✨ feat: Configurable MCP Dropdown Placeholder

[dustinhealy]

Related to #7964

Summary

This PR introduces support for configuring the default MCP server selection label via the librechat.yaml interface configuration (using a new mcpServers.placeholder field). This enables the MCPSelect label to be customized directly through YAML-based configuration, supporting better flexibility and future enhancements to MCP server UI configuration.

Changes:

• Added mcpServers.placeholder to the interface schema (packages/data-provider/src/config.ts)
• Expanded librechat.example.yaml with a new section for MCP server UI configuration
• Included mcpServers in the interface configuration loaded by api/server/services/start/interface.js
• Updated client/src/components/Chat/Input/MCPSelect.tsx to use startupConfig?.interface?.mcpServers?.placeholder

Change Type

Please delete any irrelevant options.

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Translation update

Testing

Test Configuration:

Test Steps:

1. Add the following to your librechat.yaml interface section:

   interface:
     mcpServers:
       placeholder: "Custom Label"

2. Start the LibreChat server

3. Navigate to a conversation with MCP servers available

4. Verify that the MCP dropdown shows "Custom Label" instead of the default "MCP Servers"

Fallback Testing:

1. Remove the mcpServers configuration from the YAML
2. Restart the server
3. Verify that the MCP dropdown falls back to the localized "MCP Servers" text

Configuration Validation:

• The YAML configuration should validate correctly without errors
• The interface configuration should be properly loaded and passed to the frontend

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have made pertinent documentation changes
• My changes do not introduce new warnings

[danny-avila]

follow up here, need to start from latest dev branch before beginning.

You can do:

git checkout dev
git pull origin dev
git checkout feat-mcp-env
git rebase dev

Also this will need a documentation update:
https://github.com/LibreChat-AI/librechat.ai/pulls

You can use this as a reference:
LibreChat-AI/librechat.ai#336

[danny-avila]

Just waiting on docs for this.

Make sure you make dev branch target for PRs