Merge pull request #3171 from pocharlies-org/feat/btw-boundary

feat(btw): make a btw session a side question, not a continuation of the parent

packages/ui/src/components/chat/ChatInput.tsx

@@ -35,7 +35,8 @@ import {
 import { ReviewFlowDialog, type ReviewFlowExecution } from '@/components/session/ReviewFlowDialog';
 import { BtwPanel } from './btw/BtwPanel';
 import { useBtwPanelState } from './btw/useBtwPanelState';
-import { destroyBtwSession, startBtwSession, type BtwSessionRef } from '@/lib/btw';
+import { wasPromotedBtwSession } from '@/lib/sessionBtwMetadata';
+import { BTW_BOUNDARY_INSTRUCTION, BTW_PROMOTION_NOTICE, destroyBtwSession, startBtwSession, type BtwSessionRef } from '@/lib/btw';
 import { AttachedFilesList, AttachedVSCodeFileChips, ActiveEditorFileSuggestion } from './FileAttachment';
 import { lazyWithChunkRecovery } from '@/lib/chunkLoadRecovery';
 import type { ToolPopupContent } from './message/types';
@@ -340,6 +341,10 @@ const ChatInputComponent: React.FC<ChatInputProps> = ({
         [btwDirectory, btwSessionId, currentSessionId],
     );
     const isBtwActive = Boolean(btwSessionRef) && !btwPanel.collapsed;
+    // A session promoted out of `/btw` keeps the boundary instructions in its
+    // transcript — there is no way to delete a message part — so it has to say
+    // they no longer apply.
+    const isPromotedBtwSession = wasPromotedBtwSession(btwPanel.parentSession);
     const activeRuntimeKey = getRuntimeKey();
     const chatDraftIdentity = React.useMemo(
         () => createChatDraftIdentity(
@@ -1129,7 +1134,14 @@ const ChatInputComponent: React.FC<ChatInputProps> = ({
             composerText: !queuedOnly && inputSnapshot.hasContent ? inputSnapshot.message : null,
             composerAttachments: attachedFiles,
             inlineComments: drafts,
-            syntheticTexts: syntheticParts?.map((part) => part.text) ?? [],
+            // btw mode: the boundary rides with every send, not just the
+            // first one, so the inherited transcript stays reference material
+            // for the whole side conversation.
+            syntheticTexts: [
+                ...(isBtwActive ? [BTW_BOUNDARY_INSTRUCTION] : []),
+                ...(isPromotedBtwSession ? [BTW_PROMOTION_NOTICE] : []),
+                ...(syntheticParts?.map((part) => part.text) ?? []),
+            ],
             linkedIssue: linkedIssue
                 ? { number: linkedIssue.number, title: linkedIssue.title, url: linkedIssue.url, contextText: linkedIssue.contextText }
                 : null,

packages/ui/src/components/chat/btw/useBtwPanelState.ts

@@ -5,6 +5,8 @@ import { getBtwBoundaryMessageID, getBtwSessionID } from '@/lib/sessionBtwMetada
 import { useBtwStore } from '@/stores/useBtwStore';
 
 export type BtwPanelState = {
+  /** The session the composer is in — the one `/btw` would fork. */
+  parentSession: Session | null;
   /** The active fork for this parent, or null when no panel should exist. */
   btwSessionId: string | null;
   btwSession: Session | null;
@@ -40,6 +42,7 @@ export function useBtwPanelState(
   const destroying = Boolean(uiState?.destroying);
   const btwSessionId = btwSession && !destroying ? linkedBtwSessionId : null;
   return {
+    parentSession: parentSession ?? null,
     btwSessionId,
     btwSession: btwSessionId ? btwSession : null,
     // SAFETY: the SDK Session type omits the server's `directory` field; this