diff --git a/frontend/src/components/inventory/CantripFilter.tsx b/frontend/src/components/inventory/CantripFilter.tsx
new file mode 100644
index 00000000..be02e415
--- /dev/null
+++ b/frontend/src/components/inventory/CantripFilter.tsx
@@ -0,0 +1,46 @@
+import { useState } from 'react';
+import { CANTRIP_GROUPS } from './constants';
+import type { InventorySearch } from './useInventorySearch';
+import { Group } from './FilterSidebar';
+
+export function CantripFilter({ search }: { search: InventorySearch }) {
+  const { filters: f, update } = search;
+  const [q, setQ] = useState('');
+  const toggle = (v: string) => update({
+    cantrips: f.cantrips.includes(v) ? f.cantrips.filter(x => x !== v) : [...f.cantrips, v],
+  });
+  const match = (label: string) => label.toLowerCase().includes(q.toLowerCase());
+
+  return (
+    <Group title="Cantrips" defaultOpen badge={f.cantrips.length || undefined}>
+      <input className="inv-minisearch" placeholder="find cantrip… e.g. invuln"
+        value={q} onChange={e => setQ(e.target.value)} />
+      {f.cantrips.length > 0 && (<>
+        <div className="inv-subhead">Selected</div>
+        {f.cantrips.map(v => (
+          <label className="inv-fitem inv-gold" key={v}>
+            <input type="checkbox" checked onChange={() => toggle(v)} />
+            {v.replace(/^Legendary /, '')}
+          </label>
+        ))}
+      </>)}
+      {CANTRIP_GROUPS.map(g => {
+        const items = g.items.filter(i => !f.cantrips.includes(i.value) && match(i.label));
+        if (!items.length) return null;
+        return (
+          <div key={g.group}>
+            <div className="inv-subhead">{g.group}</div>
+            {items.map(i => (
+              <label className="inv-fitem" key={i.value}>
+                <input type="checkbox" checked={false} onChange={() => toggle(i.value)} /> {i.label}
+              </label>
+            ))}
+          </div>
+        );
+      })}
+      <div className="inv-subhead">Any-tier spell search</div>
+      <input className="inv-minisearch" placeholder="spell name contains… e.g. Epic Invuln"
+        value={f.spellContains} onChange={e => update({ spellContains: e.target.value })} />
+    </Group>
+  );
+}
diff --git a/frontend/src/components/inventory/CharacterFilter.tsx b/frontend/src/components/inventory/CharacterFilter.tsx
new file mode 100644
index 00000000..f0f1fb68
--- /dev/null
+++ b/frontend/src/components/inventory/CharacterFilter.tsx
@@ -0,0 +1,48 @@
+import { useEffect, useMemo, useState } from 'react';
+import { apiFetch } from '../../api/client';
+import type { InventorySearch } from './useInventorySearch';
+import { Group } from './FilterSidebar';
+
+export function CharacterFilter({ search }: { search: InventorySearch }) {
+  const { filters: f, update } = search;
+  const [names, setNames] = useState<string[]>([]);
+  const [online, setOnline] = useState<Set<string>>(new Set());
+  const [q, setQ] = useState('');
+
+  useEffect(() => {
+    apiFetch<{ characters: Array<{ character_name: string }> }>('/inv/characters/list')
+      .then(r => setNames(r.characters.map(c => c.character_name).sort((a, b) => a.localeCompare(b))))
+      .catch(() => {});
+    apiFetch<{ players: Array<{ character_name: string }> }>('/live')
+      .then(r => setOnline(new Set(r.players.map(p => p.character_name))))
+      .catch(() => {});
+  }, []);
+
+  const checked = useMemo(() =>
+    f.characters === 'all' ? new Set(names) : new Set(f.characters), [f.characters, names]);
+  const toggle = (n: string) => {
+    const next = new Set(checked);
+    if (next.has(n)) next.delete(n); else next.add(n);
+    update({ characters: next.size === names.length ? 'all' : [...next] });
+  };
+  const shown = names.filter(n => n.toLowerCase().includes(q.toLowerCase()));
+
+  return (
+    <Group title="Characters" defaultOpen
+      badge={f.characters === 'all' ? 'All' : f.characters.length}>
+      <input className="inv-minisearch" placeholder="filter characters…"
+        value={q} onChange={e => setQ(e.target.value)} />
+      <div className="inv-links">
+        <span className="inv-linky" onClick={() => update({ characters: 'all' })}>All</span>{' · '}
+        <span className="inv-linky" onClick={() => update({ characters: [] })}>None</span>{' · '}
