cadcraft/crates/mcp/src/server.rs

//! JSON-RPC 2.0 framing and the MCP lifecycle / tools / resources methods.

use std::io::{BufRead, Write};

use serde_json::{Value, json};

use crate::backend::Backend;
use crate::tools::{call_tool, tool_definitions};

pub const PROTOCOL_VERSION: &str = "2025-06-18";
const SUPPORTED: &[&str] = &[PROTOCOL_VERSION, "2025-03-26", "2024-11-05"];

const PARSE_ERROR: i64 = -32700;
const INVALID_REQUEST: i64 = -32600;
const METHOD_NOT_FOUND: i64 = -32601;
const INVALID_PARAMS: i64 = -32602;

const INSTRUCTIONS: &str = "CADCraft is a 2D/3D CAD drafting app (an AutoCAD clone). Coordinates are drawing units, \
y up. Fastest path: command_line with AutoCAD syntax, e.g. `line 0,0 10,0 10,5 c`, `circle 5,2.5 1`, \
`rectang 0,0 4,3`, `offset 1` then pick, `zoom e`. Or execute with JSON (list_commands shows ids and params). \
Verify with inspect_drawing / query_entities and look with render.";

pub struct Server {
    backend: Box<dyn Backend>,
    initialized: bool,
}

fn response(id: Value, result: Value) -> Value {
    json!({"jsonrpc": "2.0", "id": id, "result": result})
}
fn error(id: Value, code: i64, message: impl Into<String>) -> Value {
    json!({"jsonrpc": "2.0", "id": id, "error": {"code": code, "message": message.into()}})
}

impl Server {
    pub fn new(backend: Box<dyn Backend>) -> Self {
        Self { backend, initialized: false }
    }
    pub fn backend(&mut self) -> &mut dyn Backend {
        self.backend.as_mut()
    }
    pub fn is_initialized(&self) -> bool {
        self.initialized
    }

    /// Serve newline-delimited JSON-RPC until `input` closes (logs to stderr only).
    pub fn serve(&mut self, input: impl BufRead, mut output: impl Write) -> std::io::Result<()> {
        for line in input.lines() {
            let line = line?;
            if let Some(reply) = self.handle_line(&line) {
                output.write_all(reply.as_bytes())?;
                output.write_all(b"\n")?;
                output.flush()?;
            }
        }
        Ok(())
    }

    pub fn handle_line(&mut self, line: &str) -> Option<String> {
        let line = line.trim();
        if line.is_empty() {
            return None;
        }
        let reply = match serde_json::from_str::<Value>(line) {
            Ok(Value::Array(batch)) => {
                let replies: Vec<Value> = batch.into_iter().filter_map(|m| self.handle(m)).collect();
                (!replies.is_empty()).then_some(Value::Array(replies))
            }
            Ok(msg) => self.handle(msg),
            Err(e) => Some(error(Value::Null, PARSE_ERROR, format!("parse error: {e}"))),
        };
        reply.map(|r| r.to_string())
    }

    pub fn handle(&mut self, msg: Value) -> Option<Value> {
        let Value::Object(o) = &msg else { return Some(error(Value::Null, INVALID_REQUEST, "message must be an object")) };
        let id = o.get("id").cloned();
        let Some(method) = o.get("method").and_then(Value::as_str) else {
            if o.contains_key("result") || o.contains_key("error") {
                return None;
            }
            return Some(error(id.unwrap_or(Value::Null), INVALID_REQUEST, "missing `method`"));
        };
        let params = o.get("params").cloned().unwrap_or(Value::Null);
        let Some(id) = id else {
            if method == "notifications/initialized" {
                self.initialized = true;
            }
            return None;
        };
        Some(match self.request(method, &params) {
            Ok(r) => response(id, r),
            Err((code, m)) => error(id, code, m),
        })
    }

    fn request(&mut self, method: &str, params: &Value) -> Result<Value, (i64, String)> {
        match method {
            "initialize" => {
                let asked = params.get("protocolVersion").and_then(Value::as_str).unwrap_or(PROTOCOL_VERSION);
                let version = if SUPPORTED.contains(&asked) { asked } else { PROTOCOL_VERSION };
                Ok(json!({
                    "protocolVersion": version,
                    "capabilities": {"tools": {"listChanged": false}, "resources": {"listChanged": false}},
                    "serverInfo": {"name": "cadcraft", "title": "CADCraft", "version": env!("CARGO_PKG_VERSION")},
                    "instructions": format!("{INSTRUCTIONS} Backend: {}.", self.backend.describe()),
                }))
            }
            "ping" => Ok(json!({})),
            "tools/list" => Ok(tool_definitions(self.backend.has_ui())),
            "tools/call" => {
                let name = params.get("name").and_then(Value::as_str).ok_or((INVALID_PARAMS, "missing tool `name`".to_string()))?;
                let args = params.get("arguments").cloned().unwrap_or(json!({}));
                Ok(call_tool(self.backend.as_mut(), name, &args))
            }
            "resources/list" => Ok(json!({"resources": [
                {"uri": "cadcraft://drawing", "name": "Active drawing", "mimeType": "application/json"},
                {"uri": "cadcraft://commands", "name": "Command catalog", "mimeType": "application/json"},
            ]})),
            "resources/read" => {
                let uri = params.get("uri").and_then(Value::as_str).unwrap_or("");
                let v = match uri {
                    "cadcraft://drawing" => self.backend.call("drawing.inspect", json!({})),
                    "cadcraft://commands" => self.backend.call("engine.commands", json!({})),
                    _ => return Err((-32002, format!("unknown resource `{uri}`"))),
                }
                .map_err(|e| (-32603, e))?;
                Ok(json!({"contents": [{"uri": uri, "mimeType": "application/json", "text": v.to_string()}]}))
            }
            _ => Err((METHOD_NOT_FOUND, format!("method `{method}` not found"))),
        }
    }
}