diff --git a/backend/src/index.js b/backend/src/index.js
index df87765..d56adac 100644
--- a/backend/src/index.js
+++ b/backend/src/index.js
@@ -80,54 +80,55 @@ app.delete('/api/todos/:id', (req, res) => {
   }
 })
 
-/** POST /api/agent — run AI agent; body: { prompt, context?, contextNodes?, connection? }; returns { markdown }.
- *  connection (from Settings): { provider, baseURL?, model?, apiKey? }. If omitted, uses env (OPENAI_API_KEY / AI_BASE_URL / AI_MODEL).
- */
+/** Build OpenAI client and full prompt from request body. Returns { openai, modelId, fullPrompt } or { error }. */
+function buildAgentRequest(body) {
+  const { prompt, context, contextNodes, connection: conn, reasoning } = body ?? {}
+  const reasoningEnabled = Boolean(reasoning)
+  let baseURL = process.env.AI_BASE_URL?.trim() || null
+  let apiKey = process.env.OPENAI_API_KEY?.trim() || null
+  let modelId = process.env.AI_MODEL?.trim() || (baseURL ? 'local-model' : 'gpt-4o-mini')
+  if (conn && typeof conn === 'object') {
+    const c = conn
+    const provider = c.provider === 'openai' ? 'openai' : 'local'
+    if (provider === 'local') {
+      baseURL = (typeof c.baseURL === 'string' && c.baseURL.trim()) ? c.baseURL.trim() : baseURL
+      apiKey = (typeof c.apiKey === 'string' && c.apiKey.trim()) ? c.apiKey.trim() : (apiKey || 'lm-studio')
+    } else {
+      baseURL = null
+      apiKey = (typeof c.apiKey === 'string' && c.apiKey.trim()) ? c.apiKey.trim() : apiKey
+    }
+    if (typeof c.model === 'string' && c.model.trim()) modelId = c.model.trim()
+  }
+  if (!baseURL && !apiKey) {
+    return { error: 'No AI configured. Set connection in Settings (AI) or env: OPENAI_API_KEY or AI_BASE_URL.' }
+  }
+  const basePrompt = [
+    typeof prompt === 'string' ? prompt : 'No prompt provided.',
+    context && typeof context === 'string' ? `\n\nAdditional context:\n${context}` : '',
+    Array.isArray(contextNodes) && contextNodes.length > 0
+      ? `\n\nContext from connected nodes:\n${contextNodes.map((n) => (n.content != null ? n.content : `${n.id}: (no content)`)).join('\n\n')}`
+      : '',
+  ].join('')
+  const fullPrompt = reasoningEnabled
+    ? basePrompt + '\n\nRespond in exactly two markdown sections. First: "## Reasoning" with your step-by-step reasoning. Then: "## Output" with only the final answer. No preamble.'
+    : basePrompt + '\n\nRespond with structured markdown only. No preamble.'
+  const { createOpenAI } = require('@ai-sdk/openai')
+  const openai = createOpenAI({
+    apiKey: apiKey || 'lm-studio',
+    ...(baseURL && { baseURL, compatibility: 'compatible' }),
+  })
+  return { openai, modelId, fullPrompt }
+}
+
+/** POST /api/agent — run AI agent; body: { prompt, context?, contextNodes?, connection? }; returns { markdown }. */
 app.post('/api/agent', async (req, res) => {
   try {
-    const body = req.body ?? {}
-    const { prompt, context, contextNodes, connection: conn } = body
-
-    let baseURL = process.env.AI_BASE_URL?.trim() || null
-    let apiKey = process.env.OPENAI_API_KEY?.trim() || null
-    let modelId = process.env.AI_MODEL?.trim() || (baseURL ? 'local-model' : 'gpt-4o-mini')
-
-    if (conn && typeof conn === 'object') {
-      const c = conn
-      const provider = c.provider === 'openai' ? 'openai' : 'local'
-      if (provider === 'local') {
-        baseURL = (typeof c.baseURL === 'string' && c.baseURL.trim()) ? c.baseURL.trim() : baseURL
-        apiKey = (typeof c.apiKey === 'string' && c.apiKey.trim()) ? c.apiKey.trim() : (apiKey || 'lm-studio')
-      } else {
-        baseURL = null
-        apiKey = (typeof c.apiKey === 'string' && c.apiKey.trim()) ? c.apiKey.trim() : apiKey
-      }
-      if (typeof c.model === 'string' && c.model.trim()) modelId = c.model.trim()
-    }
-
-    if (!baseURL && !apiKey) {
-      return res.status(503).json({
-        error: 'No AI configured. Set connection in Settings (AI) or env: OPENAI_API_KEY or AI_BASE_URL.',
-      })
-    }
-
-    const fullPrompt = [
-      typeof prompt === 'string' ? prompt : 'No prompt provided.',
-      context && typeof context === 'string' ? `\n\nAdditional context:\n${context}` : '',
-      Array.isArray(contextNodes) && contextNodes.length > 0
-        ? `\n\nContext from connected nodes:\n${contextNodes.map((n) => (n.content != null ? n.content : `${n.id}: (no content)`)).join('\n\n')}`
-        : '',
-    ].join('')
-
+    const built = buildAgentRequest(req.body)
+    if (built.error) return res.status(503).json({ error: built.error })
     const { generateText } = await import('ai')
-    const { createOpenAI } = await import('@ai-sdk/openai')
-    const openai = createOpenAI({
-      apiKey: apiKey || 'lm-studio',
-      ...(baseURL && { baseURL, compatibility: 'compatible' }),
-    })
     const result = await generateText({
-      model: openai(modelId),
-      prompt: fullPrompt + '\n\nRespond with structured markdown only. No preamble.',
+      model: built.openai(built.modelId),
+      prompt: built.fullPrompt,
     })
     const markdown = result?.text ?? ''
     res.json({ markdown })
@@ -137,6 +138,25 @@ app.post('/api/agent', async (req, res) => {
   }
 })
 
