diff --git a/packages/opencode/src/cli/cmd/tui/app.tsx b/packages/opencode/src/cli/cmd/tui/app.tsx
index 5e42b189b..32c70b6d0 100644
--- a/packages/opencode/src/cli/cmd/tui/app.tsx
+++ b/packages/opencode/src/cli/cmd/tui/app.tsx
@@ -68,6 +68,7 @@ import { createTuiAttention } from "@/cli/cmd/tui/attention"
 import { FormatError, FormatUnknownError } from "@/cli/error"
 import { CommandPaletteProvider, useCommandPalette } from "./context/command-palette"
 import { OpencodeKeymapProvider, registerOpencodeKeymap, useBindings, useOpencodeKeymap } from "./keymap"
+import { DiffViewer } from "./routes/diff"
 
 import type { EventSource } from "./context/sdk"
 import { DialogVariant } from "./component/dialog-variant"
@@ -102,6 +103,7 @@ const appBindingCommands = [
   "theme.switch",
   "theme.switch_mode",
   "theme.mode.lock",
+  "diff.open",
   "help.show",
   "docs.open",
   "app.debug",
@@ -352,6 +354,7 @@ function App(props: { onSnapshot?: () => Promise<string[]> }) {
     renderer.clearSelection()
   }
   const [terminalTitleEnabled, setTerminalTitleEnabled] = createSignal(kv.get("terminal_title_enabled", true))
+  const [diffOpen, setDiffOpen] = createSignal(false)
   const [pasteSummaryEnabled, setPasteSummaryEnabled] = createSignal(
     kv.get("paste_summary_enabled", !sync.data.config.experimental?.disable_paste_summary),
   )
@@ -663,6 +666,16 @@ function App(props: { onSnapshot?: () => Promise<string[]> }) {
         },
         category: "System",
       },
+      {
+        name: "diff.open",
+        title: "Open diff viewer",
+        slashName: "diff",
+        run: () => {
+          setDiffOpen(true)
+          dialog.clear()
+        },
+        category: "VCS",
+      },
       {
         name: "help.show",
         title: "Help",
@@ -962,6 +975,9 @@ function App(props: { onSnapshot?: () => Promise<string[]> }) {
           <TuiPluginRuntime.Slot name="app_bottom" />
         </box>
         <TuiPluginRuntime.Slot name="app" />
+        <Show when={diffOpen()}>
+          <DiffViewer onClose={() => setDiffOpen(false)} />
+        </Show>
       </Show>
       <StartupLoading ready={ready} />
     </box>
diff --git a/packages/opencode/src/cli/cmd/tui/config/keybind.ts b/packages/opencode/src/cli/cmd/tui/config/keybind.ts
index bd26cd5d9..2837ca851 100644
--- a/packages/opencode/src/cli/cmd/tui/config/keybind.ts
+++ b/packages/opencode/src/cli/cmd/tui/config/keybind.ts
@@ -67,6 +67,7 @@ export const Definitions = {
   sidebar_toggle: keybind("<leader>b", "Toggle sidebar"),
   scrollbar_toggle: keybind("none", "Toggle session scrollbar"),
   status_view: keybind("<leader>s", "View status"),
+  diff_open: keybind("<leader>d", "Open diff viewer"),
 
   session_export: keybind("<leader>x", "Export session to editor"),
   session_copy: keybind("none", "Copy session transcript"),
@@ -251,6 +252,7 @@ export const CommandMap = {
   sidebar_toggle: "session.sidebar.toggle",
   scrollbar_toggle: "session.toggle.scrollbar",
   status_view: "opencode.status",
+  diff_open: "diff.open",
   session_export: "session.export",
   session_copy: "session.copy",
   session_new: "session.new",
diff --git a/packages/opencode/src/cli/cmd/tui/simulation-mcp.ts b/packages/opencode/src/cli/cmd/tui/simulation-mcp.ts
index 37183d239..ee2c7b6e7 100644
--- a/packages/opencode/src/cli/cmd/tui/simulation-mcp.ts
+++ b/packages/opencode/src/cli/cmd/tui/simulation-mcp.ts
@@ -152,6 +152,12 @@ const ScriptActionSchema = z.union([
 
 const ScriptSchema = z.union([z.array(ScriptActionSchema), z.object({ actions: z.array(ScriptActionSchema) })])
 const TargetSchema = z.union([z.string(), z.array(z.string()).min(1), z.literal("all")])
+const BackendFetchSchema = z.object({
+  url: z.string(),
+  method: z.string().optional(),
+  headers: z.record(z.string(), z.string()).optional(),
+  body: z.unknown().optional(),
+})
 const remoteInstances = new Map<string, RemoteInstance>()
 
 function currentBuffer(renderer: CliRenderer): RenderBuffer {
@@ -264,6 +270,27 @@ async function control(options: Options, method: string, pathname: string, body?
   throw new Error(typeof data?.error === "string" ? data.error : `Simulation control request failed: ${response.status}`)
 }
 
+async function backendFetch(options: Options, input: z.infer<typeof BackendFetchSchema>) {
+  const running = current(options)
+  const headers = new Headers(input.headers)
+  const body =
+    input.body === undefined ? undefined : typeof input.body === "string" ? input.body : JSON.stringify(input.body)
+  if (body !== undefined && !headers.has("content-type") && typeof input.body !== "string") headers.set("content-type", "application/json")
+
+  const response = await (running.controlFetch ?? fetch)(new URL(input.url, running.controlUrl), {
+    method: input.method ?? (body === undefined ? "GET" : "POST"),
+    headers,
+    body,
+  })
+  return {
+    url: response.url,
+    status: response.status,
+    ok: response.ok,
+    headers: Object.fromEntries(response.headers.entries()),
+    body: await response.text(),
+  }
+}
+
 async function mcpRequest(url: string, method: string, params: unknown, timeout = 500) {
   const response = await fetch(url, {
     method: "POST",
@@ -431,6 +458,14 @@ function createServer(options: Options) {
     await current(options).harness.renderOnce()
     return toolResult(snapshot(options))
   })
+  server.registerTool(
+    "simulation_backend_fetch",
+    {
+      description: "Proxy one fetch request through the simulated backend and return the raw response.",
+      inputSchema: BackendFetchSchema,
+    },
+    async (input) => toolResult(await backendFetch(options, input)),
+  )
   server.registerTool(
     "simulation_action_execute",
     {