fix(desktop): route /btw through prompt.btw so the answer reaches the chat

Desktop sent /btw through the slash worker, which printed the answer after
process_command returned, so only the acknowledgement ever showed. Use the
TUI's prompt.btw RPC and persist btw.complete on the originating session.

Co-authored-by: SsSs <w-kwan@hotmail.com>
Co-authored-by: kokhlo <konstantin.khlopkov93@gmail.com>
Co-authored-by: xxxigm <tuancanhnguyen706@gmail.com>

apps/desktop/src/app/session/hooks/use-message-stream/gateway-event/status.ts

@@ -41,6 +41,35 @@ export function handleStatusEvent(ctx: GatewayEventContext): boolean {
     return true
   }
 
+  if (event.type === 'btw.complete') {
+    // prompt.btw answers a side question and emits this on the originating
+    // session. Persistent transcript line, matching the TUI's `[btw "q"]`
+    // — without it Desktop only ever showed the acknowledgement (#99065).
+    const text = coerceGatewayText(payload?.text).trim()
+
+    if (text && sessionId) {
+      const taskId = String(payload?.task_id ?? '').trim()
+      const question = coerceGatewayText(payload?.question).trim()
+      const header = `[btw${question ? ` "${question}"` : ''}${taskId ? ` (${taskId})` : ''}]`
+
+      flushQueuedDeltas(sessionId)
+      updateSessionState(sessionId, state => ({
+        ...state,
+        messages: [
+          ...state.messages,
+          {
+            id: `btw-complete-${taskId || Date.now()}`,
+            role: 'system',
+            parts: [textPart(`${header}\n${text}`, occurredAt)],
+            timestamp: occurredAt
+          }
+        ]
+      }))
+    }
+
+    return true
+  }
+
   if (event.type === 'review.summary') {
     // Self-improvement background review saved something to memory/skills
     // and emitted a persistent summary (Python formats it as

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

@@ -497,6 +497,52 @@ export function useSlashCommand(deps: SlashCommandDeps) {
         branch: async () => {
           await branchCurrentSession()
         },
+        // /btw uses prompt.btw (the TUI's path). It must NOT go through
+        // runExec: the slash worker prints the answer after process_command
+        // returns, past the stdout capture window (#99065). The RPC replies
+        // with a task id; the answer arrives later as btw.complete.
+        btw: async ctx => {
+          const question = ctx.arg.trim()
+
+          const resolved = await withSlashOutput(ctx)
+
+          if (!resolved) {
+            return
+          }
+
+          const { render: renderSlashOutput, sessionId } = resolved
+
+          if (!question) {
+            renderSlashOutput(
+              'Usage: /btw <question> — answered from a snapshot of this conversation without interrupting it.'
+            )
+
+            return
+          }
+
+          try {
+            const result = await requestGateway<{ task_id?: string }>('prompt.btw', {
+              session_id: sessionId,
+              text: question
+            })
+
+            renderSlashOutput(
+              result.task_id
+                ? `btw ${result.task_id} — answering from a conversation snapshot`
+                : 'btw — answering from a conversation snapshot'
+            )
+          } catch (err) {
+            // Older gateways without the dedicated RPC still have the
+            // slash-worker route — same compatibility fallback as runRpc.
+            if (isMissingRpcMethod(err)) {
+              await runExec(ctx)
+
+              return
+            }
+
+            renderSlashOutput(`error: ${err instanceof Error ? err.message : String(err)}`)
+          }
+        },
         // /compress (alias /compact) runs the gateway's dedicated
         // session.compress RPC — the TUI's path
         // (ui-tui/src/app/slash/commands/session.ts). It must NOT go through

apps/desktop/src/lib/chat-messages/types.ts

@@ -94,6 +94,8 @@ export type GatewayEventPayload = {
   // clarify.request
   request_id?: string
   question?: string
+  // btw.complete / background.complete — id of the side/background task
+  task_id?: string
   choices?: string[] | null
   multi_select?: boolean
   // clarify.request batch form: questions replaces question/choices, and

apps/desktop/src/lib/desktop-slash-commands.ts

@@ -55,6 +55,7 @@ export interface DesktopThemeCommandOption {
 export type DesktopActionId =
   | 'branch'
   | 'browser'
+  | 'btw'
   | 'compress'
   | 'handoff'
   | 'hatch'
@@ -240,6 +241,15 @@ const DESKTOP_COMMAND_SPECS: readonly DesktopCommandSpec[] = [
     surface: action('compress'),
     argumentMode: 'text'
   },
+  // /btw must be an action (prompt.btw RPC), not exec: the slash worker
+  // prints the answer after process_command returns, so Desktop only ever
+  // saw the acknowledgement (#99065). The answer arrives as btw.complete.
+  {
+    name: '/btw',
+    description: 'Ask a side question about this conversation without interrupting it',
+    surface: action('btw'),
+    argumentMode: 'text'
+  },
   {
     name: '/pet',
     description: 'Toggle or adopt a petdex mascot (/pet, /pet list, /pet boba)',