diff --git a/web/src/components/editor/EditorPanel.tsx b/web/src/components/editor/EditorPanel.tsx
index 8e5adaa..4aa227f 100644
--- a/web/src/components/editor/EditorPanel.tsx
+++ b/web/src/components/editor/EditorPanel.tsx
@@ -1,16 +1,81 @@
-import Editor, { type OnValidate } from "@monaco-editor/react";
-import { useState } from "react";
+import Editor, { type OnMount, type OnValidate } from "@monaco-editor/react";
+import { useEffect, useRef, useState } from "react";
 
-import type { MockFileNode } from "@/pages/workspace/mock-data";
+import { inferLanguage, useFileRead, useFileWrite } from "@/lib/api/files";
 
 interface EditorPanelProps {
-  file?: MockFileNode;
+  workspaceId: string | null;
+  path: string | null;
+  language?: string;
 }
 
-export function EditorPanel({ file }: EditorPanelProps) {
+export function EditorPanel({ workspaceId, path, language }: EditorPanelProps) {
+  const { data, isLoading, error: readError } = useFileRead(workspaceId, path);
+  const write = useFileWrite();
+
+  const [buffer, setBuffer] = useState("");
+  const [savedContent, setSavedContent] = useState("");
+  const dirty = buffer !== savedContent;
+  const saveInProgress = write.isPending;
+
+  // Sync editor buffer with fetched content when the file changes or loads.
+  useEffect(() => {
+    if (data) {
+      setBuffer(data.content);
+      setSavedContent(data.content);
+    }
+  }, [data]);
+
+  const editorLanguage =
+    language ??
+    (path ? inferLanguage(path.split("/").pop() ?? path) : undefined);
+
+  // Saving state refs so the key handler always sees the latest values.
+  const bufferRef = useRef(buffer);
+  const savedContentRef = useRef(savedContent);
+  const pathRef = useRef(path);
+  const workspaceIdRef = useRef(workspaceId);
+  useEffect(() => {
+    bufferRef.current = buffer;
+    savedContentRef.current = savedContent;
+    pathRef.current = path;
+    workspaceIdRef.current = workspaceId;
+  }, [buffer, savedContent, path, workspaceId]);
+
+  // Wire Cmd/Ctrl+S to save the current buffer.
+  const handleMount: OnMount = (editor, monaco) => {
+    editor.addCommand(
+      monaco.KeyMod.CtrlCmd | monaco.KeyCode.KEY_S,
+      () => {
+        const currentBuffer = bufferRef.current;
+        const currentSaved = savedContentRef.current;
+        const currentPath = pathRef.current;
+        const currentWorkspaceId = workspaceIdRef.current;
+        if (
+          currentWorkspaceId &&
+          currentPath &&
+          currentBuffer !== currentSaved
+        ) {
+          write.mutate(
+            {
+              workspaceId: currentWorkspaceId,
+              path: currentPath,
+              content: currentBuffer,
+            },
+            {
+              onSuccess: () => setSavedContent(currentBuffer),
+            },
+          );
+        }
+      },
+    );
+  };
+
+  // Resetting dirty state on file switch is acceptable for this task because
+  // each file gets its own EditorPanel mount via the `key={path}` prop.
   const [, setMarkers] = useState<Parameters<OnValidate>[0]>([]);
 
-  if (!file || file.kind !== "file") {
+  if (!path) {
     return (
       <section className="flex h-full w-full items-center justify-center bg-background text-sm text-muted-foreground">
         <div className="text-center">
@@ -21,19 +86,49 @@ export function EditorPanel({ file }: EditorPanelProps) {
   }
 
   return (
-    <section className="h-full w-full overflow-hidden bg-background">
-      <Editor
-        key={file.path}
-        theme="vs-dark"
-        language={file.language ?? "typescript"}
-        value={file.content ?? ""}
-        onValidate={setMarkers}
-        options={{
-          automaticLayout: true,
-          minimap: { enabled: false },
-          readOnly: false,
-        }}
-      />
+    <section className="flex h-full w-full flex-col overflow-hidden bg-background">
+      <header className="flex h-8 shrink-0 items-center justify-between border-b border-border bg-sidebar px-3 text-xs">
+        <div className="flex items-center gap-2 truncate">
+          <span className={dirty ? "text-foreground" : "text-muted-foreground"}>
+            {dirty ? "●" : "•"}
+          </span>
+          <span className="truncate text-foreground">{path}</span>
+        </div>
+        <div className="flex items-center gap-2">
+          {saveInProgress && (
+            <span className="text-muted-foreground">Saving…</span>
+          )}
+          {write.isError && (
+            <span className="text-destructive">{write.error.message}</span>
+          )}
+        </div>
+      </header>
+      {readError && (
+        <div className="shrink-0 border-b border-destructive/30 bg-destructive/10 px-3 py-1.5 text-xs text-destructive">
+          {readError.message}
+        </div>
+      )}
+      {isLoading && (
+        <div className="shrink-0 border-b border-border bg-muted px-3 py-1.5 text-xs text-muted-foreground">
+          Loading…
+        </div>
+      )}
+      <div className="min-h-0 flex-1">
+        <Editor
+          key={path}
+          theme="vs-dark"
+          language={editorLanguage ?? "plaintext"}
+          value={buffer}
+          onChange={(value) => setBuffer(value ?? "")}
+          onValidate={setMarkers}
+          onMount={handleMount}
+          options={{
+            automaticLayout: true,
+            minimap: { enabled: false },
+            readOnly: false,
+          }}
+        />
+      </div>
     </section>
   );
 }
diff --git a/web/src/components/layout/WorkspaceShell.tsx b/web/src/components/layout/WorkspaceShell.tsx
index fdc8062..a841b9a 100644
--- a/web/src/components/layout/WorkspaceShell.tsx
+++ b/web/src/components/layout/WorkspaceShell.tsx
@@ -8,33 +8,18 @@ import { TerminalPanel } from "@/components/terminal/TerminalPanel";
 import { FileExplorerPanel } from "@/components/workspace/FileExplorerPanel";
 import { StatusBar } from "@/components/workspace/StatusBar";
 import { WorkspaceSelector } from "@/components/workspace/WorkspaceSelector";
-import { inferLanguage } from "@/lib/api/files";
 import { useWorkspaceUiStore } from "@/lib/store/workspace-ui-store";
-import type { MockFileNode } from "@/pages/workspace/mock-data";
 
