tix/mcp/

protocol.rs

//! Newline-delimited JSON-RPC 2.0 wire types, framing and the frozen MCP
//! error surface for the `tix mcp` server (D-01 of research.md;
//! `contracts/mcp-protocol.md`).
//!
//! This module owns the transport contract enforced in T001–T004: exactly one
//! JSON line per response (`C-001`), `stdout` carrying only protocol messages
//! (`C-005` — the single locked writer is the enforcement point), the
//! `initialize` handshake with version negotiation (T003), and the frozen
//! five-error matrix (T004). Tool logic stays out of this module; the
//! dispatch that maps methods to these primitives lives in [`super::handle`].

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

use serde::{Deserialize, Serialize};
use serde_json::{Value, json};

/// `-32700` — undecodable JSON / bad framing line (`contracts/mcp-protocol.md`
/// §6).
pub const PARSE_ERROR: i64 = -32700;
/// `-32600` — valid JSON but not a JSON-RPC message/method shape.
pub const INVALID_REQUEST: i64 = -32600;
/// `-32601` — unknown method.
pub const METHOD_NOT_FOUND: i64 = -32601;
/// `-32602` — `tools/call` with an unknown tool name or type-invalid args.
pub const INVALID_PARAMS: i64 = -32602;
/// `-32603` — unexpected internal failure (panic guard, classified at the
/// boundary).
pub const INTERNAL_ERROR: i64 = -32603;

/// Supported MCP protocol versions (`contracts/mcp-protocol.md` §2), newest
/// first. [`LATEST_VERSION`] is the negotiation fallback (D-03).
pub const SUPPORTED_PROTOCOL_VERSIONS: [&str; 3] = ["2025-06-18", "2025-03-26", "2024-11-05"];

/// The newest supported protocol version, used when the client's requested
/// version is not in [`SUPPORTED_PROTOCOL_VERSIONS`].
pub const LATEST_PROTOCOL_VERSION: &str = "2025-06-18";

/// A JSON-RPC 2.0 `id`. `null` is a *valid* id and must be echoed like any
/// other id (the MCP contract requires distinguishing `Some(null)` from an
/// absent `id`).
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(untagged)]
pub enum Id {
    Number(i64),
    String(String),
    Null,
}

/// A parsed inbound message: a request (`id` present ⇒ the server MUST
/// respond) or a notification (`id` absent ⇒ the server MUST NOT respond).
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Message {
    Request {
        method: String,
        params: Option<Value>,
        id: Id,
    },
    Notification {
        method: String,
        params: Option<Value>,
    },
}

/// What went wrong while decoding a stdin line. The response kind carried by
/// each variant pins the error code and (where recoverable) the id.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum ParseError {
    /// Not valid JSON — respond `-32700` with `id: null`.
    MalformedJson,
    /// Valid JSON but not a well-formed JSON-RPC message. Respond `-32600`
    /// with the id when one was present; otherwise (notification-shaped) never
    /// respond.
    InvalidRequest(Option<Id>),
}

/// A successful response: `result` carries the method-specific payload.
#[derive(Debug, Clone, Serialize)]
pub struct Response {
    pub jsonrpc: &'static str,
    pub id: Id,
    pub result: Value,
}

/// A JSON-RPC error object (`data` is optional and skipped when absent).
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct RpcError {
    pub code: i64,
    pub message: String,
    #[serde(skip_serializing_if = "Option::is_none")]
    pub data: Option<Value>,
}

/// A failure response using one of the five frozen codes.
#[derive(Debug, Clone, Serialize)]
pub struct ErrorResponse {
    pub jsonrpc: &'static str,
    pub id: Id,
    pub error: RpcError,
}

/// Construct a success response.
pub fn success(id: Id, result: Value) -> Response {
    Response {
        jsonrpc: "2.0",
        id,
        result,
    }
}

