diff --git a/web/features/agent-v2/agent-detail/configure/__tests__/page.spec.tsx b/web/features/agent-v2/agent-detail/configure/__tests__/page.spec.tsx
index 2d462575853..00418b9b62a 100644
--- a/web/features/agent-v2/agent-detail/configure/__tests__/page.spec.tsx
+++ b/web/features/agent-v2/agent-detail/configure/__tests__/page.spec.tsx
@@ -370,6 +370,7 @@ vi.mock('../components/preview/build-chat', async () => {
       clearChatList?: boolean
       conversationId?: string | null
       controllerRef?: Ref<{ stop: () => void }>
+      disabled?: boolean
       onConversationComplete?: (conversationId: string) => void
       onConversationIdChange?: (conversationId: string) => void
       onBeforeSpeechToText?: () => Promise<unknown>
@@ -397,6 +398,7 @@ vi.mock('../components/preview/build-chat', async () => {
           </button>
           <button
             type="button"
+            disabled={props.disabled}
             onClick={() => {
               void props
                 .onSaveDraftBeforeRun?.()
@@ -755,6 +757,58 @@ describe('AgentConfigurePage', () => {
       expect(urlUpdate?.searchParams.get('source')).toBe('shared-link')
     })
 
+    it('should show an editable composer and an empty Build chat while starting a fresh Build session', async () => {
+      const user = userEvent.setup()
+      const refreshBuildConversation = createDeferredPromise<{
+        debug_conversation_has_messages: boolean
+        debug_conversation_id: string
+        debug_conversation_message_count: number
+      }>()
+      mocks.refreshDebugConversation.mockReturnValueOnce(refreshBuildConversation.promise)
+      mocks.queryState.composer = {
+        data: {
+          agent_soul: {
+            prompt: {
+              system_prompt: 'draft prompt',
+            },
+          },
+        },
+        isFetching: false,
+        isError: false,
+        isPending: false,
+        isSuccess: true,
+        refetch: vi.fn(),
+      }
+
+      render(
+        <QueryClientProvider client={new QueryClient()}>
+          <AgentConfigureComposerScopeHarness />
+        </QueryClientProvider>,
+      )
+
+      await user.click(screen.getByRole('button', { name: 'build mode' }))
+
+      expect(screen.getByRole('region', { name: 'orchestrate-panel' })).toHaveTextContent(
+        'readonly:no',
+      )
+      expect(screen.getByRole('region', { name: 'orchestrate-panel' })).toHaveTextContent(
+        'publish:yes',
+      )
+      expect(screen.getByRole('region', { name: 'build-chat' })).toHaveTextContent('build:none')
+      expect(screen.queryByRole('status', { name: 'appApi.loading' })).not.toBeInTheDocument()
+      expect(screen.getByRole('button', { name: 'send build message' })).toBeDisabled()
+
+      refreshBuildConversation.resolve({
+        debug_conversation_has_messages: false,
+        debug_conversation_id: 'debug-conversation-new',
+        debug_conversation_message_count: 0,
+      })
+
+      await waitFor(() => {
+        expect(screen.getByRole('button', { name: 'send build message' })).toBeEnabled()
+      })
+    })
+
     it('should confirm before discarding an existing build draft and switching to preview', async () => {
       const user = userEvent.setup()
       mocks.queryState.composer = {
diff --git a/web/features/agent-v2/agent-detail/configure/components/composer-session.tsx b/web/features/agent-v2/agent-detail/configure/components/composer-session.tsx
index 3d7d4d6e5f7..fb1b7cfe275 100644
--- a/web/features/agent-v2/agent-detail/configure/components/composer-session.tsx
+++ b/web/features/agent-v2/agent-detail/configure/components/composer-session.tsx
@@ -310,13 +310,10 @@ function AgentConfigurePageComposerContent({
   const queryClient = useQueryClient()
   const showBuildDraftBar = buildDraft.isActive
   const resetBuildChatState = useCallback(async () => {
-    try {
-      await onRefreshDebugConversationAsync()
-    } finally {
-      setCompletedBuildConversationId(null)
-      setConversationId({ mode: 'build', conversationId: null })
-      setClearChatByMode((current) => ({ ...current, build: true }))
-    }
+    setCompletedBuildConversationId(null)
+    setConversationId({ mode: 'build', conversationId: null })
+    setClearChatByMode((current) => ({ ...current, build: true }))
+    await onRefreshDebugConversationAsync()
   }, [onRefreshDebugConversationAsync, setClearChatByMode, setConversationId])
   const rebaseComposerDraftFromSoulConfig = useCallback(
     (agentSoulConfig?: AgentSoulConfig) => {
@@ -458,16 +455,11 @@ function AgentConfigurePageComposerContent({
