feat(sandbox): 使用逐调用截止时间驱动原生 MCP 会话

2026-09-09 00:49:14 +08:00
parent 12bcc8b1fa
commit 7c7a637e38
6 changed files with 553 additions and 5 deletions
@@ -620,6 +620,15 @@ mod tests {
    }
    #[test]
    fn real_container_cannot_reach_ipv4_or_ipv6_loopback_listeners() {
+        real_native_protocol_probes(false);
+    }
+    #[cfg(feature = "desktop")]
+    #[test]
+    #[ignore = "real MCP tools/call 60-second deadline acceptance; run explicitly"]
+    fn real_mcp_tool_deadline_terminates_hung_server_and_host_can_save() {
+        real_native_protocol_probes(true);
+    }
+    fn real_native_protocol_probes(_mcp_deadline: bool) {
        use std::{
            net::{TcpListener, UdpSocket},
            os::windows::fs::OpenOptionsExt,
@@ -856,6 +865,138 @@ mod tests {
                    "from host broker"
                );
            }
+            let mut mcp_modes = vec![
+                "mcp",
+                "mcp_wrong_id",
+                "mcp_cancel",
+                "mcp_remote_error",
+                "mcp_bad_version",
+            ];
+            if _mcp_deadline {
+                mcp_modes.push("mcp_deadline");
+            }
+            for mode in mcp_modes {
+                use std::sync::{
+                    atomic::{AtomicBool, Ordering},
+                    Arc,
+                };
+                let mut mcp = claims.clone();
+                mcp.arguments = vec![mode.into()];
+                mcp.expires_at_ms = 120_000;
+                let permit = authority.issue(&mcp, 1).unwrap();
+                let prepared = context
+                    .prepare(&authority, &permit, &mcp, &bound_entry, &broker, 2)
+                    .unwrap();
+                let (suspended, io) = prepared
+                    .create_suspended_with_stdio(&profile, &bound_entry)
+                    .unwrap();
+                let running = unsafe { suspended.resume().unwrap() };
+                let mut session = crate::extension_mcp::Session::new(&running, io).unwrap();
+                let cancel = Arc::new(AtomicBool::new(false));
+                assert_eq!(
+                    session
+                        .call_tool("echo", serde_json::json!({}), &cancel)
+                        .unwrap_err()
+                        .code,
+                    "EXTENSION_MCP_NOT_INITIALIZED"
+                );
+                if mode == "mcp_bad_version" {
+                    assert_eq!(
+                        session.initialize(&cancel).unwrap_err().code,
+                        "EXTENSION_MCP_INITIALIZATION_INVALID"
+                    );
+                } else {
+                    assert_eq!(
+                        session.initialize(&cancel).unwrap(),
+                        crate::extension_mcp::PROTOCOL_VERSION
+                    );
+                    assert_eq!(
+                        session.list_tools(None, &cancel).unwrap()["tools"][0]["name"],
+                        "echo"
+                    );
+                    let cancellation = if mode == "mcp_cancel" {
+                        let flag = Arc::clone(&cancel);
+                        Some(std::thread::spawn(move || {
+                            std::thread::sleep(std::time::Duration::from_millis(100));
+                            flag.store(true, Ordering::Release);
+                        }))
+                    } else {
+                        None
+                    };
+                    let tool_started = std::time::Instant::now();
+                    let result = session.call_tool("echo", serde_json::json!({}), &cancel);
+                    match mode {
+                        "mcp_deadline" => {
+                            assert_eq!(
+                                result.unwrap_err().code,
+                                "EXTENSION_TOOL_DEADLINE_EXCEEDED"
+                            );
+                            let elapsed = tool_started.elapsed();
+                            assert!(
+                                elapsed >= std::time::Duration::from_secs(59)
+                                    && elapsed < std::time::Duration::from_secs(65)
+                            );
+                            eprintln!("real MCP tools/call deadline: {elapsed:?}");
+                            let vault = tempfile::tempdir().unwrap();
+                            let mut workspace =
+                                crate::workspace::Workspace::open(vault.path()).unwrap();
+                            workspace
+                                .write(
+                                    "after-timeout.md",
+                                    "",
+                                    b"Host save after actual MCP deadline",
+                                    "local",
+                                )
+                                .unwrap();
+                            drop(workspace);
+                            let mut workspace =
+                                crate::workspace::Workspace::open(vault.path()).unwrap();
+                            assert_eq!(
+                                workspace.read("after-timeout.md").unwrap().content,
+                                "Host save after actual MCP deadline"
+                            );
+                        }
+                        "mcp_wrong_id" => assert_eq!(
+                            result.unwrap_err().code,
+                            "EXTENSION_MCP_RESPONSE_ID_MISMATCH"
+                        ),
+                        "mcp_cancel" => {
