diff --git a/package-lock.json b/package-lock.json
index 0eb062d..9f82601 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "codepilot",
-  "version": "0.8.0",
+  "version": "0.9.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "codepilot",
-      "version": "0.8.0",
+      "version": "0.9.0",
       "dependencies": {
         "@anthropic-ai/claude-agent-sdk": "^0.2.33",
         "@hugeicons/core-free-icons": "^3.1.1",
diff --git a/package.json b/package.json
index 37a9b45..0ab5592 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "codepilot",
-  "version": "0.8.0",
+  "version": "0.9.0",
   "private": true,
   "author": {
     "name": "op7418",
diff --git a/src/components/layout/AppShell.tsx b/src/components/layout/AppShell.tsx
index 7d4d5e2..055ff86 100644
--- a/src/components/layout/AppShell.tsx
+++ b/src/components/layout/AppShell.tsx
@@ -8,13 +8,25 @@ import { ChatListPanel } from "./ChatListPanel";
 import { RightPanel } from "./RightPanel";
 import { ResizeHandle } from "./ResizeHandle";
 import { UpdateDialog } from "./UpdateDialog";
-import { PanelContext, type PanelContent } from "@/hooks/usePanel";
+import { DocPreview } from "./DocPreview";
+import { PanelContext, type PanelContent, type PreviewViewMode } from "@/hooks/usePanel";
 import { UpdateContext, type UpdateInfo } from "@/hooks/useUpdate";
 
 const CHATLIST_MIN = 180;
 const CHATLIST_MAX = 400;
 const RIGHTPANEL_MIN = 200;
 const RIGHTPANEL_MAX = 480;
+const DOCPREVIEW_MIN = 320;
+const DOCPREVIEW_MAX = 800;
+
+/** Extensions that default to "rendered" view mode */
+const RENDERED_EXTENSIONS = new Set([".md", ".mdx", ".html", ".htm"]);
+
+function defaultViewMode(filePath: string): PreviewViewMode {
+  const dot = filePath.lastIndexOf(".");
+  const ext = dot >= 0 ? filePath.slice(dot).toLowerCase() : "";
+  return RENDERED_EXTENSIONS.has(ext) ? "rendered" : "source";
+}
 
 const LG_BREAKPOINT = 1024;
 const CHECK_INTERVAL = 8 * 60 * 60 * 1000; // 8 hours
@@ -77,9 +89,37 @@ export function AppShell({ children }: { children: React.ReactNode }) {
   const [streamingSessionId, setStreamingSessionId] = useState("");
   const [pendingApprovalSessionId, setPendingApprovalSessionId] = useState("");
 
+  // --- Doc Preview state ---
+  const [previewFile, setPreviewFileRaw] = useState<string | null>(null);
+  const [previewViewMode, setPreviewViewMode] = useState<PreviewViewMode>("source");
+  const [docPreviewWidth, setDocPreviewWidth] = useState(() => {
+    if (typeof window === "undefined") return 480;
+    return parseInt(localStorage.getItem("codepilot_docpreview_width") || "480");
+  });
+
+  const setPreviewFile = useCallback((path: string | null) => {
+    setPreviewFileRaw(path);
+    if (path) {
+      setPreviewViewMode(defaultViewMode(path));
+    }
+  }, []);
+
+  const handleDocPreviewResize = useCallback((delta: number) => {
+    setDocPreviewWidth((w) => Math.min(DOCPREVIEW_MAX, Math.max(DOCPREVIEW_MIN, w - delta)));
+  }, []);
+  const handleDocPreviewResizeEnd = useCallback(() => {
+    setDocPreviewWidth((w) => {
+      localStorage.setItem("codepilot_docpreview_width", String(w));
+      return w;
+    });
+  }, []);
+
   // Auto-open panel on chat detail routes, close on others
   useEffect(() => {
     setPanelOpenRaw(isChatDetailRoute);
+    if (!isChatDetailRoute) {
+      setPreviewFileRaw(null);
+    }
   }, [isChatDetailRoute]);
 
   const setPanelOpen = useCallback((open: boolean) => {
@@ -186,8 +226,12 @@ export function AppShell({ children }: { children: React.ReactNode }) {
       setStreamingSessionId,
       pendingApprovalSessionId,
       setPendingApprovalSessionId,
+      previewFile,
+      setPreviewFile,
+      previewViewMode,
+      setPreviewViewMode,
     }),
-    [panelOpen, setPanelOpen, panelContent, workingDirectory, sessionId, sessionTitle, streamingSessionId, pendingApprovalSessionId]
+    [panelOpen, setPanelOpen, panelContent, workingDirectory, sessionId, sessionTitle, streamingSessionId, pendingApprovalSessionId, previewFile, setPreviewFile, previewViewMode]
   );
 
