MCP: a bridged server starts without the app and connects once it runs (#1298)

The bridged MCP server starts when no app is listening, says so in its instructions and in each tool error, and connects on the first call after the app starts. A batch that could not be sent says the app is not running. docs/mcp.md describes it.

apps/solidcraft-cli/tests/mcp_connect.rs

@@ -1,18 +1,83 @@
-//! `mcp --connect` to a port where no app listens tells the user how to start one (#270).
+//! `mcp --connect` to a port where no app listens yet (#270): the server still starts, its tools
+//! say how to start the app, and once the app runs they reach it with that launch's token.
 
+use std::io::{BufRead, BufReader, Write};
 use std::net::TcpListener;
 use std::process::{Command, Stdio};
 
+use serde_json::{Value, json};
+
 #[test]
-fn mcp_connect_without_a_listening_app_names_the_control_flag() {
+fn mcp_connect_serves_before_the_app_runs_and_reaches_it_once_it_does() {
     // A port that was free a moment ago: nothing listens on it.
     let port = TcpListener::bind("127.0.0.1:0").unwrap().local_addr().unwrap().port();
-    let output = Command::new(env!("CARGO_BIN_EXE_solidcraft-cli"))
-        .args(["mcp", "--connect", &format!("127.0.0.1:{port}")])
-        .stdin(Stdio::null())
-        .output()
+    let addr = format!("127.0.0.1:{port}");
+    let control_dir = std::env::temp_dir().join(format!("solidcraft-mcp-connect-{}-{port}", std::process::id()));
+    std::fs::create_dir_all(&control_dir).unwrap();
+    let mut child = Command::new(env!("CARGO_BIN_EXE_solidcraft-cli"))
+        .args(["mcp", "--connect", &addr, "--allow-any-path"])
+        .env("SOLIDCRAFT_CONTROL_DIR", &control_dir)
+        .env_remove("SOLIDCRAFT_CONTROL_TOKEN")
+        .stdin(Stdio::piped())
+        .stdout(Stdio::piped())
+        .stderr(Stdio::null())
+        .spawn()
         .unwrap();
-    let stderr = String::from_utf8_lossy(&output.stderr);
-    assert!(!output.status.success(), "{stderr}");
-    assert!(stderr.contains("--control"), "say how to start the app with a control port: {stderr}");
+    let mut stdin = child.stdin.take().unwrap();
+    let mut stdout = BufReader::new(child.stdout.take().unwrap());
+    let mut ask = |id: u64, method: &str, params: Value| -> Value {
+        writeln!(stdin, "{}", json!({"jsonrpc": "2.0", "id": id, "method": method, "params": params})).unwrap();
+        let mut line = String::new();
+        stdout.read_line(&mut line).unwrap();
+        serde_json::from_str(&line).unwrap_or_else(|e| panic!("{e}: {line:?}"))
+    };
+    let measure = json!({"name": "measure", "arguments": {}});
+    let text = |r: &Value| r["result"]["content"][0]["text"].as_str().unwrap_or_default().to_string();
+
+    // The server answers without the app, and its instructions say the app isn't running.
+    let init = ask(1, "initialize", json!({"protocolVersion": "2025-06-18"}));
+    let instructions = init["result"]["instructions"].as_str().unwrap_or_default();
+    assert!(instructions.contains("not running") && instructions.contains(&format!("--control {port}")), "{instructions}");
+    let tools = ask(2, "tools/list", json!({}));
+    assert!(tools["result"]["tools"].as_array().is_some_and(|t| !t.is_empty()), "{tools}");
+
+    // A tool call says the app is not running and how to start it.
+    let r = ask(3, "tools/call", measure.clone());
+    assert_eq!(r["result"]["isError"], true, "{r}");
+    assert!(text(&r).contains("not running") && text(&r).contains(&format!("solidcraft --control {port}")), "{r}");
+
+    // The app starts: a token file for this launch, and its control port.
+    let token = "0123456789abcdef".repeat(4);
+    std::fs::write(control_dir.join(format!("{port}.token")), &token).unwrap();
+    let listener = TcpListener::bind(&addr).unwrap();
+    let app = std::thread::spawn(move || {
+        let (stream, _) = listener.accept().unwrap();
+        let mut line = String::new();
+        BufReader::new(stream.try_clone().unwrap()).read_line(&mut line).unwrap();
+        let req: Value = serde_json::from_str(&line).unwrap();
+        let reply = if req["token"] == json!(token) {
+            json!({"id": req["id"], "ok": true, "result": {"bodies": [], "method": req["method"]}})
+        } else {
+            json!({"id": req["id"], "ok": false, "error": format!("unauthorized: token {}", req["token"])})
+        };
+        writeln!(&stream, "{reply}").unwrap();
+    });
+    let r = ask(4, "tools/call", measure);
+    assert_ne!(r["result"]["isError"], true, "{r}");
+    assert!(text(&r).contains("engine.execute"), "{r}");
+    app.join().unwrap();
+
+    drop(stdin);
+    assert!(child.wait().unwrap().success());
+    std::fs::remove_dir_all(&control_dir).ok();
+}
+
+#[test]
+fn mcp_connect_without_a_port_is_refused_at_start() {
+    for addr in ["localhost", "127.0.0.1:0"] {
+        let output = Command::new(env!("CARGO_BIN_EXE_solidcraft-cli")).args(["mcp", "--connect", addr]).stdin(Stdio::null()).output().unwrap();
+        let stderr = String::from_utf8_lossy(&output.stderr);
+        assert!(!output.status.success(), "{addr}: {stderr}");
+        assert!(stderr.contains("HOST:PORT"), "{addr}: {stderr}");
+    }
 }