feat(web-ui): non-blocking historical session switch; debug log server

AGENTS.md

@@ -115,3 +115,26 @@ In `agentic/agents/`:
 1. Create agent file
 2. Define prompt in `prompts/`
 3. Register in `registry.rs`
+
+## Frontend Debugging
+
+A local log receiver server is available at `scripts/debug-log-server.mjs`.
+
+**Start the server:**
+```bash
+node scripts/debug-log-server.mjs
+# Listens on http://127.0.0.1:7469, writes logs to debug-agent.log
+```
+
+**Instrument code (one-liner fetch):**
+```typescript
+fetch('http://127.0.0.1:7469/log',{method:'POST',headers:{'Content-Type':'application/json'},body:JSON.stringify({location:'file.ts:LINE',message:'desc',data:{k:v},timestamp:Date.now()})}).catch(()=>{});
+```
+
+**Clear logs between runs:**
+```bash
+# Via HTTP
+curl -X POST http://127.0.0.1:7469/clear
+```
+
+Logs are written to `debug-agent.log` in project root as NDJSON. The agent reads this file directly — no copy-paste needed.

scripts/debug-log-server.mjs

@@ -0,0 +1,86 @@
+/**
+ * Debug Log Receiver Server
+ * Receives POST requests from fetch-based instrumentation and writes to a log file.
+ *
+ * Usage:
+ *   node scripts/debug-log-server.mjs [port] [logfile]
+ *
+ * Defaults:
+ *   port    = 7469
+ *   logfile = debug-agent.log (in project root)
+ *
+ * Frontend fetch template (copy into your code):
+ *   fetch('http://127.0.0.1:7469/log', {method:'POST', headers:{'Content-Type':'application/json'}, body: JSON.stringify({location:'file.ts:LINE', message:'desc', data:{k:v}, timestamp:Date.now()})}).catch(()=>{});
+ */
+
+import http from 'http';
+import fs from 'fs';
+import path from 'path';
+import { fileURLToPath } from 'url';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+const ROOT = path.resolve(__dirname, '..');
+
+const PORT = parseInt(process.argv[2] ?? '7469', 10);
+const LOG_FILE = path.resolve(ROOT, process.argv[3] ?? 'debug-agent.log');
+
+// Clear log file on start
+fs.writeFileSync(LOG_FILE, '', 'utf8');
+console.log(`[debug-log-server] started on http://127.0.0.1:${PORT}`);
+console.log(`[debug-log-server] writing logs to ${LOG_FILE}`);
+console.log(`[debug-log-server] fetch template:`);
+console.log(`  fetch('http://127.0.0.1:${PORT}/log', {method:'POST', headers:{'Content-Type':'application/json'}, body: JSON.stringify({location:'file.ts:LINE', message:'desc', data:{}, timestamp:Date.now()})}).catch(()=>{});\n`);
+
+const server = http.createServer((req, res) => {
+  // CORS headers so browser can POST from any origin
+  res.setHeader('Access-Control-Allow-Origin', '*');
+  res.setHeader('Access-Control-Allow-Methods', 'POST, OPTIONS');
+  res.setHeader('Access-Control-Allow-Headers', 'Content-Type');
+
+  if (req.method === 'OPTIONS') {
+    res.writeHead(204);
+    res.end();
+    return;
+  }
+
+  if (req.method === 'POST' && req.url === '/log') {
+    let body = '';
+    req.on('data', chunk => { body += chunk; });
+    req.on('end', () => {
+      try {
+        const payload = JSON.parse(body);
+        const entry = JSON.stringify({ ...payload, _receivedAt: new Date().toISOString() });
+        fs.appendFileSync(LOG_FILE, entry + '\n', 'utf8');
+
+        // Pretty print to console for quick monitoring
+        const loc = payload.location ?? '?';
+        const msg = payload.message ?? '';
+        const data = payload.data ? JSON.stringify(payload.data) : '';
+        console.log(`[LOG] ${loc} | ${msg} | ${data}`);
+
+        res.writeHead(200, { 'Content-Type': 'application/json' });
+        res.end(JSON.stringify({ ok: true }));
+      } catch (e) {
+        res.writeHead(400);
+        res.end(JSON.stringify({ error: 'invalid json' }));
+      }
+    });
+    return;
+  }
+
+  // /clear - clear log file
+  if (req.method === 'POST' && req.url === '/clear') {
+    fs.writeFileSync(LOG_FILE, '', 'utf8');
+    console.log('[debug-log-server] log file cleared');
+    res.writeHead(200);
+    res.end(JSON.stringify({ ok: true }));
+    return;
+  }
+
+  res.writeHead(404);
+  res.end();
+});
+
+server.listen(PORT, '127.0.0.1', () => {
+  console.log('[debug-log-server] ready\n');
+});