           textGenerationModelList={textGenerationModelList}
           draftSavedAt={draftSavedAt}
           isPublishing={isPublishing}
-          readOnly={
-            isViewingVersion ||
-            buildDraft.isActive ||
-            buildDraftActionsDisabled ||
-            isEnteringBuildMode
-          }
+          readOnly={isViewingVersion || buildDraft.isActive || buildDraftActionsDisabled}
           selectedVersionSnapshot={isViewingVersion ? activeConfigSnapshot : undefined}
           isBuildDraftActive={buildDraft.isActive}
           buildDraftChangedKeys={buildDraft.changedKeys}
-          showPublishBar={!buildDraft.isActive && !isEnteringBuildMode}
+          showPublishBar={!buildDraft.isActive}
           workflowReferencesEnabled={agentQuery.isSuccess}
           bottomAction={
             showBuildDraftBar ? (
@@ -519,7 +511,7 @@ function AgentConfigurePageComposerContent({
             />
           }
           chat={
-            buildDraft.isPending || isEnteringBuildMode ? (
+            buildDraft.isPending ? (
               <Loading type="app" />
             ) : (
               <AgentConfigureRightPanelChat
@@ -532,6 +524,7 @@ function AgentConfigurePageComposerContent({
                 clearChatList={clearChatByMode[rightPanelChatMode]}
                 controllerRef={rightPanelChatControllerRef}
                 conversationIds={conversationIds}
+                disabled={isEnteringBuildMode}
                 mode={rightPanelChatMode}
                 speechToTextDraftType={
                   rightPanelChatMode === 'build' && buildDraft.isActive ? 'debug_build' : 'draft'
diff --git a/web/features/agent-v2/agent-detail/configure/components/preview/chat-runtime.tsx b/web/features/agent-v2/agent-detail/configure/components/preview/chat-runtime.tsx
index 8ec147c0418..72468b5d18a 100644
--- a/web/features/agent-v2/agent-detail/configure/components/preview/chat-runtime.tsx
+++ b/web/features/agent-v2/agent-detail/configure/components/preview/chat-runtime.tsx
@@ -30,6 +30,7 @@ export type AgentChatRuntimeProps = {
   clearChatList: boolean
   controllerRef?: Ref<AgentPreviewChatController>
   conversationId?: string | null
+  disabled?: boolean
   draftType?: 'debug_build'
   speechToTextDraftType?: 'draft' | 'debug_build'
   inputPlaceholder: string
@@ -57,6 +58,7 @@ export function AgentChatRuntime({
   clearChatList,
   controllerRef,
   conversationId,
+  disabled,
   draftType,
   speechToTextDraftType,
   inputPlaceholder,
@@ -136,6 +138,7 @@ export function AgentChatRuntime({
       clearChatList={clearChatList}
       controllerRef={controllerRef}
       conversationId={conversationId}
+      disabled={disabled}
       draftType={draftType}
       speechToTextDraftType={speechToTextDraftType}
       initialChatTree={initialChatTree}
diff --git a/web/features/agent-v2/agent-detail/configure/components/preview/chat-session.tsx b/web/features/agent-v2/agent-detail/configure/components/preview/chat-session.tsx
index af1ea49507a..7f5eb32bb95 100644
--- a/web/features/agent-v2/agent-detail/configure/components/preview/chat-session.tsx
+++ b/web/features/agent-v2/agent-detail/configure/components/preview/chat-session.tsx
@@ -34,6 +34,7 @@ export function AgentPreviewChatSession({
   clearChatList,
   controllerRef,
   conversationId,
+  disabled,
   draftType,
   speechToTextDraftType,
   initialChatTree,
@@ -61,6 +62,7 @@ export function AgentPreviewChatSession({
   clearChatList: boolean
   controllerRef?: Ref<AgentPreviewChatController>
   conversationId?: string | null
+  disabled?: boolean
   draftType?: 'debug_build'
   speechToTextDraftType?: 'draft' | 'debug_build'
   initialChatTree: ChatItemInTree[]
@@ -115,8 +117,11 @@ export function AgentPreviewChatSession({
       files?: FileEntity[],
       isRegenerate: boolean = false,
       parentAnswer: ChatItem | null = null,
-    ) => conversationRef.current?.send(message, files, isRegenerate, parentAnswer),
-    [],
+    ) => {
+      if (disabled) return
+      return conversationRef.current?.send(message, files, isRegenerate, parentAnswer)
+    },
+    [disabled],
   )
   useImperativeHandle(
     controllerRef,
@@ -141,7 +146,7 @@ export function AgentPreviewChatSession({
     <ChatInputArea
       botName={agentName || 'Agent'}
       customPlaceholder={inputPlaceholder}
-      disabled={isEmptyChat && isResponding}
+      disabled={disabled || (isEmptyChat && isResponding)}
       // Build chat opts out so it does not steal focus from the configure editor.