+/** POST /api/agent/stream — same as /api/agent but streams plain text (markdown) chunks. */
+app.post('/api/agent/stream', async (req, res) => {
+  try {
+    const built = buildAgentRequest(req.body)
+    if (built.error) return res.status(503).json({ error: built.error })
+    const { streamText } = await import('ai')
+    const result = streamText({
+      model: built.openai(built.modelId),
+      prompt: built.fullPrompt,
+    })
+    res.setHeader('Content-Type', 'text/plain; charset=utf-8')
+    res.setHeader('Transfer-Encoding', 'chunked')
+    result.pipeTextStreamToResponse(res)
+  } catch (err) {
+    console.error('Agent stream error:', err)
+    res.status(500).json({ error: err?.message ?? 'Agent request failed' })
+  }
+})
+
 /** Health check for Docker / orchestration */
 app.get('/health', (req, res) => {
   res.status(200).json({ ok: true })
diff --git a/frontend/src/components/nodes/agent/AgentNode.tsx b/frontend/src/components/nodes/agent/AgentNode.tsx
index 6ce4f88..c516357 100644
--- a/frontend/src/components/nodes/agent/AgentNode.tsx
+++ b/frontend/src/components/nodes/agent/AgentNode.tsx
@@ -18,10 +18,13 @@ import FlowContext from '@/lib/graph/flowContext'
 import { useSyncConnectionStatus } from '@/lib/graph/nodeLifecycle'
 import { getNodeType } from '@/lib/graph/nodeRegistry'
 import { Bot } from 'lucide-react'
+import { Checkbox } from '@/components/ui/checkbox'
 
 export type AgentNodeData = {
   /** Additional context for the agent (configuration). */
   context?: string
+  /** When true, agent returns reasoning in a "## Reasoning" section and output in "## Output". */
+  reasoning?: boolean
   /** Cached output from last run; set by connected Rendering node. Not displayed on this node. */
   outputMarkdown?: string
   /** Signature of inputs from last successful run; set by Rendering node when it runs the agent. Used as cache. */
@@ -42,6 +45,12 @@ function AgentNodeComponent({ id, data, width, height, selected }: Props) {
       : undefined
 
   const contextText = data?.context ?? ''
+  const reasoningEnabled = data?.reasoning ?? false
+
+  const onReasoningChange = useCallback(
+    (checked: boolean) => updateData({ reasoning: checked }),
+    [updateData]
+  )
 
   const connectedSources = useMemo(() => {
     return sourceIds.map((sid) => {
@@ -87,6 +96,21 @@ function AgentNodeComponent({ id, data, width, height, selected }: Props) {
           <NodeMenubar nodeId={id} nodeType="agent" />
         </div>
         <div className="flex flex-col gap-3 p-2 min-h-0 flex-1">
+          <div className="flex flex-col gap-1.5">
+            <label htmlFor={`agent-reasoning-${id}`} className="flex items-center gap-2 text-xs font-medium text-foreground cursor-pointer">
+              <Checkbox
+                id={`agent-reasoning-${id}`}
+                checked={reasoningEnabled}
+                onCheckedChange={(c) => onReasoningChange(c === true)}
+                className="nodrag nopan"
+                aria-label="Enable reasoning"
+              />
+              Enable reasoning
+            </label>
+            <p className="text-xs text-muted-foreground">
+              When enabled, the model will output a Reasoning section and an Output section; the renderer shows them separately.
+            </p>
+          </div>
           <div className="flex flex-col gap-1.5">
             <label className="text-xs font-medium text-foreground">Context</label>
             <textarea
diff --git a/frontend/src/components/nodes/agent/renderingLogic.ts b/frontend/src/components/nodes/agent/renderingLogic.ts
index 7d7cf18..3e1bb6b 100644
--- a/frontend/src/components/nodes/agent/renderingLogic.ts
+++ b/frontend/src/components/nodes/agent/renderingLogic.ts
@@ -52,13 +52,29 @@ function buildSourceSignature(
   return JSON.stringify({ sourceIds: sourceIds.slice().sort(), configContents })
 }
 
