diff --git a/docs/decisions.md b/docs/decisions.md
index b01205535..74ccb02c0 100644
--- a/docs/decisions.md
+++ b/docs/decisions.md
@@ -99,6 +99,7 @@ in-file entries.
 - [2026-07-18 — Unsupported PlaybackOrder is loud at build time; a declared support matrix and tripwire test make new orders safe by construction (#403)](#2026-07-18--unsupported-playbackorder-is-loud-at-build-time-a-declared-support-matrix-and-tripwire-test-make-new-orders-safe-by-construction-403)
 - [2026-07-18 — CI build-once was measured and rejected; keep the #420 tree-skip](#2026-07-18--ci-build-once-was-measured-and-rejected-keep-the-420-tree-skip)
 - [2026-07-18 — Never-scanned `LastScan` surfaces as null at the API boundary, not the 0001-01-01 MinValue sentinel (#409)](#2026-07-18--never-scanned-lastscan-surfaces-as-null-at-the-api-boundary-not-the-0001-01-01-minvalue-sentinel-409)
+- [2026-07-19 — WeightedShuffle SPA: weights edited on the multi-collection, order offered only on classic MultiCollection schedule items; fair-share is a reset not a mode (#404)](#2026-07-19--weightedshuffle-spa-weights-edited-on-the-multi-collection-order-offered-only-on-classic-multicollection-schedule-items-fair-share-is-a-reset-not-a-mode-404)
 