+        <span className="inv-linky" onClick={() => update({ characters: names.filter(n => online.has(n)) })}>Online</span>
+      </div>
+      {shown.map(n => (
+        <label className="inv-fitem" key={n}>
+          <input type="checkbox" checked={checked.has(n)} onChange={() => toggle(n)} />
+          {n} {online.has(n) && <span className="inv-online" />}
+        </label>
+      ))}
+    </Group>
+  );
+}
diff --git a/frontend/src/components/inventory/FilterSidebar.tsx b/frontend/src/components/inventory/FilterSidebar.tsx
index 006eed02..caeeba78 100644
--- a/frontend/src/components/inventory/FilterSidebar.tsx
+++ b/frontend/src/components/inventory/FilterSidebar.tsx
@@ -1,3 +1,133 @@
-// Stub — replaced in Task 4
+import React, { useEffect, useState } from 'react';
 import type { InventorySearch } from './useInventorySearch';
-export function FilterSidebar(_: { search: InventorySearch }) { return <div className="inv-sidebar" />; }
+import { CharacterFilter } from './CharacterFilter';
+import { CantripFilter } from './CantripFilter';
+import { ARMOR_SLOTS, JEWELRY_SLOTS, RATING_DEFS, WEAPON_TYPES } from './constants';
+import type { ItemType } from './types';
+import { apiFetch } from '../../api/client';
+
+export function Group(props: {
+  title: string; badge?: string | number; defaultOpen?: boolean; children: React.ReactNode;
+}) {
+  const [open, setOpen] = useState(props.defaultOpen ?? false);
+  return (
+    <div className={`inv-grp${open ? ' inv-open' : ''}`}>
+      <div className="inv-grp-head" onClick={() => setOpen(o => !o)}>
+        <span className="inv-arrow">▶</span> {props.title}
+        {props.badge ? <span className="inv-badge">{props.badge}</span> : null}
+      </div>
+      {open && <div className="inv-grp-body">{props.children}</div>}
+    </div>
+  );
+}
+
+export function FilterSidebar({ search }: { search: InventorySearch }) {
+  const { filters: f, update } = search;
+  const [showArmorSlots, setShowArmorSlots] = useState(false);
+  const [allRatings, setAllRatings] = useState(false);
+  const [sets, setSets] = useState<Array<{ id: string; item_count: number }>>([]);
+  const [setQuery, setSetQuery] = useState('');
+  useEffect(() => {
+    apiFetch<{ sets: Array<{ id: string; item_count: number }> }>('/inv/sets/list')
+      .then(r => setSets(r.sets)).catch(() => {});
+  }, []);
+
+  const toggleSlot = (s: string) => update({
+    slots: f.slots.includes(s) ? f.slots.filter(x => x !== s) : [...f.slots, s],
+  });
+  const setRating = (param: string, v: string) => update({
+    ratings: { ...f.ratings, [param]: v === '' ? '' : Number(v) },
+  });
+  const ratingCount = Object.values(f.ratings).filter(v => v !== '').length;
+  const stateCount = [f.equippedOnly, f.bonded, f.attuned, f.rare].filter(Boolean).length;
+  const reqCount = [f.maxLevel, f.minValue, f.minWorkmanship, f.maxBurden].filter(v => v !== '').length;
+  const types: Array<[ItemType, string]> = [['all', 'All items'], ['armor', 'Armor'],
+    ['jewelry', 'Jewelry'], ['weapon', 'Weapons'], ['clothing', 'Clothing'],
+    ['shirt', 'Shirts'], ['pants', 'Pants']];
+
+  return (
+    <div className="inv-sidebar">
+      <CharacterFilter search={search} />
+      <Group title="Item type" defaultOpen badge={f.itemType !== 'all' ? 1 : undefined}>
+        {types.map(([v, label]) => (
+          <label className="inv-fitem" key={v}>
+            <input type="radio" name="inv-t" checked={f.itemType === v}
+              onChange={() => update({ itemType: v, weaponType: '' })} /> {label}
+          </label>
+        ))}
+        {f.itemType === 'weapon' && (
+          <select className="inv-minisearch" value={f.weaponType}
+            onChange={e => update({ weaponType: e.target.value })}>
+            {WEAPON_TYPES.map(w => <option key={w.value} value={w.value}>{w.label}</option>)}
+          </select>
+        )}
+      </Group>
+      <Group title="Slots" defaultOpen badge={f.slots.length || undefined}>
+        {JEWELRY_SLOTS.map(s => (
+          <label className="inv-fitem" key={s}>
+            <input type="checkbox" checked={f.slots.includes(s)} onChange={() => toggleSlot(s)} /> {s}
+          </label>
+        ))}
+        <span className="inv-linky" onClick={() => setShowArmorSlots(v => !v)}>
