Allow failed agent turns to be forked (#2063)

* fix(chat): allow failed turns to be forked

Failed turns can end in Paseo-generated timeline items without provider
message IDs. Anchor forks to canonical timeline positions while retaining
legacy boundaries for compatibility.

Place the fork summary before the new prompt and delimit it as structured
chat history.

* fix(chat): tighten fork boundary handling

packages/app/src/agent-stream/view.tsx

@@ -142,6 +142,7 @@ function renderStreamItemWithTurnFooter(input: {
   content: ReactNode;
   layoutItem: StreamLayoutItem;
   strategy: TurnContentStrategy;
+  supportsTimelineCursor: boolean;
   onForkAssistantTurn?: AssistantTurnForkHandler;
 }): ReactNode {
   if (!input.content) {
@@ -155,6 +156,7 @@ function renderStreamItemWithTurnFooter(input: {
       items={footerHost.items}
       timing={footerHost.timing}
       startIndex={footerHost.startIndex}
+      supportsTimelineCursor={input.supportsTimelineCursor}
       onForkAssistantTurn={input.onForkAssistantTurn}
     />
   ) : null;
@@ -282,6 +284,7 @@ function buildChatHistoryAttachment(input: {
       serverId: input.serverId,
       agentId: input.agentId,
       boundaryMessageId: input.payload.boundaryMessageId,
+      boundaryCursor: input.payload.boundaryCursor,
       itemCount: input.payload.itemCount,
     },
   };
@@ -369,6 +372,10 @@ const AgentStreamViewComponent = forwardRef<AgentStreamViewHandle, AgentStreamVi
         !readOnly &&
         state.sessions[resolvedServerId]?.serverInfo?.features?.agentForkContext === true,
     );
+    const supportsAgentForkContextCursor = useSessionStore(
+      (state) =>
+        state.sessions[resolvedServerId]?.serverInfo?.features?.agentForkContextCursor === true,
+    );
 
     const workspaceRoot = context.cwd?.trim() || "";
     const { requestDirectoryListing } = useFileExplorerActions({
@@ -466,7 +473,7 @@ const AgentStreamViewComponent = forwardRef<AgentStreamViewHandle, AgentStreamVi
     });
 
     const handleForkAssistantTurn: AssistantTurnForkHandler = useStableEvent(
-      async ({ target, boundaryMessageId }) => {
+      async ({ target, boundary }) => {
         try {
           if (!supportsAgentForkContext) {
             toast?.error(t("message.actions.forkUnavailable"));
@@ -478,10 +485,7 @@ const AgentStreamViewComponent = forwardRef<AgentStreamViewHandle, AgentStreamVi
           const draftSetup = buildForkDraftSetup(context);
           const prepareForkDraft = async () => {
             const draftId = generateDraftId();
-            const payload = await client.buildAgentForkContext(
-              agentId,
-              boundaryMessageId ? { boundaryMessageId } : {},
-            );
+            const payload = await client.buildAgentForkContext(agentId, boundary);
             const attachment = buildChatHistoryAttachment({
               draftId,
               serverId: resolvedServerId,
@@ -854,10 +858,17 @@ const AgentStreamViewComponent = forwardRef<AgentStreamViewHandle, AgentStreamVi
           content,
           layoutItem,
           strategy: streamRenderStrategy,
+          supportsTimelineCursor: supportsAgentForkContextCursor,
           onForkAssistantTurn: readOnly ? undefined : handleForkAssistantTurn,
         });
       },
-      [handleForkAssistantTurn, readOnly, renderStreamItemContent, streamRenderStrategy],
+      [
+        handleForkAssistantTurn,
+        readOnly,
+        renderStreamItemContent,
+        streamRenderStrategy,
+        supportsAgentForkContextCursor,
+      ],
     );
 
     const pendingPermissionItems = useMemo(
@@ -882,6 +893,7 @@ const AgentStreamViewComponent = forwardRef<AgentStreamViewHandle, AgentStreamVi
             inFlightTurnStartedAt={baseRenderModel.turnTiming.runningStartedAt}
             host={bottomTurnFooterHost}
             strategy={streamRenderStrategy}
+            supportsTimelineCursor={supportsAgentForkContextCursor}
             onForkAssistantTurn={readOnly ? undefined : handleForkAssistantTurn}
           />
         ) : null,
@@ -892,6 +904,7 @@ const AgentStreamViewComponent = forwardRef<AgentStreamViewHandle, AgentStreamVi
         baseRenderModel.turnTiming.runningStartedAt,
         bottomTurnFooterHost,
         streamRenderStrategy,
+        supportsAgentForkContextCursor,
       ],
     );
     const renderModel = useMemo<AgentStreamRenderModel>(() => {