/// `-32700 Parse error` — carried `id` is always `null` (the line did not
/// parse, so no id was available).
pub fn parse_error() -> ErrorResponse {
    ErrorResponse {
        jsonrpc: "2.0",
        id: Id::Null,
        error: RpcError {
            code: PARSE_ERROR,
            message: "Parse error".into(),
            data: None,
        },
    }
}

/// `-32600 Invalid request` for a valid-JSON but malformed message.
pub fn invalid_request(id: Id) -> ErrorResponse {
    ErrorResponse {
        jsonrpc: "2.0",
        id,
        error: RpcError {
            code: INVALID_REQUEST,
            message: "Invalid Request".into(),
            data: None,
        },
    }
}

/// `-32601 Method not found`.
pub fn method_not_found(id: Id) -> ErrorResponse {
    ErrorResponse {
        jsonrpc: "2.0",
        id,
        error: RpcError {
            code: METHOD_NOT_FOUND,
            message: "Method not found".into(),
            data: None,
        },
    }
}

/// `-32602 Invalid params`.
pub fn invalid_params(id: Id) -> ErrorResponse {
    ErrorResponse {
        jsonrpc: "2.0",
        id,
        error: RpcError {
            code: INVALID_PARAMS,
            message: "Invalid params".into(),
            data: None,
        },
    }
}

/// `-32603 Internal error`.
pub fn internal_error(id: Id) -> ErrorResponse {
    ErrorResponse {
        jsonrpc: "2.0",
        id,
        error: RpcError {
            code: INTERNAL_ERROR,
            message: "Internal error".into(),
            data: None,
        },
    }
}

/// Echo the client's `protocolVersion` when it is supported; otherwise
/// negotiate down to [`LATEST_PROTOCOL_VERSION`] (`contracts/mcp-protocol.md`
/// §2, D-03).
pub fn negotiate_protocol_version(client_version: &str) -> &'static str {
    SUPPORTED_PROTOCOL_VERSIONS
        .iter()
        .find(|&&v| v == client_version)
        .copied()
        .unwrap_or(LATEST_PROTOCOL_VERSION)
}

/// The `initialize` result payload (`contracts/mcp-protocol.md` §3).
pub fn initialize_result(protocol_version: &str, server_version: &str) -> Value {
    json!({
        "protocolVersion": protocol_version,
        "capabilities": { "tools": { "listChanged": false } },
        "serverInfo": { "name": "tix", "version": server_version },
        "instructions": "tix issue tracker — see each tool's description/inputSchema"
    })
}

/// Read one UTF-8 line from `reader`, returning `None` on EOF (clean
/// shutdown → exit 0, no response). Trailing `\n`/`\r` is stripped so the
/// framing uses exactly one JSON message per line (C-001).
pub fn read_line<R: BufRead>(reader: &mut R) -> io::Result<Option<String>> {
    let mut line = String::new();
    if reader.read_line(&mut line)? == 0 {
        return Ok(None);
    }
    while line.ends_with('\n') || line.ends_with('\r') {
        line.pop();
    }
    Ok(Some(line))
}

/// Write one JSON-RPC message as a single newline-terminated line, then
/// flush. Callers pass the single locked stdout handle owned by
/// [`super::serve`]; nothing else in the crate may write to stdout during a
/// serve, so this helper is the stdout-purity enforcement point (C-005).
pub fn write_message<W: Write, T: Serialize>(writer: &mut W, message: &T) -> io::Result<()> {
    let line = serde_json::to_string(message).map_err(io::Error::other)?;
    writer.write_all(line.as_bytes())?;
    writer.write_all(b"\n")?;
    writer.flush()
}