+/** Parse agent response into reasoning and output when format is "## Reasoning" / "## Output". */
+function parseReasoningAndOutput(fullMarkdown: string): { reasoning?: string; output: string } {
+  const normalized = fullMarkdown.trim()
+  const outputMatch = normalized.match(/\s*##\s*Output\s*/i)
+  if (!outputMatch) return { output: fullMarkdown }
+  const idx = outputMatch.index! + outputMatch[0].length
+  const output = normalized.slice(idx).trim()
+  const beforeOutput = normalized.slice(0, outputMatch.index).trim()
+  const reasoningMatch = beforeOutput.match(/\s*##\s*Reasoning\s*/i)
+  const reasoning = reasoningMatch
+    ? beforeOutput.slice(reasoningMatch.index! + reasoningMatch[0].length).trim()
+    : beforeOutput
+  return { reasoning: reasoning || undefined, output: output || fullMarkdown }
+}
+
 export const agentRenderingLogic = {
   defaultUpdateMode: 'manual' as const,
   getResolvedContent: async (context: SourceRenderingLogicContext): Promise<ResolvedContentResult> => {
-    const { nodes, sourceNodeId, setNodes, aiConnection } = context
+    const { nodes, sourceNodeId, setNodes, aiConnection, onStreamingStart, onStreamingChunk } = context
     const sourceNode = nodes.find((n) => n.id === sourceNodeId)
-    const agentData = sourceNode?.data as { context?: string; outputMarkdown?: string } | undefined
+    const agentData = sourceNode?.data as { context?: string; outputMarkdown?: string; reasoning?: boolean } | undefined
     const contextText = agentData?.context ?? ''
+    const reasoningEnabled = Boolean(agentData?.reasoning)
     const sourceIds = (context.edges
       .filter((e) => e.target === sourceNodeId)
       .map((e) => e.source)) as string[]
@@ -94,8 +110,61 @@ export const agentRenderingLogic = {
         content: serializeNodeForContext(nodes, sid),
       }))
       const sourceSignature = buildSourceSignature(nodes, sourceIds)
+      const useStream = Boolean(onStreamingChunk)
 
       try {
+        if (useStream) {
+          onStreamingStart?.()
+          const res = await fetch('/api/agent/stream', {
+            method: 'POST',
+            headers: { 'Content-Type': 'application/json' },
+            body: JSON.stringify({
+              prompt,
+              context: contextText.trim() || undefined,
+              contextNodes,
+              connection,
+              reasoning: reasoningEnabled,
+            }),
+          })
+          if (!res.ok) {
+            const text = await res.text().catch(() => '')
+            let msg = `Request failed: ${res.status}`
+            try {
+              const json = text ? JSON.parse(text) : {}
+              if (typeof (json as { error?: string }).error === 'string') msg = (json as { error: string }).error
+            } catch {
+              if (text) msg = text.slice(0, 200)
+            }
+            setAgentData({ outputMarkdown: undefined })
+            throw new Error(msg)
+          }
+          const reader = res.body?.getReader()
+          const decoder = new TextDecoder()
+          let markdown = ''
+          try {
+            if (reader) {
+              while (true) {
+                const { done, value } = await reader.read()
+                if (done) break
+                const chunk = decoder.decode(value, { stream: true })
+                markdown += chunk
+                onStreamingChunk?.(chunk)
+              }
+            }
+          } finally {
+            reader?.cancel()
+          }
+          setAgentData({
+            outputMarkdown: markdown,
+            lastRunSourceSignature: sourceSignature,
+          })
+          if (reasoningEnabled) {
+            const { reasoning: reasoningText, output } = parseReasoningAndOutput(markdown)
+            return { resolved: output, outputTypeId: 'markdown' as const, reasoning: reasoningText }
+          }
+          return { resolved: markdown, outputTypeId: 'markdown' }
+        }
+
         const res = await fetch('/api/agent', {
           method: 'POST',
           headers: { 'Content-Type': 'application/json' },
@@ -104,6 +173,7 @@ export const agentRenderingLogic = {
             context: contextText.trim() || undefined,
             contextNodes,
             connection,
+            reasoning: reasoningEnabled,
           }),
         })
         const json = await res.json().catch(() => ({}))
@@ -117,6 +187,10 @@ export const agentRenderingLogic = {
           outputMarkdown: markdown,
           lastRunSourceSignature: sourceSignature,
         })
+        if (reasoningEnabled) {
+          const { reasoning: reasoningText, output } = parseReasoningAndOutput(markdown)
+          return { resolved: output, outputTypeId: 'markdown' as const, reasoning: reasoningText }
+        }
         return { resolved: markdown, outputTypeId: 'markdown' }
       } catch (err: unknown) {
         setAgentData({ outputMarkdown: undefined })
@@ -126,6 +200,10 @@ export const agentRenderingLogic = {
 