 ---
 
@@ -1942,3 +1943,42 @@ internal scanner code that coalesces to `SystemTime.MinValueUtc` for its own non
 `LibrariesScreen.tsx` was removed — both call sites revert to a plain null/truthy check now that the API
 is honest. (Follow-up option, not done here: have `MediaSourceRepository` write `null` instead of
 `MinValue` so the data is clean at rest too; the read coercion makes that non-urgent.)
+
+## 2026-07-19 — WeightedShuffle SPA: weights edited on the multi-collection, order offered only on classic MultiCollection schedule items; fair-share is a reset not a mode (#404)
+
+The UI half of #70 (backend + API shipped in PR #402). No new endpoint or DTO — `weight` was already on
+`MultiCollectionItemRequest`/`…ResponseModel` and `WeightedShuffle` already in the `PlaybackOrder` enum; this
+is purely SPA (+ docs).
+
+- **Two surfaces, split by where each datum lives.** The per-source *weights* are a property of the
+  `MultiCollection`, so they are edited in the multi-collection editor (`/app/multi-collections`), not on the
+  schedule item. The *order* (`WeightedShuffle`) is a property of the schedule item, so it is added to the
+  classic schedule item's Playback Order options. Putting weights on the schedule item was rejected: the same
+  multi-collection can be referenced by many schedule items, and weights are shared, not per-reference.
+- **`WeightedShuffle` is offered ONLY for `collectionType === 'MultiCollection'`** (`itemRules.ts`
+  `MULTI_COLLECTION_ORDERS`) — a *meaningfulness* scope, not a rejection mirror. Weights only exist on
+  multi-collection members, so the order only does something with 2+ weighted sources; on a single collection
+  the classic engine still **accepts** it (verified by live-E2E: `PUT /schedules/{id}/items` with a plain
+  Collection + `WeightedShuffle` → 200) but it degrades to fair-share (≈ `Shuffle`), a confusing no-op — so
+  the SPA doesn't offer it there. The *rejection* the #70 entry describes is on the separate **playlist and
+  block** write paths (different engines); the playlist/block editors keep their own order lists (not
+  `MULTI_COLLECTION_ORDERS`) and already omit `WeightedShuffle`, so nothing extra was needed there.
+- **`WeightedShuffle` joins `ShuffleInOrder` in the `fillWithGroup` exclusion.** `fillWithGroupModeEligible`
+  already excluded `ShuffleInOrder`; WeightedShuffle is excluded for the same structural reason —
+  `PlayoutBuilder` splits a fill-with-group item into per-group enumerators scheduled one group at a time,
+  which is incompatible with WeightedShuffle's *whole-multi-collection* per-source share of airtime. Offering
+  the combination would silently not do what the name says.
+- **Fair-share is a "Reset to fair share" button, not a stateful toggle.** decisions.md 2026-07-17 established
+  that fair-share is not a separate mode — it is `WeightedShuffle` with all weights left at the default 1. So
+  there is nothing to toggle *on*: the button just sets every source's weight to 1 (disabled when already
+  there). A persistent toggle was rejected because "fair share" has no stored representation distinct from
+  "the weights all happen to be 1".
+- **Weights round-trip through the editor draft or they silently reset.** The multi-collection PUT replaces
+  the whole item list, so `weight` is read in `itemsFromMultiCollection` and written in `toItemRequest`;
+  dropping it would reset every source to 1 on the next unrelated save (rename, add a source). Covered by a
+  round-trip test. This is the general replace-all-DTO trap now recorded in `spa-conventions.md` §4.
+- **Percentages are display-only; the wire format is the integer weight.** A 3:1 weight shows as 75% / 25%,
+  computed from the clamped weights; the request always carries the integer relative share (1..1000). The
+  weight field is held as a string in the editor draft so it edits smoothly, clamped to the API validator's
+  1..1000 on blur and again at save — an out-of-range value never reaches the server as a raw 400. `Input`
+  gained `min`/`max`/`inputMode`/`onBlur` passthroughs for this (reusable by #425's weight UI).
diff --git a/docs/domain-model.md b/docs/domain-model.md
index 593a1a9fa..01ee195a6 100644
--- a/docs/domain-model.md
+++ b/docs/domain-model.md
@@ -66,11 +66,11 @@ Channel (1) ──< Playout (0..N per channel; ChannelPlayoutSource distinguishe
 | **FillerPreset** | A reusable filler definition: `FillerKind` (PreRoll/MidRoll/PostRoll/Tail/Fallback; also `GuideMode=99`, `DecoDefault=100`) × `FillerMode` (None/Duration/Count/Pad/RandomCount) over a collection/media-item/multi-collection/smart-collection/playlist source, with an optional `Expression` (NCalc). Referenced from `ProgramScheduleItem` (Pre/Mid/Post/Tail/FallbackFillerId) and `Channel.FallbackFillerId`. | `FillerPreset`, `FillerKind`, `FillerMode` | `/app/filler-presets` |
 | **Clock-boundary padding** (#77) | Snapping a schedule to clean `:00/:15/:30`-style guide times is **not a separate feature** — it is `FillerMode.Pad` + `PadToNearestMinute` on a `FillerPreset` (Classic), or the `pad_to_next`/`pad_until` YAML instructions (Sequential). Block playouts are inherently clock-anchored via `TemplateItem.StartTime`. The EPG reflects the padded boundary automatically (`ChannelGuideProjector` coalesces trailing filler into the programme window). No one-click per-channel toggle yet — deferred to a UI follow-up blocked on #388. See `decisions.md` 2026-07-17. | `FillerPreset` (`Pad`) | `/app/filler-presets` |
 | **Seasonal / date-conditional scheduling** (#73) | Holiday/seasonal channels are **not a separate feature** — they are the existing date predicate on `IAlternateScheduleItem`, implemented by `ProgramScheduleAlternate` (Classic) and `PlayoutTemplate` (Block), evaluated by `AlternateScheduleSelector.GetScheduleForDate` (first match by `Index`, catch-all last). **Leaving `StartYear`/`EndYear` empty makes the range repeat every year** — the "set once, works every December" switch; explicit years (required in pairs) mean a one-off window and disable wrap-around detection. Wrap-around (Nov→Feb) and invalid/leap dates (Feb 31) are handled. No *soft* prioritization primitive exists (binary first-match-wins); that ask belongs to #70's weighting work. See `channels.md` → "Recipe: seasonal / holiday programming" and `decisions.md` 2026-07-17. | `IAlternateScheduleItem`, `ProgramScheduleAlternate`, `PlayoutTemplate` | `/app/playouts/{id}/alternate-schedules`, `/app/playouts/{id}/templates` |
