diff --git a/src/commands/connect.ts b/src/commands/connect.ts
index ec72f6a..b34dd57 100644
--- a/src/commands/connect.ts
+++ b/src/commands/connect.ts
@@ -8,6 +8,7 @@ import { requireToken, resolveApiBase, resolveAppBase } from '../lib/config'
 import { runAction } from '../lib/output'
 import { sessionUrl } from '../lib/urls'
 import { makeOpenSocket, resolveWsBase } from '../lib/stream'
+import { applyDetectedThemeMode } from '../lib/terminalBackground'
 import { ConnectApp } from '../ui/ConnectApp'
 import { canHostSessionsUi, defaultStartRequest, runSessionsUi } from '../ui/launch'
 
@@ -99,6 +100,8 @@ export async function runConnect(
   const [{ session }, me] = await Promise.all([
     client.sessions.get(sessionId),
     client.me(),
+    // Pick the palette for this terminal's background before the first frame.
+    applyDetectedThemeMode(),
   ])
   const c = connectability(session)
   // --no-input forces watch-only even when the session would accept messages.
diff --git a/src/lib/terminalBackground.ts b/src/lib/terminalBackground.ts
new file mode 100644
index 0000000..018f705
--- /dev/null
+++ b/src/lib/terminalBackground.ts
@@ -0,0 +1,91 @@
+import { applyThemeMode, type ThemeMode } from './theme'
+
+// Which palette to render: ask the terminal what its background is, the same
+// way Claude Code's "auto" theme does. Order of trust:
+//
+//   1. OSC 11 — the terminal reports its actual background color. Supported
+//      by every mainstream emulator (iTerm2, Terminal.app, kitty, WezTerm,
+//      Ghostty, Windows Terminal, VS Code); tmux passes it through.
+//   2. COLORFGBG — a legacy env hint ("15;0" = light-on-dark) set by rxvt and
+//      a few others. Stale after a mid-session theme change, but better than
+//      guessing.
+//   3. dark — the brand's main mode and the safe default when nothing answers.
+//
+// Called once at each UI entry point, BEFORE the first Ink render, so no frame
+// ever paints in the wrong palette and nothing needs to re-render on the
+// answer.
+
+// OSC 11 reply: `\x1b]11;rgb:RRRR/GGGG/BBBB` terminated by BEL or ST, where
+// each channel is 1-4 hex digits scaled to 16 bits.
+const OSC_REPLY = /\x1b\]11;rgb:([0-9a-f]{1,4})\/([0-9a-f]{1,4})\/([0-9a-f]{1,4})/i
+
+function channelToUnit(hex: string): number {
+  return parseInt(hex, 16) / (16 ** hex.length - 1)
+}
+
+export function modeFromOscReply(reply: string): ThemeMode | null {
+  const m = OSC_REPLY.exec(reply)
+  if (!m) return null
+  const [r, g, b] = [m[1]!, m[2]!, m[3]!].map(channelToUnit) as [number, number, number]
+  // Perceived luminance (Rec. 601). The cut is the midpoint: a background
+  // brighter than half is a light theme.
+  return 0.299 * r + 0.587 * g + 0.114 * b > 0.5 ? 'light' : 'dark'
+}
+
+// COLORFGBG is "fg;bg" or "fg;default;bg"; the LAST field is the background's
+// ANSI-16 index. 7 and 15 are the whites; everything else is dark or unknown.
+export function modeFromColorFgBg(value: string | undefined): ThemeMode | null {
+  const bg = value?.trim().split(';').pop()
+  if (!bg || !/^\d+$/.test(bg)) return null
+  return bg === '7' || bg === '15' ? 'light' : 'dark'
+}
+
+function queryOsc11(timeoutMs = 200): Promise<ThemeMode | null> {
+  return new Promise((resolve) => {
+    const { stdin, stdout } = process
+    if (!stdin.isTTY || !stdout.isTTY || typeof stdin.setRawMode !== 'function') {
+      resolve(null)
+      return
+    }
+    const wasRaw = stdin.isRaw
+    let buffer = ''
+    let settled = false
+    const finish = (mode: ThemeMode | null): void => {
+      if (settled) return
+      settled = true
+      clearTimeout(timer)
+      stdin.off('data', onData)
+      stdin.setRawMode(wasRaw)
+      // Ink attaches its own stdin handling after this; leave the stream
+      // paused so a bare detection doesn't hold the process open.
+      stdin.pause()
+      resolve(mode)
+    }
+    const onData = (chunk: Buffer): void => {
+      buffer += chunk.toString('latin1')
+      const mode = modeFromOscReply(buffer)
+      if (mode) finish(mode)
+      else if (buffer.length > 64) finish(null)
+    }
+    const timer = setTimeout(() => finish(null), timeoutMs)
+    stdin.setRawMode(true)
+    stdin.resume()
+    stdin.on('data', onData)
+    stdout.write('\x1b]11;?\x1b\\')
+  })
+}
+
+// Detect and apply, once, at a UI entry point. Never throws — a terminal that
+// answers strangely just keeps the dark default.
+export async function applyDetectedThemeMode(): Promise<ThemeMode> {
+  let mode: ThemeMode | null = null
+  try {
+    mode = await queryOsc11()
+  } catch {
+    mode = null
+  }
+  mode ??= modeFromColorFgBg(process.env.COLORFGBG)
+  mode ??= 'dark'
+  applyThemeMode(mode)
+  return mode
+}
diff --git a/src/lib/theme.ts b/src/lib/theme.ts
index 3e358d1..06301f5 100644
--- a/src/lib/theme.ts
+++ b/src/lib/theme.ts
@@ -1,19 +1,21 @@
 import chalk from 'chalk'
 
