Building an MCP Server in Elixir (revelry.co)

The post describes building a custom MCP (Model-Call Protocol) server in Elixir to move from single-pass RAG pipelines to agentic tool-calling, where an LLM autonomously decides which tools to call, inspects results, and chains further calls until it has what it needs. The authors note MCP (popularized by Anthropic) standardizes that agentic pattern; they evaluated Hermes but implemented their own server because their multi-tenant permission model, integrations, and desire for deep control required tight coupling with existing user/team security. They haven’t exposed this as an external service—tools remain internal to keep security, formatting, and performance guarantees. Technically, the implementation defines a Tool behaviour with core callbacks (name, description, parameters, execute) returning structured results (type, content, metadata, status) so each tool can include source metadata for citations. Example tools include semantic document query (embeddings → vector DB search with team/user filters), web search, and document context. An MCP Client GenServer manages the tool registry, enabled-tool set, and routes execution with user/team context and feature-flag gating. Key production practices: per-tool access control, user-configurable enabled tools, live UI visibility of tool calls, and end-to-end tracing and citation for auditability. The result is a more capable, debuggable, and auditable agentic workflow—well-suited to the BEAM’s concurrency model for multi-step LLM-driven interactions.