recipient, _, people = await _three_people(app_client)
+    nid = await _owners_note(app_client, "first line\n- [ ] milk")
+    await _share(app_client, nid, people["recipient"], "view")
+
+    # View: every write is a 404, the same answer a stranger gets (#1984).
+    writes = [
+        recipient.patch(f"/api/notes/{nid}", json={"body": "mine now"}),
+        recipient.post(f"/api/notes/{nid}/items", json={"text": "eggs"}),
+        recipient.patch(f"/api/notes/{nid}/items/0", json={"checked": True}),
+        recipient.post(f"/api/notes/{nid}/trash"),
+        recipient.put(f"/api/notes/{nid}/labels", json={"label_ids": []}),
+        recipient.get(f"/api/notes/{nid}/shares"),
+        recipient.post(f"/api/notes/{nid}/shares", json={"user_id": people["stranger"]}),
+    ]
+    for pending in writes:
+        assert (await pending).status_code == 404
+
+    # Sharing again changes the permission rather than adding a second grant.
+    upgraded = await _share(app_client, nid, people["recipient"], "edit")
+    assert [s["permission"] for s in (await upgraded.get_json())["shares"]] == ["edit"]
+
+    # Edit: the text and the checklist.
+    edited = await recipient.patch(f"/api/notes/{nid}", json={"body": "first line, edited #fromguest\n- [ ] milk"})
+    assert edited.status_code == 200, await edited.get_data(as_text=True)
+    assert (await edited.get_json())["labels"] == []
+    ticked = await recipient.patch(f"/api/notes/{nid}/items/0", json={"checked": True})
+    assert ticked.status_code == 200
+    assert (await recipient.post(f"/api/notes/{nid}/items", json={"text": "eggs"})).status_code == 200
+
+    # A #tag typed into someone else's note files it under the OWNER's tags.
+    owner_tags = [lb["name"] for lb in (await (await app_client.get("/api/labels")).get_json())["labels"]]
+    assert "fromguest" in owner_tags
+    assert (await (await recipient.get("/api/labels")).get_json())["labels"] == []
+
+    # Everything else stays the owner's.
+    assert (await recipient.patch(f"/api/notes/{nid}", json={"pinned": True})).status_code == 403
+    assert (await recipient.patch(f"/api/notes/{nid}", json={"body": "x", "archived": True})).status_code == 403
+    assert (await recipient.post(f"/api/notes/{nid}/trash")).status_code == 404
+    assert (await recipient.get(f"/api/notes/{nid}/revisions")).status_code == 404
+
+    body = (await (await app_client.get(f"/api/notes/{nid}")).get_json())["body"]
+    assert body.startswith("first line, edited")
+    assert "- [x] milk" in body and "eggs" in body
+
+
+async def test_unsharing_trashing_and_deleting_each_end_access(app_client, db):
+    recipient, _, people = await _three_people(app_client)
+    nid = await _owners_note(app_client)
+    share_id = (await (await _share(app_client, nid, people["recipient"])).get_json())["shares"][0]["id"]
+
+    left = await app_client.delete(f"/api/notes/{nid}/shares/{share_id}")
+    assert left.status_code == 200
+    assert (await left.get_json())["shares"] == []
+    assert (await recipient.get(f"/api/notes/{nid}")).status_code == 404
+    assert (await (await app_client.get(f"/api/notes/{nid}")).get_json())["shared"] is False
+
+    # Trashed by its owner, it leaves the recipient's board without reaching their Trash.
+    await _share(app_client, nid, people["recipient"])
+    assert (await app_client.post(f"/api/notes/{nid}/trash")).status_code == 200
+    assert nid not in await _board_ids(recipient)
+    assert await _board_ids(recipient, "filter=trash") == []
+
+    # Deleted for good, it is shared with nobody.
+    assert (await app_client.delete(f"/api/notes/{nid}")).status_code == 200
+    async with session_scope() as fresh:
+        assert await fresh.scalar(select(func.count()).select_from(Share)) == 0
+
+
+async def test_a_share_names_someone_else_on_this_instance(app_client, db):
+    recipient, _, people = await _three_people(app_client)
+    nid = await _owners_note(app_client)
+    me = (await (await app_client.get("/api/auth/me")).get_json())["id"]
+    assert (await _share(app_client, nid, me)).status_code == 400
+    assert (await _share(app_client, nid, str(uuid.uuid4()))).status_code == 400
+    assert (await _share(app_client, nid, "not-an-id")).status_code == 400
+    assert (await _share(app_client, nid, people["recipient"], "admin")).status_code == 400
+    # Only the owner shares: a recipient re-sharing gets the stranger's 404.
+    await _share(app_client, nid, people["recipient"], "edit")
+    assert (await _share(recipient, nid, people["stranger"])).status_code == 404
+    # A share someone else's note doesn't hold can't be removed through this one.
+    other = await _owners_note(app_client, "another")
+    sid = (await (await _share(app_client, nid, people["stranger"])).get_json())["shares"][-1]["id"]
+    assert (await app_client.delete(f"/api/notes/{other}/shares/{sid}")).status_code == 404