-// The Ellipsis brand palette, dark mode — the CLI's one source of color.
+// The Ellipsis brand palette — the CLI's one source of color.
 //
 // These hexes are COPIES of brand/tokens.json in the ellipsis monorepo (the
 // canonical source). This repo can't reach that file, so when a brand color
 // changes there, it has to be re-copied here by hand.
 //
-// Dark mode is the brand's main mode, which is what a terminal is, so the CLI
-// only ever renders the dark palette — there is no light variant to switch to.
+// The CLI carries BOTH brand modes and picks one at startup by asking the
+// terminal for its background (OSC 11, then COLORFGBG, then dark — see
+// lib/terminalBackground.ts). Dark is the default because it is the brand's
+// main mode and the safe guess when the terminal won't say.
 //
 // One rule carried over from the web app (landing globals.css `.dark`): the
 // accent in dark mode is BONE, not brand blue. Brand ink #175173 scores 1.79:1
-// on a dark surface — unreadable as terminal text. So emphasis is carried by
-// brightness (bone against stone), not by hue. The ▶ cursor is the one
-// exception, and takes `cursor` below.
+// on a dark surface — unreadable as terminal text. So dark emphasis is carried
+// by brightness (bone against stone), not by hue. In light mode the brand ink
+// IS legible (8.5:1 on white), so the cursor takes it there.
 //
 // EXACTLY ONE SURFACE IS PAINTED: the composer's (`inputSurface` below). The CLI
 // used to paint a canvas behind everything and lift panels onto it, which worked
@@ -35,41 +37,87 @@ import chalk from 'chalk'
 // takes `muted`, never a bare `dimColor`. (dim is fine ON TOP of an explicit
 // colour, where it only shades a known hue.)
 
-export const theme = {
-  // Type. `foreground` is body copy and doubles as the accent (see above);
-  // `muted` is every secondary string (meta, hints, timestamps) — and, since
-  // the rule above rules out a bare `dimColor`, it is also how a quiet line
-  // reads quiet. 7.4:1 on the brand charcoal, so quiet still means legible.
-  foreground: '#f0efe9',
-  muted: '#a8a59c',
+export interface Palette {
+  // Type. `foreground` is body copy and doubles as the accent; `muted` is
+  // every secondary string (meta, hints, timestamps) — and, since the rule
+  // above rules out a bare `dimColor`, it is also how a quiet line reads
+  // quiet, while staying legible on its canvas.
+  foreground: string
+  muted: string
 
