diff --git a/ui-tui/src/__tests__/textInputFastEcho.test.ts b/ui-tui/src/__tests__/textInputFastEcho.test.ts
index 6221314a062..98928d1baf1 100644
--- a/ui-tui/src/__tests__/textInputFastEcho.test.ts
+++ b/ui-tui/src/__tests__/textInputFastEcho.test.ts
@@ -178,6 +178,43 @@ describe('supportsFastEchoTerminal', () => {
     expect(supportsFastEchoTerminal({ TERM_PROGRAM: 'Apple_Terminal' } as NodeJS.ProcessEnv)).toBe(false)
   })
 
+  it('disables fast-echo inside tmux', () => {
+    expect(supportsFastEchoTerminal({ TMUX: '/tmp/tmux-1000/default,1234,0' } as NodeJS.ProcessEnv)).toBe(false)
+    expect(supportsFastEchoTerminal({ TMUX: '/private/tmp/tmux-501/default' } as NodeJS.ProcessEnv)).toBe(false)
+  })
+
+  it('tmux wins over Termux fast-echo opt-in', () => {
+    expect(
+      supportsFastEchoTerminal({
+        TMUX: '/tmp/tmux-1000/default,1234,0',
+        HERMES_TUI_TERMUX_FAST_ECHO: '1',
+        TERMUX_VERSION: '0.118.0'
+      } as NodeJS.ProcessEnv)
+    ).toBe(false)
+  })
+
+  it('keeps fast-echo enabled when TMUX is empty or unset', () => {
+    expect(supportsFastEchoTerminal({ TMUX: '' } as NodeJS.ProcessEnv)).toBe(true)
+    expect(supportsFastEchoTerminal({ TERM_PROGRAM: 'vscode' } as NodeJS.ProcessEnv)).toBe(true)
+  })
+
+  it('disables fast-echo when only a tmux-flavored TERM is present (SSH from tmux, no TMUX forwarded)', () => {
+    // OpenSSH forwards TERM but not TMUX, so a TUI on a remote host launched
+    // from inside local tmux sees TERM=tmux-256color with no TMUX var. The
+    // cursor-drift bug still applies, so fast-echo must stay off.
+    expect(supportsFastEchoTerminal({ TERM: 'tmux' } as NodeJS.ProcessEnv)).toBe(false)
+    expect(supportsFastEchoTerminal({ TERM: 'tmux-256color' } as NodeJS.ProcessEnv)).toBe(false)
+  })
+
+  it('does NOT disable fast-echo for screen-flavored TERM (GNU screen out of scope, no reported drift)', () => {
+    // GNU screen sets TERM=screen/screen-256color and has no reported drift.
+    // We must not widen the tmux guard to screen* and regress its perf.
+    expect(supportsFastEchoTerminal({ TERM: 'screen' } as NodeJS.ProcessEnv)).toBe(true)
+    expect(supportsFastEchoTerminal({ TERM: 'screen-256color' } as NodeJS.ProcessEnv)).toBe(true)
+    // And an unrelated 256color TERM must stay enabled.
+    expect(supportsFastEchoTerminal({ TERM: 'xterm-256color' } as NodeJS.ProcessEnv)).toBe(true)
+  })
+
   it('disables fast-echo by default in Termux mode', () => {
     expect(
       supportsFastEchoTerminal({ TERMUX_VERSION: '0.118.0', PREFIX: '/data/data/com.termux/files/usr' } as NodeJS.ProcessEnv)
diff --git a/ui-tui/src/components/textInput.tsx b/ui-tui/src/components/textInput.tsx
index 564484999f6..deb22914695 100644
--- a/ui-tui/src/components/textInput.tsx
+++ b/ui-tui/src/components/textInput.tsx
@@ -359,6 +359,22 @@ export function supportsFastEchoTerminal(env: NodeJS.ProcessEnv = process.env):
     return false
   }
 
+  // tmux adds a PTY multiplexing layer that desyncs stdout.write() cursor
+  // advances from its internal cursor model, causing cursor drift and ghost
+  // whitespace under the fast-echo bypass path.
+  //
+  // `TMUX` catches the local case. It is NOT forwarded over SSH, so when the
+  // TUI runs on a remote host launched from inside local tmux we only see a
+  // tmux-flavored `TERM` (tmux sets `tmux`/`tmux-256color`); match that too so
+  // remote-over-tmux sessions still fall back to the safe render path. We
+  // deliberately do NOT match `screen*`: GNU screen sets the same TERM and has
+  // no reported drift, so widening to screen would disable the optimization for
+  // those users with no evidence of a bug.
+  const term = (env.TERM ?? '').trim().toLowerCase()
+  if ((env.TMUX ?? '').trim().length > 0 || term === 'tmux' || term.startsWith('tmux-')) {
+    return false
+  }
+
   // Termux terminals are especially sensitive to bypass-path cursor drift and
   // stale paints at soft-wrap boundaries on tall/narrow viewports. Keep this
   // off by default in Termux mode; allow explicit opt-in for local debugging.