   return (
@@ -213,6 +257,18 @@ export function AppShell({ children }: { children: React.ReactNode }) {
               />
               <main className="relative flex-1 overflow-hidden">{children}</main>
             </div>
+            {isChatDetailRoute && previewFile && (
+              <ResizeHandle side="right" onResize={handleDocPreviewResize} onResizeEnd={handleDocPreviewResizeEnd} />
+            )}
+            {isChatDetailRoute && previewFile && (
+              <DocPreview
+                filePath={previewFile}
+                viewMode={previewViewMode}
+                onViewModeChange={setPreviewViewMode}
+                onClose={() => setPreviewFile(null)}
+                width={docPreviewWidth}
+              />
+            )}
             {isChatDetailRoute && panelOpen && (
               <ResizeHandle side="right" onResize={handleRightPanelResize} onResizeEnd={handleRightPanelResizeEnd} />
             )}
diff --git a/src/components/layout/DocPreview.tsx b/src/components/layout/DocPreview.tsx
new file mode 100644
index 0000000..06cea04
--- /dev/null
+++ b/src/components/layout/DocPreview.tsx
@@ -0,0 +1,274 @@
+"use client";
+
+import { useState, useEffect, useMemo } from "react";
+import { useTheme } from "next-themes";
+import { HugeiconsIcon } from "@hugeicons/react";
+import { Cancel01Icon, Copy01Icon, Tick01Icon, Loading02Icon } from "@hugeicons/core-free-icons";
+import { Button } from "@/components/ui/button";
+import { Light as SyntaxHighlighter } from "react-syntax-highlighter";
+import { atomOneDark } from "react-syntax-highlighter/dist/esm/styles/hljs";
+import { atomOneLight } from "react-syntax-highlighter/dist/esm/styles/hljs";
+import { Streamdown } from "streamdown";
+import { cjk } from "@streamdown/cjk";
+import { code } from "@streamdown/code";
+import { math } from "@streamdown/math";
+import { mermaid } from "@streamdown/mermaid";
+import type { FilePreview as FilePreviewType } from "@/types";
+
+const streamdownPlugins = { cjk, code, math, mermaid };
+
+type ViewMode = "source" | "rendered";
+
+interface DocPreviewProps {
+  filePath: string;
+  viewMode: ViewMode;
+  onViewModeChange: (mode: ViewMode) => void;
+  onClose: () => void;
+  width: number;
+}
+
+/** Extensions that support a rendered preview */
+const RENDERABLE_EXTENSIONS = new Set([".md", ".mdx", ".html", ".htm"]);
+
+function getExtension(filePath: string): string {
+  const dot = filePath.lastIndexOf(".");
+  return dot >= 0 ? filePath.slice(dot).toLowerCase() : "";
+}
+
+function isRenderable(filePath: string): boolean {
+  return RENDERABLE_EXTENSIONS.has(getExtension(filePath));
+}
+
+function isHtml(filePath: string): boolean {
+  const ext = getExtension(filePath);
+  return ext === ".html" || ext === ".htm";
+}
+
+export function DocPreview({
+  filePath,
+  viewMode,
+  onViewModeChange,
+  onClose,
+  width,
+}: DocPreviewProps) {
+  const { resolvedTheme } = useTheme();
+  const isDark = resolvedTheme === "dark";
+  const [preview, setPreview] = useState<FilePreviewType | null>(null);
+  const [loading, setLoading] = useState(true);
+  const [error, setError] = useState<string | null>(null);
+  const [copied, setCopied] = useState(false);
+
+  useEffect(() => {
+    let cancelled = false;
+
+    async function loadPreview() {
+      setLoading(true);
+      setError(null);
+      try {
+        const res = await fetch(
+          `/api/files/preview?path=${encodeURIComponent(filePath)}&maxLines=500`
+        );
+        if (!res.ok) {
+          const data = await res.json();
+          throw new Error(data.error || "Failed to load file");
+        }
+        const data = await res.json();
+        if (!cancelled) {
+          setPreview(data.preview);
+        }
+      } catch (err) {
+        if (!cancelled) {
+          setError(err instanceof Error ? err.message : "Failed to load file");
+        }
+      } finally {
+        if (!cancelled) {
+          setLoading(false);
+        }
+      }
+    }
+
+    loadPreview();
+    return () => {
+      cancelled = true;
+    };
+  }, [filePath]);
+
+  const handleCopyPath = async () => {
+    await navigator.clipboard.writeText(filePath);
+    setCopied(true);
+    setTimeout(() => setCopied(false), 2000);
+  };
+
+  const fileName = filePath.split("/").pop() || filePath;
+
+  // Build breadcrumb — show last 3 segments
+  const breadcrumb = useMemo(() => {
+    const segments = filePath.split("/").filter(Boolean);
+    const display = segments.slice(-3);
+    const prefix = display.length < segments.length ? ".../" : "";
+    return prefix + display.join("/");
+  }, [filePath]);
+
+  const canRender = isRenderable(filePath);
+
+  return (
+    <div
+      className="hidden h-full shrink-0 flex-col overflow-hidden border-l border-border/40 bg-background lg:flex"
+      style={{ width }}
+    >
+      {/* Header */}
+      <div className="flex h-10 shrink-0 items-center gap-2 px-3">
