fix(desktop): render reasoning text in the Thinking widget (#63999)

apps/desktop/src/components/assistant-ui/markdown-text.tsx

@@ -15,7 +15,6 @@ import {
   useDeferredValue,
   useEffect,
   useMemo,
-  useRef,
   useState
 } from 'react'
 
@@ -348,112 +347,6 @@ function MarkdownImage({ className, src, alt, ...props }: ComponentProps<'img'>)
   )
 }
 
-// Steady character-reveal for streaming text: decouples visible cadence from
-// bursty arrival so text flows instead of popping (cf. assistant-ui's useSmooth,
-// reimplemented for a tunable rate). Proportional drain — each frame reveals a
-// slice of the backlog so the reveal converges within ~REVEAL_DRAIN_MS whatever
-// the size; the per-frame cap stops a huge dump rendering as one slab. The loop
-// is gated on backlog, not isRunning, so a stream that completes mid-reveal
-// keeps draining its tail instead of snapping.
-const REVEAL_DRAIN_MS = 500
-const REVEAL_MAX_CHARS_PER_FRAME = 30
-// Floor between reveal commits. Each commit republishes the text context and
-// re-runs the whole Streamdown pipeline (preprocess → remend → lex → micromark
-// on the open block) over the full accumulated text — at raw rAF cadence
-// that's 60 full parses/second and was the dominant streaming cost for
-// reasoning text. ~33ms keeps the reveal visually fluid (2 frames) while
-// halving the parse work.
-const REVEAL_MIN_COMMIT_MS = 33
-
-function useSmoothReveal(text: string, isRunning: boolean): string {
-  const [displayed, setDisplayed] = useState(isRunning ? '' : text)
-  const targetRef = useRef(text)
-  const shownRef = useRef(displayed)
-  const frameRef = useRef<number | null>(null)
-  const lastTickRef = useRef(0)
-
-  shownRef.current = displayed
-  targetRef.current = text
-
-  useEffect(() => {
-    if (typeof window === 'undefined') {
-      return
-    }
-
-    // Non-extending change (regenerate / branch / history swap): restart from
-    // empty while streaming, else snap to the replacement.
-    if (!text.startsWith(shownRef.current)) {
-      shownRef.current = isRunning ? '' : text
-      setDisplayed(shownRef.current)
-    }
-
-    if (shownRef.current.length >= text.length || frameRef.current !== null) {
-      return
-    }
-
-    lastTickRef.current = performance.now()
-
-    const tick = () => {
-      const now = performance.now()
-      const dt = now - lastTickRef.current
-
-      // Skip this frame if the floor hasn't elapsed — the backlog math below
-      // is dt-proportional, so delayed commits reveal proportionally more.
-      if (dt < REVEAL_MIN_COMMIT_MS) {
-        frameRef.current = requestAnimationFrame(tick)
-
-        return
-      }
-
-      lastTickRef.current = now
-
-      const remaining = targetRef.current.length - shownRef.current.length
-
-      const add = Math.min(
-        remaining,
-        // dt-scaled so the per-commit cap stays equivalent to the old
-        // per-frame cap at any commit cadence.
-        Math.ceil((REVEAL_MAX_CHARS_PER_FRAME * dt) / 16.7),
-        Math.max(1, Math.ceil((remaining * dt) / REVEAL_DRAIN_MS))
-      )
-
-      shownRef.current = targetRef.current.slice(0, shownRef.current.length + add)
-      setDisplayed(shownRef.current)
-
-      frameRef.current = shownRef.current.length < targetRef.current.length ? requestAnimationFrame(tick) : null
-    }
-
-    frameRef.current = requestAnimationFrame(tick)
-  }, [text, isRunning])
-
-  useEffect(
-    () => () => {
-      if (frameRef.current !== null && typeof window !== 'undefined') {
-        cancelAnimationFrame(frameRef.current)
-      }
-    },
-    []
-  )
-
-  return displayed
-}
-
-// Re-publish the part context with a smooth character-reveal, above
-// DeferStreamingText so the reveal feeds the deferred markdown pipeline. Status
-// stays running while revealing so the caret persists past the underlying part
-// settling.
-function SmoothStreamingText({ children }: { children: ReactNode }) {
-  const { text, status } = useMessagePartText()
-  const isRunning = status.type === 'running'
-  const revealed = useSmoothReveal(text, isRunning)
-
-  return (
-    <TextMessagePartProvider isRunning={isRunning || revealed !== text} text={revealed}>
-      {children}
-    </TextMessagePartProvider>
-  )
-}
-
 /**
  * Re-publish the active message-part context with React's `useDeferredValue`
  * applied to the streaming text and status. The outer wrapper still re-renders
@@ -694,13 +587,14 @@ interface MarkdownTextContentProps extends MarkdownTextSurfaceProps {
 }
 
 export function MarkdownTextContent({ isRunning, text, ...surfaceProps }: MarkdownTextContentProps) {
+  // Same path as the assistant answer. A reasoning-only smoothing wrapper used
+  // to sit here but stalled its char-reveal at empty (the part stays running
+  // the whole message), blanking the Thinking widget.
   return (
     <TextMessagePartProvider isRunning={isRunning} text={text}>
-      <SmoothStreamingText>
-        <DeferStreamingText>
-          <MarkdownTextSurface {...surfaceProps} />
-        </DeferStreamingText>
-      </SmoothStreamingText>
+      <DeferStreamingText>
+        <MarkdownTextSurface {...surfaceProps} />
+      </DeferStreamingText>
     </TextMessagePartProvider>
   )
 }

apps/desktop/src/components/assistant-ui/thread/message-parts.tsx

@@ -1,4 +1,9 @@
-import { type ToolCallMessagePartProps, useAuiState } from '@assistant-ui/react'
+import {
+  type ReasoningMessagePartComponent,
+  type ToolCallMessagePartProps,
+  useAuiState,
+  useMessagePartReasoning
+} from '@assistant-ui/react'
 import { type ComponentProps, type FC, type ReactNode, useEffect, useRef, useState } from 'react'
 
 import { ClarifyTool } from '@/components/assistant-ui/clarify-tool'
@@ -174,17 +179,19 @@ const ReasoningAccordionGroup: FC<{ children?: ReactNode; endIndex: number; star
   )
 }
 
-const ReasoningTextPart: FC<{ text: string; status?: { type: string } }> = ({ text, status }) => {
-  const displayText = text.trimStart()
+// Read the part from context, same contract as MarkdownText's
+// useMessagePartText — the reasoning-only smoothing wrapper (removed) stalled
+// the char-reveal at empty, blanking the widget.
+const ReasoningTextPart: ReasoningMessagePartComponent = () => {
+  const { status, text } = useMessagePartReasoning()
   const messageRunning = useAuiState(s => s.message.status?.type === 'running')
-  const isRunning = status?.type === 'running' || messageRunning
 
   return (
     <MarkdownTextContent
       containerClassName="text-xs leading-snug text-muted-foreground/85"
       containerProps={{ 'data-slot': 'aui_reasoning-text' } as ComponentProps<'div'>}
-      isRunning={isRunning}
-      text={displayText}
+      isRunning={status.type === 'running' || messageRunning}
+      text={text.trimStart()}
     />
   )
 }