[Feat] Start empty Fast Sessions

apps/web/src/app/(sandbox)/sessions/[sessionId]/FastSessionTranscript.tsx

@@ -109,7 +109,11 @@ type PendingResponseState = {
 };
 
 type PendingResponseAction =
-  | { type: 'hydrate'; messages: TranscriptMessage[] }
+  | {
+      type: 'hydrate';
+      messages: TranscriptMessage[];
+      initialResponsePending?: boolean;
+    }
   | {
       type: 'messages';
       messages: TranscriptMessage[];
@@ -149,7 +153,8 @@ export function pendingResponseReducer(
   if (action.type === 'hydrate' || action.type === 'messages') {
     let pendingAfter =
       action.type === 'hydrate'
-        ? action.messages.length === 0
+        ? action.initialResponsePending !== false &&
+          action.messages.length === 0
           ? { id: '', ts: 0, turnSeq: -1 }
           : null
         : state.pendingAfter;
@@ -278,6 +283,7 @@ export function FastSessionTranscript({
   hasOlderMessages,
   canReply,
   initialTitle = null,
+  initialResponsePending = true,
   fallbackTitle = 'New session',
   sessionModel = null,
   sessionReasoningEffort = null,
@@ -292,6 +298,8 @@ export function FastSessionTranscript({
   hasOlderMessages?: boolean;
   canReply?: boolean;
   initialTitle?: string | null;
+  /** Whether an empty initial transcript is waiting on a scheduled kickoff. */
+  initialResponsePending?: boolean;
   fallbackTitle?: string;
   sessionModel?: string | null;
   sessionReasoningEffort?: ReasoningEffort | null;
@@ -333,7 +341,7 @@ export function FastSessionTranscript({
           latestVisibleResponse: null,
           optimisticRollback: null,
         },
-        { type: 'hydrate', messages },
+        { type: 'hydrate', messages, initialResponsePending },
       ),
   );
   const [replyError, setReplyError] = useState<string | null>(null);

apps/web/src/app/(sandbox)/sessions/[sessionId]/page.tsx

@@ -158,6 +158,9 @@ export default async function SessionDetailPage({
                   hasOlderMessages={session.hasOlderMessages}
                   canReply
                   initialTitle={unifiedSession.title}
+                  initialResponsePending={
+                    unifiedSession.sourceTrigger !== 'manual'
+                  }
                   fallbackTitle={unifiedSession.title}
                   sessionModel={session.model}
                   sessionReasoningEffort={session.reasoningEffort}

apps/web/src/components/tasks/NewTaskForm.tsx

@@ -1,6 +1,6 @@
 'use client';
 
-import { useState, useCallback, useEffect, type Ref } from 'react';
+import { useState, useCallback, useEffect, useRef, type Ref } from 'react';
 import { useRouter, useSearchParams } from 'next/navigation';
 import { toast } from 'sonner';
 
@@ -62,6 +62,7 @@ export function NewTaskForm({
   const [selectedReasoningEffort, setSelectedReasoningEffort] = useState<
     ReasoningEffort | null | undefined
   >(undefined);
+  const emptySessionConversationIdRef = useRef<string | null>(null);
 
   useEffect(() => setPromptText(promptParam), [promptParam]);
   useEffect(() => setSelectedModelOverrideId(modelParam), [modelParam]);
@@ -75,6 +76,8 @@ export function NewTaskForm({
       attachmentTexts?: string[];
       model?: string | null;
       reasoningEffort?: ReasoningEffort | null;
+      conversationId?: string;
+      empty?: true;
     }): Promise<void> => {
       // A second submit while the first is in flight would mint a second
       // session and orphan one of them.
@@ -180,11 +183,21 @@ export function NewTaskForm({
       };
 
       if (!environmentIdParam) {
-        if (
+        const isEmpty =
           !submission.description &&
           !submission.images?.length &&
-          !submission.attachmentTexts?.length
-        ) {
+          !submission.attachmentTexts?.length;
+        if (isEmpty) {
+          emptySessionConversationIdRef.current ??= crypto.randomUUID();
+          await startFastSession({
+            text: '',
+            conversationId: emptySessionConversationIdRef.current,
+            empty: true,
+            model: selectedModelOverrideId,
+            ...(selectedReasoningEffort !== undefined
+              ? { reasoningEffort: selectedReasoningEffort }
+              : {}),
+          });
           return;
         }
         await startFastSession({

apps/web/src/hooks/task-runs/useStartFastSession.ts

@@ -36,6 +36,10 @@ type StartFastSessionVariables = {
   attachmentTexts?: string[];
   model?: string | null;
   reasoningEffort?: ReasoningEffort | null;
+  /** Stable identity used to make a start retry idempotent. */
+  conversationId?: string;
+  /** Persist the Session without scheduling an initial Fast turn. */
+  empty?: true;
   /** Launch into a chosen workspace without a Fast decision. */
   pinnedLaunch?: {
     launchId: string;

apps/web/src/trpc/commands/fast-sessions/index.ts

@@ -380,6 +380,7 @@ export async function startFastSessionCommand(
     model?: string | null;
     reasoningEffort?: ReasoningEffort | null;
     conversationId?: string;
+    empty?: true;
     pinnedLaunch?: PinnedFastSessionLaunchInput;
   },
 ): Promise<{
@@ -414,6 +415,17 @@ export async function startFastSessionCommand(
   });
   const unifiedSession = await ensureSessionForFastConversation(db, session.id);
 
+  if (input.empty) {
+    await db
+      .update(sessions)
+      .set({ sourceTrigger: 'manual' })
+      .where(eq(sessions.id, unifiedSession.id));
+    return {
+      sessionId: unifiedSession.id,
+      fastConversationId: session.id,
+    };
+  }
+
   const kickoffTurnId = input.conversationId
     ? `web-kickoff:${session.id}`
     : undefined;