diff --git a/frontend/e2e/page-context-menu.spec.ts b/frontend/e2e/page-context-menu.spec.ts
index d7ea12c..f42255c 100644
--- a/frontend/e2e/page-context-menu.spec.ts
+++ b/frontend/e2e/page-context-menu.spec.ts
@@ -373,5 +373,12 @@ test.describe('page action sheet (mobile long-press)', () => {
         await expect(page.getByTestId('page-action-add-tag')).toBeVisible();
         await expect(page.getByTestId('page-action-save-image')).toBeVisible();
         await expect(page.getByTestId('page-action-copy-link')).toBeVisible();
+
+        // The editor itself opens as a bottom Sheet on mobile (not a centered
+        // Modal), so the action-sheet → editor flow stays one consistent
+        // idiom. Sheet renders a `${testid}-scrim`; Modal a `${testid}-backdrop`.
+        await page.getByTestId('page-action-add-tag').click();
+        await expect(page.getByTestId('add-tags-modal-scrim')).toBeVisible();
+        await expect(page.getByTestId('add-tags-modal-backdrop')).toHaveCount(0);
     });
 });
diff --git a/frontend/src/lib/components/AdaptiveDialog.svelte b/frontend/src/lib/components/AdaptiveDialog.svelte
new file mode 100644
index 0000000..9f27e45
--- /dev/null
+++ b/frontend/src/lib/components/AdaptiveDialog.svelte
@@ -0,0 +1,53 @@
+<script lang="ts">
+    import type { Snippet } from 'svelte';
+    import Modal from './Modal.svelte';
+    import Sheet from './Sheet.svelte';
+
+    // One overlay that adapts to the form factor: a bottom Sheet on phones,
+    // a centered Modal on desktop. Both share the same chrome contract
+    // (open / title / onClose / children / testid), so callers stay agnostic
+    // and the page-action editors present consistently with the rest of the
+    // mobile UI (long-press → action sheet → editor sheet, no Modal hop).
+    let {
+        open,
+        title,
+        onClose,
+        children,
+        size = 'md',
+        testid
+    }: {
+        open: boolean;
+        title: string;
+        onClose: () => void;
+        children: Snippet;
+        /** Desktop modal width; ignored by the mobile sheet. */
+        size?: 'sm' | 'md' | 'lg';
+        testid?: string;
+    } = $props();
+
+    // Bottom sheet under the 640px breakpoint. Guarded on matchMedia rather
+    // than $app/environment so it's SSR-safe (no window) and unit-testable
+    // (stub matchMedia). First paint defaults to the modal; the effect
+    // corrects to a sheet on phones before the dialog is opened.
+    let isMobile = $state(false);
+    $effect(() => {
+        if (typeof window === 'undefined' || !window.matchMedia) return;
+        const mql = window.matchMedia('(max-width: 640px)');
+        isMobile = mql.matches;
+        const onChange = (e: MediaQueryListEvent) => {
+            isMobile = e.matches;
+        };
+        mql.addEventListener('change', onChange);
+        return () => mql.removeEventListener('change', onChange);
+    });
+</script>
+
+{#if isMobile}
+    <Sheet {open} {title} {onClose} {testid}>
+        {@render children()}
+    </Sheet>
+{:else}
+    <Modal {open} {title} {onClose} {size} {testid}>
+        {@render children()}
+    </Modal>
+{/if}
diff --git a/frontend/src/lib/components/AdaptiveDialog.svelte.test.ts b/frontend/src/lib/components/AdaptiveDialog.svelte.test.ts
new file mode 100644
index 0000000..f2dae93
--- /dev/null
+++ b/frontend/src/lib/components/AdaptiveDialog.svelte.test.ts
@@ -0,0 +1,61 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { render, screen, cleanup } from '@testing-library/svelte';
+import { createRawSnippet } from 'svelte';
+import AdaptiveDialog from './AdaptiveDialog.svelte';
+
+afterEach(() => {
+    cleanup();
+    // @ts-expect-error — restore between tests
+    delete window.matchMedia;
+});
+
+// jsdom has no matchMedia; stub it to drive the breakpoint branch.
