OneCAD-Simulation/crates/mcp/src/server.rs

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

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

use serde_json::{Value, json};

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

/// The newest protocol revision spoken; offered when the client asks for one not known.
pub const PROTOCOL_VERSION: &str = "2025-06-18";
pub const SUPPORTED_VERSIONS: &[&str] = &[PROTOCOL_VERSION, "2025-03-26", "2024-11-05"];

/// Longest message line accepted (the control channel's limit too).
pub const MAX_LINE: usize = 16 << 20;
/// Most messages in one JSON-RPC batch (protocol revisions before 2025-06-18).
pub const MAX_BATCH_MESSAGES: usize = 100;

const PARSE_ERROR: i64 = -32700;
const INVALID_REQUEST: i64 = -32600;
const METHOD_NOT_FOUND: i64 = -32601;
const INVALID_PARAMS: i64 = -32602;
const INTERNAL_ERROR: i64 = -32603;
const RESOURCE_NOT_FOUND: i64 = -32002;

const INSTRUCTIONS: &str = "FieldCraft turns geometry into finite element models and solves them. The model is \
unit-consistent and never converts (SolidCraft geometry is in mm, so use mm-t-s-N-MPa: E in MPa, density in t/mm³, \
forces in N); Z is up. A typical analysis: `open` a .solidcraft design (or a .fieldcraft model, or a face-tagged \
.3mf); `inspect_model` for its bodies and their persistent face names; `mesh` (method tet: {body, tolerance}); \
then `execute` the model definition commands (model.set.create {name, geometry: [{body, face}]}, \
model.material.create, model.section.create, model.boundary_condition.create, model.load.create, model.step.create \
— list_commands shows each one's parameters); `check_model`; `solve` (static, frequency, explicit, fluid); \
`show_results` (field: displacement, von_mises…; its reply has the field's range) and `probe` / `reactions` for \
numbers; `screenshot` to look; `save` or `write_deck` (abaqus, nastran, lsdyna). Failures leave the model unchanged \
and name what to fix.";

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()}})
}

/// One line of at most `max` bytes (without the newline). A longer line is read to its end and
/// dropped, so the stream stays in step: `Err(InvalidData)` reports it.
fn read_line(r: &mut impl BufRead, max: usize) -> std::io::Result<Option<Vec<u8>>> {
    let mut line = Vec::new();
    let mut too_long = false;
    loop {
        let (done, used) = {
            let available = r.fill_buf()?;
            if available.is_empty() {
                if too_long {
                    return Err(std::io::Error::new(std::io::ErrorKind::InvalidData, "request too large"));
                }
                return Ok((!line.is_empty()).then_some(line));
            }
            let (part, done, used) = match available.iter().position(|b| *b == b'\n') {
                Some(i) => (available.get(..i).unwrap_or(&[]), true, i + 1),
                None => (available, false, available.len()),
            };
            if !too_long {
                line.extend_from_slice(part);
                if line.len() > max {
                    too_long = true;
                    line = Vec::new();
                }
            }
            (done, used)
        };
        r.consume(used);
        if done {
            if too_long {
                return Err(std::io::Error::new(std::io::ErrorKind::InvalidData, "request too large"));
            }
            return Ok(Some(line));
        }
    }
}

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()
    }

    /// Whether the client sent `notifications/initialized`.
    pub fn is_initialized(&self) -> bool {
        self.initialized
    }

    /// Serve newline-delimited JSON-RPC until `input` closes. Only protocol messages go to
    /// `output`; diagnostics belong on stderr.
    pub fn serve(&mut self, mut input: impl BufRead, mut output: impl Write) -> std::io::Result<()> {
        loop {
            let reply = match read_line(&mut input, MAX_LINE) {
                Ok(None) => return Ok(()),
                Ok(Some(bytes)) => match String::from_utf8(bytes) {
                    Ok(line) => self.handle_line(&line),
                    Err(_) => Some(error(Value::Null, PARSE_ERROR, "parse error: input is not UTF-8").to_string()),
                },
                Err(e) if e.kind() == std::io::ErrorKind::InvalidData => {
                    Some(error(Value::Null, INVALID_REQUEST, format!("request too large (at most {MAX_LINE} bytes a line)")).to_string())
                }
                Err(e) => return Err(e),
            };
            if let Some(reply) = reply {
                output.write_all(reply.as_bytes())?;
                output.write_all(b"\n")?;
                output.flush()?;
            }
        }
    }

