diff --git a/src/game/item-upgrade.ts b/src/game/item-upgrade.ts
new file mode 100644
index 0000000..e0c1c20
--- /dev/null
+++ b/src/game/item-upgrade.ts
@@ -0,0 +1,334 @@
+/**
+ * Diablo II Exceptional / Elite Item Base Upgrade System.
+ *
+ * Implements base item quality upgrades during TreasureClass drop generation:
+ * - Reads difficulty parameters from `Difficultylevels.txt` (UberCodeOddsNormal, UberCodeOddsGood,
+ *   UltraCodeOddsNormal, UltraCodeOddsGood for Normal, Nightmare, Hell).
+ * - Implements 1.13c disassembly logic from D2Game!6FC32380 and D2Game!6FC31880:
+ *   - qf[4] (uberCodeOdds) and qf[5] (ultraCodeOdds) checks against rng.rand(1024).
+ *   - Flags 0x04 (exceptional) and 0x10 (elite).
+ *   - Selection priority: elite ultracode -> exceptional ubercode -> original base (normcode).
+ *   - When upgraded, the item base is replaced with the upgraded base, and `isUber` is set to true.
+ */
+import type { D2Table } from './acts.ts'
+import { parseTable } from './acts.ts'
+import type { MountedArchives } from '../mpq/mount.ts'
+import type { ItemBase } from './items.ts'
+import type { D2Rng } from './d2-rng.ts'
+
+/** Canonical MPQ path of `Difficultylevels.txt`. */
+export const DIFFICULTY_LEVELS_TABLE_PATH = 'data\\global\\excel\\Difficultylevels.txt'
+
+/** Bitflag set when exceptional upgrade succeeds (genFlags |= 0x04). */
+export const ITEM_UPGRADE_FLAG_EXCEPTIONAL = 0x04
+
+/** Bitflag set when elite upgrade succeeds (genFlags |= 0x10). */
+export const ITEM_UPGRADE_FLAG_ELITE = 0x10
+
+/**
+ * Difficulty level odds parsed from `Difficultylevels.txt`.
+ */
+export interface DifficultyLevelOdds {
+  readonly name: string
+  readonly uberCodeOddsNormal: number
+  readonly uberCodeOddsGood: number
+  readonly ultraCodeOddsNormal: number
+  readonly ultraCodeOddsGood: number
+  // PascalCase aliases matching exact table column names
+  readonly UberCodeOddsNormal: number
+  readonly UberCodeOddsGood: number
+  readonly UltraCodeOddsNormal: number
+  readonly UltraCodeOddsGood: number
+}
+
+/**
+ * Case-insensitive, whitespace-tolerant Map for looking up items/difficulties by key.
+ */
+export class CaseInsensitiveMap<V> extends Map<string, V> {
+  private normalize(key: string): string {
+    return typeof key === 'string' ? key.trim().toLowerCase() : String(key).trim().toLowerCase()
+  }
+
+  override get(key: string): V | undefined {
+    return super.get(this.normalize(key))
+  }
+
+  override has(key: string): boolean {
+    return super.has(this.normalize(key))
+  }
+
+  override set(key: string, value: V): this {
+    return super.set(this.normalize(key), value)
+  }
+
+  override delete(key: string): boolean {
+    return super.delete(this.normalize(key))
+  }
+}
+
+/** Default canonical odds for standard Diablo II difficulties. */
+export const DEFAULT_DIFFICULTY_LEVEL_ODDS: Record<string, DifficultyLevelOdds> = {
+  Normal: {
+    name: 'Normal',
+    uberCodeOddsNormal: 0,
+    uberCodeOddsGood: 0,
+    ultraCodeOddsNormal: 0,
+    ultraCodeOddsGood: 0,
+    UberCodeOddsNormal: 0,
+    UberCodeOddsGood: 0,
+    UltraCodeOddsNormal: 0,
+    UltraCodeOddsGood: 0,
+  },
+  Nightmare: {
+    name: 'Nightmare',
+    uberCodeOddsNormal: 10,
+    uberCodeOddsGood: 20,
+    ultraCodeOddsNormal: 0,
+    ultraCodeOddsGood: 0,
+    UberCodeOddsNormal: 10,
+    UberCodeOddsGood: 20,
+    UltraCodeOddsNormal: 0,
+    UltraCodeOddsGood: 0,
+  },
+  Hell: {
+    name: 'Hell',
+    uberCodeOddsNormal: 20,
+    uberCodeOddsGood: 40,
+    ultraCodeOddsNormal: 30,
+    ultraCodeOddsGood: 40,
+    UberCodeOddsNormal: 20,
+    UberCodeOddsGood: 40,
+    UltraCodeOddsNormal: 30,
+    UltraCodeOddsGood: 40,
+  },
+}
+
+/**
+ * Parsed collection of difficulty level upgrade odds from `Difficultylevels.txt`.