   // The ▶ cursor, and nothing else — which, with no highlight bar to fall back
-  // on, is now the ONLY thing that says "you are here". Bone-on-stone was too
-  // quiet a step to find at a glance, so the cursor carries HUE as well as
-  // brightness: cyan is the one hue not already spoken for (green = done, amber
-  // = working, red = failed), so it never reads as a status.
-  cursor: '#5fd3e0',
+  // on, is the ONLY thing that says "you are here". It carries HUE as well as
+  // brightness: a hue not already spoken for (green = done, amber = working,
+  // red = failed), so it never reads as a status.
+  cursor: string
 
   // Status.
-  success: '#4ebc7b',
-  error: '#e5544b',
+  success: string
+  error: string
   // In-flight. brand/tokens.json has no dedicated "working" color; this is
   // syntaxLiteral, the warm amber, which is the only brand hue that reads as
   // activity without colliding with success green or error red.
-  active: '#d9bd8d',
+  active: string
 
   // Syntax, for rendered markdown in a transcript. Same values the web apps
   // use for code blocks, so a snippet reads the same in the CLI as in the docs.
+  syntaxLiteral: string
+  syntaxString: string
+}
+
+// brand/tokens.json `dark` values. muted is 7.4:1 on the brand charcoal.
+// cursor is the one non-brand hex: cyan, the hue no status owns (see above).
+export const darkPalette: Palette = {
+  foreground: '#f0efe9',
+  muted: '#a8a59c',
+  cursor: '#5fd3e0',
+  success: '#4ebc7b',
+  error: '#e5544b',
+  active: '#d9bd8d',
   syntaxLiteral: '#d9bd8d',
   syntaxString: '#c8c6bc',
 }
 
-// The composer's fill — the app's ONE painted surface (see the note above for why
-// it is the only one that can be). The brand panel step, neutralized: chalk sends
-// any hex whose channels differ to the 6x6x6 colour cube, whose darkest step
-// above black is rgb(95,95,95), so the authored warm #262523 paints as a MID GREY
-// slab on a terminal that does 256 colours but not truecolor (Terminal.app, tmux
-// without RGB, mosh, conhost). Equal channels route to the greyscale ramp
-// instead, where a near-black stays near-black. Truecolor terminals lose only the
-// warmth, which is invisible at this brightness.
-export const inputSurface = chalk.level >= 3 ? '#262523' : '#252525'
+// brand/tokens.json `light` values. cursor is the brand ink accent — in light
+// mode it is legible AND it is exactly what the accent means on the web apps
+// (interactive, never status). active mirrors dark by borrowing syntaxLiteral.
+export const lightPalette: Palette = {
+  foreground: '#1c1b17',
+  muted: '#706f66',
+  cursor: '#175173',
+  success: '#10b981',
+  error: '#dc2626',
+  active: '#8a6d2a',
+  syntaxLiteral: '#8a6d2a',
+  syntaxString: '#56544b',
+}
+
+// The live palette. MUTATED IN PLACE by applyThemeMode so every existing
+// `theme.foreground` call site follows the mode with no plumbing; the mode is
+// set once at startup, before the first render, and never after.
+export const theme: Palette = { ...darkPalette }
+
+// The composer's fill — the app's ONE painted surface (see the note above for
+// why it is the only one that can be). The brand panel step, neutralized: chalk
+// sends any hex whose channels differ to the 6x6x6 colour cube, whose darkest
+// step above black is rgb(95,95,95), so the authored warm dark #262523 paints
+// as a MID GREY slab on a terminal that does 256 colours but not truecolor
+// (Terminal.app, tmux without RGB, mosh, conhost). Equal channels route to the
+// greyscale ramp instead, where a near-black stays near-black — and, in light
+// mode, a near-white sand (brand border #e3e1d8) stays near-white. Truecolor
+// terminals lose only the warmth, which is invisible at this brightness.
+export let inputSurface = chalk.level >= 3 ? '#262523' : '#252525'
+
+export type ThemeMode = 'light' | 'dark'
+
+export function applyThemeMode(mode: ThemeMode): void {
+  Object.assign(theme, mode === 'light' ? lightPalette : darkPalette)
+  inputSurface =
+    mode === 'light'
+      ? chalk.level >= 3
+        ? '#e3e1d8'
+        : '#e4e4e4'
+      : chalk.level >= 3
+        ? '#262523'
+        : '#252525'
+}
diff --git a/src/ui/launch.tsx b/src/ui/launch.tsx
index dffa953..2824ec8 100644
--- a/src/ui/launch.tsx
+++ b/src/ui/launch.tsx
@@ -4,6 +4,7 @@ import { api } from '../lib/api'
 import { requireToken, resolveApiBase, resolveAppBase, sessionBar } from '../lib/config'
 import { repoFromCwd } from '../lib/laptop'
 import { makeOpenSocket, resolveWsBase } from '../lib/stream'
+import { applyDetectedThemeMode } from '../lib/terminalBackground'
 import type { StartAgentSessionRequest } from '../lib/types'
 import { SessionsApp } from './SessionsApp'
 