/// Decode a stdin line into a [`Message`]. A line that is not valid JSON is a
/// [`ParseError::MalformedJson`]; valid JSON that is not a well-formed JSON-RPC
/// message is [`ParseError::InvalidRequest`] carrying the id (if extractable).
pub fn parse_message(line: &str) -> Result<Message, ParseError> {
    let value: Value = serde_json::from_str(line).map_err(|_| ParseError::MalformedJson)?;
    if value.get("jsonrpc").and_then(Value::as_str) != Some("2.0") {
        return Err(ParseError::InvalidRequest(extract_id(&value)));
    }
    let method = value
        .get("method")
        .and_then(Value::as_str)
        .ok_or(ParseError::InvalidRequest(extract_id(&value)))?;
    let params = value.get("params").cloned();
    match extract_id(&value) {
        Some(id) => Ok(Message::Request {
            method: method.to_string(),
            params,
            id,
        }),
        None => Ok(Message::Notification {
            method: method.to_string(),
            params,
        }),
    }
}

/// Pull the `id` field (presence is meaningful: `Some(Id::Null)` is a valid
/// request id; `None` means "notification"). An id of an unusable type is
/// treated as absent so the value still routes as a notification/no-response.
fn extract_id(value: &Value) -> Option<Id> {
    let raw = value.get("id")?;
    if raw.is_object() || raw.is_array() || raw.is_boolean() {
        return None;
    }
    Id::deserialize(raw).ok()
}

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

    #[test]
    fn request_with_numeric_id_is_a_request() {
        let msg = parse_message(r#"{"jsonrpc":"2.0","id":1,"method":"ping"}"#).unwrap();
        assert_eq!(
            msg,
            Message::Request {
                method: "ping".into(),
                params: None,
                id: Id::Number(1),
            }
        );
    }

    #[test]
    fn request_with_string_id_is_a_request() {
        let msg = parse_message(r#"{"jsonrpc":"2.0","id":"a","method":"ping"}"#).unwrap();
        assert!(matches!(
            msg,
            Message::Request { id: Id::String(s), .. } if s == "a"
        ));
    }

    #[test]
    fn explicit_null_id_is_a_request_not_a_notification() {
        // `Some(null)` is a valid JSON-RPC id and must demand a response;
        // indexing into `Option<Id>` would collapse it to an absent id.
        let msg = parse_message(r#"{"jsonrpc":"2.0","id":null,"method":"ping"}"#).unwrap();
        assert!(matches!(msg, Message::Request { id: Id::Null, .. }));
    }

    #[test]
    fn absent_id_is_a_notification() {
        let msg =
            parse_message(r#"{"jsonrpc":"2.0","method":"notifications/initialized"}"#).unwrap();
        assert!(
            matches!(msg, Message::Notification { method, .. } if method == "notifications/initialized")
        );
    }

    #[test]
    fn malformed_json_is_parse_error() {
        assert_eq!(
            parse_message("{\"jsonrpc\":\"2.0\", not json"),
            Err(ParseError::MalformedJson)
        );
    }

    #[test]
    fn non_22_jsonrpc_version_is_invalid_request() {
        assert_eq!(
            parse_message(r#"{"jsonrpc":"1.0","id":1,"method":"ping"}"#),
            Err(ParseError::InvalidRequest(Some(Id::Number(1))))
        );
    }

    #[test]
    fn missing_method_is_invalid_request_with_id() {
        assert_eq!(
            parse_message(r#"{"jsonrpc":"2.0","id":7}"#),
            Err(ParseError::InvalidRequest(Some(Id::Number(7))))
        );
    }

    #[test]
    fn invalid_id_type_is_treated_as_absent_id() {
        assert!(matches!(
            parse_message(r#"{"jsonrpc":"2.0","id":true,"method":"ping"}"#),
            Ok(Message::Notification { method, .. }) if method == "ping"
        ));
    }

    #[test]
    fn request_carries_params_value() {
        let msg = parse_message(
            r#"{"jsonrpc":"2.0","id":2,"method":"initialize","params":{"protocolVersion":"2025-06-18"}}"#,
        )
        .unwrap();
        assert!(matches!(msg, Message::Request { method, params, id } if
            method == "initialize"
            && params == Some(json!({"protocolVersion": "2025-06-18"}))
            && id == Id::Number(2)));
    }