+ */
+export class DifficultyLevelsTable {
+  readonly all: readonly DifficultyLevelOdds[]
+  readonly byName: Map<string, DifficultyLevelOdds>
+  readonly normal: DifficultyLevelOdds
+  readonly nightmare: DifficultyLevelOdds
+  readonly hell: DifficultyLevelOdds
+  readonly Normal: DifficultyLevelOdds
+  readonly Nightmare: DifficultyLevelOdds
+  readonly Hell: DifficultyLevelOdds
+
+  constructor(all: DifficultyLevelOdds[], byName: Map<string, DifficultyLevelOdds>) {
+    this.all = all
+    this.byName = byName
+
+    const normal = byName.get('Normal') ?? all[0] ?? DEFAULT_DIFFICULTY_LEVEL_ODDS.Normal!
+    const nightmare = byName.get('Nightmare') ?? all[1] ?? DEFAULT_DIFFICULTY_LEVEL_ODDS.Nightmare!
+    const hell = byName.get('Hell') ?? all[2] ?? DEFAULT_DIFFICULTY_LEVEL_ODDS.Hell!
+
+    this.normal = normal
+    this.nightmare = nightmare
+    this.hell = hell
+    this.Normal = normal
+    this.Nightmare = nightmare
+    this.Hell = hell
+  }
+
+  get(name: string): DifficultyLevelOdds | undefined {
+    return this.byName.get(name)
+  }
+
+  [Symbol.iterator](): IterableIterator<DifficultyLevelOdds> {
+    return this.all[Symbol.iterator]()
+  }
+
+  get length(): number {
+    return this.all.length
+  }
+}
+
+/**
+ * Parses `Difficultylevels.txt` table rows into {@link DifficultyLevelsTable}.
+ *
+ * @param table - Raw parsed tab-separated D2Table.
+ * @returns {@link DifficultyLevelsTable} instance.
+ */
+export function parseDifficultyLevelsTable(table: D2Table): DifficultyLevelsTable {
+  const colMap = new Map<string, number>()
+  const lowerMap = new Map<string, number>()
+
+  table.header.forEach((name, idx) => {
+    const trimmed = name.trim()
+    if (!colMap.has(trimmed)) {
+      colMap.set(trimmed, idx)
+    }
+    const lower = trimmed.toLowerCase()
+    if (!lowerMap.has(lower)) {
+      lowerMap.set(lower, idx)
+    }
+  })
+
+  const strAt = (row: readonly string[], colName: string): string => {
+    const idx = colMap.get(colName) ?? lowerMap.get(colName.toLowerCase())
+    return idx !== undefined ? (row[idx] ?? '').trim() : ''
+  }
+
+  const numAt = (row: readonly string[], colName: string, fallback = 0): number => {
+    const raw = strAt(row, colName)
+    if (!raw) return fallback
+    const val = Number(raw)
+    return Number.isFinite(val) ? val : fallback
+  }
+
+  const all: DifficultyLevelOdds[] = []
+  const byName = new CaseInsensitiveMap<DifficultyLevelOdds>()
+
+  for (const row of table.rows) {
+    const name = strAt(row, 'Name')
+    if (!name || name === 'Expansion') {
+      continue
+    }
+
+    const uberCodeOddsNormal = numAt(row, 'UberCodeOddsNormal', 0)
+    const uberCodeOddsGood = numAt(row, 'UberCodeOddsGood', 0)
+    const ultraCodeOddsNormal = numAt(row, 'UltraCodeOddsNormal', 0)
+    const ultraCodeOddsGood = numAt(row, 'UltraCodeOddsGood', 0)
+
+    const entry: DifficultyLevelOdds = {
+      name,
+      uberCodeOddsNormal,
+      uberCodeOddsGood,
+      ultraCodeOddsNormal,
+      ultraCodeOddsGood,
+      UberCodeOddsNormal: uberCodeOddsNormal,
+      UberCodeOddsGood: uberCodeOddsGood,
+      UltraCodeOddsNormal: ultraCodeOddsNormal,
+      UltraCodeOddsGood: ultraCodeOddsGood,
+    }
+
+    all.push(entry)
+    byName.set(name, entry)
+  }
+
+  return new DifficultyLevelsTable(all, byName)
+}
+
+/**
+ * Loads and parses `Difficultylevels.txt` from mounted MPQ archives.