     const outputMarkdown =
       (sourceNode?.data as { outputMarkdown?: string } | undefined)?.outputMarkdown ?? ''
+    if (reasoningEnabled && outputMarkdown) {
+      const { reasoning: reasoningText, output } = parseReasoningAndOutput(outputMarkdown)
+      return { resolved: output, outputTypeId: 'markdown' as const, reasoning: reasoningText }
+    }
     return { resolved: outputMarkdown, outputTypeId: 'markdown' }
   },
 }
diff --git a/frontend/src/components/nodes/render/RenderingNode.tsx b/frontend/src/components/nodes/render/RenderingNode.tsx
index bc7e36a..9e35c1a 100644
--- a/frontend/src/components/nodes/render/RenderingNode.tsx
+++ b/frontend/src/components/nodes/render/RenderingNode.tsx
@@ -28,6 +28,7 @@ import { NodeHeaderTitle } from '@/components/graph/NodeHeaderTitle'
 import { NodeMenubar } from '@/components/graph/NodeMenubar'
 import { NodeStatusIndicator } from '@/components/graph/NodeStatusIndicator'
 import { MenubarCheckboxItem, MenubarItem, MenubarSeparator, MenubarSub, MenubarSubContent, MenubarSubTrigger } from '@/components/ui/menubar'
+import { Collapsible, CollapsibleContent, CollapsibleTrigger } from '@/components/ui/collapsible'
 import { Sparkles, ZoomIn, ZoomOut, RotateCcw, RotateCw, Copy, Play, ChevronDown, Loader2 } from 'lucide-react'
 import { InputHandle } from '@/components/graph/NodeHandles'
 import { TransformWrapper, TransformComponent } from 'react-zoom-pan-pinch'
@@ -55,6 +56,10 @@ export type RenderingNodeData = {
     runTrigger?: number
     /** Signature of inputs used in the last successful render. Used in manual mode to show paused (yellow) when upstream changed. */
     lastRunSourceSignature?: string
+    /** Cached render output so fullscreen can show content (set when render completes, cleared when new run starts). */
+    cachedRenderedContent?: string
+    cachedResolvedContent?: string
+    cachedReasoningContent?: string
 }
 
 const DEFAULT_VIEWPORT_WIDTH = 1200
@@ -64,14 +69,31 @@ type Props = AbstractNodeProps<RenderingNodeData>
 
 type ViewMode = 'preview' | 'raw'
 
+/** Extract <think>...</think> blocks from HTML/markdown; return main content (with blocks removed) and think content for a collapsible. */
+function parseThinkSections(html: string): { main: string; think: string } {
+    const thinkRegex = /<think>([\s\S]*?)<\/think>/gi
+    const thinkParts: string[] = []
+    let match
+    while ((match = thinkRegex.exec(html)) !== null) {
+        thinkParts.push(match[1].trim())
+    }
+    const think = thinkParts.join('\n\n')
+    const main = html.replace(thinkRegex, '').replace(/\n{3,}/g, '\n\n').trim()
+    return { main, think }
+}
+
 function RenderingNodeComponent({ id, data, width, height, selected }: Props) {
     const flowContext = useContext(FlowContext)
     const setFullscreenNodeId = flowContext?.setFullscreenNodeId
     const supportsFullscreen = getNodeType('render')?.supportsFullscreen
-    const [renderedContent, setRenderedContent] = useState<string | null>(null)
-    const [resolvedContent, setResolvedContent] = useState<string | null>(null)
+    const [renderedContent, setRenderedContent] = useState<string | null>(() => (data?.cachedRenderedContent as string | undefined) ?? null)
+    const [resolvedContent, setResolvedContent] = useState<string | null>(() => (data?.cachedResolvedContent as string | undefined) ?? null)
     const [error, setError] = useState<null | { kind: string; message: string }>(null)
     const [loading, setLoading] = useState(false)
+    const [streamingMarkdown, setStreamingMarkdown] = useState<string | null>(null)
+    const [streamingPreviewHtml, setStreamingPreviewHtml] = useState<string>('')
+    const [reasoningContent, setReasoningContent] = useState<string>(() => (data?.cachedReasoningContent as string | undefined) ?? '')
+    const [reasoningHtml, setReasoningHtml] = useState<string>('')
     const [retryCount, setRetryCount] = useState(0)
     const [viewMode, setViewMode] = useState<ViewMode>('preview')
     const [viewportFocused, setViewportFocused] = useState(false)
@@ -79,7 +101,10 @@ function RenderingNodeComponent({ id, data, width, height, selected }: Props) {
     const loadingStartedAtRef = useRef<number | null>(null)
     const minLoadingTimeoutRef = useRef<ReturnType<typeof setTimeout> | null>(null)
     const lastManualRunTriggerRef = useRef<number>(0)
+    const manualRunTriggerSyncedRef = useRef<boolean>(false)
+    const nodesEdgesRef = useRef<{ nodes: unknown[]; edges: unknown[] }>({ nodes: [], edges: [] })
     const { nodes, edges, setNodes, setEdges, sourceIds, updateData } = useAbstractNode<RenderingNodeData>(id, data ?? {})
+    nodesEdgesRef.current = { nodes, edges }
     const { aiConnection } = usePlatform()
     const viewportWidth = data?.viewportWidth ?? DEFAULT_VIEWPORT_WIDTH
     const viewportHeight = data?.viewportHeight ?? DEFAULT_VIEWPORT_HEIGHT
@@ -262,20 +287,30 @@ function RenderingNodeComponent({ id, data, width, height, selected }: Props) {
             setLoading(false)
             return
         }
-        if (effectiveUpdateMode === 'manual' && runTrigger === 0) {
-            setRenderedContent(null)
-            setResolvedContent(null)
-            setError({
-                kind: 'no-content',
-                message: 'Click Run to render.',
-            })
-            setLoading(false)
-            return
+        if (effectiveUpdateMode === 'manual') {
+            if (!manualRunTriggerSyncedRef.current) {
+                lastManualRunTriggerRef.current = runTrigger
+                manualRunTriggerSyncedRef.current = true
+                return
+            }
+            if (runTrigger === 0) {
+                const hasCachedOutput = Boolean((data?.cachedRenderedContent ?? data?.cachedResolvedContent) as string | undefined)
+                if (!hasCachedOutput) {
+                    setRenderedContent(null)
+                    setResolvedContent(null)
+                    setError({
+                        kind: 'no-content',
+                        message: 'Click Run to render.',
+                    })
+                    setLoading(false)
+                }
+                return
+            }
+            if (runTrigger === lastManualRunTriggerRef.current) {
+                return
+            }
+            lastManualRunTriggerRef.current = runTrigger
         }
-        if (effectiveUpdateMode === 'manual' && runTrigger === lastManualRunTriggerRef.current) {
-            return
-        }
-        if (effectiveUpdateMode === 'manual') lastManualRunTriggerRef.current = runTrigger
 
