fix(shell): force UTF-8 encoding for PowerShell commands on Windows

[enjoykumawat]

Fixes #20661

Problem

On Windows with a non-UTF-8 system code page (e.g. CP936 for Chinese locale), PowerShell uses the system default encoding for console output. When gemini-cli captures this output through Node.js pipes, non-ASCII characters (e.g. Chinese text) are decoded incorrectly and appear garbled.

Two issues combine to cause this:

1. PowerShell's [Console]::OutputEncoding defaults to the system code page, so bytes written to stdout use that encoding.
2. getCachedEncodingForBuffer detects the system code page via chcp and uses it for the TextDecoder, but since we now force UTF-8 output, using the system code page decoder produces garbled text.

Fix

• Add a POWERSHELL_UTF8_GUARD preamble prepended to every PowerShell command:

  [Console]::OutputEncoding = [System.Text.Encoding]::UTF8; $OutputEncoding = [System.Text.Encoding]::UTF8;

  This forces PowerShell to write UTF-8 bytes to stdout/stderr before the user's command runs.

• Rename ensurePromptvarsDisabled → applyShellPreamble to reflect its broader responsibility (it now handles both bash shopt options and PowerShell UTF-8 setup).

• In handleOutput, skip getCachedEncodingForBuffer when shell === 'powershell' and always construct the TextDecoder with 'utf-8', ensuring the forced UTF-8 output is decoded correctly regardless of the system code page.

The guard is idempotent — if the command already starts with it, it is not prepended again.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses character encoding issues encountered when capturing PowerShell output on Windows systems with non-UTF-8 default code pages. By forcing UTF-8 encoding at the shell level and updating the output decoding logic, the changes ensure that non-ASCII characters are correctly captured and displayed.

Highlights

• PowerShell UTF-8 Enforcement: Introduced a preamble that forces PowerShell to use UTF-8 encoding for stdout and stderr, preventing garbled text on non-UTF-8 Windows systems.
• Refactored Shell Preamble Logic: Renamed 'ensurePromptvarsDisabled' to 'applyShellPreamble' to support both bash-specific shopt configurations and the new PowerShell UTF-8 guard.
• Encoding Decoder Update: Updated 'handleOutput' to bypass system encoding detection and explicitly use 'utf-8' when executing PowerShell commands.

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[gemini-code-assist]

Code Review

This pull request introduces a shell preamble to force UTF-8 output encoding in PowerShell, ensuring consistent non-ASCII character handling across different system locales. It also updates the output decoder to skip system encoding detection when PowerShell is used. A critical issue was identified where the shell variable is referenced in the handleOutput closure but is not defined within that scope, which would lead to a ReferenceError at runtime.

[gemini-code-assist]

A ReferenceError is introduced in the childProcessFallback method. The variable shell is used on line 632 within the handleOutput closure, but it is not defined in the scope of childProcessFallback or the closure itself. This will cause the application to crash with a ReferenceError whenever handleOutput is invoked, leading to a reliable Denial of Service (DoS) for any shell execution that falls back to childProcessFallback. To remediate, ensure the shell variable is available in the scope of childProcessFallback, for example, by calling getShellConfiguration() here. Additionally, please note that executeWithPty (around line 1127) appears to be missing this logic, which means PTY-based executions might still suffer from encoding issues on Windows.

          const { shell } = getShellConfiguration();
          const encoding =
            shell === 'powershell' ? 'utf-8' : getCachedEncodingForBuffer(data);