| 1 | //! The loopback Streamable HTTP transcript server for the `mcp` family. |
| 2 | //! |
| 3 | //! One server per case, scripted by the case's `server` object. It speaks |
| 4 | //! just enough HTTP for any MCP client to connect to it by URL, so a child |
| 5 | //! Node process can be pointed at it exactly like at a user's server. |
| 6 | //! |
| 7 | //! Per-method answers (`initialize`, `tools/list`, `resources/list`, |
| 8 | //! `resources/templates/list`, `prompts/list`, `tools/call`, |
| 9 | //! `resources/read`, `prompts/get`): either one entry, or an array of |
| 10 | //! candidate entries. For an array the first candidate whose `match` fields |
| 11 | //! all equal the request params wins (no `match` matches anything), and a |
| 12 | //! candidate marked `"once": true` is skipped after it has answered once — |
| 13 | //! that is how a case scripts "the first `tools/list` differs from the |
| 14 | //! second". Unknown methods get JSON-RPC `-32601`. |
| 15 | //! |
| 16 | //! An entry answers with exactly one of: |
| 17 | //! |
| 18 | //! - `result` / `error`: a JSON-RPC reply. `progress` (progress |
| 19 | //! notifications) and `notifications` (arbitrary notification objects) are |
| 20 | //! sent as SSE events ahead of the reply, which then becomes SSE too. |
| 21 | //! - `hold`: never answer until the client goes away (or [`HOLD_LIMIT`]). |
| 22 | //! - `http`: a raw HTTP response, `{status, headers, body}`, instead of a |
| 23 | //! JSON-RPC one. `{{authorization}}` in `body` is replaced by the request's |
| 24 | //! `Authorization` header value (the "server echoes your credential" case), |
| 25 | //! and `{{sink_url}}` anywhere in the spec by the case's sink server URL. |
| 26 | //! - `expire_session`: the server forgets the current session and answers |
| 27 | //! `404`, without running the request. |
| 28 | //! - `generate_tools` `{count, prefix}` / `generate_pages` `{count, prefix}`: |
| 29 | //! a `tools/list` result built at request time, so a cap-sized catalog does |
| 30 | //! not have to live in a fixture. `generate_pages` serves one tool per page |
| 31 | //! and links pages with `nextCursor` `page-N`. |
| 32 | //! |
| 33 | //! [`ServerOptions`] adds server-wide behaviour: numbered sessions (the |
| 34 | //! `initialize` reply issues `session-1`, `session-2`, … and every later |
| 35 | //! request must carry the latest one, else `404`) and a required bearer token |
| 36 | //! (anything else is `401`). |
| 37 | //! |
| 38 | //! What the server records, for the golden: |
| 39 | //! |
| 40 | //! - `received`: side-effecting requests it actually dispatched |
| 41 | //! (`tools/call`, `resources/read`, `prompts/get`). A request it refused at |
| 42 | //! the HTTP layer (stale session) is not here: the server never ran it. |
| 43 | //! - `requests`: every POST, as `{rpc, session, protocol_version, |
| 44 | //! authorization, status}`. The `Authorization` value is recorded only as a |
| 45 | //! shape (`<bearer>` / `<other>`), never as bytes, so a recorded frame |
| 46 | //! cannot carry a credential. |
| 47 | |
| 48 | use std::collections::{HashMap, HashSet}; |
| 49 | use std::sync::atomic::{AtomicUsize, Ordering}; |
| 50 | use std::sync::{Arc, Mutex}; |
| 51 | use std::time::Duration; |
| 52 | |
| 53 | use serde_json::{Value, json}; |
| 54 | use tokio::io::{AsyncBufReadExt, AsyncReadExt, AsyncWriteExt, BufReader}; |
| 55 | use tokio::sync::Notify; |
| 56 | |
| 57 | /// A held `tools/call` is released after this long even if nobody cancels, |
| 58 | /// so a dispatch that ignores cancellation fails the case instead of hanging. |
| 59 | pub(super) const HOLD_LIMIT: Duration = Duration::from_secs(20); |
| 60 | |
| 61 | /// Server-wide behaviour a case opts into (`server_options`). |
| 62 | #[derive(Clone, Default)] |
| 63 | pub(super) struct ServerOptions { |