+        <div className="min-w-0 flex-1">
+          <p className="truncate text-sm font-medium">{fileName}</p>
+        </div>
+
+        {canRender && (
+          <ViewModeToggle value={viewMode} onChange={onViewModeChange} />
+        )}
+
+        <Button variant="ghost" size="icon-sm" onClick={handleCopyPath}>
+          {copied ? (
+            <HugeiconsIcon icon={Tick01Icon} className="h-3.5 w-3.5 text-green-500" />
+          ) : (
+            <HugeiconsIcon icon={Copy01Icon} className="h-3.5 w-3.5" />
+          )}
+          <span className="sr-only">Copy path</span>
+        </Button>
+
+        <Button variant="ghost" size="icon-sm" onClick={onClose}>
+          <HugeiconsIcon icon={Cancel01Icon} className="h-3.5 w-3.5" />
+          <span className="sr-only">Close preview</span>
+        </Button>
+      </div>
+
+      {/* Breadcrumb + language — subtle, no border */}
+      <div className="flex shrink-0 items-center gap-2 px-3 pb-2">
+        <p className="min-w-0 flex-1 truncate text-[11px] text-muted-foreground/60">
+          {breadcrumb}
+        </p>
+        {preview && (
+          <span className="shrink-0 text-[10px] text-muted-foreground/50">
+            {preview.language}
+          </span>
+        )}
+      </div>
+
+      {/* Content */}
+      <div className="flex-1 min-h-0 overflow-auto">
+        {loading ? (
+          <div className="flex items-center justify-center py-12">
+            <HugeiconsIcon
+              icon={Loading02Icon}
+              className="h-5 w-5 animate-spin text-muted-foreground"
+            />
+          </div>
+        ) : error ? (
+          <div className="px-4 py-8 text-center">
+            <p className="text-sm text-destructive">{error}</p>
+          </div>
+        ) : preview ? (
+          viewMode === "rendered" && canRender ? (
+            <RenderedView content={preview.content} filePath={filePath} />
+          ) : (
+            <SourceView preview={preview} isDark={isDark} />
+          )
+        ) : null}
+      </div>
+    </div>
+  );
+}
+
+/** Capsule toggle for Source / Preview view mode */
+function ViewModeToggle({
+  value,
+  onChange,
+}: {
+  value: ViewMode;
+  onChange: (v: ViewMode) => void;
+}) {
+  return (
+    <div className="flex h-6 items-center rounded-full bg-muted p-0.5 text-[11px]">
+      <button
+        className={`rounded-full px-2 py-0.5 font-medium transition-colors ${
+          value === "source"
+            ? "bg-background text-foreground shadow-sm"
+            : "text-muted-foreground hover:text-foreground"
+        }`}
+        onClick={() => onChange("source")}
+      >
+        Source
+      </button>
+      <button
+        className={`rounded-full px-2 py-0.5 font-medium transition-colors ${
+          value === "rendered"
+            ? "bg-background text-foreground shadow-sm"
+            : "text-muted-foreground hover:text-foreground"
+        }`}
+        onClick={() => onChange("rendered")}
+      >
+        Preview
+      </button>
+    </div>
+  );
+}
+
+/** Source code view using react-syntax-highlighter */
+function SourceView({ preview, isDark }: { preview: FilePreviewType; isDark: boolean }) {
+  return (
+    <div className="text-xs">
+      <SyntaxHighlighter
+        language={preview.language}
+        style={isDark ? atomOneDark : atomOneLight}
+        showLineNumbers
+        customStyle={{
+          margin: 0,
+          padding: "8px",
+          borderRadius: 0,
+          fontSize: "11px",
+          lineHeight: "1.5",
+          background: "transparent",
+        }}
+        lineNumberStyle={{
+          minWidth: "2.5em",
+          paddingRight: "8px",
+          color: isDark ? "#636d83" : "#9ca3af",
+          userSelect: "none",
+        }}
+      >
+        {preview.content}
+      </SyntaxHighlighter>
+    </div>
+  );
+}
+
+/** Rendered view for markdown / HTML files */
+function RenderedView({
+  content,
+  filePath,
+}: {
+  content: string;
+  filePath: string;
+}) {
+  if (isHtml(filePath)) {
+    return (
+      <iframe
+        srcDoc={content}
+        sandbox=""
+        className="h-full w-full border-0"
+        title="HTML Preview"
+      />
+    );
+  }
+
+  // Markdown / MDX
+  return (
+    <div className="p-4 overflow-x-hidden break-words">
+      <Streamdown
+        className="size-full [&>*:first-child]:mt-0 [&>*:last-child]:mb-0"
+        plugins={streamdownPlugins}
+      >
+        {content}
+      </Streamdown>
+    </div>
+  );
+}
diff --git a/src/components/layout/RightPanel.tsx b/src/components/layout/RightPanel.tsx
index ab04f58..e3f6586 100644
--- a/src/components/layout/RightPanel.tsx
+++ b/src/components/layout/RightPanel.tsx
@@ -12,15 +12,13 @@ import {
 } from "@/components/ui/tooltip";
 import { usePanel } from "@/hooks/usePanel";
 import { FileTree } from "@/components/project/FileTree";
-import { FilePreview } from "@/components/project/FilePreview";
 