-| **Playback order** | How a schedule item's source(s) are sequenced (`PlaybackOrder`). Note three that are easily confused: **`Shuffle`** is Fisher–Yates over the flattened items, so airtime is implicitly proportional to collection size (a 200-episode show swamps a 20-episode one). **`ShuffleInOrder`** is a balanced shuffle (keyj) that pads sources to equal length with non-emitting spacers — it plays every item exactly once per cycle, so it prevents *clumping* but leaves airtime proportional to size; it is **not** fair-share. **`WeightedShuffle`** (#70) picks a *source* by smooth weighted round-robin then takes its next item, so each source's `Weight` is its share of airtime — equal weights (the default) mean equal airtime regardless of library size, with small sources looping. Classic engine only; rejected at the write path for playlist/block items. See `decisions.md` 2026-07-17. | `PlaybackOrder`, `MultiCollectionItem.Weight`, `MultiCollectionSmartItem.Weight` | `/app/schedules` (weight UI pending, gated on #388) |
+| **Playback order** | How a schedule item's source(s) are sequenced (`PlaybackOrder`). Note three that are easily confused: **`Shuffle`** is Fisher–Yates over the flattened items, so airtime is implicitly proportional to collection size (a 200-episode show swamps a 20-episode one). **`ShuffleInOrder`** is a balanced shuffle (keyj) that pads sources to equal length with non-emitting spacers — it plays every item exactly once per cycle, so it prevents *clumping* but leaves airtime proportional to size; it is **not** fair-share. **`WeightedShuffle`** (#70) picks a *source* by smooth weighted round-robin then takes its next item, so each source's `Weight` is its share of airtime — equal weights (the default) mean equal airtime regardless of library size, with small sources looping. Classic engine only; rejected at the write path for playlist/block items. See `decisions.md` 2026-07-17. | `PlaybackOrder`, `MultiCollectionItem.Weight`, `MultiCollectionSmartItem.Weight` | `WeightedShuffle` is offered as a Playback Order **only** on classic schedule items whose source is a MultiCollection (`web/src/schedules/itemRules.ts`, #404); the per-source weights themselves are edited at `/app/multi-collections` |
 | **Watermark** | `ChannelWatermark` image overlay; attached at channel, schedule-item, block-item, deco, or playout-item level with position/size/opacity. | `ChannelWatermark`, `DecoWatermark`, `BlockItemWatermark`, `ProgramScheduleItemWatermark` | `/app/watermarks` |
 | **Collection** | Manual list of media items (`CollectionItem`). | `Collection` | `/app/collections` |
 | **SmartCollection** | Saved search — a `Query` string, no static item list. | `SmartCollection` | `/app/collections` |
-| **MultiCollection** | Combines multiple `Collection`s and/or `SmartCollection`s (with grouping via `MultiCollectionItem`/`MultiCollectionSmartItem`). Both join entities carry a per-source `Weight` (default 1) used by `PlaybackOrder.WeightedShuffle` (#70) and ignored by every other order — the two are mirrors, so a change to one belongs on the other. | `MultiCollection` | `/app/multi-collections` (#151) |
+| **MultiCollection** | Combines multiple `Collection`s and/or `SmartCollection`s (with grouping via `MultiCollectionItem`/`MultiCollectionSmartItem`). Both join entities carry a per-source `Weight` (default 1) used by `PlaybackOrder.WeightedShuffle` (#70) and ignored by every other order — the two are mirrors, so a change to one belongs on the other. The editor exposes a per-source weight input (1..1000, mirroring the API validator) with a computed % share and a "Reset to fair share" action; it round-trips `weight` from the GET because the PUT replaces the item list (#404). | `MultiCollection` | `/app/multi-collections` (#151, weight UI #404) |
 | **RerunCollection** | One source (collection/media item/multi/smart) with separate `FirstRunPlaybackOrder` vs `RerunPlaybackOrder`. | `RerunCollection` | `/app/rerun-collections` (#152) |
 | **Playlist** / **PlaylistGroup** | Ordered `PlaylistItem`s; `IsSystem` flag marks built-in/non-deletable playlists and groups. | `Playlist`, `PlaylistGroup`, `PlaylistItem` | `/app/playlists` |
 | **Media kinds** | `CollectionType` enum distinguishes container kinds (Collection/TelevisionShow/TelevisionSeason/Artist/MultiCollection/SmartCollection/Playlist/RerunFirstRun/RerunRerun/SearchQuery) from leaf media kinds (Movie/Episode/MusicVideo/OtherVideo/Song/Image/RemoteStream) plus synthetic `FakeCollection`/`FakePlaylistItem`. Concrete media entities: `Movie`, `Show`/`Season`/`Episode`, `Artist`/`MusicVideo`/`Song`, `OtherVideo`, `Image`, `RemoteStream` (`ErsatzTV.Core/Domain/MediaItem/`). | `MediaItem` subclasses | `/app/media?kind=...` |
diff --git a/docs/spa-conventions.md b/docs/spa-conventions.md
index 9414579b1..ed585a12c 100644
--- a/docs/spa-conventions.md
+++ b/docs/spa-conventions.md
@@ -172,6 +172,18 @@ fresher edit:
   ETag and the draft data come from one read and stay consistent (the fail-safe ordering; don't pair a
   list-row draft with a separately-fetched ETag). Editors that navigate back to the list on save (Playout,
   Multi/Rerun, Collection reorder) need only the 412 branch — no post-save ETag rotation.
+- **Replace-all PUT → round-trip every persisted field, even ones the editor doesn't visibly edit.** A
+  replace-all update (Multi/Rerun collection, schedule items, block items) submits the *whole* item list,
+  so any field the draft mapping drops is silently reset to its default server-side. The multi-collection
+  editor's per-source `weight` (#404) is the canonical trap: it must be read in `itemsFromMultiCollection`
+  and written in `toItemRequest`, or every source resets to weight 1 on the next save of an unrelated edit
+  (rename, add a source). When you add a field to a replace-all DTO, add it to **both** the response→draft
+  and draft→request mappings and cover the round-trip in a test.
+- **Numeric inputs with API bounds**: hold the value as a string in the draft so the field edits smoothly
+  (transient empty/partial), pass `type="number"` + `min`/`max`/`inputMode="numeric"` to `Input` for the
+  native stepper, clamp to the API's validator range on `onBlur` (immediate feedback) **and** again at save
+  (authoritative) — never let an out-of-range value reach the server and surface a raw 400. Mirror the
+  server's bound as a shared const (e.g. multi-collection weight `WEIGHT_MIN`/`WEIGHT_MAX` = 1..1000, #404).
 
