diff --git a/ui-tui/src/__tests__/activeSessionSwitcher.test.ts b/ui-tui/src/__tests__/activeSessionSwitcher.test.ts
index bf409e95b354..d598f6a834c6 100644
--- a/ui-tui/src/__tests__/activeSessionSwitcher.test.ts
+++ b/ui-tui/src/__tests__/activeSessionSwitcher.test.ts
@@ -86,10 +86,10 @@ describe('session orchestrator helpers', () => {
 
   it('turns model picker values into session-scoped draft model args', () => {
     expect(draftModelArgFromPickerValue('kimi-k2.6 --provider ollama-cloud --tui-session')).toBe(
-      'kimi-k2.6 --provider ollama-cloud'
+      'kimi-k2.6 --provider ollama-cloud --session'
     )
     expect(draftModelArgFromPickerValue('openai/gpt-5.5 --provider openai-codex --global')).toBe(
-      'openai/gpt-5.5 --provider openai-codex'
+      'openai/gpt-5.5 --provider openai-codex --session'
     )
   })
 
diff --git a/ui-tui/src/__tests__/createSlashHandler.test.ts b/ui-tui/src/__tests__/createSlashHandler.test.ts
index ca1af4cd9abe..e5f97c7e3562 100644
--- a/ui-tui/src/__tests__/createSlashHandler.test.ts
+++ b/ui-tui/src/__tests__/createSlashHandler.test.ts
@@ -209,7 +209,7 @@ describe('createSlashHandler', () => {
       confirm_expensive_model: false,
       key: 'model',
       session_id: 'sid-abc',
-      value: 'anthropic/claude-sonnet-4.6 --provider openrouter'
+      value: 'anthropic/claude-sonnet-4.6 --provider openrouter --session'
     })
   })
 
diff --git a/ui-tui/src/__tests__/orchestratorPromptSession.test.ts b/ui-tui/src/__tests__/orchestratorPromptSession.test.ts
index f9ff16f34a5c..2d670c4a5b8d 100644
--- a/ui-tui/src/__tests__/orchestratorPromptSession.test.ts
+++ b/ui-tui/src/__tests__/orchestratorPromptSession.test.ts
@@ -32,7 +32,7 @@ describe('startPromptLiveSession', () => {
         'rpc',
         {
           method: 'config.set',
-          params: { key: 'model', session_id: 'abc123', value: 'kimi-k2.6 --provider ollama-cloud' }
+          params: { key: 'model', session_id: 'abc123', value: 'kimi-k2.6 --provider ollama-cloud --session' }
         }
       ],
       ['sys', 'model → kimi-k2.6'],
diff --git a/ui-tui/src/app/slash/commands/session.ts b/ui-tui/src/app/slash/commands/session.ts
index 6b1fe55481bc..ce1c4ead945d 100644
--- a/ui-tui/src/app/slash/commands/session.ts
+++ b/ui-tui/src/app/slash/commands/session.ts
@@ -1,5 +1,5 @@
 import { attachedImageNotice, introMsg, toTranscriptMessages } from '../../../domain/messages.js'
-import { TUI_SESSION_MODEL_FLAG } from '../../../domain/slash.js'
+import { sessionScopedModelArg, TUI_SESSION_MODEL_FLAG } from '../../../domain/slash.js'
 import type {
   BackgroundStartResponse,
   ConfigGetValueResponse,
@@ -20,10 +20,6 @@ import { patchUiState } from '../../uiStore.js'
 import type { SlashCommand } from '../types.js'
 
 const TUI_SESSION_MODEL_RE = new RegExp(`(?:^|\\s)${TUI_SESSION_MODEL_FLAG}(?:\\s|$)`)
-const TUI_SESSION_STRIP_RE = new RegExp(`\\s*${TUI_SESSION_MODEL_FLAG}\\b\\s*`, 'g')
-
-const stripTuiSessionFlag = (trimmed: string) => trimmed.replace(TUI_SESSION_STRIP_RE, ' ').replace(/\s+/g, ' ').trim()
-
 const modelValueForConfigSet = (arg: string) => {
   const trimmed = arg.trim()
 