+ *
+ * @param archives - Mounted MPQ archives.
+ * @param path - Optional override MPQ path (defaults to {@link DIFFICULTY_LEVELS_TABLE_PATH}).
+ * @returns Promise resolving to {@link DifficultyLevelsTable}.
+ */
+export async function loadDifficultyLevels(
+  archives: MountedArchives,
+  path = DIFFICULTY_LEVELS_TABLE_PATH,
+): Promise<DifficultyLevelsTable> {
+  const bytes = await archives.read(path)
+  const d2Table = parseTable(bytes)
+  return parseDifficultyLevelsTable(d2Table)
+}
+
+/**
+ * ItemBase with optional tier code linkages (normcode, ubercode, ultracode).
+ */
+export interface UpgradableItemBase extends ItemBase {
+  readonly normcode?: string | undefined
+  readonly ubercode?: string | undefined
+  readonly ultracode?: string | undefined
+  readonly code?: string | undefined
+}
+
+/**
+ * Result of rolling an item base upgrade.
+ */
+export interface BaseUpgradeResult {
+  /** The resulting item base (original or exceptional/elite). */
+  readonly base: ItemBase
+  /** Whether the base was upgraded to exceptional or elite tier. */
+  readonly isUber: boolean
+}
+
+/**
+ * Checks whether a base code is valid for upgrade (non-empty and not 'xxx').
+ */
+function isValidUpgradeCode(code: unknown): code is string {
+  if (typeof code !== 'string') {
+    return false
+  }
+  const trimmed = code.trim()
+  return trimmed.length > 0 && trimmed.toLowerCase() !== 'xxx'
+}
+
+/**
+ * Rolls exceptional / elite base upgrade for an item base.
+ *
+ * Disassembly reference:
+ * - D2Game!6FC32380 (drop roll loop):
+ *   `if (qf[4] && (rng.rand(1024) < qf[4])) genFlags |= 0x04`
+ *   `if (qf[5] && (rng.rand(1024) < qf[5])) genFlags |= 0x10`
+ * - D2Game!6FC31880 (base upgrade dispatch):
+ *   - If genFlags & 0x10 (elite): check base `ultracode`. If valid and !== 'xxx', use ultracode.
+ *   - Else if genFlags & 0x04 (exceptional): check base `ubercode`. If valid and !== 'xxx', use ubercode.
+ *   - Otherwise keep original `normcode`.
+ * - When upgraded:
+ *   - Replaces item base with the exceptional/elite base (providing the new base's `qlvl` / `level`).
+ *   - `isUber` flag is set to true.
+ *
+ * @param base - Original item base (usually normal tier).
+ * @param qf4 - Uber code odds (qf[4], e.g. UberCodeOddsNormal or UberCodeOddsGood).
+ * @param qf5 - Ultra code odds (qf[5], e.g. UltraCodeOddsNormal or UltraCodeOddsGood).
+ * @param rng - Diablo II PRNG instance.
+ * @param lookupBase - Lookup function resolving a 3-4 letter item code to an {@link ItemBase}.
+ * @returns {@link BaseUpgradeResult} containing the new or original base and `isUber`.