| 64 | /// `initialize` issues `session-N`; every later request must carry the |
| 65 | /// latest one or is refused with `404`. |
| 66 | pub(super) numbered_sessions: bool, |
| 67 | /// Every request must carry `Authorization: Bearer <token>`, else `401`. |
| 68 | pub(super) require_bearer: Option<String>, |
| 69 | } |
| 70 | |
| 71 | pub(super) struct State { |
| 72 | spec: Value, |
| 73 | options: ServerOptions, |
| 74 | received: Mutex<Vec<Value>>, |
| 75 | requests: Mutex<Vec<Value>>, |
| 76 | /// Signalled when a `hold` entry starts holding a request. |
| 77 | pub(super) held: Notify, |
| 78 | sessions_issued: AtomicUsize, |
| 79 | current_session: Mutex<Option<String>>, |
| 80 | used: Mutex<HashSet<(String, usize)>>, |
| 81 | } |
| 82 | |
| 83 | pub(super) struct TranscriptServer { |
| 84 | pub(super) url: String, |
| 85 | pub(super) addr: String, |
| 86 | pub(super) state: Arc<State>, |
| 87 | task: tokio::task::JoinHandle<()>, |
| 88 | } |
| 89 | |
| 90 | impl Drop for TranscriptServer { |
| 91 | fn drop(&mut self) { |
| 92 | self.task.abort(); |
| 93 | } |
| 94 | } |
| 95 | |
| 96 | impl TranscriptServer { |
| 97 | pub(super) async fn start(spec: Value, options: ServerOptions) -> Self { |
| 98 | let listener = tokio::net::TcpListener::bind("127.0.0.1:0") |
| 99 | .await |
| 100 | .expect("bind transcript server"); |
| 101 | let addr = listener.local_addr().expect("addr").to_string(); |
| 102 | let url = format!("http://{addr}/mcp"); |
| 103 | let state = Arc::new(State { |
| 104 | spec, |
| 105 | options, |
| 106 | received: Mutex::new(Vec::new()), |
| 107 | requests: Mutex::new(Vec::new()), |
| 108 | held: Notify::new(), |
| 109 | sessions_issued: AtomicUsize::new(0), |
| 110 | current_session: Mutex::new(None), |
| 111 | used: Mutex::new(HashSet::new()), |
| 112 | }); |
| 113 | let served = Arc::clone(&state); |
| 114 | let task = tokio::spawn(async move { |
| 115 | let mut connections = tokio::task::JoinSet::new(); |
| 116 | loop { |
| 117 | let accepted = tokio::select! { |
| 118 | accepted = listener.accept() => accepted, |
| 119 | _ = connections.join_next(), if !connections.is_empty() => continue, |
| 120 | }; |
| 121 | let Ok((socket, _)) = accepted else { |
| 122 | break; |
| 123 | }; |
| 124 | let state = Arc::clone(&served); |
| 125 | connections.spawn(async move { |
| 126 | answer(socket, &state).await; |
| 127 | }); |
| 128 | } |
| 129 | }); |
| 130 | Self { |
| 131 | url, |
| 132 | addr, |
| 133 | state, |
| 134 | task, |
| 135 | } |
| 136 | } |
| 137 | |
| 138 | pub(super) fn received(&self) -> Vec<Value> { |
| 139 | self.state.received.lock().expect("log").clone() |
| 140 | } |
| 141 | |
| 142 | pub(super) fn requests(&self) -> Vec<Value> { |
| 143 | self.state.requests.lock().expect("log").clone() |
| 144 | } |
| 145 | } |
| 146 | |
| 147 | struct Request { |
| 148 | method: String, |
| 149 | headers: HashMap<String, String>, |
| 150 | body: Vec<u8>, |
| 151 | } |
| 152 | |
| 153 | async fn read_request(socket: tokio::net::TcpStream) -> Option<(Request, tokio::net::TcpStream)> { |
| 154 | let mut reader = BufReader::new(socket); |
| 155 | let mut line = String::new(); |
| 156 | if reader.read_line(&mut line).await.unwrap_or(0) == 0 { |
| 157 | return None; |
| 158 | } |
| 159 | let method = line |
| 160 | .split_whitespace() |
| 161 | .next() |
| 162 | .unwrap_or_default() |
| 163 | .to_string(); |
| 164 | let mut headers = HashMap::new(); |
| 165 | loop { |
| 166 | line.clear(); |
| 167 | if reader.read_line(&mut line).await.unwrap_or(0) == 0 { |
| 168 | return None; |
| 169 | } |
| 170 | if line == "\r\n" || line == "\n" { |
| 171 | break; |
| 172 | } |
| 173 | if let Some((name, value)) = line.split_once(':') { |
| 174 | headers.insert(name.trim().to_ascii_lowercase(), value.trim().to_string()); |
| 175 | } |
| 176 | } |