+          {showArmorSlots ? 'hide' : 'show'} armor slots {showArmorSlots ? '▴' : '▾'}
+        </span>
+        {showArmorSlots && ARMOR_SLOTS.map(s => (
+          <label className="inv-fitem" key={s}>
+            <input type="checkbox" checked={f.slots.includes(s)} onChange={() => toggleSlot(s)} /> {s}
+          </label>
+        ))}
+      </Group>
+      <CantripFilter search={search} />
+      <Group title="Ratings" badge={ratingCount || undefined}>
+        {RATING_DEFS.filter(r => allRatings || r.common).map(r => (
+          <div className="inv-range" key={r.param}>
+            <label>{r.label} ≥</label>
+            <input type="number" value={f.ratings[r.param] ?? ''} placeholder="min"
+              onChange={e => setRating(r.param, e.target.value)} />
+          </div>
+        ))}
+        <span className="inv-linky" onClick={() => setAllRatings(v => !v)}>
+          {allRatings ? 'common ratings ▴' : `all ${RATING_DEFS.length} ratings ▾`}
+        </span>
+      </Group>
+      <Group title="Equipment sets" badge={f.itemSet ? 1 : undefined}>
+        <input className="inv-minisearch" placeholder="find set…" value={setQuery}
+          onChange={e => setSetQuery(e.target.value)} />
+        <label className="inv-fitem">
+          <input type="radio" name="inv-set" checked={f.itemSet === ''}
+            onChange={() => update({ itemSet: '' })} /> Any set
+        </label>
+        {sets.filter(s => s.id.toLowerCase().includes(setQuery.toLowerCase())).map(s => (
+          <label className="inv-fitem" key={s.id}>
+            <input type="radio" name="inv-set" checked={f.itemSet === s.id}
+              onChange={() => update({ itemSet: s.id })} /> {s.id} <span className="inv-dim">({s.item_count})</span>
+          </label>
+        ))}
+      </Group>
+      <Group title="Item state" badge={stateCount || undefined}>
+        <label className="inv-fitem"><input type="checkbox" checked={f.equippedOnly}
+          onChange={e => update({ equippedOnly: e.target.checked })} /> Equipped only</label>
+        <label className="inv-fitem"><input type="checkbox" checked={f.bonded}
+          onChange={e => update({ bonded: e.target.checked })} /> Bonded</label>
+        <label className="inv-fitem"><input type="checkbox" checked={f.attuned}
+          onChange={e => update({ attuned: e.target.checked })} /> Attuned</label>
+        <label className="inv-fitem"><input type="checkbox" checked={f.rare}
+          onChange={e => update({ rare: e.target.checked })} /> Rare</label>
+      </Group>
+      <Group title="Reqs & value" badge={reqCount || undefined}>
+        <div className="inv-range"><label>Wield lvl ≤</label>
+          <input type="number" value={f.maxLevel} placeholder="max"
+            onChange={e => update({ maxLevel: e.target.value === '' ? '' : Number(e.target.value) })} /></div>
+        <div className="inv-range"><label>Value ≥</label>
+          <input type="number" value={f.minValue} placeholder="min"
+            onChange={e => update({ minValue: e.target.value === '' ? '' : Number(e.target.value) })} /></div>
+        <div className="inv-range"><label>Workmanship ≥</label>
+          <input type="number" value={f.minWorkmanship} placeholder="min"
+            onChange={e => update({ minWorkmanship: e.target.value === '' ? '' : Number(e.target.value) })} /></div>
+        <div className="inv-range"><label>Burden ≤</label>
+          <input type="number" value={f.maxBurden} placeholder="max"
+            onChange={e => update({ maxBurden: e.target.value === '' ? '' : Number(e.target.value) })} /></div>
+      </Group>
+    </div>
+  );
+}
diff --git a/frontend/src/styles/inventory.css b/frontend/src/styles/inventory.css
index 5df15221..22689c07 100644
--- a/frontend/src/styles/inventory.css
+++ b/frontend/src/styles/inventory.css
@@ -86,3 +86,9 @@
 .inv-hint { color: #555; font-size: 10px; margin-top: 12px; }
 .inv-closex { float: right; color: #666; cursor: pointer; font-size: 14px; }
 .inv-closex:hover { color: #f88; }
+
+/* ── FilterSidebar additions (Task 4) ── */
+.inv-dim { color: #666; font-size: 10px; }
+.inv-online { width: 6px; height: 6px; border-radius: 50%; background: #4c4; display: inline-block; margin-left: 4px; }
+.inv-links { margin-bottom: 4px; font-size: 10px; }
+.inv-gold { color: #fc6; }