 ## 5. Artwork rendering
 
diff --git a/web/src/components/forms.tsx b/web/src/components/forms.tsx
index 831836409..f96049bf2 100644
--- a/web/src/components/forms.tsx
+++ b/web/src/components/forms.tsx
@@ -1,4 +1,4 @@
-import type { ChangeEvent, CSSProperties, MouseEvent, ReactNode } from 'react';
+import type { ChangeEvent, CSSProperties, FocusEvent, MouseEvent, ReactNode } from 'react';
 import { Check, ChevronDown, Minus } from 'lucide-react';
 import { Spinner } from './feedback';
 
@@ -112,6 +112,13 @@ export interface InputProps {
   type?: string;
   fullWidth?: boolean;
   style?: CSSProperties;
+  // Native <input> passthroughs — added for numeric fields (e.g. the multi-collection weight input,
+  // #404): min/max bound the stepper, inputMode hints the mobile keyboard, onBlur normalizes on commit.
+  min?: number;
+  max?: number;
+  inputMode?: 'numeric' | 'decimal' | 'text';
+  onBlur?: (e: FocusEvent<HTMLInputElement>) => void;
+  ariaLabel?: string;
 }
 
 export function Input({
@@ -126,7 +133,12 @@ export function Input({
   size = 'md',
   type = 'text',
   fullWidth = true,
-  style
+  style,
+  min,
+  max,
+  inputMode,
+  onBlur,
+  ariaLabel
 }: InputProps) {
   return (
     <label className={classNames('ctv-field', fullWidth && 'ctv-field-full')}>
@@ -146,8 +158,13 @@ export function Input({
           type={type}
           value={value}
           onChange={onChange}
+          onBlur={onBlur}
           placeholder={placeholder}
           disabled={disabled}
+          min={min}
+          max={max}
+          inputMode={inputMode}
+          aria-label={ariaLabel}
         />
         {trailing && <span className="ctv-field-trailing">{trailing}</span>}
       </span>
diff --git a/web/src/schedules/itemRules.test.ts b/web/src/schedules/itemRules.test.ts
index ad92709da..2bb6b7a66 100644
--- a/web/src/schedules/itemRules.test.ts
+++ b/web/src/schedules/itemRules.test.ts
@@ -31,8 +31,25 @@ function draft(overrides: Partial<DraftItem> = {}): DraftItem {
 }
 
 describe('playbackOrderOptions', () => {
-  it('MultiCollection offers Shuffle + ShuffleInOrder only', () => {
-    expect(playbackOrderOptions('MultiCollection')).toEqual(['Shuffle', 'ShuffleInOrder']);
+  it('MultiCollection offers Shuffle + ShuffleInOrder + WeightedShuffle only', () => {
+    expect(playbackOrderOptions('MultiCollection')).toEqual(['Shuffle', 'ShuffleInOrder', 'WeightedShuffle']);
+  });
+
+  it('WeightedShuffle is offered ONLY on MultiCollection (needs per-source weights)', () => {
+    const others: DraftItem['collectionType'][] = [
+      'Collection',
+      'SmartCollection',
+      'SearchQuery',
+      'TelevisionShow',
+      'TelevisionSeason',
+      'Artist',
+      'Playlist',
+      'RerunFirstRun',
+      'RerunRerun'
+    ];
+    for (const t of others) {
+      expect(playbackOrderOptions(t)).not.toContain('WeightedShuffle');
+    }
   });
 
   it('Collection / SmartCollection / SearchQuery offer the collection-like set incl. Marathon', () => {
@@ -160,10 +177,13 @@ describe('fillWithGroupModeEligible', () => {
     expect(fillWithGroupModeEligible(draft({ playoutMode: 'Duration', playbackOrder: 'Random', collectionType: 'SmartCollection' }))).toBe(true);
   });
 
-  it('ineligible for One/Flood, ShuffleInOrder, or non-collection types', () => {
+  it('ineligible for One/Flood, ShuffleInOrder, WeightedShuffle, or non-collection types', () => {
     expect(fillWithGroupModeEligible(draft({ playoutMode: 'One', collectionType: 'Collection' }))).toBe(false);
     expect(fillWithGroupModeEligible(draft({ playoutMode: 'Flood', collectionType: 'Collection' }))).toBe(false);
     expect(fillWithGroupModeEligible(draft({ playoutMode: 'Multiple', playbackOrder: 'ShuffleInOrder', collectionType: 'Collection' }))).toBe(false);
+    // WeightedShuffle joins ShuffleInOrder in the exclusion (#404): fillWithGroup schedules per-group,
+    // which conflicts with WeightedShuffle's whole-multi-collection per-source airtime share.
+    expect(fillWithGroupModeEligible(draft({ playoutMode: 'Multiple', playbackOrder: 'WeightedShuffle', collectionType: 'MultiCollection' }))).toBe(false);
     expect(fillWithGroupModeEligible(draft({ playoutMode: 'Multiple', playbackOrder: 'Shuffle', collectionType: 'TelevisionShow' }))).toBe(false);
     expect(fillWithGroupModeEligible(draft({ playoutMode: 'Multiple', playbackOrder: 'None', collectionType: 'Playlist' }))).toBe(false);
   });
diff --git a/web/src/schedules/itemRules.ts b/web/src/schedules/itemRules.ts
index be2081d5f..61740a54e 100644
--- a/web/src/schedules/itemRules.ts
+++ b/web/src/schedules/itemRules.ts
@@ -43,7 +43,13 @@ export function nextDraftKey(): string {
 
 // ---- Option lists --------------------------------------------------------
 
-const MULTI_COLLECTION_ORDERS: PlaybackOrder[] = ['Shuffle', 'ShuffleInOrder'];
+// WeightedShuffle (#70) is offered ONLY for MultiCollection: it distributes airtime across per-source
+// weights, which live on MultiCollectionItem/SmartItem, so it is only *meaningful* with 2+ weighted
+// sources. On a single collection the classic engine still accepts it but it degrades to fair-share
+// (≈ Shuffle), a confusing no-op — hence MultiCollection-only here. (Distinct concern: the playlist and
+// block editors — which keep their own order lists, not this one — must also omit it because those
+// write paths reject it outright; docs/decisions.md 2026-07-17.)
+const MULTI_COLLECTION_ORDERS: PlaybackOrder[] = ['Shuffle', 'ShuffleInOrder', 'WeightedShuffle'];
 const COLLECTION_LIKE_ORDERS: PlaybackOrder[] = ['Chronological', 'Random', 'Shuffle', 'ShuffleInOrder', 'Marathon'];
 const TV_SHOW_ORDERS: PlaybackOrder[] = ['Chronological', 'SeasonEpisode', 'Random', 'Shuffle', 'MultiEpisodeShuffle'];
 const SEASON_ARTIST_ORDERS: PlaybackOrder[] = ['Chronological', 'Random', 'Shuffle'];
@@ -153,11 +159,14 @@ export function shufflePlaylistItemsVisible(item: DraftItem): boolean {
   return item.collectionType === 'Playlist';
 }
 
