[BUG] Unable to set long system prompt using `--system-prompt` on Windows Native

[blouflashdb]

Environment

• Platform (select one):
  • Anthropic API
  • AWS Bedrock
  • Google Vertex AI
  • Other:
• Claude CLI version: 1.0.51 (Claude Code)
• Operating System: Windows 11
• Terminal: pwsh

Bug Description

The claude command, along with a massive --system-prompt argument, exceeds the character limit for a single command that can be passed to the underlying CreateProcess API in Windows, which is typically 8,191 characters.

Steps to Reproduce

1. Create a long system prompt
2. Run: claude -p "Build a REST API" --system-prompt "Your very long system prompt"

Expected Behavior

Claude code is able to read the system prompt from a file. For example by implementing a --system-prompt-file cli argument

Actual Behavior

Command failed with ENAMETOOLONG: claude -p --system-prompt "You are Kilo Code, ..." --verbose --output-format stream-json --disallowedTools "Task,Bash,Glob,Grep,LS,exit_plan_mode,Read,Edit,MultiEdit,Write,NotebookRead,NotebookEdit,WebFetch,TodoRead,TodoWrite,WebSearch" --max-turns 1 --model claude-sonnet-4-20250514
spawn ENAMETOOLONG

Additional Context

Discovered using Kilocode with the Claude Code Provider on Windows Native.

[BarreiroT]

Hey! I was about to open a similar issue. It's worth mentioning it's more immediately apparent on Windows because of the lower limits, but it's also possible to hit E2BIG or the MAX_ARG_STRLEN on MacOS/Linux.
It would be great if the system prompt could be a file uri or we had a dedicated flag to pass a file path.

[daniel-lxs]

Also it would be interesting to simply send the system prompt via stdin as well.

[ant-kurt]

Pointing to a file or using stdin are interesting ideas - let me take this back to the team to discuss!

[ant-kurt]

We're adding --system-prompt-file in v1.0.55