src/web-ui/src/flow_chat/services/flow-chat-manager/SessionModule.ts

@@ -258,73 +258,8 @@ export async function switchChatSession(
 ): Promise<void> {
   try {
     const session = context.flowChatStore.getState().sessions.get(sessionId);
-
-    if (session?.isHistorical) {
-      try {
-        const workspacePath = requireSessionWorkspacePath(session.workspacePath, sessionId);
-
-        await context.flowChatStore.loadSessionHistory(
-          sessionId,
-          workspacePath,
-          undefined,
-          session.remoteConnectionId,
-          session.remoteSshHost
-        );
-
-        try {
-          await agentAPI.restoreSession(
-            sessionId,
-            workspacePath,
-            session.remoteConnectionId,
-            session.remoteSshHost
-          );
-
-          context.flowChatStore.setState(prev => {
-            const newSessions = new Map(prev.sessions);
-            const sess = newSessions.get(sessionId);
-            if (sess) {
-              newSessions.set(sessionId, { ...sess, isHistorical: false });
-            }
-            return { ...prev, sessions: newSessions };
-          });
-        } catch (restoreError: any) {
-          log.warn('Historical session restore failed, creating new session', { sessionId, error: restoreError });
-          const currentSession = context.flowChatStore.getState().sessions.get(sessionId);
-          if (currentSession) {
-            await agentAPI.createSession({
-              sessionId: sessionId,
-              sessionName: currentSession.title || `Session ${sessionId.slice(0, 8)}`,
-              agentType: currentSession.mode || 'agentic',
-              workspacePath,
-              remoteConnectionId: currentSession.remoteConnectionId,
-              remoteSshHost: currentSession.remoteSshHost,
-              config: {
-                modelName: currentSession.config.modelName || 'auto',
-                enableTools: true,
-                safeMode: true,
-                remoteConnectionId: currentSession.remoteConnectionId,
-                remoteSshHost: currentSession.remoteSshHost,
-              }
-            });
-
-            context.flowChatStore.setState(prev => {
-              const newSessions = new Map(prev.sessions);
-              const sess = newSessions.get(sessionId);
-              if (sess) {
-                newSessions.set(sessionId, { ...sess, isHistorical: false });
-              }
-              return { ...prev, sessions: newSessions };
-            });
-          }
-        }
-      } catch (error) {
-        log.error('Failed to load session history', { sessionId, error });
-        notificationService.warning('Failed to load session history, showing empty session', {
-          duration: 3000
-        });
-      }
-    }
-
+
+    // Switch UI immediately so the user sees the new session without waiting for history load.
     context.flowChatStore.switchSession(sessionId);
 
     touchSessionActivity(
@@ -335,6 +270,29 @@ export async function switchChatSession(
     ).catch(error => {
       log.debug('Failed to touch session activity', { sessionId, error });
     });
+
+    if (session?.isHistorical) {
+      // Load history in the background — do not block the UI.
+      (async () => {
+        try {
+          const workspacePath = requireSessionWorkspacePath(session.workspacePath, sessionId);
+
+          // loadSessionHistory internally calls restoreSession + loadSessionTurns.
+          await context.flowChatStore.loadSessionHistory(
+            sessionId,
+            workspacePath,
+            undefined,
+            session.remoteConnectionId,
+            session.remoteSshHost
+          );
+        } catch (error) {
+          log.error('Failed to load session history', { sessionId, error });
+          notificationService.warning('Failed to load session history, showing empty session', {
+            duration: 3000
+          });
+        }
+      })();
+    }
   } catch (error) {
     log.error('Failed to switch chat session', { sessionId, error });
     notificationService.error('Failed to switch session', {