+ */
+export function rollBaseUpgrade(
+  base: ItemBase,
+  qf4: number,
+  qf5: number,
+  rng: D2Rng,
+  lookupBase: (code: string) => ItemBase | undefined,
+): BaseUpgradeResult {
+  let genFlags = 0
+
+  if (qf4 > 0 && rng.rand(1024) < qf4) {
+    genFlags |= ITEM_UPGRADE_FLAG_EXCEPTIONAL
+  }
+
+  if (qf5 > 0 && rng.rand(1024) < qf5) {
+    genFlags |= ITEM_UPGRADE_FLAG_ELITE
+  }
+
+  const uBase = base as Partial<UpgradableItemBase>
+  const rawUltra = uBase.ultracode
+  const rawUber = uBase.ubercode
+
+  const ultracode = isValidUpgradeCode(rawUltra) ? rawUltra.trim() : undefined
+  const ubercode = isValidUpgradeCode(rawUber) ? rawUber.trim() : undefined
+
+  let targetCode: string | undefined
+  if ((genFlags & ITEM_UPGRADE_FLAG_ELITE) && ultracode) {
+    targetCode = ultracode
+  } else if ((genFlags & ITEM_UPGRADE_FLAG_EXCEPTIONAL) && ubercode) {
+    targetCode = ubercode
+  }
+
+  if (targetCode) {
+    const upgraded = lookupBase(targetCode) ?? lookupBase(targetCode.toLowerCase())
+    if (upgraded) {
+      return { base: upgraded, isUber: true }
+    }
+    // If elite was selected but lookup failed, fall back to exceptional if rolled and valid
+    if (
+      (genFlags & ITEM_UPGRADE_FLAG_ELITE) &&
+      (genFlags & ITEM_UPGRADE_FLAG_EXCEPTIONAL) &&
+      ubercode &&
+      targetCode !== ubercode
+    ) {
+      const fallbackUber = lookupBase(ubercode) ?? lookupBase(ubercode.toLowerCase())
+      if (fallbackUber) {
+        return { base: fallbackUber, isUber: true }
+      }
+    }
+  }
+
+  return { base, isUber: false }
+}
diff --git a/tests/item-upgrade.test.ts b/tests/item-upgrade.test.ts
new file mode 100644
index 0000000..27f8652
--- /dev/null
+++ b/tests/item-upgrade.test.ts
@@ -0,0 +1,438 @@
+import { describe, it, expect, beforeAll, vi } from 'vitest'
+import * as fs from 'fs'
+import { MountedArchives } from '../src/mpq/mount.ts'
+import { MpqArchive } from '../src/mpq/archive.ts'
+import { fileSource } from '../src/mpq/file-source.ts'
+import type { D2Table } from '../src/game/acts.ts'
+import { parseTable } from '../src/game/acts.ts'
+import type { ItemBase } from '../src/game/items.ts'
+import { D2Rng } from '../src/game/d2-rng.ts'
+import { loadArmor, ArmorTable } from '../src/game/armor.ts'
+import { loadWeapons, type WeaponTable } from '../src/game/weapons.ts'
+import {
+  DIFFICULTY_LEVELS_TABLE_PATH,
+  ITEM_UPGRADE_FLAG_EXCEPTIONAL,
+  ITEM_UPGRADE_FLAG_ELITE,
+  DifficultyLevelsTable,
+  parseDifficultyLevelsTable,
+  loadDifficultyLevels,
+  rollBaseUpgrade,
+  type UpgradableItemBase,
+} from '../src/game/item-upgrade.ts'
+
+const hasD2 = fs.existsSync('samples/d2/d2data.mpq')
+
+/**
+ * Creates a minimal fallback D2Table for Difficultylevels.txt
+ * matching the canonical 1.13c table values.