-// fillWithGroupMode is eligible iff Multiple/Duration && playbackOrder != ShuffleInOrder &&
-// collectionType in {Collection, MultiCollection, SmartCollection}.
+// fillWithGroupMode is eligible iff Multiple/Duration && playbackOrder not in {ShuffleInOrder,
+// WeightedShuffle} && collectionType in {Collection, MultiCollection, SmartCollection}.
+// WeightedShuffle joins ShuffleInOrder in the exclusion: fillWithGroup splits the multi-collection
+// into per-group enumerators scheduled one group at a time (PlayoutBuilder), which is incompatible
+// with WeightedShuffle's whole-collection per-source share of airtime (#70/#404).
 export function fillWithGroupModeEligible(item: DraftItem): boolean {
   const modeOk = item.playoutMode === 'Multiple' || item.playoutMode === 'Duration';
-  const orderOk = item.playbackOrder !== 'ShuffleInOrder';
+  const orderOk = item.playbackOrder !== 'ShuffleInOrder' && item.playbackOrder !== 'WeightedShuffle';
   const typeOk =
     item.collectionType === 'Collection' ||
     item.collectionType === 'MultiCollection' ||
diff --git a/web/src/screens/MultiCollectionsScreen.test.tsx b/web/src/screens/MultiCollectionsScreen.test.tsx
index 44623f727..cf4ab1e3d 100644
--- a/web/src/screens/MultiCollectionsScreen.test.tsx
+++ b/web/src/screens/MultiCollectionsScreen.test.tsx
@@ -122,7 +122,7 @@ describe('MultiCollectionsScreen', () => {
       );
       expect(postCall).toBeDefined();
       expect(JSON.parse(String(postCall?.[1]?.body))).toEqual({
-        items: [{ collectionId: 5, playbackOrder: 'Chronological', scheduleAsGroup: false, smartCollectionId: null }],
+        items: [{ collectionId: 5, playbackOrder: 'Chronological', scheduleAsGroup: false, smartCollectionId: null, weight: 1 }],
         name: 'New Bundle'
       });
     });
@@ -155,13 +155,132 @@ describe('MultiCollectionsScreen', () => {
       expect(parsed.name).toBe('Saturday Bundle');
       expect(parsed.items).toEqual(
         expect.arrayContaining([
-          { collectionId: 5, playbackOrder: 'Chronological', scheduleAsGroup: false, smartCollectionId: null },
-          { collectionId: null, playbackOrder: 'Chronological', scheduleAsGroup: true, smartCollectionId: 9 }
+          { collectionId: 5, playbackOrder: 'Chronological', scheduleAsGroup: false, smartCollectionId: null, weight: 1 },
+          { collectionId: null, playbackOrder: 'Chronological', scheduleAsGroup: true, smartCollectionId: 9, weight: 1 }
         ])
       );
     });
   });
 