+                            assert_eq!(result.unwrap_err().code, "EXTENSION_MCP_CANCELLED");
+                            cancellation.unwrap().join().unwrap();
+                        }
+                        "mcp_remote_error" => {
+                            assert_eq!(result.unwrap_err().code, "EXTENSION_MCP_REMOTE_ERROR");
+                            assert_eq!(
+                                session
+                                    .call_tool("echo", serde_json::json!({}), &cancel)
+                                    .unwrap()["content"][0]["text"],
+                                "native MCP success"
+                            );
+                        }
+                        _ => {
+                            assert_eq!(result.unwrap()["content"][0]["text"], "native MCP success");
+                            assert!(session.take_tools_changed());
+                            assert!(!session.take_tools_changed());
+                        }
+                    }
+                    if mode == "mcp_cancel" || mode == "mcp_wrong_id" {
+                        assert_eq!(
+                            session.list_tools(None, &cancel).unwrap_err().code,
+                            "EXTENSION_MCP_SESSION_FAILED"
+                        );
+                    }
+                }
+                session.shutdown().unwrap();
+                assert!(running
+                    .wait(std::time::Duration::from_secs(5))
+                    .unwrap()
+                    .is_some());
+                let started = std::time::Instant::now();
+                while running.active_test_processes().unwrap() != 0 {
+                    assert!(started.elapsed() < std::time::Duration::from_secs(5));
+                    std::thread::sleep(std::time::Duration::from_millis(5));
+                }
+            }
            for mode in ["wait_tree", "stderr_flood", "stdout_flood"] {
                let mut io_claims = claims.clone();
                io_claims.arguments = vec![mode.into()];
@@ -28,6 +28,8 @@ pub enum Event {
 }
 struct State {
    stopped: AtomicBool,
+    #[cfg(test)]
+    writing: AtomicBool,
    error: Mutex<Option<String>>,
    job: Job,
 }
@@ -106,6 +108,8 @@ impl Pump {
    pub(crate) fn start(io: HostIo, job: Job) -> Result<Self> {
        let state = Arc::new(State {
            stopped: AtomicBool::new(false),
+            #[cfg(test)]
+            writing: AtomicBool::new(false),
            error: Mutex::new(None),
            job,
        });
@@ -130,7 +134,11 @@ impl Pump {
                    match writes.recv_timeout(Duration::from_millis(20)) {
                        Ok(frame) => {
                            state.check()?;
+                            #[cfg(test)]
+                            state.writing.store(true, Ordering::Release);
                            write_frame(&mut input, &frame)?;
+                            #[cfg(test)]
+                            state.writing.store(false, Ordering::Release);
                        }
                        Err(RecvTimeoutError::Timeout) => {}
                        Err(RecvTimeoutError::Disconnected) => return Ok(()),
@@ -201,7 +209,15 @@ impl Pump {
    }
    /// Nonblocking admission; at most one pending write plus one in progress.
    pub fn send(&self, frame: Vec<u8>) -> Result<()> {
+        self.send_wait(frame, Duration::ZERO)
+    }
+    /// Bounded admission for serial protocol notifications immediately followed
+    /// by a request; retries retain the same frame, without allocating copies.
+    pub(crate) fn send_wait(&self, mut frame: Vec<u8>, timeout: Duration) -> Result<()> {
        self.state.check()?;
+        if timeout > Duration::from_secs(1) {
+            return Err(HostError::new("EXTENSION_IO_TIMEOUT_INVALID"));
+        }
        if frame.is_empty()
            || frame.len() > MAX_FRAME_BYTES
            || frame.contains(&b'\n')
@@ -209,11 +225,25 @@ impl Pump {
        {
            return Err(HostError::new("EXTENSION_PIPE_INVALID_FRAME"));
        }
-        self.input
+        let sender = self
+            .input
            .as_ref()
-            .ok_or_else(|| HostError::new("EXTENSION_IO_INPUT_CLOSED"))?
-            .try_send(frame)
-            .map_err(|_| HostError::new("EXTENSION_IO_INPUT_BACKPRESSURE"))
+            .ok_or_else(|| HostError::new("EXTENSION_IO_INPUT_CLOSED"))?;
+        let started = Instant::now();
+        loop {
+            self.state.check()?;
+            match sender.try_send(frame) {
+                Ok(()) => return Ok(()),
+                Err(mpsc::TrySendError::Disconnected(_)) => {
+                    return Err(HostError::new("EXTENSION_IO_INPUT_CLOSED"))
+                }
+                Err(mpsc::TrySendError::Full(value)) => frame = value,
+            }
+            if started.elapsed() >= timeout {
+                return Err(HostError::new("EXTENSION_IO_INPUT_BACKPRESSURE"));
+            }
+            std::thread::sleep(Duration::from_millis(1));
+        }
    }
    pub fn close_input(&mut self) {
        self.input.take();
@@ -318,7 +348,7 @@ mod tests {
                    );
                    pending != 0
                });
-                if pending {
+                if pending && pump.state.writing.load(Ordering::Acquire) {
                    break;
                }
                assert!(
@@ -0,0 +1,325 @@
+//! Serial MCP session over an already-authorized native instance. Package trust,
+//! tool consent/schema validation and registry routing remain Host responsibilities.
