feat(mcp): Add environment variable expansion support for MCP server configurations #535
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…configurations
- Add env_expansion utility module with support for ${VAR} syntax
- Update CustomToolClient to expand environment variables in command, args, and env
- Support expansion in MCP server command, arguments, and environment variables
- Add comprehensive error handling for missing environment variables
- Add tests for environment variable expansion functionality
This enables dynamic MCP server configurations using system environment
variables, improving security by avoiding hardcoded credentials and
supporting environment-specific configurations.
🤖 Assisted by [Amazon Q Developer](https://aws.amazon.com/q/developer)
…nsion The functionality is self-explanatory and doesn't need explicit comments mentioning the feature in every location where it's used. 🤖 Assisted by [Amazon Q Developer](https://aws.amazon.com/q/developer)
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Summary
This PR adds support for environment variable expansion in MCP (Model Context Protocol) server configurations, allowing users to use ${VAR_NAME} syntax in command paths, arguments, and
environment variables.
Changes Made
Core Implementation
• New module: crates/chat-cli/src/util/env_expansion.rs
• Implements expand_env_vars() function with regex-based ${VAR_NAME} pattern matching
• Provides specialized functions for expanding variables in commands, arguments, and environment maps
• Comprehensive error handling for missing environment variables
Integration
• Modified: crates/chat-cli/src/cli/chat/tools/custom_tool.rs
• Integrated environment variable expansion into CustomToolClient::from_config()
• Expands variables in command paths, arguments, and environment variables before MCP client creation
• Added proper error propagation with descriptive error messages
Testing
• Unit tests in env_expansion.rs covering:
• Simple and complex variable expansion scenarios
• Missing variable error handling
• HashMap and vector expansion utilities
• Integration tests in custom_tool.rs covering:
• Full MCP configuration expansion workflow
• Mixed expansion scenarios (some vars expanded, some static)
• Error cases with missing variables
• Additional test file: tests/mcp_env_expansion_test.rs with comprehensive mock testing
Use Cases
This feature enables users to:
• Store sensitive credentials in environment variables: "env": {"API_KEY": "${MY_API_KEY}"}
• Use dynamic paths: "command": "${PYTHON_PATH}"
• Configure server ports: "args": ["--port", "${SERVER_PORT}"]
• Maintain environment-specific configurations without hardcoding values
Example Configuration
json
{
"command": "${PYTHON_PATH}",
"args": ["-m", "server", "--port", "${SERVER_PORT}"],
"env": {
"API_KEY": "${MY_API_KEY}",
"CONFIG_PATH": "${HOME}/.config/myapp"
}
}
Error Handling
• Clear error messages when environment variables are missing
• Fails fast during MCP client initialization to prevent runtime issues
• Preserves original configuration values when expansion fails
Backward Compatibility
• Fully backward compatible - configurations without environment variables work unchanged
• No breaking changes to existing MCP server configurations
• Optional feature that only activates when ${VAR_NAME} patterns are detected
🤖 Assisted by Amazon Q Developer