+  // Weighted multi-collection fixture reused by the round-trip tests: 3:1 split (Favorites : Action).
+  const weightedMulti = [
+    {
+      id: 1,
+      items: [
+        { collectionId: 5, name: 'Favorites', playbackOrder: 'Chronological', scheduleAsGroup: false, smartCollectionId: null, weight: 3 },
+        { collectionId: null, name: 'Action', playbackOrder: 'Chronological', scheduleAsGroup: true, smartCollectionId: 9, weight: 1 }
+      ],
+      name: 'Saturday Bundle'
+    }
+  ];
+
+  it('preserves weights when only an unrelated field is edited (the silent-reset trap)', async () => {
+    // The canonical #404 trap: the replace-all PUT resubmits the whole item list, so editing ONLY the
+    // name must still round-trip each source's weight untouched — dropping it in the draft mapping
+    // would silently reset every weight to 1 on this save (docs/decisions.md 2026-07-17).
+    const fetchMock = mockApi({
+      multi: weightedMulti,
+      onRequest: (url, method) =>
+        url === '/api/v1/multi-collections/1' && method === 'PUT'
+          ? jsonResponse({ id: 1, items: [], name: 'Weekend Bundle' })
+          : null
+    });
+
+    render(<MultiCollectionsScreen />);
+    fireEvent.click(await screen.findByText('Saturday Bundle'));
+    await screen.findByLabelText('Weight for Favorites');
+
+    // Edit ONLY the name — never touch a weight input.
+    fireEvent.change(screen.getByPlaceholderText('Multi-collection name'), { target: { value: 'Weekend Bundle' } });
+    fireEvent.click(screen.getByRole('button', { name: 'Save multi-collection' }));
+
+    await waitFor(() => {
+      const putCall = fetchMock.mock.calls.find(
+        ([u, init]) => u === '/api/v1/multi-collections/1' && (init?.method ?? '').toUpperCase() === 'PUT'
+      );
+      expect(putCall).toBeDefined();
+      const parsed = JSON.parse(String(putCall?.[1]?.body));
+      expect(parsed.name).toBe('Weekend Bundle');
+      // Both original weights survive unchanged — NOT reset to 1.
+      expect(parsed.items).toEqual(
+        expect.arrayContaining([
+          { collectionId: 5, playbackOrder: 'Chronological', scheduleAsGroup: false, smartCollectionId: null, weight: 3 },
+          { collectionId: null, playbackOrder: 'Chronological', scheduleAsGroup: true, smartCollectionId: 9, weight: 1 }
+        ])
+      );
+    });
+  });
+
+  it('round-trips per-source weights on save (no silent reset), and edits them', async () => {
+    // Weighted multi-collection: the API returns each item's weight and the update REPLACES the item
+    // list, so the editor must carry weight through untouched or every source silently resets to 1
+    // (docs/decisions.md 2026-07-17, #404).
+    const weighted = weightedMulti;
+
+    const fetchMock = mockApi({
+      multi: weighted,
+      onRequest: (url, method) =>
+        url === '/api/v1/multi-collections/1' && method === 'PUT'
+          ? jsonResponse({ id: 1, items: [], name: 'Saturday Bundle' })
+          : null
+    });
+
+    render(<MultiCollectionsScreen />);
+    fireEvent.click(await screen.findByText('Saturday Bundle'));
+
+    // The persisted weights hydrate the inputs (75% / 25% for a 3:1 split).
+    const favoritesWeight = await screen.findByLabelText('Weight for Favorites');
+    expect((favoritesWeight as HTMLInputElement).value).toBe('3');
+    expect(screen.getByText('75%')).toBeInTheDocument();
+    expect(screen.getByText('25%')).toBeInTheDocument();
+
+    // Edit one weight; an out-of-range value clamps to the API ceiling on blur (mirrors the validator).
+    fireEvent.change(favoritesWeight, { target: { value: '5000' } });
+    fireEvent.blur(favoritesWeight);
+    expect((favoritesWeight as HTMLInputElement).value).toBe('1000');
+
+    fireEvent.click(screen.getByRole('button', { name: 'Save multi-collection' }));
+
+    await waitFor(() => {
+      const putCall = fetchMock.mock.calls.find(
+        ([u, init]) => u === '/api/v1/multi-collections/1' && (init?.method ?? '').toUpperCase() === 'PUT'
+      );
+      expect(putCall).toBeDefined();
+      const parsed = JSON.parse(String(putCall?.[1]?.body));
+      // Favorites clamped to 1000; Action's weight of 1 rides through unchanged (not reset).
+      expect(parsed.items).toEqual(
+        expect.arrayContaining([
+          { collectionId: 5, playbackOrder: 'Chronological', scheduleAsGroup: false, smartCollectionId: null, weight: 1000 },
+          { collectionId: null, playbackOrder: 'Chronological', scheduleAsGroup: true, smartCollectionId: 9, weight: 1 }
+        ])
+      );
+    });
+  });
+
+  it('resets all weights to fair share', async () => {
+    const weighted = [
+      {
+        id: 1,
+        items: [
+          { collectionId: 5, name: 'Favorites', playbackOrder: 'Chronological', scheduleAsGroup: false, smartCollectionId: null, weight: 3 },
+          { collectionId: null, name: 'Action', playbackOrder: 'Chronological', scheduleAsGroup: true, smartCollectionId: 9, weight: 1 }
+        ],
+        name: 'Saturday Bundle'
+      }
+    ];
+
+    mockApi({ multi: weighted });
+    render(<MultiCollectionsScreen />);
+    fireEvent.click(await screen.findByText('Saturday Bundle'));
+
+    const favoritesWeight = (await screen.findByLabelText('Weight for Favorites')) as HTMLInputElement;
+    expect(favoritesWeight.value).toBe('3');
+
+    fireEvent.click(screen.getByRole('button', { name: 'Reset to fair share' }));
+    expect(favoritesWeight.value).toBe('1');
+    expect((screen.getByLabelText('Weight for Action') as HTMLInputElement).value).toBe('1');
+  });
+
   it('confirms and DELETEs a multi-collection', async () => {
     const fetchMock = mockApi();
 
diff --git a/web/src/screens/MultiCollectionsScreen.tsx b/web/src/screens/MultiCollectionsScreen.tsx
index e7e71c267..d0ff0360d 100644
--- a/web/src/screens/MultiCollectionsScreen.tsx
+++ b/web/src/screens/MultiCollectionsScreen.tsx
@@ -70,20 +70,53 @@ function useMultiCollectionsData() {
 