@@ -32,7 +28,7 @@ const modelValueForConfigSet = (arg: string) => {
   }
 
   if (TUI_SESSION_MODEL_RE.test(trimmed)) {
-    return stripTuiSessionFlag(trimmed)
+    return sessionScopedModelArg(trimmed)
   }
 
   return trimmed
diff --git a/ui-tui/src/app/useMainApp.ts b/ui-tui/src/app/useMainApp.ts
index 50a94c9d45a9..761f4a851afb 100644
--- a/ui-tui/src/app/useMainApp.ts
+++ b/ui-tui/src/app/useMainApp.ts
@@ -9,6 +9,7 @@ import { hasLeadGap, prevRenderedMsg } from '../domain/blockLayout.js'
 import { SECTION_NAMES, sectionMode } from '../domain/details.js'
 import { attachedImageNotice, imageTokenMeta } from '../domain/messages.js'
 import { composeTabTitle, fmtCwdBranch, shortCwd } from '../domain/paths.js'
+import { sessionScopedModelArg } from '../domain/slash.js'
 import { type GatewayClient } from '../gatewayClient.js'
 import type {
   ClarifyRespondResponse,
@@ -116,7 +117,7 @@ export async function startPromptLiveSession({
     return null
   }
 
-  const requestedModel = modelArg?.trim()
+  const requestedModel = modelArg ? sessionScopedModelArg(modelArg) : ''
 
   if (requestedModel) {
     const result = await rpc<ConfigSetResponse>('config.set', { key: 'model', session_id: sid, value: requestedModel })
diff --git a/ui-tui/src/components/activeSessionSwitcher.tsx b/ui-tui/src/components/activeSessionSwitcher.tsx
index af4fbbb27fb1..4ac3385420c1 100644
--- a/ui-tui/src/components/activeSessionSwitcher.tsx
+++ b/ui-tui/src/components/activeSessionSwitcher.tsx
@@ -1,7 +1,7 @@
 import { Box, Text, useInput, useStdout } from '@hermes/ink'
 import { useCallback, useEffect, useRef, useState } from 'react'
 
-import { TUI_SESSION_MODEL_FLAG } from '../domain/slash.js'
+import { sessionScopedModelArg } from '../domain/slash.js'
 import type { GatewayClient } from '../gatewayClient.js'
 import type {
   SessionActiveItem,
@@ -205,18 +205,7 @@ export const closeFallbackAfterClose = (
 }
 
 export const draftModelArgFromPickerValue = (value: string) => {
-  const parts = value.trim().split(/\s+/).filter(Boolean)
-  const kept: string[] = []
-
-  for (const part of parts) {
-    if (part === TUI_SESSION_MODEL_FLAG || part === '--global') {
-      continue
-    }
-
-    kept.push(part)
-  }
-
-  return kept.join(' ')
+  return sessionScopedModelArg(value)
 }
 
 export const draftModelNameFromArg = (value: string) => {
diff --git a/ui-tui/src/domain/slash.ts b/ui-tui/src/domain/slash.ts
index 42962ae69d4c..b86c34d134cc 100644
--- a/ui-tui/src/domain/slash.ts
+++ b/ui-tui/src/domain/slash.ts
@@ -1,6 +1,13 @@
-/** Appended to `/model` args from the TUI picker for session scope; stripped in `session` slash before `config.set`. */
+/** Appended by TUI pickers; converted to the backend's `--session` flag before `config.set`. */
 export const TUI_SESSION_MODEL_FLAG = '--tui-session'
 
+export const sessionScopedModelArg = (value: string) => {
+  const parts = value.trim().split(/\s+/).filter(Boolean)
+  const kept = parts.filter(part => part !== TUI_SESSION_MODEL_FLAG && part !== '--global' && part !== '--session')
+
+  return kept.length ? `${kept.join(' ')} --session` : ''
+}
+
 export const looksLikeSlashCommand = (text: string) => /^\/[^\s/]*(?:\s|$)/.test(text)
 
 export const parseSlashCommand = (cmd: string) => {