src/mcp/server.rs

This file serves your ToolRegistry over MCP under the mcp feature, so your tools become usable by any MCP client. It exposes rmcp types deliberately, because you are embedding a real server.

Key items

McpServerAdapter::new(registry, context, server_name, server_version) is Clone. Your registry is served live since 0.4.1. The adapter reads it on every tools/list and every call, and registry() hands you the shared handle, so mutations you make surface on the next request.

You no longer rebuild the adapter to serve a changing tool set.

serve_stdio(self) is your canonical interactive transport. The returned RunningService has already completed the handshake, .waiting() blocks until stdin closes, and you cancel through its token.

Other transports go through adapter.serve(transport) using rmcp’s ServiceExt.

Behavior notes

Your server advertises the tools capability only: no resources, prompts, sampling, or logging, and no listChanged.

list_tools maps every schema through the validation layer, see convert.rs. Invalid-schema tools are omitted with a warning, and external $refs are refused, never fetched.

In call_tool, an unknown name returns MCP METHOD_NOT_FOUND naming what is registered. Everything else your tool reports, soft error, hard failure, or cancellation, arrives as a tool-level is_error result.

Cancellation drops in-flight tool futures, so your served tools must be cancellation-safe.

Structured emission joined in 0.4.1. A tool result you mark DisplayHint::Json has its parseable-object payload emitted as structuredContent beside the text lane. The emission parses the payload’s last text part, so a prose-plus-JSON multipart round-trips.

A hint that lies, meaning prose or a non-object, keeps the text lane with a warning.

On rmcp ≥ 3.4 the adapter follows rmcp’s ServerInfo to ServerConfig rename. Behavior is unchanged, but a downstream build naming the deprecated alias now fails under -D warnings.

Deep dive: MCP.