 /* ---------- editor ---------- */
 
-// One row in the editor: a manual collection or a smart collection, plus its
-// schedule-as-group flag. Names are resolved from the fetched collection lists.
+// Per-source weight bounds. Must mirror the API's MultiCollectionItemWeight validator (1..1000) so a
+// client-side value never provokes a raw 400 (docs/decisions.md 2026-07-17, #404).
+const WEIGHT_MIN = 1;
+const WEIGHT_MAX = 1000;
+
+// Coerce the editor's free-text weight to a valid integer share. Empty / non-numeric / out-of-range
+// all snap to the [1,1000] bound — 1 (the floor, = fair share) when the field is blank or invalid.
+function clampWeight(raw: string): number {
+  const parsed = Math.trunc(Number(raw));
+  if (!Number.isFinite(parsed)) {
+    return WEIGHT_MIN;
+  }
+  return Math.min(WEIGHT_MAX, Math.max(WEIGHT_MIN, parsed));
+}
+
+// One row in the editor: a manual collection or a smart collection, plus its schedule-as-group flag
+// and per-source weight. `weight` is held as a string so the number field edits smoothly (transient
+// empty / partial values are allowed); it is clamped to the API bounds on blur and again at save.
+// Names are resolved from the fetched collection lists.
 interface DraftItem {
   id: number;
   kind: 'manual' | 'smart';
   name: string;
   scheduleAsGroup: boolean;
+  weight: string;
 }
 
 function itemsFromMultiCollection(mc: MultiCollection): DraftItem[] {
   return mc.items.map((item) =>
     item.collectionId != null
-      ? { id: item.collectionId, kind: 'manual', name: item.name, scheduleAsGroup: item.scheduleAsGroup }
-      : { id: item.smartCollectionId ?? 0, kind: 'smart', name: item.name, scheduleAsGroup: item.scheduleAsGroup }
+      ? {
+          id: item.collectionId,
+          kind: 'manual',
+          name: item.name,
+          scheduleAsGroup: item.scheduleAsGroup,
+          // Round-trip the persisted weight — the update REPLACES the item list, so dropping it here
+          // would silently reset every weight to 1 on save (the API returns it for exactly this
+          // reason). Default 1 for rows created before weights existed (docs/decisions.md, #404).
+          weight: String(item.weight ?? WEIGHT_MIN)
+        }
+      : {
+          id: item.smartCollectionId ?? 0,
+          kind: 'smart',
+          name: item.name,
+          scheduleAsGroup: item.scheduleAsGroup,
+          weight: String(item.weight ?? WEIGHT_MIN)
+        }
   );
 }
 
@@ -92,7 +125,8 @@ function toItemRequest(item: DraftItem): MultiCollectionItemRequest {
     collectionId: item.kind === 'manual' ? item.id : null,
     playbackOrder: 'Chronological',
     scheduleAsGroup: item.scheduleAsGroup,
-    smartCollectionId: item.kind === 'smart' ? item.id : null
+    smartCollectionId: item.kind === 'smart' ? item.id : null,
+    weight: clampWeight(item.weight)
   };
 }
 
@@ -162,7 +196,10 @@ function MultiCollectionEditor({
     }
 
     const collection = collections.find((entry) => entry.id === id);
-    setItems((current) => [...current, { id, kind: 'manual', name: collection?.name ?? `#${id}`, scheduleAsGroup: false }]);
+    setItems((current) => [
+      ...current,
+      { id, kind: 'manual', name: collection?.name ?? `#${id}`, scheduleAsGroup: false, weight: String(WEIGHT_MIN) }
+    ]);
     setSelectedCollection('');
   };
 
@@ -173,7 +210,10 @@ function MultiCollectionEditor({
     }
 
     const collection = smartCollections.find((entry) => entry.id === id);
-    setItems((current) => [...current, { id, kind: 'smart', name: collection?.name ?? `#${id}`, scheduleAsGroup: false }]);
+    setItems((current) => [
+      ...current,
+      { id, kind: 'smart', name: collection?.name ?? `#${id}`, scheduleAsGroup: false, weight: String(WEIGHT_MIN) }
+    ]);
     setSelectedSmart('');
   };
 
@@ -187,6 +227,28 @@ function MultiCollectionEditor({
     );
   };
 
+  // Free-text while typing (allow transient empty / partial); the value is clamped to the API bounds
+  // on blur (normalizeWeight) and again at save (clampWeight in toItemRequest).
+  const setWeight = (target: DraftItem, next: string) => {
+    setItems((current) =>
+      current.map((item) => (item.kind === target.kind && item.id === target.id ? { ...item, weight: next } : item))
+    );
+  };
+
+  const normalizeWeight = (target: DraftItem) => {
+    setItems((current) =>
+      current.map((item) =>
+        item.kind === target.kind && item.id === target.id ? { ...item, weight: String(clampWeight(item.weight)) } : item
+      )
+    );
+  };
+
+  // Fair share = every source at weight 1 (docs/decisions.md 2026-07-17: fair-share is not a separate
+  // mode, just WeightedShuffle with equal weights — so this resets, it does not write a different order).
+  const resetToFairShare = () => {
+    setItems((current) => current.map((item) => ({ ...item, weight: String(WEIGHT_MIN) })));
+  };
+
   const trimmedName = name.trim();
 
   const save = async () => {
@@ -240,6 +302,15 @@ function MultiCollectionEditor({
 
   const sortedItems = [...items].sort((a, b) => a.name.localeCompare(b.name, undefined, { sensitivity: 'base' }));
 