+function stubMatchMedia(matches: boolean) {
+    window.matchMedia = vi.fn().mockImplementation((media: string) => ({
+        matches,
+        media,
+        onchange: null,
+        addEventListener: vi.fn(),
+        removeEventListener: vi.fn(),
+        addListener: vi.fn(),
+        removeListener: vi.fn(),
+        dispatchEvent: vi.fn()
+    })) as unknown as typeof window.matchMedia;
+}
+
+const body = createRawSnippet(() => ({
+    render: () => `<p data-testid="dlg-body">hello</p>`
+}));
+
+describe('AdaptiveDialog', () => {
+    it('renders a centered Modal on desktop (the scrim is the Modal backdrop)', async () => {
+        stubMatchMedia(false);
+        render(AdaptiveDialog, {
+            props: { open: true, title: 'Tag this page', onClose: () => {}, testid: 'dlg', children: body }
+        });
+        // Modal exposes a `${testid}-backdrop`; Sheet exposes `${testid}-scrim`.
+        expect(await screen.findByTestId('dlg-backdrop')).toBeTruthy();
+        expect(screen.queryByTestId('dlg-scrim')).toBeNull();
+        expect(screen.getByRole('dialog', { name: 'Tag this page' })).toBeTruthy();
+        expect(screen.getByTestId('dlg-body')).toBeTruthy();
+    });
+
+    it('renders a bottom Sheet on mobile', async () => {
+        stubMatchMedia(true);
+        render(AdaptiveDialog, {
+            props: { open: true, title: 'Tag this page', onClose: () => {}, testid: 'dlg', children: body }
+        });
+        // The effect flips to mobile after mount; poll for the Sheet scrim.
+        expect(await screen.findByTestId('dlg-scrim')).toBeTruthy();
+        expect(screen.queryByTestId('dlg-backdrop')).toBeNull();
+        expect(screen.getByRole('dialog', { name: 'Tag this page' })).toBeTruthy();
+    });
+
+    it('renders nothing when closed', () => {
+        stubMatchMedia(false);
+        render(AdaptiveDialog, {
+            props: { open: false, title: 'Tag this page', onClose: () => {}, testid: 'dlg', children: body }
+        });
+        expect(screen.queryByRole('dialog')).toBeNull();
+    });
+});
diff --git a/frontend/src/lib/components/AddToCollectionModal.svelte b/frontend/src/lib/components/AddToCollectionModal.svelte
index 37f9499..d1d8d04 100644
--- a/frontend/src/lib/components/AddToCollectionModal.svelte
+++ b/frontend/src/lib/components/AddToCollectionModal.svelte
@@ -1,5 +1,5 @@
 <script lang="ts">
-    import Modal from './Modal.svelte';
+    import AdaptiveDialog from './AdaptiveDialog.svelte';
     import {
         addMangaToCollection,
         createCollection,
@@ -166,7 +166,7 @@
     }
 </script>
 
-<Modal {open} {onClose} title="Add to collection" size="md" testid="add-to-collection-modal">
+<AdaptiveDialog {open} {onClose} title="Add to collection" size="md" testid="add-to-collection-modal">
     {#if loading}
         <p class="status">Loading your collections…</p>
     {:else if error}
@@ -235,7 +235,7 @@
             <span>{creating ? 'Creating…' : 'Create + add'}</span>
         </button>
     </form>
-</Modal>
+</AdaptiveDialog>
 
 <style>
     .status {
diff --git a/frontend/src/routes/manga/[id]/chapter/[chapter_id]/+page.svelte b/frontend/src/routes/manga/[id]/chapter/[chapter_id]/+page.svelte
index c59893f..ea12a06 100644
--- a/frontend/src/routes/manga/[id]/chapter/[chapter_id]/+page.svelte
+++ b/frontend/src/routes/manga/[id]/chapter/[chapter_id]/+page.svelte
@@ -11,7 +11,7 @@
     import { readerFullscreen } from '$lib/reader-fullscreen.svelte';
     import { session } from '$lib/session.svelte';
     import Sheet from '$lib/components/Sheet.svelte';
-    import Modal from '$lib/components/Modal.svelte';
+    import AdaptiveDialog from '$lib/components/AdaptiveDialog.svelte';
     import SegmentedControl from '$lib/components/SegmentedControl.svelte';
     import TapZone from '$lib/components/TapZone.svelte';
     import PageContextMenu from '$lib/components/PageContextMenu.svelte';
@@ -1577,7 +1577,7 @@
                 if (activePageId) void loadPageSummary(activePageId);
             }}
         />
-        <Modal
+        <AdaptiveDialog
             open={tagsModalOpen}
             title="Tag this page"
             onClose={() => (tagsModalOpen = false)}
@@ -1590,7 +1590,7 @@
                     activePageTags = tags;
                 }}
             />
-        </Modal>
+        </AdaptiveDialog>
     {/if}
 {/if}