diff --git a/src/backend/services/share/ShareService.test.ts b/src/backend/services/share/ShareService.test.ts
index 759278c24..5a6089f01 100644
--- a/src/backend/services/share/ShareService.test.ts
+++ b/src/backend/services/share/ShareService.test.ts
@@ -806,7 +806,35 @@ describe('ShareService', () => {
                 recipient: { email: third.email },
                 mode: 'manage',
             }),
-        ).rejects.toMatchObject({ statusCode: 403 });
+        ).rejects.toMatchObject({
+            statusCode: 403,
+            legacyCode: 'cannot_delegate_manage',
+        });
+
+        // What they can do is unchanged.
+        await expect(
+            share(delegate.actor, {
+                uid: file.uuid,
+                recipient: { email: third.email },
+                mode: 'write',
+            }),
+        ).resolves.toMatchObject({ mode: 'write' });
+    });
+
+    it('tells a stranger nothing when they ask to grant `manage`', async () => {
+        const owner = await makeUser();
+        const stranger = await makeUser();
+        const third = await makeUser();
+        const file = await makeFile(owner.user);
+
+        // No access at all, so the refusal must not confirm the file exists.
+        await expect(
+            share(stranger.actor, {
+                uid: file.uuid,
+                recipient: { email: third.email },
+                mode: 'manage',
+            }),
+        ).rejects.not.toMatchObject({ legacyCode: 'cannot_delegate_manage' });
     });
 
     it('leaves a delegate alone when their authority survives another issuer', async () => {
diff --git a/src/backend/services/share/ShareService.ts b/src/backend/services/share/ShareService.ts
index 9e6ce17da..6327ae06d 100644
--- a/src/backend/services/share/ShareService.ts
+++ b/src/backend/services/share/ShareService.ts
@@ -1720,6 +1720,22 @@ export class ShareService extends PuterService {
         // given, rather than everything its user owns.
         if (allowed && (await this.#hasOwnReach(actor, entry, mode))) return;
 
+        // Only for someone who can already share here, so it leaks nothing.
+        if (mode === MANAGE_PERM_PREFIX) {
+            const canDelegateAccess =
+                await this.services.permission.canManagePermission(
+                    userRelatedActor(actor),
+                    entryPermissionForMode(entry.uuid, 'write'),
+                );
+            if (canDelegateAccess) {
+                throw new HttpError(
+                    403,
+                    'Only the owner can grant edit & share access',
+                    { legacyCode: 'cannot_delegate_manage' },
+                );
+            }
+        }
+
         const safe = await this.services.acl.getSafeAclError(
             actor,
             this.#descriptorFor(entry),
diff --git a/src/gui/src/UI/Dashboard/UIShareModal.js b/src/gui/src/UI/Dashboard/UIShareModal.js
index 65e4217b8..9e84a55b2 100644
--- a/src/gui/src/UI/Dashboard/UIShareModal.js
+++ b/src/gui/src/UI/Dashboard/UIShareModal.js
@@ -19,7 +19,7 @@
 
 import path from '../../lib/path.js';
 import item_icon from '../../helpers/item_icon.js';
-import { owner_of_path } from '../../helpers/path_owner.js';
+import { is_owned_by_me, owner_of_path } from '../../helpers/path_owner.js';
 import { invalidate_shared_roots } from '../../helpers/shared_access.js';
 import { icons } from '../../helpers/actionIcons.js';
 import { mode_label, options_for } from '../../helpers/share_modes.js';
@@ -115,6 +115,8 @@ export default function UIShareModal ({ items, path: item_path, name, owner, fse
         }));
     const target_paths = targets.map((item) => item.path);
     const total = targets.length;
+    // Strictest item decides: one borrowed item withholds it for the rest.
+    const allow_manage = target_paths.every((p) => is_owned_by_me(p));
     const is_multi = total > 1;
     // Nothing to share: an empty selection is a caller's mistake, not a dialog.
     if ( total === 0 ) return { close: () => {} };
@@ -159,7 +161,7 @@ export default function UIShareModal ({ items, path: item_path, name, owner, fse
                         <div class="share-modal-add-row">
                             <input type="text" class="share-modal-recipient" autocomplete="off" autocapitalize="off" spellcheck="false" enterkeyhint="send"
                                 placeholder="${i18n('share_add_people')}" aria-label="${i18n('share_add_people')}" />
-                            <select class="share-modal-mode" aria-label="${i18n('share_access_level')}">${options_for('read')}</select>
+                            <select class="share-modal-mode" aria-label="${i18n('share_access_level')}">${options_for('read', { allow_manage })}</select>
                         </div>
                         <button type="submit" class="share-modal-submit" disabled>
                             <span class="share-modal-spinner" aria-hidden="true"></span>
@@ -318,7 +320,7 @@ export default function UIShareModal ({ items, path: item_path, name, owner, fse
             // The accessible names carry the person: a list where every row
             // reads as bare "Access level" / "Remove access" leaves a screen
             // reader user unable to tell whose grant a control changes.
-            row += `<select class="share-modal-row-mode" data-key="${key}" aria-label="${i18n('share_access_level_for', { recipient: group.name })}">${options_for(group.mode)}</select>`;
+            row += `<select class="share-modal-row-mode" data-key="${key}" aria-label="${i18n('share_access_level_for', { recipient: group.name })}">${options_for(group.mode, { allow_manage })}</select>`;
         } else {
             const fixed_mode = group.pending ? group.pendingMode : group.inheritedMode;
             row += `<span class="share-modal-row-tag">${fixed_mode ? mode_label(fixed_mode) : i18n('share_access_mixed')}</span>`;
diff --git a/src/gui/src/UI/UIWindowShare.js b/src/gui/src/UI/UIWindowShare.js
index b2bf737ed..a0352b8bb 100644
--- a/src/gui/src/UI/UIWindowShare.js
+++ b/src/gui/src/UI/UIWindowShare.js
@@ -20,7 +20,7 @@
 import UIWindow from './UIWindow.js';
 import UIAlert from './UIAlert.js';
 import path from '../lib/path.js';
-import { owner_of_path } from '../helpers/path_owner.js';
+import { is_owned_by_me, owner_of_path } from '../helpers/path_owner.js';
 import { invalidate_shared_roots } from '../helpers/shared_access.js';
 import { icons } from '../helpers/actionIcons.js';
 import { mode_label, options_for } from '../helpers/share_modes.js';
@@ -41,6 +41,8 @@ async function UIWindowShare (options) {
     const item_name = options.name ?? path.basename(item_path);
     const item_owner =
         options.owner ?? owner_of_path(item_path) ?? window.user.username;
+    // A delegate passes on access, never the authority to pass it on.
+    const allow_manage = is_owned_by_me(item_path);
 
