feat: enable Claude Code provider to run natively on Windows

[SannidhyaSah]

Description

Fixes #5042

This PR enables the Claude Code provider to run natively on Windows by implementing a stdin-based approach for passing both system prompt and messages. This eliminates the Windows command-line length limitation (8191 characters) that was preventing Claude Code from working on Windows systems.

Background

Claude Code was recently added with native Windows support, but the integration in Roo Code was hitting Windows' command-line argument length limitations when passing large system prompts. The previous implementation passed the system prompt as a command-line argument, which would fail on Windows when the prompt exceeded ~8KB.

Changes Made

• Modified `src/integrations/claude-code/run.ts`: Refactored to always pass both `systemPrompt` and `messages` via stdin as a single JSON object
• Updated tests in `src/integrations/claude-code/tests/run.spec.ts`: Updated test expectations to reflect the new stdin-based approach
• Removed platform-specific code: The solution is now platform-agnostic, eliminating the need for Windows-specific workarounds

Technical Details

The key insight was that the Claude Code CLI accepts input via stdin (though this is undocumented). By passing both the system prompt and messages together through stdin, we:

1. Completely bypass Windows command-line length limitations
2. Simplify the code by removing platform-specific logic
3. Make the implementation more robust and future-proof

Testing

• All existing tests pass
• Updated tests for stdin-based message passing
• Manual testing completed:
  • Verified Claude Code provider works on Windows without WSL
  • Tested with large system prompts that previously failed
  • Confirmed functionality remains intact on Linux/macOS

Verification of Acceptance Criteria

• Claude Code no longer gets stuck at "API Request" on Windows
• Works natively on Windows without requiring WSL
• Handles large system prompts without command-line limitations
• Maintains compatibility with Linux and macOS

Checklist

• Code follows project style guidelines
• Self-review completed
• Comments added for complex logic
• Documentation updated (if needed)
• No breaking changes
• Tests updated to reflect new behavior

Additional Notes

This fix not only resolves the immediate Windows issue but also provides a more elegant solution that works consistently across all platforms. The stdin approach is cleaner and more scalable than trying to work around platform-specific command-line limitations."

---

Important

Refactor Claude Code provider to use stdin for input, bypassing Windows command-line length limits and simplifying platform-specific logic.

• Behavior:
  • Refactor runProcess() in run.ts to pass systemPrompt and messages via stdin as JSON, bypassing Windows command-line length limits.
  • Remove --system-prompt from command-line arguments in run.ts.
• Testing:
  • Update tests in run.spec.ts to verify stdin-based input handling and ensure no --system-prompt in args.
  • Add tests for handling stdin write and access errors gracefully.
• Misc:
  • Remove platform-specific code, making the solution platform-agnostic.

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[daniel-lxs]

Thank you @SannidhyaSah !

It seems like stdin only accepts user messages:
https://docs.anthropic.com/en/docs/claude-code/sdk#streaming-json-input

I don't think I have seen issues with the way we send the system prompt currently.

[SannidhyaSah]

Thank you @SannidhyaSah !

It seems like stdin only accepts user messages:
https://docs.anthropic.com/en/docs/claude-code/sdk#streaming-json-input

I don't think I have seen issues with the way we send the system prompt currently.

@daniel-lxs the problem was windows only has a limit of 8192 characters that can be sent in the terminal. And that meant the claude code provider wasn't really working as the system prompt wasn't being sent . Our system prompt is 131k charactes. I tinkered with it to make it work and was able to get it running with this approach. Basically it's either this . Or not working at all . For now ..

[SannidhyaSah]

this was a comparatively simpler task but it was able to do it . did not seem to encounter any errors in this run. it was using the sonnet 4.0 model here . that said .. we should probably test it more. let me know if you guys find anything

It seems that Claude Code doesn't support overwriting the system prompt with stdin

[daniel-lxs]

Hey! I did some testing on this PR to see how it handles system prompts via stdin.

It works - both the system prompt and messages are processed correctly when passed through stdin, which should help with the Windows command-line length issue. However, I noticed that this approach increases token usage compared to the current implementation.

When comparing token usage between the current main branch (which uses the --system-prompt flag) and this PR (which passes everything through stdin), I saw a clear increase in tokens consumed:

A few concerns beyond the token cost:

• This behavior is undocumented. The Claude Code docs say stdin is limited to user messages, yet { "systemPrompt": "...", "messages": [...] } seems to work.
• Neither this PR nor the current implementation uses --input-format=stream-json, which is the only officially documented way to pass input via stdin.
• It's unclear how the CLI handles this under the hood. The increased token usage suggests it might be duplicating or reprocessing the system prompt.
• Since this relies on undocumented behavior, it could break in a future Claude Code update without warning.

While this does solve the Windows shell limit, the extra token usage is a significant tradeoff. We might want to explore other options like only enabling this on Windows since it will at least allow Windows users to use Claude Code.

[blouflashdb]

Related: anthropics/claude-code#3411

[daniel-lxs]

This PR successfully enables Claude Code on Windows by working around the command-line length limitation.

Note: This is a temporary workaround using undocumented stdin behavior for system prompts. Be aware that on Windows, the system prompt may be counted twice in token usage - once from the stdin data and once if Claude Code still processes the default system prompt internally.

Approving as it unblocks Windows users while a more permanent solution can be explored with the Claude Code team.