| 177 | let content_length = headers |
| 178 | .get("content-length") |
| 179 | .and_then(|value| value.parse().ok()) |
| 180 | .unwrap_or(0usize); |
| 181 | let mut body = vec![0u8; content_length]; |
| 182 | reader.read_exact(&mut body).await.ok()?; |
| 183 | Some(( |
| 184 | Request { |
| 185 | method, |
| 186 | headers, |
| 187 | body, |
| 188 | }, |
| 189 | reader.into_inner(), |
| 190 | )) |
| 191 | } |
| 192 | |
| 193 | fn reason(status: u16) -> &'static str { |
| 194 | match status { |
| 195 | 200 => "OK", |
| 196 | 202 => "Accepted", |
| 197 | 307 => "Temporary Redirect", |
| 198 | 400 => "Bad Request", |
| 199 | 401 => "Unauthorized", |
| 200 | 404 => "Not Found", |
| 201 | 405 => "Method Not Allowed", |
| 202 | 500 => "Internal Server Error", |
| 203 | _ => "Status", |
| 204 | } |
| 205 | } |
| 206 | |
| 207 | struct Reply { |
| 208 | status: u16, |
| 209 | headers: Vec<(String, String)>, |
| 210 | body: String, |
| 211 | } |
| 212 | |
| 213 | impl Reply { |
| 214 | fn new(status: u16, body: impl Into<String>) -> Self { |
| 215 | Self { |
| 216 | status, |
| 217 | headers: Vec::new(), |
| 218 | body: body.into(), |
| 219 | } |
| 220 | } |
| 221 | |
| 222 | fn header(mut self, name: &str, value: impl Into<String>) -> Self { |
| 223 | self.headers.push((name.to_string(), value.into())); |
| 224 | self |
| 225 | } |
| 226 | |
| 227 | fn bytes(&self) -> Vec<u8> { |
| 228 | let mut head = format!("HTTP/1.1 {} {}\r\n", self.status, reason(self.status)); |
| 229 | for (name, value) in &self.headers { |
| 230 | head.push_str(&format!("{name}: {value}\r\n")); |
| 231 | } |
| 232 | head.push_str(&format!( |
| 233 | "Content-Length: {}\r\nConnection: close\r\n\r\n{}", |
| 234 | self.body.len(), |
| 235 | self.body |
| 236 | )); |
| 237 | head.into_bytes() |
| 238 | } |
| 239 | } |
| 240 | |
| 241 | /// The shape of an `Authorization` header, never its bytes. |
| 242 | fn authorization_shape(value: Option<&String>) -> Value { |
| 243 | match value { |
| 244 | None => Value::Null, |
| 245 | Some(value) if value.to_ascii_lowercase().starts_with("bearer ") => json!("<bearer>"), |
| 246 | Some(_) => json!("<other>"), |
| 247 | } |
| 248 | } |
| 249 | |
| 250 | async fn answer(socket: tokio::net::TcpStream, state: &State) { |
| 251 | let Some((request, mut socket)) = read_request(socket).await else { |
| 252 | return; |
| 253 | }; |
| 254 | if request.method != "POST" { |
| 255 | // No server-initiated stream: the spec's answer for a GET. |
| 256 | let _ = socket |
| 257 | .write_all(b"HTTP/1.1 405 Method Not Allowed\r\nAllow: POST\r\nContent-Length: 0\r\nConnection: close\r\n\r\n") |
| 258 | .await; |
| 259 | return; |
| 260 | } |
| 261 | let Ok(message) = serde_json::from_slice::<Value>(&request.body) else { |
| 262 | let _ = socket |
| 263 | .write_all( |
| 264 | b"HTTP/1.1 400 Bad Request\r\nContent-Length: 0\r\nConnection: close\r\n\r\n", |
| 265 | ) |
| 266 | .await; |
| 267 | return; |
| 268 | }; |
| 269 | let rpc_method = message["method"].as_str().unwrap_or_default().to_string(); |
| 270 | let params = message.get("params").cloned().unwrap_or(Value::Null); |
| 271 | let session = request.headers.get("mcp-session-id").cloned(); |
| 272 | let authorization = request.headers.get("authorization").cloned(); |
| 273 | let record = |status: u16| { |
| 274 | state.requests.lock().expect("log").push(json!({ |
| 275 | "rpc": rpc_method, |
| 276 | "session": session, |
| 277 | "protocol_version": request.headers.get("mcp-protocol-version"), |
| 278 | "authorization": authorization_shape(authorization.as_ref()), |
| 279 | "status": status, |
| 280 | })); |
| 281 | }; |
| 282 | let finish = async |socket: &mut tokio::net::TcpStream, reply: Reply| { |
| 283 | record(reply.status); |
| 284 | let _ = socket.write_all(&reply.bytes()).await; |
| 285 | let _ = socket.shutdown().await; |
| 286 | }; |
