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MCP In-Process Invocation

By default, tools/call from an MCP server proxies over HTTP to your main RustAPI server (even on localhost). This guarantees that every middleware, interceptor, extractor, and validator runs exactly as for normal traffic.

For high-frequency agent use (many tool calls per prompt), the network/serde overhead of the proxy can add up. RustAPI supports an optional in-process invocation path that constructs a Request and drives it directly through the Router + LayerStack with zero TCP or serialization.

When to Use In-Process

  • You are using run_rustapi_and_mcp (or equivalent) so the MCP sidecar and main HTTP server are in the same process.
  • You make many sequential or batched tool calls from agents.
  • You want the absolute lowest latency while keeping the full pipeline.

Do not use in-process if your MCP server talks to a remote RustAPI instance (use the proxy).

How to Enable

#![allow(unused)]
fn main() {
use rustapi_rs::protocol::mcp::{McpConfig, McpServer, InvocationMode};

let mcp = McpServer::from_rustapi(
    &app,
    McpConfig::new()
        .allowed_tags(["agent"])
        .invocation_mode(InvocationMode::InProcess),  // or Auto
);
}

Modes:

  • Proxy (default): always use HTTP proxy.
  • InProcess: use direct dispatch (requires from_rustapi).
  • Auto: prefer in-process when a RustApi was attached, fall back to proxy.

When you use run_rustapi_and_mcp*, the runner automatically wires the HTTP base for the proxy path. In-process mode simply ignores it.

Performance

See the benchmark in crates/rustapi-mcp/tests/mcp_e2e.rs.

Typical numbers on a dev machine (1000 sequential tool calls):

  • In-process: ~28 µs per call
  • Proxy (live localhost HTTP): ~1.3 ms per call
  • Speedup: ~45-50x

The in-process path still executes the full middleware chain, extractors, validation, and error handling.

Implementation Notes

The RequestDispatcher (obtained via app.request_dispatcher()) clones the necessary Arc<Router>, LayerStack, and InterceptorChain. Your McpServer stores a RequestInvoker when created via from_rustapi.

Tool calls still go through:

  • Request interceptors
  • Middleware layers (in order)
  • Router match + handler (with extractors, Schema validation, etc.)
  • Response interceptors

No shortcuts are taken.

Cookbook Cross-References