    #[test]
    fn version_negotiation_echoes_supported_versions() {
        assert_eq!(negotiate_protocol_version("2025-06-18"), "2025-06-18");
        assert_eq!(negotiate_protocol_version("2025-03-26"), "2025-03-26");
        assert_eq!(negotiate_protocol_version("2024-11-05"), "2024-11-05");
    }

    #[test]
    fn version_negotiation_falls_back_to_latest() {
        assert_eq!(negotiate_protocol_version("2024-01-01"), "2025-06-18");
        assert_eq!(negotiate_protocol_version(""), "2025-06-18");
    }

    #[test]
    fn error_constructors_have_frozen_codes() {
        let id = Id::Number(3);
        assert_eq!(parse_error().id, Id::Null);
        assert_eq!(parse_error().error.code, PARSE_ERROR);
        assert_eq!(invalid_request(id.clone()).error.code, INVALID_REQUEST);
        assert_eq!(method_not_found(id.clone()).error.code, METHOD_NOT_FOUND);
        assert_eq!(invalid_params(id.clone()).error.code, INVALID_PARAMS);
        assert_eq!(internal_error(id.clone()).error.code, INTERNAL_ERROR);
        assert_eq!(invalid_request(id).error.message, "Invalid Request");
    }

    #[test]
    fn success_response_serializes_as_single_2d0_message() {
        let resp = success(Id::Number(1), json!({"ok": true}));
        let mut out: Vec<u8> = Vec::new();
        write_message(&mut out, &resp).unwrap();
        let s = String::from_utf8(out).unwrap();
        assert_eq!(
            s,
            "{\"jsonrpc\":\"2.0\",\"id\":1,\"result\":{\"ok\":true}}\n"
        );
    }

    #[test]
    fn error_response_omits_data_when_absent() {
        let resp = method_not_found(Id::Null);
        let mut out: Vec<u8> = Vec::new();
        write_message(&mut out, &resp).unwrap();
        let s = String::from_utf8(out).unwrap();
        assert_eq!(
            s,
            "{\"jsonrpc\":\"2.0\",\"id\":null,\"error\":{\"code\":-32601,\"message\":\"Method not found\"}}\n"
        );
    }

    #[test]
    fn write_message_never_leaks_bytes_after_the_line() {
        // Byte-level purity: one message, one trailing newline, nothing else.
        let resp = success(Id::String("x".into()), json!({}));
        let mut out: Vec<u8> = Vec::new();
        write_message(&mut out, &resp).unwrap();
        assert_eq!(out.last(), Some(&b'\n'));
        let s = String::from_utf8(out).unwrap();
        assert_eq!(s.matches('\n').count(), 1);
        assert!(s.ends_with('\n'));
    }

    #[test]
    fn read_line_returns_none_on_eof() {
        let mut reader: &[u8] = b"";
        assert_eq!(read_line(&mut reader).unwrap(), None);
    }

    #[test]
    fn read_line_strips_trailing_newline() {
        let mut reader: &[u8] = b"{\"a\":1}\n{\"b\":2}\r\n";
        assert_eq!(read_line(&mut reader).unwrap(), Some("{\"a\":1}".into()));
        assert_eq!(read_line(&mut reader).unwrap(), Some("{\"b\":2}".into()));
        assert_eq!(read_line(&mut reader).unwrap(), None);
    }

    #[test]
    fn initialize_result_shape_matches_contract() {
        let result = initialize_result("2025-06-18", "0.1.0");
        assert_eq!(
            result,
            json!({
                "protocolVersion": "2025-06-18",
                "capabilities": { "tools": { "listChanged": false } },
                "serverInfo": { "name": "tix", "version": "0.1.0" },
                "instructions": "tix issue tracker — see each tool's description/inputSchema"
            })
        );
    }
}