    /// One line in, at most one line out (notifications get no reply).
    pub fn handle_line(&mut self, line: &str) -> Option<String> {
        let line = line.trim();
        if line.is_empty() {
            return None;
        }
        if line.len() > MAX_LINE {
            return Some(error(Value::Null, INVALID_REQUEST, "request too large").to_string());
        }
        let reply = match serde_json::from_str::<Value>(line) {
            Ok(Value::Array(batch)) if batch.is_empty() => Some(error(Value::Null, INVALID_REQUEST, "empty batch")),
            Ok(Value::Array(batch)) if batch.len() > MAX_BATCH_MESSAGES => {
                Some(error(Value::Null, INVALID_REQUEST, format!("batch too large (at most {MAX_BATCH_MESSAGES} messages)")))
            }
            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())
    }

    /// One message in, its reply out (none for notifications and responses).
    pub fn handle(&mut self, msg: Value) -> Option<Value> {
        let Value::Object(o) = &msg else { return Some(error(Value::Null, INVALID_REQUEST, "a message must be an object")) };
        let id = o.get("id").cloned();
        if let Some(id) = &id
            && !(id.is_string() || id.is_i64() || id.is_u64())
        {
            return Some(error(Value::Null, INVALID_REQUEST, "`id` must be a string or an integer"));
        }
        let Some(method) = o.get("method").and_then(Value::as_str) else {
            // A response to a request of ours (none are sent) or junk.
            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_VERSIONS.contains(&asked) { asked } else { PROTOCOL_VERSION };
                Ok(json!({
                    "protocolVersion": version,
                    "capabilities": {"tools": {"listChanged": false}, "resources": {"listChanged": false}},
                    "serverInfo": {"name": "fieldcraft", "title": "FieldCraft", "version": env!("CARGO_PKG_VERSION")},
                    "instructions": format!("{INSTRUCTIONS} Backend: {}.", self.backend.describe()),
                }))
            }
            "ping" => Ok(json!({})),
            "tools/list" => Ok(json!({"tools": tool_definitions()})),
            "tools/call" => {
                let name = params.get("name").and_then(Value::as_str).ok_or((INVALID_PARAMS, "missing tool `name`".to_string()))?;
                let args = match params.get("arguments") {
                    None | Some(Value::Null) => json!({}),
                    Some(a) => a.clone(),
                };
                check_arguments(name, &args).map_err(|e| (INVALID_PARAMS, e))?;
                Ok(call_tool(self.backend.as_mut(), name, &args))
            }
            "resources/list" => Ok(json!({"resources": [
                {"uri": "fieldcraft://model", "name": "Model", "description": "model.info: node and element counts, units, definitions", "mimeType": "application/json"},
                {"uri": "fieldcraft://commands", "name": "Command catalog", "description": "every command with its parameter docs", "mimeType": "application/json"},
            ]})),
            "resources/templates/list" => Ok(json!({"resourceTemplates": []})),
            "prompts/list" => Ok(json!({"prompts": []})),
            "resources/read" => {
                let uri = params.get("uri").and_then(Value::as_str).unwrap_or("");
                let v = match uri {
                    "fieldcraft://model" => self.backend.execute("model.info", json!({})),
                    "fieldcraft://commands" => self.backend.commands(),
                    _ => return Err((RESOURCE_NOT_FOUND, format!("unknown resource `{}`", uri.chars().take(200).collect::<String>()))),
                }
                .map_err(|e| (INTERNAL_ERROR, e))?;
                Ok(json!({"contents": [{"uri": uri, "mimeType": "application/json", "text": v.to_string()}]}))
            }
            _ => Err((METHOD_NOT_FOUND, format!("method `{}` not found", method.chars().take(200).collect::<String>()))),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn lines_are_capped_and_the_stream_stays_in_step() {
        let mut input = Vec::new();
        input.extend(vec![b'x'; 100]);
        input.extend(b"\nshort\n");
        input.extend(vec![b'y'; 30]);
        let mut r = std::io::BufReader::with_capacity(8, input.as_slice());
        assert_eq!(read_line(&mut r, 10).unwrap_err().kind(), std::io::ErrorKind::InvalidData);
        assert_eq!(read_line(&mut r, 10).unwrap().unwrap(), b"short");
        assert!(read_line(&mut r, 10).is_err(), "a last line without newline that is too long");
        assert!(read_line(&mut r, 10).unwrap().is_none());
    }
}