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.

crates/mcp/src/backend.rs

@@ -35,12 +35,18 @@ const BATCH_TIMEOUT: Duration = Duration::from_secs(3600);
 /// (`solidcraft --control PORT`).
 pub struct Remote {
     addr: String,
+    port: u16,
     conn: Option<(BufReader<TcpStream>, TcpStream)>,
     next_id: u64,
-    /// The app's control token, sent with every request (docs/control-protocol.md).
-    token: Option<String>,
+    /// `--token`, when given. Otherwise each request looks the token up again, since every
+    /// launch of the app makes a new one (docs/control-protocol.md).
+    explicit_token: Option<String>,
 }
 
+/// How a tool error starts when no app listens at the bridged address. The agent reads it, and
+/// `batch` uses it to tell "never sent" from "sent, reply lost".
+pub(crate) const NOT_RUNNING: &str = "no SolidCraft app is listening at";
+
 impl Remote {
     /// Connect to the app at `addr`, with its control token found the usual way
     /// (`SOLIDCRAFT_CONTROL_TOKEN`, else the app's token file for that port).
@@ -50,17 +56,31 @@ impl Remote {
 
     /// [`Remote::connect`] with the token given (`--token`); `None` finds it the usual way.
     pub fn connect_with_token(addr: &str, token: Option<&str>) -> std::io::Result<Self> {
-        let port = addr.rsplit(':').next().and_then(|p| p.parse::<u16>().ok()).unwrap_or(0);
-        let token = solidcraft_engine::control_auth::client_token(port, token);
-        let mut r = Self { addr: addr.to_string(), conn: None, next_id: 1, token };
+        let mut r = Self::new(addr, token);
         r.reconnect()?;
         Ok(r)
     }
 
+    /// A bridge to the app at `addr` that connects when a call needs it: an MCP server started
+    /// before the app keeps its tools, and they reach the app once it runs.
+    pub fn new(addr: &str, token: Option<&str>) -> Self {
+        let port = addr.rsplit(':').next().and_then(|p| p.parse::<u16>().ok()).unwrap_or(0);
+        let explicit_token = token.map(str::to_string);
+        Self { addr: addr.to_string(), port, conn: None, next_id: 1, explicit_token }
+    }
+
+    /// Connect now, if the app is there (`new` does not); the error says why not.
+    pub fn try_connect(&mut self) -> std::io::Result<()> {
+        self.reconnect()
+    }
+
+    /// Failing to reach the app at all is `NotConnected`, so [`Remote::explain`] can tell it from
+    /// a connection lost during a call.
     fn reconnect(&mut self) -> std::io::Result<()> {
         self.conn = None;
+        let not_connected = |e: std::io::Error| std::io::Error::new(std::io::ErrorKind::NotConnected, e);
         let mut last = std::io::Error::new(std::io::ErrorKind::NotFound, format!("cannot resolve {}", self.addr));
-        for sa in self.addr.to_socket_addrs()? {
+        for sa in self.addr.to_socket_addrs().map_err(not_connected)? {
             match TcpStream::connect_timeout(&sa, Duration::from_millis(800)) {
                 Ok(s) => {
                     s.set_nodelay(true).ok();
@@ -72,7 +92,23 @@ impl Remote {
                 Err(e) => last = e,
             }
         }
-        Err(last)
+        Err(not_connected(last))
+    }
+
+    /// The tool error for a failed call: whether the app is not running (and how to start it) or
+    /// went away during the call.
+    fn explain(&self, e: &std::io::Error) -> String {
+        let (addr, port) = (&self.addr, self.port);
+        if e.kind() == std::io::ErrorKind::NotConnected {
+            format!(
+                "{NOT_RUNNING} {addr} ({e}): it is not running, or it quit. Start it with `solidcraft --control {port}` \
+                 (or ask the user to); the next call connects to it"
+            )
+        } else {
+            format!(
+                "the SolidCraft app at {addr} did not answer ({e}): it may have quit or crashed during this call, after                  running it or not. Inspect the design before sending a change again; if the app quit, start it with                  `solidcraft --control {port}`"
+            )
+        }
     }
 
     fn roundtrip(&mut self, line: &str) -> std::io::Result<String> {
@@ -130,19 +166,22 @@ impl Backend for Remote {
             o.insert("allow_paths".into(), json!(scope.roots().iter().map(|r| r.to_string_lossy()).collect::<Vec<_>>()));
         }
         let mut msg = json!({"id": id, "method": method, "params": params});
-        if let Some(t) = &self.token {
+        // The token goes with every request (an app restarted mid-session has a new one).
+        if let Some(t) = solidcraft_engine::control_auth::client_token(self.port, self.explicit_token.as_deref()) {
             msg["token"] = json!(t);
         }
         let line = msg.to_string();
         let reply = if method == "engine.batch" {
-            self.send_batch(&line).map_err(|e| format!("SolidCraft at {}: {e}", self.addr))?
+            self.send_batch(&line)
         } else {
-            match self.roundtrip(&line) {
-                Ok(r) => r,
-                // One retry on a fresh connection (the app may have restarted).
-                Err(_) => self.roundtrip(&line).map_err(|e| format!("SolidCraft at {}: {e}", self.addr))?,
-            }
-        };
+            // One retry on a fresh connection (the app may have restarted). When nothing listens
+            // any more, the app went away, maybe while running this request: say that, not
+            // just "not running".
+            self.roundtrip(&line).or_else(|lost| {
+                self.roundtrip(&line).map_err(|e| if e.kind() == std::io::ErrorKind::NotConnected && lost.kind() != e.kind() { lost } else { e })
+            })
+        }
+        .map_err(|e| self.explain(&e))?;
         let v: Value = serde_json::from_str(&reply).map_err(|e| format!("bad reply from the app: {e}"))?;
         if v.get("ok").and_then(Value::as_bool) == Some(true) {
             Ok(v.get("result").cloned().unwrap_or(Value::Null))
@@ -154,7 +193,16 @@ impl Backend for Remote {
         true
     }
     fn describe(&self) -> String {
-        format!("connected to the SolidCraft app at {} (the user watches the viewport)", self.addr)
+        if self.conn.is_some() {
+            format!("connected to the SolidCraft app at {} (the user watches the viewport)", self.addr)
+        } else {
+            // Still "connected to the SolidCraft app": the skills tell bridged from headless by it.
+            format!(
+                "connected to the SolidCraft app at {} once it runs (the user watches the viewport). It was not running \
+                 when this server started; until it is (`solidcraft --control {}`), tool calls say so",
+                self.addr, self.port
+            )
+        }
     }
 }