+ */
+function createMinimalDifficultyLevelsTable(): D2Table {
+  const header = [
+    'Name',
+    'ResistPenalty',
+    'DeathExpPenalty',
+    'UberCodeOddsNormal',
+    'UberCodeOddsGood',
+    'UltraCodeOddsNormal',
+    'UltraCodeOddsGood',
+  ]
+  const rows = [
+    ['Normal', '0', '0', '0', '0', '0', '0'],
+    ['Nightmare', '-40', '5', '10', '20', '0', '0'],
+    ['Hell', '-100', '10', '20', '40', '30', '40'],
+  ]
+  return { header, rows }
+}
+
+describe('Item Base Upgrade System (Issue #103)', () => {
+  let diffTable: DifficultyLevelsTable
+  let armorTable: ArmorTable | undefined
+  let weaponTable: WeaponTable | undefined
+
+  beforeAll(async () => {
+    if (hasD2) {
+      const archives = new MountedArchives()
+      for (const name of ['d2data.mpq', 'd2exp.mpq', 'Patch_D2.mpq']) {
+        archives.add(name, await MpqArchive.open(await fileSource(`samples/d2/${name}`)))
+      }
+      diffTable = await loadDifficultyLevels(archives)
+      armorTable = await loadArmor(archives)
+      weaponTable = await loadWeapons(archives)
+    } else {
+      diffTable = parseDifficultyLevelsTable(createMinimalDifficultyLevelsTable())
+    }
+  })
+
+  describe('Difficultylevels.txt loader & parser', () => {
+    it('defines canonical MPQ path', () => {
+      expect(DIFFICULTY_LEVELS_TABLE_PATH).toBe('data\\global\\excel\\Difficultylevels.txt')
+    })
+
+    it('loads and parses DifficultyLevelsTable with correct values', () => {
+      expect(diffTable).toBeInstanceOf(DifficultyLevelsTable)
+      expect(diffTable.length).toBeGreaterThanOrEqual(3)
+
+      // Normal: 0, 0, 0, 0
+      const normal = diffTable.normal
+      expect(normal).toBeDefined()
+      expect(normal.name).toBe('Normal')
+      expect(normal.uberCodeOddsNormal).toBe(0)
+      expect(normal.uberCodeOddsGood).toBe(0)
+      expect(normal.ultraCodeOddsNormal).toBe(0)
+      expect(normal.ultraCodeOddsGood).toBe(0)
+      expect(normal.UberCodeOddsNormal).toBe(0)
+      expect(normal.UberCodeOddsGood).toBe(0)
+      expect(normal.UltraCodeOddsNormal).toBe(0)
+      expect(normal.UltraCodeOddsGood).toBe(0)
+
+      // Nightmare: 10, 20, 0, 0
+      const nightmare = diffTable.nightmare
+      expect(nightmare).toBeDefined()
+      expect(nightmare.name).toBe('Nightmare')
+      expect(nightmare.uberCodeOddsNormal).toBe(10)
+      expect(nightmare.uberCodeOddsGood).toBe(20)
+      expect(nightmare.ultraCodeOddsNormal).toBe(0)
+      expect(nightmare.ultraCodeOddsGood).toBe(0)
+      expect(nightmare.UberCodeOddsNormal).toBe(10)
+      expect(nightmare.UberCodeOddsGood).toBe(20)
+      expect(nightmare.UltraCodeOddsNormal).toBe(0)
+      expect(nightmare.UltraCodeOddsGood).toBe(0)
+
+      // Hell: 20, 40, 30, 40
+      const hell = diffTable.hell
+      expect(hell).toBeDefined()
+      expect(hell.name).toBe('Hell')
+      expect(hell.uberCodeOddsNormal).toBe(20)
+      expect(hell.uberCodeOddsGood).toBe(40)
+      expect(hell.ultraCodeOddsNormal).toBe(30)
+      expect(hell.ultraCodeOddsGood).toBe(40)
+      expect(hell.UberCodeOddsNormal).toBe(20)
+      expect(hell.UberCodeOddsGood).toBe(40)
+      expect(hell.UltraCodeOddsNormal).toBe(30)
+      expect(hell.UltraCodeOddsGood).toBe(40)
+    })
+
+    it('supports case-insensitive lookup by difficulty name', () => {
+      expect(diffTable.get('normal')?.uberCodeOddsNormal).toBe(0)
+      expect(diffTable.get('Nightmare')?.uberCodeOddsGood).toBe(20)
+      expect(diffTable.get('HELL')?.ultraCodeOddsGood).toBe(40)
+    })
+
+    it('exposes direct properties for Normal, Nightmare, and Hell', () => {
+      expect(diffTable.Normal.name).toBe('Normal')
