Fix: keep the conversation when an agent changes project folder

CHANGELOG.md

@@ -203,6 +203,12 @@ for every approved plan. Desktop's version follows the engine generation, so it
   user's decision instead of showing retry choices or reporting an unexpected failure afterward.
 - **Approval and recovery cards stay out of persisted transcript history.** They are live controls,
   not conversation messages, so stale actions are not replayed into a restored session.
+- **Changing an agent's project folder no longer erases the conversation.** Pointing an agent at a
+  different folder rebuilt its session from scratch and cleared everything said up to that moment,
+  so a folder change part-way through a task lost all context. The conversation is now kept. The
+  agent still repoints its tools, system prompt, and project skills at the new folder, and is told
+  the folder moved so it re-reads files rather than reusing paths from before the change. Only
+  folder changes from here on benefit — conversations already cleared cannot be recovered.
 
 ### Test coverage
 330 Desktop tests pass. New host-level coverage exercises deferred plan execution, instruction

src/MandoCode.Desktop/Controls/ChatTabView.Explorer.cs

@@ -1,4 +1,4 @@
-using System.Collections.ObjectModel;
+using System.Collections.ObjectModel;
 using System.Text.Json;
 using MandoCode.Models;
 using MandoCode.Desktop.Services;
@@ -269,7 +269,7 @@ private async void OpenFolderButton_Click(object sender, RoutedEventArgs e)
         if (folder == null) return;
 
         _transcript.Append(_html.Info($"Project root changed to: {folder.Path}"));
-        _transcript.Append(_html.Dim("Rebuilding the AI session for the new project…"));
+        _transcript.Append(_html.Dim(TranscriptHtmlBuilder.ProjectSwitchNotice));
 
         // Retargets THIS tab only — its own ProjectRootAccessor, file cache, and kernel.
         // Other agents keep working in their own folders.

src/MandoCode.Desktop/Services/AgentSession.cs

@@ -1,4 +1,4 @@
-using MandoCode.Desktop.ViewModels;
+using MandoCode.Desktop.ViewModels;
 using MandoCode.Models;
 using MandoCode.Services;
 using Microsoft.Extensions.DependencyInjection;
@@ -284,14 +284,35 @@ public void ResetConfigToDefaults(ConfigCoordinator configs)
         _persistedConfigJson = AgentConfigStore.Fingerprint(Config);
     }
 
-    /// <summary>Repoints this tab at a different project folder and rebuilds its AI session.</summary>
+    /// <summary>
+    /// Repoints this tab at a different project folder, KEEPING the conversation. Changing folders
+    /// is a navigation step inside one piece of work — "now look at this repo" — not the start of a
+    /// new one, so everything said up to here still applies.
+    /// </summary>
     public async Task ChangeProjectRootAsync(string folder)
     {
         // The tab keeps its name ("Agent N" or a user rename) across a folder change — the folder
         // is shown in the header, so the label doesn't need to track it.
         ProjectRoot.ProjectRoot = folder;
         FileProvider.RefreshCache();
-        await Ai.ReinitializeAsync(Config);
+
+        // The new folder brings its own project skills, and the skill index is baked into the
+        // system prompt — so the rescan has to happen BEFORE the prompt is recomposed below.
+        Skills.Reload();
+
+        // RefreshSettingsAsync, not ReinitializeAsync: both rebuild the system prompt, the agent,
+        // and the MCP tool set, but ReinitializeAsync ends in ClearHistoryAsync — which is what
+        // used to wipe the conversation on every folder change. The tools themselves need no
+        // rebuild to follow the move: they hold the live ProjectRootAccessor mutated above, not a
+        // copied path. Same trade as ChatController.RefreshFromConfigAsync.
+        await Ai.RefreshSettingsAsync(Config);
+
+        // The model has just been handed a new working folder while still holding a conversation
+        // about the old one. Without this it keeps resolving remembered paths against a root that
+        // moved out from under it — the history survives, but silently goes stale.
+        Ai.AppendUserNote(
+            $"[Project root changed to: {folder}. Earlier messages refer to the previous folder — " +
+            "re-read any file you need rather than reusing paths or contents from before this point.]");
     }
 
     private static string FolderLabel(string path)

src/MandoCode.Desktop/Services/TranscriptHtmlBuilder.cs

@@ -1,4 +1,4 @@
-using System.Net;
+using System.Net;
 using System.Text;
 using MandoCode.Models;
 using MandoCode.Services;
@@ -91,6 +91,11 @@ public string ApprovalNotice(string text, string state = "success") =>
     public string PlanStarted(int totalSteps) =>
         PlanActivity("Executing plan", $"Preparing {totalSteps} step{(totalSteps == 1 ? "" : "s")}");
 
+    /// <summary>The "switching folders" progress line. Lives here as a const because
+    /// <see cref="IsEphemeralStatus"/> matches on it verbatim — reworded in one place only, the
+    /// notice would silently start replaying on session restore.</summary>
+    public const string ProjectSwitchNotice = "Switching to the new project — this conversation is kept…";
+
     /// <summary>True for blocks that describe LIVE session state (status chips: connection,
     /// model ready, MCP counts, pending offers) rather than conversation history. Session
     /// restore replays journaled transcripts — replaying a dead process's state pills next
@@ -102,7 +107,7 @@ public static bool IsEphemeralStatus(string blockHtml) =>
         || blockHtml.StartsWith("<div class=\"panel checkpoint-card\"", StringComparison.Ordinal)
         // Boot/progress narration — true only while it was happening. ("Project root
         // changed to: X" is deliberately NOT here: that's a real event, kept as history.)
-        || blockHtml.Contains(">Rebuilding the AI session for the new project…<", StringComparison.Ordinal)
+        || blockHtml.Contains($">{ProjectSwitchNotice}<", StringComparison.Ordinal)
         || blockHtml.Contains(">✓ Ready.<", StringComparison.Ordinal)
         || ModelNoticeReplay.IsTransient(blockHtml);