fix: force UTF-8 output encoding for PowerShell on Windows

packages/core/src/shell.ts

@@ -163,6 +163,17 @@ function info(file: string): Item {
   }
 }
 
+// Windows PowerShell 5.1 encodes its own output with the legacy OEM code page
+// (e.g. CP936 on zh-CN systems) when stdout is redirected, while opencode
+// decodes process output as UTF-8. Force UTF-8 before the user command runs.
+const PS_UTF8_PREAMBLE =
+  "[Console]::OutputEncoding = [System.Text.UTF8Encoding]::new(); $OutputEncoding = [System.Text.UTF8Encoding]::new(); "
+
+export function psCommand(command: string) {
+  if (process.platform !== "win32") return command
+  return PS_UTF8_PREAMBLE + command
+}
+
 export function args(file: string, command: string, cwd: string) {
   const n = name(file)
   if (n === "nu" || n === "fish") return ["-c", command]
@@ -195,7 +206,7 @@ export function args(file: string, command: string, cwd: string) {
     ]
   }
   if (n === "cmd") return ["/c", command]
-  if (ps(file)) return ["-NoProfile", "-Command", command]
+  if (ps(file)) return ["-NoProfile", "-Command", psCommand(command)]
   return ["-c", command]
 }
 

packages/core/test/shell.test.ts

@@ -63,6 +63,23 @@ describe("shell", () => {
     expect(zsh.at(-1)).toBe("/tmp")
   })
 
+  test("psCommand wraps command with UTF-8 preamble on Windows", () => {
+    const result = Shell.psCommand("echo hi")
+    if (process.platform === "win32") {
+      expect(result.startsWith("[Console]::OutputEncoding")).toBe(true)
+      expect(result.endsWith("echo hi")).toBe(true)
+    } else {
+      expect(result).toBe("echo hi")
+    }
+  })
+
+  test("applies psCommand to PowerShell args", () => {
+    const psArgs = Shell.args("powershell", "echo hi", "C:\\tmp")
+    expect(psArgs[0]).toBe("-NoProfile")
+    expect(psArgs[1]).toBe("-Command")
+    expect(String(psArgs.at(-1))).toContain("echo hi")
+  })
+
   if (process.platform === "win32") {
     test("rejects blacklisted shells case-insensitively", async () => {
       await withShell("NU.EXE", async () => {

packages/opencode/src/tool/shell.ts

@@ -292,12 +292,16 @@ const ask = Effect.fn("ShellTool.ask")(function* (ctx: Tool.Context, scan: Scan,
 
 function cmd(shell: string, command: string, cwd: string, env: NodeJS.ProcessEnv) {
   if (process.platform === "win32" && Shell.ps(shell)) {
-    return ChildProcess.make(shell, ["-NoLogo", "-NoProfile", "-NonInteractive", "-Command", command], {
-      cwd,
-      env,
-      stdin: "ignore",
-      detached: false,
-    })
+    return ChildProcess.make(
+      shell,
+      ["-NoLogo", "-NoProfile", "-NonInteractive", "-Command", Shell.psCommand(command)],
+      {
+        cwd,
+        env,
+        stdin: "ignore",
+        detached: false,
+      },
+    )
   }
 
   return ChildProcess.make(command, [], {