         runIdRef.current += 1
         const thisRunId = runIdRef.current
@@ -287,43 +322,59 @@ function RenderingNodeComponent({ id, data, width, height, selected }: Props) {
             loadingStartedAtRef.current = Date.now()
             setLoading(true)
             setError(null)
+            setRenderedContent(null)
+            setResolvedContent(null)
+            setStreamingMarkdown(null)
+            setReasoningContent('')
+            updateData({ cachedRenderedContent: undefined, cachedResolvedContent: undefined, cachedReasoningContent: undefined })
             try {
+                const { nodes: ctxNodes, edges: ctxEdges } = nodesEdgesRef.current
                 const context = {
-                    nodes,
-                    edges,
+                    nodes: ctxNodes,
+                    edges: ctxEdges,
                     sourceNodeId: srcId!,
                     renderNodeId: id,
                     viewportWidth,
                     viewportHeight,
                     setNodes: setNodes ?? undefined,
                     aiConnection,
+                    ...(isAgentSource && {
+                        onStreamingStart: () => setStreamingMarkdown(''),
+                        onStreamingChunk: (chunk: string) => setStreamingMarkdown((prev) => (prev ?? '') + chunk),
+                    }),
                 } as SourceRenderingLogicContext
-                const { resolved, outputTypeId } = await logic.getResolvedContent(context)
-                if (cancelled || thisRunId !== runIdRef.current) return
+                const { resolved, outputTypeId, reasoning } = await logic.getResolvedContent(context)
+                if (thisRunId !== runIdRef.current) return
                 setResolvedContent(resolved)
+                setReasoningContent(reasoning ?? '')
                 const typeRenderer = getConfigType(outputTypeId)
                 const renderOptions = outputTypeId === 'wireframe' ? { width: viewportWidth, height: viewportHeight } : undefined
                 const htmlOrSvg = await typeRenderer.render(resolved, renderOptions)
-                if (cancelled || thisRunId !== runIdRef.current) return
+                if (thisRunId !== runIdRef.current) return
                 setRenderedContent(htmlOrSvg)
                 setError(null)
-                if (isManualMode) {
-                    updateData({ lastRunSourceSignature: signatureForThisRun })
-                }
+                updateData({
+                    cachedRenderedContent: htmlOrSvg,
+                    cachedResolvedContent: resolved,
+                    cachedReasoningContent: reasoning ?? '',
+                    ...(isManualMode ? { lastRunSourceSignature: signatureForThisRun } : {}),
+                })
             } catch (err: any) {
                 if (!cancelled && thisRunId === runIdRef.current) {
                     setRenderedContent(null)
+                    setReasoningContent('')
                     setError({ kind: 'render', message: err?.message ?? 'Render error' })
                 }
             } finally {
-                if (!cancelled && thisRunId === runIdRef.current) {
+                setStreamingMarkdown(null)
+                if (thisRunId === runIdRef.current) {
                     const startedAt = loadingStartedAtRef.current ?? 0
                     const elapsed = Date.now() - startedAt
                     const remaining = Math.max(0, 1000 - elapsed)
                     if (remaining > 0) {
                         minLoadingTimeoutRef.current = setTimeout(() => {
                             minLoadingTimeoutRef.current = null
-                            if (!cancelled && thisRunId === runIdRef.current) {
+                            if (thisRunId === runIdRef.current) {
                                 setLoading(false)
                             }
                         }, remaining)
@@ -349,13 +400,14 @@ function RenderingNodeComponent({ id, data, width, height, selected }: Props) {
         run()
         return () => {
             cancelled = true
+            setStreamingMarkdown(null)
             if (minLoadingTimeoutRef.current != null) {
                 clearTimeout(minLoadingTimeoutRef.current)
                 minLoadingTimeoutRef.current = null
             }
-            setLoading(false)
+            // Do not setLoading(false) here: the in-flight run() will set it in finally. Clearing it here would hide the Run button spinner when the effect re-runs (e.g. agent updates nodes).
         }
-    }, [id, srcId, srcNode?.type, effectiveUpdateMode, runTrigger, sourceContent, sourceSignature, configSignature, edgesSignature, variablesSignature, functionsSignature, dataSignature, viewportWidth, viewportHeight, retryCount, nodes, edges, updateData, setNodes, aiConnection])
+    }, [id, srcId, srcNode?.type, effectiveUpdateMode, runTrigger, sourceContent, sourceSignature, configSignature, edgesSignature, variablesSignature, functionsSignature, dataSignature, viewportWidth, viewportHeight, retryCount, updateData, setNodes, aiConnection, isAgentSource])
 