     let h = '';
     h += '<div class="share-dialog">';
@@ -51,7 +53,7 @@ async function UIWindowShare (options) {
     h += '<div class="share-dialog-row">';
     h += `<input class="share-recipient" id="share-recipient" type="text" autocomplete="off" spellcheck="false"
                  placeholder="${html_encode(i18n('share_add_people'))}" />`;
-    h += `<select class="share-mode">${options_for('read')}</select>`;
+    h += `<select class="share-mode">${options_for('read', { allow_manage })}</select>`;
     h += '</div>';
     h += `<button class="share-btn button button-primary button-block button-normal">${i18n('share')}</button>`;
 
@@ -143,7 +145,7 @@ async function UIWindowShare (options) {
             }
             rows += '<div class="share-row">';
             rows += `<span class="share-row-who">${holder}</span>`;
-            rows += `<select class="share-row-mode-select" data-holder="${holder}">${options_for(share.mode)}</select>`;
+            rows += `<select class="share-row-mode-select" data-holder="${holder}">${options_for(share.mode, { allow_manage })}</select>`;
             rows += `<button class="share-revoke" data-holder="${holder}" title="${html_encode(i18n('share_remove_access'))}" aria-label="${html_encode(i18n('share_remove_access'))}">${icons.trash}</button>`;
             rows += '</div>';
         }
diff --git a/src/gui/src/helpers/share_modes.js b/src/gui/src/helpers/share_modes.js
index 8415da8df..1c4d7f1f8 100644
--- a/src/gui/src/helpers/share_modes.js
+++ b/src/gui/src/helpers/share_modes.js
@@ -48,11 +48,17 @@ export const mode_label = (mode) => {
  * unselectable placeholder, so a batch of mixed modes can't be read as one of
  * them, and picking a real mode is what levels them.
  *
+ * `allow_manage: false` drops "Can edit & share", bar a row already on it.
+ *
  * @param {string|null} current
+ * @param {{ allow_manage?: boolean }} [options]
  * @returns {string} HTML-safe markup
  */
-export const options_for = (current) => {
-    const listed = MODES
+export const options_for = (current, { allow_manage = true } = {}) => {
+    const offered = MODES.filter(
+        (mode) => allow_manage || mode !== 'manage' || mode === current,
+    );
+    const listed = offered
         .map(
             (mode) =>
                 `<option value="${html_encode(mode)}"${mode === current ? ' selected' : ''}>${mode_label(mode)}</option>`,
diff --git a/src/gui/src/helpers/share_modes.test.js b/src/gui/src/helpers/share_modes.test.js
index a55fc554b..b693b356f 100644
--- a/src/gui/src/helpers/share_modes.test.js
+++ b/src/gui/src/helpers/share_modes.test.js
@@ -57,6 +57,19 @@ describe('options_for', () => {
         expect(html).not.toContain('<option value="read" selected>');
     });
 
+    it('withholds `manage` from someone who cannot grant it', () => {
+        // Offering it to a delegate is a dead end the server refuses.
+        const html = options_for('read', { allow_manage: false });
+        expect(values(html)).toEqual(['read', 'write']);
+        expect(html).not.toContain('value="manage"');
+    });
+
+    it('still shows a row already set to `manage`, so opening the dialog does not downgrade it', () => {
+        const html = options_for('manage', { allow_manage: false });
+        expect(values(html)).toEqual(MODES);
+        expect(html).toContain('<option value="manage" selected>');
+    });
+
     it('keeps an out-of-band mode instead of rounding it to read', () => {
         const html = options_for('see');
         expect(values(html)).toEqual(['see', ...MODES]);