| 287 | |
| 288 | if let Some(token) = &state.options.require_bearer |
| 289 | && authorization.as_deref() != Some(format!("Bearer {token}").as_str()) |
| 290 | { |
| 291 | let reply = Reply::new(401, r#"{"error":"unauthorized"}"#) |
| 292 | .header("WWW-Authenticate", "Bearer realm=\"conformance\"") |
| 293 | .header("Content-Type", "application/json"); |
| 294 | finish(&mut socket, reply).await; |
| 295 | return; |
| 296 | } |
| 297 | let is_initialize = rpc_method == "initialize"; |
| 298 | if state.options.numbered_sessions && !is_initialize { |
| 299 | let current = state.current_session.lock().expect("session").clone(); |
| 300 | if current.is_none() || current != session { |
| 301 | finish(&mut socket, Reply::new(404, "session not found")).await; |
| 302 | return; |
| 303 | } |
| 304 | } |
| 305 | let Some(id) = message.get("id").cloned() else { |
| 306 | // Notifications (initialized, cancelled, progress) are accepted. |
| 307 | finish(&mut socket, Reply::new(202, "")).await; |
| 308 | return; |
| 309 | }; |
| 310 | |
| 311 | let entry = scripted_entry(state, &rpc_method, ¶ms); |
| 312 | if entry.get("expire_session").and_then(Value::as_bool) == Some(true) { |
| 313 | *state.current_session.lock().expect("session") = None; |
| 314 | finish(&mut socket, Reply::new(404, "session not found")).await; |
| 315 | return; |
| 316 | } |
| 317 | match rpc_method.as_str() { |
| 318 | "tools/call" => state.received.lock().expect("log").push(json!({ |
| 319 | "method": "tools/call", |
| 320 | "name": params["name"], |
| 321 | "arguments": params.get("arguments").cloned().unwrap_or(Value::Null), |
| 322 | })), |
| 323 | "resources/read" => state |
| 324 | .received |
| 325 | .lock() |
| 326 | .expect("log") |
| 327 | .push(json!({ "method": "resources/read", "uri": params["uri"] })), |
| 328 | "prompts/get" => state |
| 329 | .received |
| 330 | .lock() |
| 331 | .expect("log") |
| 332 | .push(json!({ "method": "prompts/get", "name": params["name"] })), |
| 333 | _ => {} |
| 334 | } |
| 335 | if entry.get("hold").and_then(Value::as_bool) == Some(true) { |
| 336 | state.held.notify_one(); |
| 337 | // Answer nothing until the client goes away (or the hold limit). |
| 338 | let mut byte = [0u8; 1]; |
| 339 | let _ = tokio::time::timeout(HOLD_LIMIT, socket.read(&mut byte)).await; |
| 340 | return; |
| 341 | } |
| 342 | if let Some(http) = entry.get("http") { |
| 343 | let status = http["status"].as_u64().unwrap_or(500) as u16; |
| 344 | let body = http["body"].as_str().unwrap_or_default().replace( |
| 345 | "{{authorization}}", |
| 346 | authorization.as_deref().unwrap_or("<none>"), |
| 347 | ); |
| 348 | let mut reply = Reply::new(status, body); |
| 349 | if let Some(headers) = http["headers"].as_object() { |
| 350 | for (name, value) in headers { |
| 351 | reply = reply.header(name, value.as_str().unwrap_or_default()); |
| 352 | } |
| 353 | } |
| 354 | finish(&mut socket, reply).await; |
| 355 | return; |
| 356 | } |
| 357 | let body = match entry.get("error") { |
| 358 | Some(error) => json!({ "jsonrpc": "2.0", "id": id, "error": error }), |
| 359 | None => json!({ "jsonrpc": "2.0", "id": id, "result": entry["result"] }), |
| 360 | }; |
| 361 | let mut reply = if entry.get("progress").is_some() || entry.get("notifications").is_some() { |
| 362 | let token = params |
| 363 | .pointer("/_meta/progressToken") |
| 364 | .cloned() |
| 365 | .unwrap_or_else(|| json!("conformance")); |
| 366 | let mut sse = String::new(); |
| 367 | for step in entry |
| 368 | .get("progress") |
| 369 | .and_then(Value::as_array) |
| 370 | .into_iter() |
| 371 | .flatten() |
| 372 | { |
| 373 | let notification = json!({ |
| 374 | "jsonrpc": "2.0", |
| 375 | "method": "notifications/progress", |
| 376 | "params": { "progressToken": token, "progress": step["progress"], "total": step["total"] }, |