+  // Percentages are a DISPLAY concern (the wire format is integer relative shares) — a 3:1 weight
+  // shows as 75% / 25% (#404). Computed from clamped weights so a transient out-of-range edit never
+  // skews the shares mid-type.
+  const totalWeight = items.reduce((sum, item) => sum + clampWeight(item.weight), 0);
+  const weightPercent = (item: DraftItem): number =>
+    totalWeight > 0 ? Math.round((clampWeight(item.weight) / totalWeight) * 100) : 0;
+  // Fair share = every source already at weight 1; nothing to reset in that state.
+  const alreadyFairShare = items.length > 0 && items.every((item) => clampWeight(item.weight) === WEIGHT_MIN);
+
   return (
     <div className="ctv-collections">
       <div className="ctv-channels-actionbar">
@@ -316,33 +387,72 @@ function MultiCollectionEditor({
         {sortedItems.length === 0 ? (
           <div className="ctv-collections-empty">No collections added yet.</div>
         ) : (
-          sortedItems.map((item, index) => (
-            <div
-              className="ctv-settings-flush-row"
-              key={`${item.kind}:${item.id}`}
-              style={index === 0 ? { borderTop: 'none' } : undefined}
-            >
-              <Layers aria-hidden="true" color="var(--ctv-accent)" size={15} />
-              <span className="ctv-settings-flush-row-main ctv-settings-flush-row-title">{item.name}</span>
-              <Badge tone="neutral">{item.kind === 'smart' ? 'Smart' : 'Manual'}</Badge>
-              <label className="ctv-collections-order-toggle" title="Schedule as group">
-                <Switch
-                  checked={item.scheduleAsGroup}
-                  onChange={(next) => setScheduleAsGroup(item, next)}
+          <>
+            {sortedItems.map((item, index) => (
+              <div
+                className="ctv-settings-flush-row"
+                key={`${item.kind}:${item.id}`}
+                style={index === 0 ? { borderTop: 'none' } : undefined}
+              >
+                <Layers aria-hidden="true" color="var(--ctv-accent)" size={15} />
+                <span className="ctv-settings-flush-row-main ctv-settings-flush-row-title">{item.name}</span>
+                <Badge tone="neutral">{item.kind === 'smart' ? 'Smart' : 'Manual'}</Badge>
+                <div
+                  className="ctv-collections-weight"
+                  title="Relative share of airtime under the Weighted Shuffle playback order"
+                >
+                  <span className="ctv-collections-weight-label" aria-hidden="true">
+                    Weight
+                  </span>
+                  <Input
+                    ariaLabel={`Weight for ${item.name}`}
+                    fullWidth={false}
+                    inputMode="numeric"
+                    max={WEIGHT_MAX}
+                    min={WEIGHT_MIN}
+                    onBlur={() => normalizeWeight(item)}
+                    onChange={(event) => setWeight(item, event.target.value)}
+                    size="sm"
+                    style={{ width: 68 }}
+                    type="number"
+                    value={item.weight}
+                  />
+                  <span className="ctv-collections-weight-pct">{weightPercent(item)}%</span>
+                </div>
+                <label className="ctv-collections-order-toggle" title="Schedule as group">
+                  <Switch
+                    checked={item.scheduleAsGroup}
+                    onChange={(next) => setScheduleAsGroup(item, next)}
+                    size="sm"
+                  />
+                  <span>Schedule as group</span>
+                </label>
+                <IconButton
+                  onClick={() => removeItem(item)}
                   size="sm"
-                />
-                <span>Schedule as group</span>
-              </label>
-              <IconButton
-                onClick={() => removeItem(item)}
+                  title={`Remove ${item.name}`}
+                  variant="ghost"
+                >
+                  <Trash2 aria-hidden="true" size={14} />
+                </IconButton>
+              </div>
+            ))}
+            <div className="ctv-settings-flush-footer ctv-collections-weight-footer">
+              <span className="ctv-collections-weight-hint">
+                Weights set each source&rsquo;s share of airtime under the <strong>Weighted Shuffle</strong> playback
+                order (set on the schedule item). Equal weights = fair share.
+              </span>
+              <span className="ctv-channels-spacer" />
+              <Button
+                disabled={alreadyFairShare}
+                onClick={resetToFairShare}
                 size="sm"
-                title={`Remove ${item.name}`}
                 variant="ghost"
               >
-                <Trash2 aria-hidden="true" size={14} />
-              </IconButton>
+                Reset to fair share
+              </Button>
             </div>
-          ))
+          </>
         )}
       </Card>
 
diff --git a/web/src/shell.css b/web/src/shell.css
index 9a4698741..f89d611b2 100644
--- a/web/src/shell.css
+++ b/web/src/shell.css
@@ -4369,6 +4369,37 @@ body {
   cursor: pointer;
 }
 
+/* per-source weight input + computed share (#404) */
+.ctv-collections-weight {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--space-3, 6px);
+  color: var(--text-secondary);
+  font-size: var(--text-xs, 12px);
+}
+
+.ctv-collections-weight-label {
+  color: var(--text-secondary);
+}
+
+.ctv-collections-weight-pct {
+  min-width: 34px;
+  text-align: right;
+  font: var(--text-2xs) / 1 var(--font-mono);
+  color: var(--text-disabled);
+}
+
+.ctv-collections-weight-footer {
+  gap: var(--space-6, 12px);
+}
+
+.ctv-collections-weight-hint {
+  flex: 1;
+  min-width: 0;
+  font-size: var(--text-xs, 12px);
+  color: var(--text-secondary);
+}
+
 .ctv-collections-query {
   margin-top: 3px;
   font: var(--text-2xs) / 1.3 var(--font-mono);