 interface RightPanelProps {
   width?: number;
 }
 
 export function RightPanel({ width }: RightPanelProps) {
-  const { panelOpen, setPanelOpen, workingDirectory, sessionId, sessionTitle, setSessionTitle } = usePanel();
-  const [previewPath, setPreviewPath] = useState<string | null>(null);
+  const { panelOpen, setPanelOpen, workingDirectory, sessionId, sessionTitle, setSessionTitle, previewFile, setPreviewFile } = usePanel();
   const [isEditingName, setIsEditingName] = useState(false);
   const [editName, setEditName] = useState("");
   const inputRef = useRef<HTMLInputElement>(null);
@@ -68,6 +66,15 @@ export function RightPanel({ width }: RightPanelProps) {
     }
   }, [handleSaveName]);
 
+  const handleFileSelect = useCallback((path: string) => {
+    // Toggle: clicking the same file closes the preview
+    if (previewFile === path) {
+      setPreviewFile(null);
+    } else {
+      setPreviewFile(path);
+    }
+  }, [previewFile, setPreviewFile]);
+
   if (!panelOpen) {
     return (
       <div className="flex flex-col items-center gap-2 bg-background p-2">
@@ -88,14 +95,6 @@ export function RightPanel({ width }: RightPanelProps) {
     );
   }
 
-  const handleFileSelect = (path: string) => {
-    setPreviewPath(path);
-  };
-
-  const handleBackToTree = () => {
-    setPreviewPath(null);
-  };
-
   return (
     <aside className="hidden h-full shrink-0 flex-col overflow-hidden bg-background lg:flex" style={{ width: width ?? 288 }}>
       {/* Header */}
@@ -153,18 +152,14 @@ export function RightPanel({ width }: RightPanelProps) {
         {/* Divider */}
         <div className="h-px bg-border/50" />
 
-        {/* Files */}
+        {/* Files — always show FileTree */}
         <div className="flex flex-col min-h-0">
           <span className="text-[11px] font-medium uppercase tracking-wider text-muted-foreground/70 mb-1.5 block">Files</span>
           <div className="overflow-hidden">
-            {previewPath ? (
-              <FilePreview filePath={previewPath} onBack={handleBackToTree} />
-            ) : (
-              <FileTree
-                workingDirectory={workingDirectory}
-                onFileSelect={handleFileSelect}
-              />
-            )}
+            <FileTree
+              workingDirectory={workingDirectory}
+              onFileSelect={handleFileSelect}
+            />
           </div>
         </div>
 
diff --git a/src/hooks/usePanel.ts b/src/hooks/usePanel.ts
index 92e5872..f614df3 100644
--- a/src/hooks/usePanel.ts
+++ b/src/hooks/usePanel.ts
@@ -4,6 +4,8 @@ import { createContext, useContext } from "react";
 
 export type PanelContent = "files" | "tasks";
 
+export type PreviewViewMode = "source" | "rendered";
+
 export interface PanelContextValue {
   panelOpen: boolean;
   setPanelOpen: (open: boolean) => void;
@@ -19,6 +21,10 @@ export interface PanelContextValue {
   setStreamingSessionId: (id: string) => void;
   pendingApprovalSessionId: string;
   setPendingApprovalSessionId: (id: string) => void;
+  previewFile: string | null;
+  setPreviewFile: (path: string | null) => void;
+  previewViewMode: PreviewViewMode;
+  setPreviewViewMode: (mode: PreviewViewMode) => void;
 }
 
 export const PanelContext = createContext<PanelContextValue | null>(null);