+      expect(diffTable.Nightmare.name).toBe('Nightmare')
+      expect(diffTable.Hell.name).toBe('Hell')
+    })
+  })
+
+  describe('rollBaseUpgrade logic', () => {
+    // Mock base items for isolated testing
+    const capNormal: UpgradableItemBase = {
+      id: 'cap',
+      name: 'Cap',
+      kind: 'armor',
+      invWidth: 2,
+      invHeight: 2,
+      maxStack: 1,
+      value: 12,
+      damage: 0,
+      defense: 5,
+      tags: ['helm'],
+      level: 1, // qlvl 1
+      normcode: 'cap',
+      ubercode: 'xap',
+      ultracode: 'uap',
+    }
+
+    const warHatExceptional: UpgradableItemBase = {
+      id: 'xap',
+      name: 'War Hat',
+      kind: 'armor',
+      invWidth: 2,
+      invHeight: 2,
+      maxStack: 1,
+      value: 800,
+      damage: 0,
+      defense: 53,
+      tags: ['helm'],
+      level: 34, // qlvl 34
+      normcode: 'cap',
+      ubercode: 'xap',
+      ultracode: 'uap',
+    }
+
+    const shakoElite: UpgradableItemBase = {
+      id: 'uap',
+      name: 'Shako',
+      kind: 'armor',
+      invWidth: 2,
+      invHeight: 2,
+      maxStack: 1,
+      value: 2400,
+      damage: 0,
+      defense: 141,
+      tags: ['helm'],
+      level: 58, // qlvl 58
+      normcode: 'cap',
+      ubercode: 'xap',
+      ultracode: 'uap',
+    }
+
+    const mockLookup = (code: string): ItemBase | undefined => {
+      const normalized = code.trim().toLowerCase()
+      if (normalized === 'cap') return capNormal
+      if (normalized === 'xap') return warHatExceptional
+      if (normalized === 'uap') return shakoElite
+      return undefined
+    }
+
+    it('Normal difficulty: odds are 0 -> base never upgrades and consumes 0 RNG rolls', () => {
+      const rng = new D2Rng(12345)
+      const randSpy = vi.spyOn(rng, 'rand')
+
+      // Test with Normal difficulty odds (0, 0)
+      const result = rollBaseUpgrade(capNormal, 0, 0, rng, mockLookup)
+      expect(result.base).toBe(capNormal)
+      expect(result.base.id).toBe('cap')
+      expect(result.base.level).toBe(1)
+      expect(result.isUber).toBe(false)
+      expect(randSpy).not.toHaveBeenCalled()
+
+      // Repeat multiple times with different seeds
+      for (let seed = 1; seed <= 50; seed++) {
+        const testRng = new D2Rng(seed)
+        const res = rollBaseUpgrade(
+          capNormal,
+          diffTable.normal.uberCodeOddsNormal,
+          diffTable.normal.ultraCodeOddsNormal,
+          testRng,
+          mockLookup,
+        )
+        expect(res.base.id).toBe('cap')
+        expect(res.isUber).toBe(false)
+      }
+    })
+
+    it('Test odds = 1024 -> base 100% upgrades to ubercode / ultracode', () => {
+      // 1. Exceptional 100% upgrade (qf4 = 1024, qf5 = 0)
+      const rng1 = new D2Rng(999)
+      const resultUber = rollBaseUpgrade(capNormal, 1024, 0, rng1, mockLookup)
+      expect(resultUber.base).toBe(warHatExceptional)
+      expect(resultUber.base.id).toBe('xap')
+      expect(resultUber.isUber).toBe(true)
+
+      // 2. Elite 100% upgrade (qf4 = 0, qf5 = 1024)
+      const rng2 = new D2Rng(888)
+      const resultUltra = rollBaseUpgrade(capNormal, 0, 1024, rng2, mockLookup)
+      expect(resultUltra.base).toBe(shakoElite)
+      expect(resultUltra.base.id).toBe('uap')
+      expect(resultUltra.isUber).toBe(true)
+
+      // 3. Both 100% (qf4 = 1024, qf5 = 1024) -> elite takes priority
+      const rng3 = new D2Rng(777)
+      const resultBoth = rollBaseUpgrade(capNormal, 1024, 1024, rng3, mockLookup)
+      expect(resultBoth.base).toBe(shakoElite)
+      expect(resultBoth.base.id).toBe('uap')
+      expect(resultBoth.isUber).toBe(true)
+    })
+
+    it('Handle empty or "xxx" gracefully without upgrading', () => {
