MCP client

The non-default mcp-client feature lets acorn-lib initialize configured MCP servers, discover approved tools, and invoke them through child-process or Streamable HTTP transports. The ACORN CLI enables this feature for acorn mcp.

cargo add acorn-lib --no-default-features --features mcp-client

Application configuration owns every server transport, environment-variable reference, tool allowlist, declared effect, timeout, and output limit. The top-level mcp map names servers, each server’s allowed map names tools, timeout is measured in seconds, and max_output uses a unit-bearing memory value such as 256KB. Callers select only a configured server alias, tool name, and JSON-object arguments:

#![allow(unused)]
fn main() {
use acorn::io::api::operation::InvocationContext;
use acorn::io::config::ApplicationConfiguration;
use acorn::io::mcp::client::McpClient;
use serde_json::json;

async fn example(configuration: &ApplicationConfiguration) -> color_eyre::Result<()> {
    let client = McpClient::try_from(configuration)?;
    let result = client
        .call_tool(
            "research-tools",
            "resolve_identifier",
            json!({"identifier": "https://doi.org/10.1000/example"}),
            &InvocationContext::default(),
        )
        .await?;
    if result.is_error == Some(true) {
        // The MCP exchange succeeded and the remote tool reported an error.
    }
    Ok(())
 }
}

list_tools returns only tools that are configured, currently advertised, and authorized by the supplied invocation context. call_tool preserves the complete typed rmcp::model::CallToolResult; a tool-level isError value is not converted into a transport error.

The client and ACORN’s JSON-RPC operation registry share the same transport-neutral effects policy. Credentials and process effects are unavailable; inference requires explicit inference authorization; mutations require mutation authorization; and network reads are denied offline. Streamable HTTP itself is unavailable offline. The effective result limit is the smaller of the tool configuration and caller context limits.

Sessions are intentionally one operation long. Initialization, paginated catalog discovery, requests, cancellation, output bounds, and shutdown all have fixed limits. Configuration stores environment-variable names, never resolved bearer tokens or child values. Child processes run without a shell or inherited environment, and remote HTTP requires TLS except on loopback.

The client is not a general MCP proxy. Register an ACORN operation handler explicitly when an application should call a remote tool; enabling mcp-client does not add remote tools to any inbound catalog or transport.