fix(desktop): make English number formatting deterministic

apps/desktop/src/app/session/hooks/use-prompt-actions/utils.test.ts

@@ -1,5 +1,5 @@
 import type { AppendMessage } from '@assistant-ui/react'
-import { describe, expect, it } from 'vitest'
+import { describe, expect, it, vi } from 'vitest'
 
 import type { ChatMessage } from '@/lib/chat-messages'
 
@@ -207,6 +207,18 @@ describe('renderRpcResult', () => {
       )
     })
 
+    it('pins English usage copy to en-US number formatting', () => {
+      const localeSpy = vi.spyOn(Number.prototype, 'toLocaleString').mockImplementation(function (this: number) {
+        return String(this)
+      })
+
+      renderRpcResult({ calls: 12, input: 1_234, output: 56, total: 1_290 }, 'usage')
+
+      expect(localeSpy).toHaveBeenCalledTimes(4)
+      expect(localeSpy.mock.calls).toEqual([['en-US'], ['en-US'], ['en-US'], ['en-US']])
+      localeSpy.mockRestore()
+    })
+
     it('appends credits_lines when present', () => {
       const body = renderRpcResult(
         {

apps/desktop/src/app/session/hooks/use-prompt-actions/utils.ts

@@ -271,7 +271,7 @@ export function renderRpcResult(response: unknown, name: string): string {
     const total = Number(r.total ?? 0)
 
     const lines: string[] = [
-      `Usage: ${calls.toLocaleString()} calls · ${input.toLocaleString()} in / ${output.toLocaleString()} out · ${total.toLocaleString()} total`
+      `Usage: ${calls.toLocaleString('en-US')} calls · ${input.toLocaleString('en-US')} in / ${output.toLocaleString('en-US')} out · ${total.toLocaleString('en-US')} total`
     ]
 
     if (Array.isArray(r.credits_lines)) {

apps/desktop/src/app/settings/billing/billing-amounts.test.ts

@@ -0,0 +1,27 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+
+import { formatMoney } from './billing-amounts'
+
+afterEach(() => {
+  vi.restoreAllMocks()
+})
+
+describe('formatMoney', () => {
+  it('pins hard-coded USD copy to en-US formatting', () => {
+    const OriginalNumberFormat = Intl.NumberFormat
+    const seenLocales: Array<Intl.LocalesArgument | undefined> = []
+
+    vi.spyOn(Intl, 'NumberFormat').mockImplementation(
+      class {
+        constructor(locales?: Intl.LocalesArgument, options?: Intl.NumberFormatOptions) {
+          seenLocales.push(locales)
+
+          return new OriginalNumberFormat(locales ?? 'en-DE', options)
+        }
+      } as typeof Intl.NumberFormat
+    )
+
+    expect(formatMoney(25)).toBe('$25')
+    expect(seenLocales).toEqual(['en-US'])
+  })
+})

apps/desktop/src/app/settings/billing/billing-amounts.ts

@@ -114,7 +114,7 @@ export function formatMoney(value?: null | number | string): string {
     return EMPTY_BILLING_VALUE
   }
 
-  return new Intl.NumberFormat(undefined, {
+  return new Intl.NumberFormat('en-US', {
     currency: 'USD',
     maximumFractionDigits: amount % 1 === 0 ? 0 : 2,
     minimumFractionDigits: amount % 1 === 0 ? 0 : 2,

apps/desktop/src/components/assistant-ui/tool/fallback-model.test.ts

@@ -1,4 +1,4 @@
-import { afterEach, describe, expect, it } from 'vitest'
+import { afterEach, describe, expect, it, vi } from 'vitest'
 
 import { setRuntimeI18nLocale } from '@/i18n'
 
@@ -372,6 +372,15 @@ describe('clampForDisplay', () => {
     expect(clamped).toContain('5,000 more characters truncated')
     expect(clamped).toContain('Copy')
   })
+
+  it('pins the English truncation copy to en-US number formatting', () => {
+    const localeSpy = vi.spyOn(Number.prototype, 'toLocaleString').mockReturnValue('5,000')
+
+    clampForDisplay('x'.repeat(MAX_TOOL_RENDER_CHARS + 5_000))
+
+    expect(localeSpy).toHaveBeenCalledWith('en-US')
+    localeSpy.mockRestore()
+  })
 })
 
 // A large tool result (e.g. a 100KB read_file during a `/learn` run) must not

apps/desktop/src/components/assistant-ui/tool/fallback-model/format.ts

@@ -55,7 +55,7 @@ export function clampForDisplay(value: string, max = MAX_TOOL_RENDER_CHARS): str
 
   const omitted = value.length - max
 
-  return `${value.slice(0, max)}\n\n… ${omitted.toLocaleString()} more characters truncated — use Copy for the full output.`
+  return `${value.slice(0, max)}\n\n… ${omitted.toLocaleString('en-US')} more characters truncated — use Copy for the full output.`
 }
 
 export function prettyJson(value: unknown): string {