[Feature Request]: Add support for custom system prompt templates

Environment Information

• Operating System: Ubuntu 22.04 / macOS Sonoma / Windows 11
• Programming Language: Python 3.11 / Node.js 20.x
• Claude Code Version: Latest

Problem Description

Claude Code currently does not support loading custom system prompt templates from external files. Users who want to customize the system behavior for different project types need to manually edit prompts or use workarounds.

Reproduction Steps

1. Try to use a custom system prompt template with Claude Code
2. There's no command-line flag or configuration option to specify a custom prompt file
3. The default system prompt is always used

Expected Behavior

Claude Code should support loading custom system prompt templates from external files, allowing users to define different prompts for different project types or use cases.

Actual Behavior

• No --system-prompt-file or similar flag available
• Custom prompts cannot be loaded from external files
• Users must modify the default prompt manually

Error Log

N/A - This is a feature request

Minimal Reproduction Code

Currently not possible - no flag exists:

# This doesn't work:
claude --system-prompt-file ./my-prompt.txt

Initial Analysis

1. Claude Code lacks a mechanism to load external prompt templates
2. This limits flexibility for users with specific use cases
3. A simple file-based template system would solve this

Suggested Solutions

1. Add a --system-prompt-file command-line flag that reads a .txt/.md file
2. Support a claude.json config option for prompt template path
3. Allow template variables like {{project_name}}, {{language}} for dynamic prompts
4. Support multiple preset templates for common use cases (code review, debugging, documentation)

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Reported by Financier-Nuri from Unum AI