@@ -51,7 +52,8 @@ export async function runSessionsUi(options: SessionsUiOptions): Promise<void> {
   const client = api()
   const token = requireToken()
   const openSocket = makeOpenSocket(token, resolveWsBase(resolveApiBase()))
-  const me = await client.me()
+  // Pick the palette for this terminal's background before the first frame.
+  const [me] = await Promise.all([client.me(), applyDetectedThemeMode()])
 
   // No screen-clearing dance: the chat prints its settled transcript into THIS
   // terminal's scrollback (see ConnectApp), so the conversation grows down the
diff --git a/test/terminal-background.test.ts b/test/terminal-background.test.ts
new file mode 100644
index 0000000..e070a7a
--- /dev/null
+++ b/test/terminal-background.test.ts
@@ -0,0 +1,49 @@
+import { describe, expect, it } from 'vitest'
+import { modeFromColorFgBg, modeFromOscReply } from '../src/lib/terminalBackground'
+import { applyThemeMode, darkPalette, lightPalette, theme } from '../src/lib/theme'
+
+describe('modeFromOscReply', () => {
+  it('reads a light background from a 16-bit-per-channel reply', () => {
+    expect(modeFromOscReply('\x1b]11;rgb:ffff/ffff/ffff\x07')).toBe('light')
+    expect(modeFromOscReply('\x1b]11;rgb:fdfd/f6f6/e3e3\x1b\\')).toBe('light')
+  })
+
+  it('reads a dark background', () => {
+    expect(modeFromOscReply('\x1b]11;rgb:1c1c/1b1b/1a1a\x07')).toBe('dark')
+    expect(modeFromOscReply('\x1b]11;rgb:0000/0000/0000\x07')).toBe('dark')
+  })
+
+  it('handles short channel widths', () => {
+    expect(modeFromOscReply('\x1b]11;rgb:ff/ff/ff\x07')).toBe('light')
+    expect(modeFromOscReply('\x1b]11;rgb:0/0/0\x07')).toBe('dark')
+  })
+
+  it('rejects anything that is not an OSC 11 color reply', () => {
+    expect(modeFromOscReply('')).toBeNull()
+    expect(modeFromOscReply('\x1b[6n')).toBeNull()
+    expect(modeFromOscReply('\x1b]11;?\x07')).toBeNull()
+  })
+})
+
+describe('modeFromColorFgBg', () => {
+  it('reads the background from the last field', () => {
+    expect(modeFromColorFgBg('0;15')).toBe('light')
+    expect(modeFromColorFgBg('15;0')).toBe('dark')
+    expect(modeFromColorFgBg('0;default;7')).toBe('light')
+  })
+
+  it('returns null when unset or malformed', () => {
+    expect(modeFromColorFgBg(undefined)).toBeNull()
+    expect(modeFromColorFgBg('')).toBeNull()
+    expect(modeFromColorFgBg('default')).toBeNull()
+  })
+})
+
+describe('applyThemeMode', () => {
+  it('swaps the live palette in place, so existing imports follow', () => {
+    applyThemeMode('light')
+    expect(theme.foreground).toBe(lightPalette.foreground)
+    applyThemeMode('dark')
+    expect(theme.foreground).toBe(darkPalette.foreground)
+  })
+})