feat: add systemPrompt property to ModeConfig for custom system prompt override

[daniel-lxs]

Summary

Add a new systemPrompt property to ModeConfig that allows custom modes to override the default system prompt sections (CAPABILITIES, OBJECTIVE, TOOL USE, etc.) with custom content.

Problem

Currently, custom modes can only customize:

• roleDefinition - The mode's identity/persona
• customInstructions - Mode-specific behavioral instructions

The middle sections of the system prompt are hardcoded and cannot be disabled or replaced except by creating a file at .roo/system-prompt-{mode-slug}. Users want the ability to define a minimal/custom system prompt directly in their mode configuration without creating external files. This is useful for creating specialized modes (e.g., MCP-only modes) where the default "code assistant" framing is unnecessary.

Solution

Added a new optional systemPrompt property to the ModeConfig schema. When provided, it replaces the middle sections of the generated prompt with the custom content.

Final prompt structure when systemPrompt is provided:

roleDefinition + systemPrompt + customInstructions

Instead of:

roleDefinition + MARKDOWN + TOOL USE + MCP SERVERS + CAPABILITIES + MODES + RULES + SYSTEM INFO + OBJECTIVE + customInstructions

Changes

• packages/types/src/mode.ts - Added systemPrompt to schema with JSDoc
• src/core/prompts/system.ts - Implemented the override logic
• src/core/prompts/__tests__/system-prompt.spec.ts - Added 4 new tests

Priority Order

1. File-based (.roo/system-prompt-{mode}) - highest priority
2. Config systemPrompt property - second priority
3. Default generated prompt - fallback

Usage Example

{
  "slug": "mcp-only",
  "name": "MCP Only Mode",
  "roleDefinition": "You are an MCP assistant.",
  "groups": ["mcp"],
  "systemPrompt": "Use MCP tools to accomplish tasks. You have access to connected MCP servers.",
  "customInstructions": "Focus on efficiency."
}

Testing

• All 5104 tests pass
• 4 new tests added for systemPrompt functionality

Closes EXT-557

---

Important

Adds systemPrompt to ModeConfig for custom prompt overrides, with priority handling and tests.

• Behavior:
  • Adds systemPrompt property to ModeConfig schema in mode.ts for custom prompt content.
  • Overrides default prompt sections (e.g., TOOL USE, CAPABILITIES) when systemPrompt is provided.
  • Priority: file-based > config systemPrompt > default prompt.
• Implementation:
  • system.ts: Implements logic to use systemPrompt from config or file.
  • system-prompt.spec.ts: Adds tests for systemPrompt functionality, including priority handling.
• Testing:
  • 4 new tests added for systemPrompt in system-prompt.spec.ts.
  • All existing tests pass.

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[roomote]

Rooviewer   See task on Roo Cloud

Review complete. No issues found.

The implementation correctly:

• Adds the systemPrompt property to the ModeConfig schema with proper JSDoc
• Implements the priority order (file-based > config systemPrompt > default)
• Includes comprehensive test coverage for all scenarios

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