src/mcp.rs

This file connects you to MCP servers under the mcp feature, discovers their tools, and adapts them as loopctl tools. It is built on the rmcp library, hidden from your public client surface.

Key items

Your McpClient offers stdio(CommandSpec), http_sse(endpoint) plus http_sse_with_client, in_process(server), and from_service(...). Call reconnect(&StreamRetryConfig) when a link drops.

CommandSpec { program, args, env, cwd } uses struct-literal construction, with no builder to chase.

McpToolProvider::connect(client, name_prefix) snapshots the server’s tools for you. Then use register_into(&registry), which takes &ToolRegistry since 0.4.1, plus with_call_timeout which defaults to 60s, refresh(), tools(), and client().

Since 0.4.1 you also get sync_into(&registry).await, which re-lists the server and mirrors the diff into a live registry. It returns an McpSyncReport { added, removed, untouched } with the sorted diff. Call with_structured_policy(StructuredContentPolicy::Error | Lenient) to set the structured-content policy, where Error is the default.

McpTool::matches compares the full observable contract for you: schema, description, the read-only and destructive annotations, call timeout, and the structured policy.

McpTool is your adapted Tool: server schema verbatim, is_concurrency_safe false, and is_read_only taken from readOnlyHint.

McpError covers Handshake, Protocol, and EmptyToolError.

Behavior notes

Your prefix namespaces names client-side as prefix__tool, and the un-prefixed name is what tools/call forwards.

Exceed the call timeout and you get a soft error naming the tool and its budget, so a wedged server costs you one result, not the loop.

Stdio children die with your client handle, since rmcp kills them with a 3s grace. In-process needs a tokio runtime and cannot reconnect.

Refresh does not retro-update tools you already registered, and duplicate names after prefixing collapse with the first kept. sync_into is the retro-updating path for your live registries.

sync_into reconciles against the registration baseline, not the discovery snapshot. A refresh between setup and sync still sheds dropped tools, and a server-side change a refresh already pulled still re-registers.

A dropped name is removed only while the target still holds this provider’s registration, through registration_matches. A foreign tool that overwrote the shared name is left alone with a warning.

A server-declared compilable outputSchema turns on per-call structuredContent validation. A valid payload rides as a pretty-printed JSON part with DisplayHint::Json, and an invalid one hits the policy.

Under Error you get a soft error naming the violated keyword, and under Lenient a warning and passthrough. An uncompilable schema is omitted at discovery with a warning, so the tool stays listed but unvalidated. Schema-less tools keep the JSON-stringified note byte-for-byte.

Deep dive: MCP.