 
     const dimensions =
@@ -363,20 +415,70 @@ function RenderingNodeComponent({ id, data, width, height, selected }: Props) {
             ? { width, height }
             : undefined
 
+    // Render streaming agent markdown to HTML for preview
+    useEffect(() => {
+        if (streamingMarkdown === null) {
+            setStreamingPreviewHtml('')
+            return
+        }
+        let cancelled = false
+        import('marked').then(async ({ marked }) => {
+            if (cancelled) return
+            const html = typeof marked.parse === 'function' ? await marked.parse(streamingMarkdown) : (marked as (s: string) => string)(streamingMarkdown)
+            const str = typeof html === 'string' ? html : String(html)
+            if (!cancelled) setStreamingPreviewHtml(str)
+        }).catch(() => {
+            if (!cancelled) setStreamingPreviewHtml(streamingMarkdown)
+        })
+        return () => { cancelled = true }
+    }, [streamingMarkdown])
+
+    // Render reasoning markdown to HTML for collapsible section
+    useEffect(() => {
+        if (!reasoningContent) {
+            setReasoningHtml('')
+            return
+        }
+        let cancelled = false
+        import('marked').then(async ({ marked }) => {
+            if (cancelled) return
+            const html = typeof marked.parse === 'function' ? await marked.parse(reasoningContent) : (marked as (s: string) => string)(reasoningContent)
+            const str = typeof html === 'string' ? html : String(html)
+            if (!cancelled) setReasoningHtml(str)
+        }).catch(() => {
+            if (!cancelled) setReasoningHtml(reasoningContent)
+        })
+        return () => { cancelled = true }
+    }, [reasoningContent])
+
     // Kroki and other SVG sources may prepend <?xml ... ?> so we detect by presence of <svg> tag
     const isSvgOutput = Boolean(renderedContent?.trim() && /<svg[\s>]/i.test(renderedContent.trim()))
 
-    /** Strip remaining Nunjucks tags from resolved content for display in raw view (so tags don't show as literal lines). */
+    /** Split final markdown HTML into <think> (collapsible) and main. Only for non-image output. */
+    const renderedThinkSplit = useMemo(() => {
+        if (outputType === 'image' || !renderedContent) return { main: '', think: '' }
+        return parseThinkSections(renderedContent)
+    }, [renderedContent, outputType])
+
+    /** Split streaming content into <think> (collapsible) and main. Uses HTML when ready, else raw markdown. */
+    const streamingThinkSplit = useMemo(() => {
+        if (streamingMarkdown === null) return { main: '', think: '' }
+        const src = streamingPreviewHtml || streamingMarkdown
+        return parseThinkSections(src)
+    }, [streamingMarkdown, streamingPreviewHtml])
+
+    /** Raw markdown/text for Raw view: resolved content when set, otherwise streaming accumulation (so raw updates while streaming). */
     const rawDisplayContent = useMemo(() => {
-        if (resolvedContent == null) return ''
-        return resolvedContent
+        const src = resolvedContent != null ? resolvedContent : (streamingMarkdown ?? '')
+        if (!src) return ''
+        return src
             .replace(/\{%[\s\S]*?%\}/g, '')
             .replace(/\{\{[\s\S]*?\}\}/g, '')
             .replace(/\{#[\s\S]*?#\}/g, '')
             .replace(/(\r?\n)\s*(\r?\n)/g, '$1$2')
             .replace(/^\s*\n|\n\s*$/g, (m) => (m === '\n' ? '\n' : ''))
             .trim()
-    }, [resolvedContent])
+    }, [resolvedContent, streamingMarkdown])
 
