ADR-003: MCP Integration via stdio Subprocess 

Context 

hn/typo3-mcp-server implements the Model Context Protocol over stdio. It is designed to be launched as a subprocess by an MCP host. The MCP specification also defines HTTP+SSE as a transport option.

Alternatives considered:

• HTTP+SSE transport: Would require hn/typo3-mcp-server to run as a persistent HTTP server, adding deployment complexity and changing its operational model.
• Direct PHP function calls: Would require forking or reimplementing the MCP server logic inside nr_mcp_agent, coupling the two extensions tightly.
• stdio subprocess: Uses hn/typo3-mcp-server exactly as designed, with zero modifications.

Decision 

Connect to hn/typo3-mcp-server by spawning it as a stdio subprocess. McpConnection manages the process lifecycle (start, communication, shutdown). McpToolProvider translates between the agent loop and the MCP protocol.

Consequences 

• hn/typo3-mcp-server is used without modification.
• The MCP connection is process-local: each CLI processing job spawns its own MCP server instance.
• The stdio transport is synchronous within the agent loop, which is sufficient given that processing already runs in a CLI subprocess (see ADR-002: CLI-Based Message Processing).
• MCP is an optional dependency: if hn/typo3-mcp-server is not installed, the extension works without tool-calling capability.