       // oxlint-disable-next-line jsx-a11y/no-autofocus
       autoFocus={isEmptyChat ? inputAutoFocus : undefined}
diff --git a/web/features/agent-v2/agent-detail/configure/use-agent-configure-build-draft.ts b/web/features/agent-v2/agent-detail/configure/use-agent-configure-build-draft.ts
index 96811dc0b21..ab83bf70f2a 100644
--- a/web/features/agent-v2/agent-detail/configure/use-agent-configure-build-draft.ts
+++ b/web/features/agent-v2/agent-detail/configure/use-agent-configure-build-draft.ts
@@ -365,10 +365,10 @@ export function useAgentConfigureBuildDraftActions({
 
   const startFreshBuildSession = useCallback(async () => {
     cancelBuildDraftRefresh()
+    setSoulSourceOverride('draft')
     try {
       await resetBuildChatSession()
       forceCheckoutBeforeNextBuildRunRef.current = true
-      setSoulSourceOverride('draft')
       return true
     } catch {
       toast.error(tCommon(($) => $['api.actionFailed']))
diff --git a/web/features/agent-v2/agent-detail/configure/use-agent-configure-session-controller.ts b/web/features/agent-v2/agent-detail/configure/use-agent-configure-session-controller.ts
index 9c2114d61b7..2a29e712ca3 100644
--- a/web/features/agent-v2/agent-detail/configure/use-agent-configure-session-controller.ts
+++ b/web/features/agent-v2/agent-detail/configure/use-agent-configure-session-controller.ts
@@ -214,21 +214,23 @@ export function useAgentConfigureSessionController({
           if (pendingBuildModeTransitionRef.current) return
 
           setIsEnteringBuildMode(true)
+          modeRef.current = 'build'
+          rotateBuildCallbackGeneration(true)
+          const buildSessionStart = startFreshBuildSession()
+          const modeChange = (async () => onModeChange(nextMode))()
           const transition = (async () => {
-            try {
-              const started = await startFreshBuildSession()
-              if (!started || modeRef.current !== 'preview') return
+            const [buildSessionResult, modeChangeResult] = await Promise.allSettled([
+              buildSessionStart,
+              modeChange,
+            ])
+            const buildSessionStarted =
+              buildSessionResult.status === 'fulfilled' && buildSessionResult.value
+            const modeChanged = modeChangeResult.status === 'fulfilled'
+            if ((buildSessionStarted && modeChanged) || modeRef.current !== 'build') return
 
-              modeRef.current = 'build'
-              rotateBuildCallbackGeneration(true)
-              try {
-                await onModeChange(nextMode)
-              } catch (error) {
-                modeRef.current = 'preview'
-                rotateBuildCallbackGeneration(false)
-                throw error
-              }
-            } catch {}
+            modeRef.current = 'preview'
+            rotateBuildCallbackGeneration(false)
+            await Promise.resolve(onModeChange('preview')).catch(() => undefined)
           })()
           pendingBuildModeTransitionRef.current = transition
           void transition.finally(() => {