| 377 | }); |
| 378 | sse.push_str(&format!("event: message\ndata: {notification}\n\n")); |
| 379 | } |
| 380 | for notification in entry |
| 381 | .get("notifications") |
| 382 | .and_then(Value::as_array) |
| 383 | .into_iter() |
| 384 | .flatten() |
| 385 | { |
| 386 | sse.push_str(&format!("event: message\ndata: {notification}\n\n")); |
| 387 | } |
| 388 | sse.push_str(&format!("event: message\ndata: {body}\n\n")); |
| 389 | Reply::new(200, sse).header("Content-Type", "text/event-stream") |
| 390 | } else { |
| 391 | Reply::new(200, body.to_string()).header("Content-Type", "application/json") |
| 392 | }; |
| 393 | if is_initialize { |
| 394 | let session = if state.options.numbered_sessions { |
| 395 | let number = state.sessions_issued.fetch_add(1, Ordering::SeqCst) + 1; |
| 396 | let issued = format!("session-{number}"); |
| 397 | *state.current_session.lock().expect("session") = Some(issued.clone()); |
| 398 | issued |
| 399 | } else { |
| 400 | "conformance-session".to_string() |
| 401 | }; |
| 402 | reply = reply.header("Mcp-Session-Id", session); |
| 403 | } |
| 404 | finish(&mut socket, reply).await; |
| 405 | } |
| 406 | |
| 407 | /// The transcript's answer for one request: one entry, or the first usable |
| 408 | /// candidate of an array (see the module doc). Unknown methods get -32601. |
| 409 | fn scripted_entry(state: &State, method: &str, params: &Value) -> Value { |
| 410 | let not_found = |
| 411 | || json!({ "error": { "code": -32601, "message": format!("method not found: {method}") } }); |
| 412 | let Some(entry) = state.spec.get(method) else { |
| 413 | return not_found(); |
| 414 | }; |
| 415 | let Some(candidates) = entry.as_array() else { |
| 416 | return expand_generated(entry.clone(), params); |
| 417 | }; |
| 418 | for (index, candidate) in candidates.iter().enumerate() { |
| 419 | let matches = candidate["match"].as_object().is_none_or(|fields| { |
| 420 | fields |
| 421 | .iter() |
| 422 | .all(|(key, value)| params.get(key) == Some(value)) |
| 423 | }); |
| 424 | if !matches { |
| 425 | continue; |
| 426 | } |
| 427 | if candidate.get("once").and_then(Value::as_bool) == Some(true) |
| 428 | && !state |
| 429 | .used |
| 430 | .lock() |
| 431 | .expect("used") |
| 432 | .insert((method.to_string(), index)) |
| 433 | { |
| 434 | continue; |
| 435 | } |
| 436 | return expand_generated(candidate.clone(), params); |
| 437 | } |
| 438 | json!({ "error": { "code": -32602, "message": format!("no scripted {method} answer for {params}") } }) |
| 439 | } |
| 440 | |
| 441 | fn generated_tool(prefix: &str, index: usize) -> Value { |
| 442 | json!({ "name": format!("{prefix}-{index:05}"), "inputSchema": { "type": "object" } }) |
| 443 | } |
| 444 | |
| 445 | fn expand_generated(mut entry: Value, params: &Value) -> Value { |
| 446 | if let Some(spec) = entry.get("generate_tools") { |
| 447 | let count = spec["count"].as_u64().unwrap_or(0) as usize; |
| 448 | let prefix = spec["prefix"].as_str().unwrap_or("tool"); |
| 449 | let tools: Vec<Value> = (0..count) |
| 450 | .map(|index| generated_tool(prefix, index)) |
| 451 | .collect(); |
| 452 | entry["result"] = json!({ "tools": tools }); |
| 453 | } else if let Some(spec) = entry.get("generate_pages") { |
| 454 | let count = spec["count"].as_u64().unwrap_or(1) as usize; |
| 455 | let prefix = spec["prefix"].as_str().unwrap_or("tool"); |
| 456 | let page = params["cursor"] |
| 457 | .as_str() |
| 458 | .and_then(|cursor| cursor.strip_prefix("page-")) |
| 459 | .and_then(|page| page.parse::<usize>().ok()) |
| 460 | .unwrap_or(1); |
| 461 | let mut result = json!({ "tools": [generated_tool(prefix, page)] }); |
| 462 | if page < count { |
| 463 | result["nextCursor"] = json!(format!("page-{}", page + 1)); |
| 464 | } |
| 465 | entry["result"] = result; |
| 466 | } |
| 467 | entry |
| 468 | } |
| 469 |