     /** Process SVG HTML so it keeps aspect ratio and fills the viewport (used only for display, not download). */
     const displayContent = useMemo(() => {
@@ -653,7 +755,7 @@ function RenderingNodeComponent({ id, data, width, height, selected }: Props) {
                         />
                     </div>
                     <div className="min-h-0 flex-1 flex flex-col">
-                        {incomingIds.length === 0 ? (
+                        {incomingIds.length === 0 && !renderedContent && !streamingMarkdown && !loading ? (
                             <Empty className="min-h-0 flex-1">
                                 <EmptyHeader>
                                     <EmptyMedia variant="icon">
@@ -700,8 +802,6 @@ function RenderingNodeComponent({ id, data, width, height, selected }: Props) {
                                     </Button>
                                 </div>
                             )
-                        ) : loading ? (
-                            <div className="flex min-h-0 flex-1 items-center justify-center text-xs text-muted-foreground">Rendering…</div>
                         ) : viewMode === 'raw' ? (
                             <div ref={rawEditorContainerRef} className="relative min-h-0 flex-1 w-full nodrag nopan overflow-hidden border-t border-input">
                                 <Button
@@ -732,6 +832,7 @@ function RenderingNodeComponent({ id, data, width, height, selected }: Props) {
                                 />
                             </div>
                         ) : renderedContent ? (
+                            /* Show final content as soon as it's ready (even if still loading), so streamed preview isn't replaced by "Rendering…" */
                             outputType === 'image' ? (
                                 <div
                                     className="rendering-viewport nodrag nopan relative min-h-0 flex-1 w-full min-w-0 overflow-hidden bg-white dark:bg-secondary rounded outline-none"
@@ -815,11 +916,76 @@ function RenderingNodeComponent({ id, data, width, height, selected }: Props) {
                                     </TransformWrapper>
                                 </div>
                             ) : (
-                                <div
-                                    className="rendering-markdown min-h-0 flex-1 w-full overflow-auto p-3 text-sm [&_h1]:text-xl [&_h2]:text-lg [&_h3]:text-base [&_ul]:list-disc [&_ol]:list-decimal [&_ul]:pl-5 [&_ol]:pl-5 [&_p]:my-2 [&_pre]:bg-muted [&_pre]:p-2 [&_pre]:rounded"
-                                    dangerouslySetInnerHTML={{ __html: renderedContent }}
-                                />
+                                <div className="min-h-0 flex-1 flex flex-col overflow-hidden">
+                                    {reasoningHtml ? (
+                                        <Collapsible defaultOpen={false} className="group shrink-0 border-b border-border">
+                                            <CollapsibleTrigger className="flex w-full items-center justify-between px-3 py-2 text-left text-xs font-medium text-muted-foreground hover:bg-muted/50 nodrag nopan">
+                                                Reasoning
+                                                <ChevronDown className="size-3.5 shrink-0 transition-transform group-data-[state=open]:rotate-180" />
+                                            </CollapsibleTrigger>
+                                            <CollapsibleContent>
+                                                <div
+                                                    className="rendering-markdown max-h-48 overflow-auto p-3 text-sm [&_h1]:text-lg [&_h2]:text-base [&_h3]:text-sm [&_ul]:list-disc [&_ol]:list-decimal [&_ul]:pl-5 [&_ol]:pl-5 [&_p]:my-1.5 [&_pre]:bg-muted [&_pre]:p-2 [&_pre]:rounded text-muted-foreground"
+                                                    dangerouslySetInnerHTML={{ __html: reasoningHtml }}
+                                                />
+                                            </CollapsibleContent>
+                                        </Collapsible>
+                                    ) : null}
+                                    {renderedThinkSplit.think ? (
+                                        <Collapsible defaultOpen={false} className="group shrink-0 border-b border-border">
+                                            <CollapsibleTrigger className="flex w-full items-center justify-between px-3 py-2 text-left text-xs font-medium text-muted-foreground hover:bg-muted/50 nodrag nopan">
+                                                Thinking
+                                                <ChevronDown className="size-3.5 shrink-0 transition-transform group-data-[state=open]:rotate-180" />
+                                            </CollapsibleTrigger>
+                                            <CollapsibleContent>
+                                                <div
+                                                    className="rendering-markdown max-h-48 overflow-auto p-3 text-sm [&_h1]:text-lg [&_h2]:text-base [&_h3]:text-sm [&_ul]:list-disc [&_ol]:list-decimal [&_ul]:pl-5 [&_ol]:pl-5 [&_p]:my-1.5 [&_pre]:bg-muted [&_pre]:p-2 [&_pre]:rounded text-muted-foreground"
+                                                    dangerouslySetInnerHTML={{ __html: renderedThinkSplit.think }}
