[aidan] chat: isolate branch history, fresh session on edit/switch (#62)

backend/apps/agents/agent_manager.py

@@ -1768,27 +1768,20 @@ class AgentManager:
             except Exception as e:
                 logger.debug(f"thinking_level param injection skipped: {e}")
 
-            # MCPActivate fresh-restart path: when the session has prior
-            # turns AND the user just activated a new MCP, the bundled CLI
-            # won't re-read mcp_servers from a `resume + fork_session`
-            # combo (the transport snapshot from the original launch is
-            # what serves tool schemas). Symptom: model calls hallucinated
-            # names like `Searchgmail`/`Listemails` instead of the real
-            # `mcp__google-workspace__query_gmail_emails` because it
-            # never received the schemas. Soft restart: drop resume +
-            # sdk_session_id, replay history via the prompt, let the SDK
-            # build a clean transport with the activated server in its
-            # mcp_servers dict from the start. Costs one cold-start TTFT
-            # (~200-400ms) on the auto-continuation turn; that turn is
-            # already happening anyway because pending_continuation fires
-            # right after MCPActivate.
-            if session.needs_fresh_session and session.sdk_session_id:
-                logger.info(
-                    f"[MCP-DEBUG] Fresh-session restart for {session_id}: dropping "
-                    f"sdk_session_id={session.sdk_session_id} so the new MCP servers "
-                    f"({session.active_mcps}) take effect."
-                )
-                session.sdk_session_id = None
+            # Fresh-restart path: some session changes must not reuse the
+            # CLI's resume transcript. MCPActivate needs a new transport so
+            # tool schemas are reread; branch edits/switches need the model
+            # to see only _get_branch_messages(session), not facts from the
+            # old branch's SDK transcript. Soft restart: drop resume +
+            # sdk_session_id, replay local history via the prompt, let the
+            # SDK build a clean session from the current app state.
+            if session.needs_fresh_session:
+                if session.sdk_session_id:
+                    logger.info(
+                        f"Fresh-session restart for {session_id}: dropping "
+                        f"sdk_session_id={session.sdk_session_id}; active_mcps={session.active_mcps}"
+                    )
+                    session.sdk_session_id = None
                 session.needs_fresh_session = False
                 session.needs_fork = False  # superseded by the fresh restart
 
@@ -3569,6 +3562,7 @@ class AgentManager:
         )
         session.branches[new_branch_id] = new_branch
         session.active_branch_id = new_branch_id
+        session.needs_fresh_session = True
 
 
         edited_msg = Message(
@@ -3617,6 +3611,7 @@ class AgentManager:
         if branch_id not in session.branches:
             raise ValueError(f"Branch {branch_id} not found")
         session.active_branch_id = branch_id
+        session.needs_fresh_session = True
         await ws_manager.send_to_session(session_id, "agent:branch_switched", {
             "session_id": session_id,
             "active_branch_id": branch_id,

frontend/src/app/pages/AgentChat/AgentChat.tsx

@@ -927,6 +927,7 @@ const AgentChat: React.FC<AgentChatProps> = ({ sessionId: sessionIdProp, onClose
   }
 
   const isActive = session.status === 'running' || session.status === 'waiting_approval' || session.status === 'draft';
+  const branchNavLocked = agentBusy || hasStreaming;
   const statusStyle = STATUS_STYLES[session.status] || { color: c.text.tertiary, bg: c.bg.secondary };
 
   return (
@@ -1386,11 +1387,14 @@ const AgentChat: React.FC<AgentChatProps> = ({ sessionId: sessionIdProp, onClose
                           ? {
                               currentIndex: Math.max(0, currentBranchIdx),
                               totalBranches: siblings.length,
+                              disabled: branchNavLocked,
                               onPrevious: () => {
+                                if (branchNavLocked) return;
                                 const prevBranch = siblings[Math.max(0, currentBranchIdx - 1)];
                                 if (prevBranch && id) dispatch(switchBranch({ sessionId: id, branchId: prevBranch }));
                               },
                               onNext: () => {
+                                if (branchNavLocked) return;
                                 const nextBranch = siblings[Math.min(siblings.length - 1, currentBranchIdx + 1)];
                                 if (nextBranch && id) dispatch(switchBranch({ sessionId: id, branchId: nextBranch }));
                               },

frontend/src/app/pages/AgentChat/shell/MessageActionBar.tsx

@@ -21,6 +21,7 @@ import FeedbackDialog, { Sentiment } from './FeedbackDialog';
 interface BranchNavProps {
   currentIndex: number;
   totalBranches: number;
+  disabled?: boolean;
   onPrevious: () => void;
   onNext: () => void;
 }
@@ -108,7 +109,7 @@ const MessageActionBar: React.FC<Props> = ({
               <IconButton
                 size="small"
                 onClick={branchNav.onPrevious}
-                disabled={branchNav.currentIndex === 0}
+                disabled={branchNav.disabled || branchNav.currentIndex === 0}
                 sx={btnSx(c)}
               >
                 <ChevronLeftIcon sx={{ fontSize: 16 }} />
@@ -127,7 +128,7 @@ const MessageActionBar: React.FC<Props> = ({
               <IconButton
                 size="small"
                 onClick={branchNav.onNext}
-                disabled={branchNav.currentIndex === branchNav.totalBranches - 1}
+                disabled={branchNav.disabled || branchNav.currentIndex === branchNav.totalBranches - 1}
                 sx={btnSx(c)}
               >
                 <ChevronRightIcon sx={{ fontSize: 16 }} />
@@ -199,6 +200,7 @@ export default React.memo(MessageActionBar, (prev, next) => (
   && !!prev.onBranch === !!next.onBranch
   && prev.branchNav?.currentIndex === next.branchNav?.currentIndex
   && prev.branchNav?.totalBranches === next.branchNav?.totalBranches
+  && prev.branchNav?.disabled === next.branchNav?.disabled
   && prev.messageId === next.messageId
   && prev.sessionId === next.sessionId
 ));

frontend/src/shared/ws/WebSocketManager.ts

@@ -22,7 +22,7 @@ import {
   setTurnLabel,
   clearTurnLabel,
 } from '../state/agentsSlice';
-import { streamStart, streamDelta, streamEnd } from '../state/streamingSlice';
+import { streamStart, streamDelta, streamEnd, clearStreamingForSession } from '../state/streamingSlice';
 import { addBrowserCardFromBackend, removeBrowserCard, setBrowserCardPosition, setGlowingBrowserCards, GRID_GAP } from '../state/dashboardLayoutSlice';
 import { upsertOutput } from '../state/outputsSlice';
 import { getAuthToken } from '../config';
@@ -493,6 +493,7 @@ class WebSocketManager {
           // before its own aux call lands.
           if (session_id && (data.status === 'completed' || data.status === 'error' || data.status === 'stopped')) {
             store.dispatch(clearTurnLabel(session_id));
+            store.dispatch(clearStreamingForSession(session_id));
           }
         }
         // Clean spawned browser cards when the PARENT finishes, never per