+      const rng = new D2Rng(42)
+
+      // Base with 'xxx' for both ubercode and ultracode
+      const xxxBase: UpgradableItemBase = {
+        id: 'xxxitem',
+        name: 'No Upgrades Item',
+        kind: 'armor',
+        invWidth: 1,
+        invHeight: 1,
+        maxStack: 1,
+        value: 10,
+        damage: 0,
+        defense: 1,
+        tags: [],
+        level: 5,
+        normcode: 'xxxitem',
+        ubercode: 'xxx',
+        ultracode: 'XXX',
+      }
+
+      // Even with 100% odds, it must not upgrade
+      const resXxx = rollBaseUpgrade(xxxBase, 1024, 1024, rng, () => undefined)
+      expect(resXxx.base).toBe(xxxBase)
+      expect(resXxx.isUber).toBe(false)
+
+      // Base with empty strings
+      const emptyBase: UpgradableItemBase = {
+        ...xxxBase,
+        ubercode: '',
+        ultracode: '   ',
+      }
+      const resEmpty = rollBaseUpgrade(emptyBase, 1024, 1024, rng, () => undefined)
+      expect(resEmpty.base).toBe(emptyBase)
+      expect(resEmpty.isUber).toBe(false)
+
+      // Base without ubercode or ultracode fields
+      const regularItemBase: ItemBase = {
+        id: 'gold',
+        name: 'Gold',
+        kind: 'misc',
+        invWidth: 1,
+        invHeight: 1,
+        maxStack: 50000,
+        value: 1,
+        damage: 0,
+        defense: 0,
+        tags: [],
+        level: 1,
+      }
+      const resReg = rollBaseUpgrade(regularItemBase, 1024, 1024, rng, mockLookup)
+      expect(resReg.base).toBe(regularItemBase)
+      expect(resReg.isUber).toBe(false)
+
+      // Ultracode is 'xxx' but ubercode is valid: falls back to exceptional
+      const mixedBase: UpgradableItemBase = {
+        ...capNormal,
+        ubercode: 'xap',
+        ultracode: 'xxx',
+      }
+      const resMixed = rollBaseUpgrade(mixedBase, 1024, 1024, rng, mockLookup)
+      expect(resMixed.base).toBe(warHatExceptional)
+      expect(resMixed.base.id).toBe('xap')
+      expect(resMixed.isUber).toBe(true)
+
+      // Lookup returns undefined (unrecognized code): gracefully keeps original base
+      const unresolvableBase: UpgradableItemBase = {
+        ...capNormal,
+        ubercode: 'nonexistent',
+        ultracode: 'unknown',
+      }
+      const resUnresolved = rollBaseUpgrade(unresolvableBase, 1024, 1024, rng, () => undefined)
+      expect(resUnresolved.base).toBe(unresolvableBase)
+      expect(resUnresolved.isUber).toBe(false)
+    })
+
+    it('RNG consumption: exactly two rand(1024) rolls', () => {
+      const rng = new D2Rng(100)
+      const randSpy = vi.spyOn(rng, 'rand')
+
+      // Call with non-zero qf4 and qf5
+      rollBaseUpgrade(capNormal, 20, 40, rng, mockLookup)
+
+      expect(randSpy).toHaveBeenCalledTimes(2)
+      expect(randSpy).toHaveBeenNthCalledWith(1, 1024)
+      expect(randSpy).toHaveBeenNthCalledWith(2, 1024)
+    })
+
+    it('Switch to new base qlvl and sets isUber = true', () => {
+      const rng = new D2Rng(123)
+
+      // 1. Exceptional upgrade changes base to War Hat (qlvl 34, isUber = true)
+      const resExceptional = rollBaseUpgrade(capNormal, 1024, 0, rng, mockLookup)
+      expect(resExceptional.isUber).toBe(true)
+      expect(resExceptional.base.level).toBe(34) // Switches to War Hat qlvl
+      expect(resExceptional.base.defense).toBe(53)
+
+      // 2. Elite upgrade changes base to Shako (qlvl 58, isUber = true)
+      const resElite = rollBaseUpgrade(capNormal, 0, 1024, rng, mockLookup)
+      expect(resElite.isUber).toBe(true)
+      expect(resElite.base.level).toBe(58) // Switches to Shako qlvl
+      expect(resElite.base.defense).toBe(141)
+
+      // 3. Non-upgraded base retains original qlvl and isUber = false