+                                                />
+                                            </CollapsibleContent>
+                                        </Collapsible>
+                                    ) : null}
+                                    <div
+                                        className="rendering-markdown min-h-0 flex-1 w-full overflow-auto p-3 text-sm [&_h1]:text-xl [&_h2]:text-lg [&_h3]:text-base [&_ul]:list-disc [&_ol]:list-decimal [&_ul]:pl-5 [&_ol]:pl-5 [&_p]:my-2 [&_pre]:bg-muted [&_pre]:p-2 [&_pre]:rounded"
+                                        dangerouslySetInnerHTML={{ __html: renderedThinkSplit.main }}
+                                    />
+                                </div>
                             )
+                        ) : loading && viewMode === 'preview' && streamingMarkdown !== null ? (
+                            <div className="min-h-0 flex-1 flex flex-col overflow-hidden">
+                                {streamingThinkSplit.think ? (
+                                    <Collapsible defaultOpen={false} className="group shrink-0 border-b border-border">
+                                        <CollapsibleTrigger className="flex w-full items-center justify-between px-3 py-2 text-left text-xs font-medium text-muted-foreground hover:bg-muted/50 nodrag nopan">
+                                            Thinking
+                                            <ChevronDown className="size-3.5 shrink-0 transition-transform group-data-[state=open]:rotate-180" />
+                                        </CollapsibleTrigger>
+                                        <CollapsibleContent>
+                                            {streamingPreviewHtml ? (
+                                                <div
+                                                    className="rendering-markdown max-h-48 overflow-auto p-3 text-sm [&_h1]:text-lg [&_h2]:text-base [&_h3]:text-sm [&_ul]:list-disc [&_ol]:list-decimal [&_ul]:pl-5 [&_ol]:pl-5 [&_p]:my-1.5 [&_pre]:bg-muted [&_pre]:p-2 [&_pre]:rounded text-muted-foreground"
+                                                    dangerouslySetInnerHTML={{ __html: streamingThinkSplit.think }}
+                                                />
+                                            ) : (
+                                                <pre className="rendering-markdown max-h-48 overflow-auto whitespace-pre-wrap break-words p-3 text-sm font-sans text-muted-foreground">
+                                                    {streamingThinkSplit.think}
+                                                </pre>
+                                            )}
+                                        </CollapsibleContent>
+                                    </Collapsible>
+                                ) : null}
+                                {streamingPreviewHtml ? (
+                                    <div
+                                        className="rendering-markdown min-h-0 flex-1 w-full overflow-auto p-3 text-sm [&_h1]:text-xl [&_h2]:text-lg [&_h3]:text-base [&_ul]:list-disc [&_ol]:list-decimal [&_ul]:pl-5 [&_ol]:pl-5 [&_p]:my-2 [&_pre]:bg-muted [&_pre]:p-2 [&_pre]:rounded"
+                                        dangerouslySetInnerHTML={{ __html: streamingThinkSplit.main || streamingPreviewHtml }}
+                                    />
+                                ) : (
+                                    <pre className="min-h-0 flex-1 w-full overflow-auto whitespace-pre-wrap break-words p-3 text-sm font-sans">
+                                        {streamingThinkSplit.main || streamingMarkdown}
+                                    </pre>
+                                )}
+                            </div>
+                        ) : loading ? (
+                            <div className="flex min-h-0 flex-1 items-center justify-center text-xs text-muted-foreground">Rendering…</div>
                         ) : null}
                     </div>
                 </BaseNodeContent>
diff --git a/frontend/src/lib/graph/sourceRenderingLogic.ts b/frontend/src/lib/graph/sourceRenderingLogic.ts
index 8637190..1566af4 100644
--- a/frontend/src/lib/graph/sourceRenderingLogic.ts
+++ b/frontend/src/lib/graph/sourceRenderingLogic.ts
@@ -20,14 +20,21 @@ export type SourceRenderingLogicContext = {
   setNodes?: (updater: (nodes: { id: string; type?: string; data?: unknown }[]) => { id: string; type?: string; data?: unknown }[]) => void
   /** Optional: AI connection for agent source (used when Rendering node runs the agent). */
   aiConnection?: unknown
+  /** Optional: called when agent streaming starts (Rendering node can show streaming preview). */
+  onStreamingStart?: () => void
+  /** Optional: called with each text chunk during agent stream (Rendering node can update preview). */
+  onStreamingChunk?: (chunk: string) => void
 }
 
 /**
  * Result of getResolvedContent: resolved string plus the config type to use for final render.
+ * When the source is an agent with reasoning enabled, reasoning may be set for a collapsible section.
  */
 export type ResolvedContentResult = {
   resolved: string
   outputTypeId: ConfigTypeId
+  /** Optional reasoning section (e.g. agent with reasoning enabled); renderer shows it in a collapsible. */
+  reasoning?: string
 }
 
 /**