 interface WorkspaceShellProps {
   workspaceId: string;
 }
 
-function fileNodeFromPath(path: string): MockFileNode {
-  const name = path.split("/").pop() ?? path;
-  return {
-    name,
-    path,
-    kind: "file",
-    language: inferLanguage(name),
-  };
-}
-
 export function WorkspaceShell({ workspaceId }: WorkspaceShellProps) {
   const activeFilePath = useWorkspaceUiStore((s) => s.activeFilePath);
   const sidebarCollapsed = useWorkspaceUiStore((s) => s.sidebarCollapsed);
   const terminalVisible = useWorkspaceUiStore((s) => s.terminalVisible);
   const toggleSidebar = useWorkspaceUiStore((s) => s.toggleSidebar);
   const toggleTerminal = useWorkspaceUiStore((s) => s.toggleTerminal);
-  const activeFile = activeFilePath
-    ? fileNodeFromPath(activeFilePath)
-    : undefined;
 
   return (
     <div className="flex h-full w-full flex-col bg-background text-foreground">
@@ -86,7 +71,7 @@ export function WorkspaceShell({ workspaceId }: WorkspaceShellProps) {
             <PanelGroup direction="vertical">
               {/* Editor */}
               <Panel>
-                <EditorPanel file={activeFile} />
+                <EditorPanel workspaceId={workspaceId} path={activeFilePath || null} />
               </Panel>
 
               {/* Terminal panel */}
diff --git a/web/src/lib/api/files.ts b/web/src/lib/api/files.ts
index b4193a2..5f1be9c 100644
--- a/web/src/lib/api/files.ts
+++ b/web/src/lib/api/files.ts
@@ -1,4 +1,10 @@
-import { useQuery, type UseQueryResult } from "@tanstack/react-query";
+import {
+  useMutation,
+  useQuery,
+  useQueryClient,
+  type UseMutationResult,
+  type UseQueryResult,
+} from "@tanstack/react-query";
 
 import { apiClient } from "@/lib/api/client";
 
@@ -31,6 +37,11 @@ export function inferLanguage(name: string): string | undefined {
   return ext ? LANGUAGE_BY_EXTENSION[ext] : undefined;
 }
 
+export interface ReadFileResponse {
+  path: string;
+  content: string;
+}
+
 function fileListQueryKey(workspaceId: string, path: string) {
   return ["workspaces", workspaceId, "files", path || "."] as const;
 }
@@ -50,6 +61,35 @@ async function fetchFileList(
   return (await res.json()) as FileEntry[];
 }
 
+export function fileReadQueryKey(workspaceId: string, path: string) {
+  return ["workspaces", workspaceId, "files", "read", path] as const;
+}
+
+async function fetchFileRead(
+  workspaceId: string,
+  path: string,
+): Promise<ReadFileResponse> {
+  const url =
+    `/api/workspaces/${encodeURIComponent(workspaceId)}/files/read?` +
+    new URLSearchParams({ path }).toString();
+  const res = await apiClient(url);
+  if (!res.ok) {
+    throw new Error(`Failed to read file: ${res.status}`);
+  }
+  return (await res.json()) as ReadFileResponse;
+}
+
+export function useFileRead(
+  workspaceId: string | null,
+  path: string | null,
+): UseQueryResult<ReadFileResponse, Error> {
+  return useQuery({
+    queryKey: fileReadQueryKey(workspaceId ?? "", path ?? ""),
+    queryFn: () => fetchFileRead(workspaceId!, path!),
+    enabled: !!workspaceId && !!path,
+  });
+}
+
 export function useFileList(
   workspaceId: string | null,
   path: string,
@@ -62,3 +102,29 @@ export function useFileList(
     enabled: !!workspaceId && enabled,
   });
 }
+
+export function useFileWrite(): UseMutationResult<
+  void,
+  Error,
+  { path: string; content: string; workspaceId: string }
+> {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: async ({ workspaceId, path, content }) => {
+      const url = `/api/workspaces/${encodeURIComponent(workspaceId)}/files/write`;
+      const res = await apiClient(url, {
+        method: "PUT",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ path, content }),
+      });
+      if (!res.ok) {
+        throw new Error(`Failed to save file: ${res.status}`);
+      }
+    },
+    onSuccess: (_, variables) => {
+      queryClient.invalidateQueries({
+        queryKey: fileReadQueryKey(variables.workspaceId, variables.path),
+      });
+    },
+  });
+}
diff --git a/web/src/pages/workspace/mock-data.ts b/web/src/pages/workspace/mock-data.ts
deleted file mode 100644
index 15d00a8..0000000
--- a/web/src/pages/workspace/mock-data.ts
+++ /dev/null
@@ -1,8 +0,0 @@
-export interface MockFileNode {
-  name: string;
-  path: string;
-  kind: "file" | "dir";
-  language?: string;
-  content?: string;
-  children?: MockFileNode[];
-}