+      const resNormal = rollBaseUpgrade(capNormal, 0, 0, rng, mockLookup)
+      expect(resNormal.isUber).toBe(false)
+      expect(resNormal.base.level).toBe(1) // Cap qlvl
+    })
+
+    if (hasD2) {
+      it('integrates with real Armor.txt bases and verifies tier progression', () => {
+        expect(armorTable).toBeDefined()
+        const cap = armorTable!.get('cap')
+        expect(cap).toBeDefined()
+        expect(cap?.ubercode).toBe('xap')
+        expect(cap?.ultracode).toBe('uap')
+        expect(cap?.level).toBe(1)
+
+        const lookup = (code: string) => armorTable!.get(code)
+        const rng = new D2Rng(54321)
+
+        // Upgrade to exceptional
+        const uberResult = rollBaseUpgrade(cap!, 1024, 0, rng, lookup)
+        expect(uberResult.isUber).toBe(true)
+        expect(uberResult.base.id).toBe('xap')
+        expect(uberResult.base.level).toBe(34) // War Hat qlvl
+
+        // Upgrade to elite
+        const eliteResult = rollBaseUpgrade(cap!, 0, 1024, rng, lookup)
+        expect(eliteResult.isUber).toBe(true)
+        expect(eliteResult.base.id).toBe('uap')
+        expect(eliteResult.base.level).toBe(58) // Shako qlvl
+      })
+
+      it('integrates with real Weapons.txt bases and verifies tier progression', () => {
+        expect(weaponTable).toBeDefined()
+        const handAxe = weaponTable!.get('hax')
+        expect(handAxe).toBeDefined()
+        expect(handAxe?.ubercode).toBe('9ha')
+        expect(handAxe?.ultracode).toBe('7ha')
+        expect(handAxe?.level).toBe(3)
+
+        const lookup = (code: string) => weaponTable!.get(code)
+        const rng = new D2Rng(65432)
+
+        // Upgrade to exceptional (Hatchet)
+        const uberResult = rollBaseUpgrade(handAxe!, 1024, 0, rng, lookup)
+        expect(uberResult.isUber).toBe(true)
+        expect(uberResult.base.id).toBe('9ha')
+        expect(uberResult.base.level).toBe(31) // Hatchet qlvl
+
+        // Upgrade to elite (Tomahawk)
+        const eliteResult = rollBaseUpgrade(handAxe!, 0, 1024, rng, lookup)
+        expect(eliteResult.isUber).toBe(true)
+        expect(eliteResult.base.id).toBe('7ha')
+        expect(eliteResult.base.level).toBe(54) // Tomahawk qlvl
+      })
+    }
+  })
+
+  describe('Disassembly alignment verification (D2Game!6FC32380 & 6FC31880)', () => {
+    it('defines bitflags 0x04 for exceptional and 0x10 for elite', () => {
+      expect(ITEM_UPGRADE_FLAG_EXCEPTIONAL).toBe(0x04)
+      expect(ITEM_UPGRADE_FLAG_ELITE).toBe(0x10)
+    })
+
+    it('verifies rand < odds threshold condition with specific seed', () => {
+      const base: UpgradableItemBase = {
+        id: 'tst',
+        name: 'Test',
+        kind: 'armor',
+        invWidth: 1,
+        invHeight: 1,
+        maxStack: 1,
+        value: 10,
+        damage: 0,
+        defense: 1,
+        tags: [],
+        level: 1,
+        ubercode: 'xtst',
+        ultracode: 'utst',
+      }
+      const xtst: ItemBase = { ...base, id: 'xtst', level: 25 }
+      const utst: ItemBase = { ...base, id: 'utst', level: 50 }
+      const lookup = (c: string) => (c === 'xtst' ? xtst : c === 'utst' ? utst : undefined)
+
+      // Test seed where we know exact rolls
+      const rng = new D2Rng(0)
+      const roll1 = rng.rand(1024)
+      const roll2 = rng.rand(1024)
+
+      // Reset seed to replay exact rolls
+      const testRng = new D2Rng(0)
+      // Set qf4 to roll1 (so roll1 < qf4 is false) and qf5 to roll2 + 1 (so roll2 < qf5 is true)
+      const res = rollBaseUpgrade(base, roll1, roll2 + 1, testRng, lookup)
+      expect(res.base.id).toBe('utst')
+      expect(res.isUber).toBe(true)
+    })
+  })
+})