+use crate::{
+    extension_io::{Event, Pump},
+    extension_process::Running,
+    extension_stdio::HostIo,
+    workspace::{HostError, Result},
+};
+use serde::Deserialize;
+use serde_json::{json, Value};
+use std::{
+    sync::atomic::{AtomicBool, Ordering},
+    time::{Duration, Instant},
+};
+pub const PROTOCOL_VERSION: &str = "2025-11-25";
+const VERSIONS: [&str; 4] = [PROTOCOL_VERSION, "2025-06-18", "2025-03-26", "2024-11-05"];
+fn present<'de, D: serde::Deserializer<'de>, T: Deserialize<'de>>(
+    deserializer: D,
+) -> std::result::Result<Option<T>, D::Error> {
+    T::deserialize(deserializer).map(Some)
+}
+#[derive(Deserialize)]
+#[serde(deny_unknown_fields)]
+struct Envelope {
+    jsonrpc: String,
+    #[serde(default, deserialize_with = "present")]
+    id: Option<Value>,
+    #[serde(default, deserialize_with = "present")]
+    method: Option<String>,
+    #[serde(default, deserialize_with = "present")]
+    params: Option<Value>,
+    #[serde(default, deserialize_with = "present")]
+    result: Option<Value>,
+    #[serde(default, deserialize_with = "present")]
+    error: Option<RemoteError>,
+}
+#[derive(Deserialize)]
+#[serde(deny_unknown_fields)]
+struct RemoteError {
+    code: i64,
+    message: String,
+    data: Option<Value>,
+}
+fn invalid() -> HostError {
+    HostError::new("EXTENSION_MCP_PROTOCOL_INVALID")
+}
+fn decode(bytes: &[u8]) -> Result<Envelope> {
+    let value: Envelope = serde_json::from_slice(bytes).map_err(|_| invalid())?;
+    if value.jsonrpc != "2.0" || value.params.as_ref().is_some_and(|v| !v.is_object()) {
+        return Err(invalid());
+    }
+    if let Some(id) = &value.id {
+        if id.as_i64().is_none() && !id.as_str().is_some_and(|s| !s.is_empty() && s.len() <= 128) {
+            return Err(invalid());
+        }
+    }
+    if let Some(method) = &value.method {
+        if method.is_empty()
+            || method.len() > 256
+            || method.chars().any(char::is_control)
+            || value.result.is_some()
+            || value.error.is_some()
+        {
+            return Err(invalid());
+        }
+    } else if value.id.is_none()
+        || value.params.is_some()
+        || value.result.is_some() == value.error.is_some()
+    {
+        return Err(invalid());
+    }
+    if value.result.as_ref().is_some_and(|v| !v.is_object()) {
+        return Err(invalid());
+    }
+    if let Some(error) = &value.error {
+        if error.message.len() > 4096
+            || error.code < i32::MIN as i64
+            || error.code > i32::MAX as i64
+        {
+            return Err(invalid());
+        }
+        // Error data is never propagated or logged; it may contain secrets.
+        let _ = &error.data;
+    }
+    Ok(value)
+}
+pub struct Session<'a, 'p> {
+    process: &'a Running<'p>,
+    pump: Pump,
+    next: u64,
+    ready: bool,
+    tools: bool,
+    failed: bool,
+    tools_changed: bool,
+}
+impl<'a, 'p> Session<'a, 'p> {
+    pub fn new(process: &'a Running<'p>, io: HostIo) -> Result<Self> {
+        Ok(Self {
+            process,
+            pump: process.start_io(io)?,
+            next: 1,
+            ready: false,
+            tools: false,
+            failed: false,
+            tools_changed: false,
+        })
+    }
+    pub fn initialize(&mut self, cancel: &AtomicBool) -> Result<String> {
+        if self.ready {
+            return Err(HostError::new("EXTENSION_MCP_ALREADY_INITIALIZED"));
+        }
+        let value = self.request("initialize", json!({"protocolVersion":PROTOCOL_VERSION,"capabilities":{},"clientInfo":{"name":"OpenNexus","version":env!("CARGO_PKG_VERSION")}}), Duration::from_secs(10), cancel)?;
+        let version = value["protocolVersion"]
+            .as_str()
+            .filter(|v| VERSIONS.contains(v));
+        let info = value["serverInfo"].as_object();
+        let capabilities = value["capabilities"].as_object();
+        if version.is_none()
+            || info.is_none()
+            || capabilities.is_none()
+            || !value["serverInfo"]["name"]
+                .as_str()
+                .is_some_and(|s| !s.is_empty() && s.len() <= 256)
+            || !value["serverInfo"]["version"]
+                .as_str()
+                .is_some_and(|s| !s.is_empty() && s.len() <= 128)
+            || value["capabilities"]
+                .get("tools")
+                .is_some_and(|v| !v.is_object())
+        {
+            self.abort();
+            return Err(HostError::new("EXTENSION_MCP_INITIALIZATION_INVALID"));
+        }
+        self.tools = value["capabilities"].get("tools").is_some();
+        if let Err(error) = self.send(json!({"jsonrpc":"2.0","method":"notifications/initialized"}))
+        {
+            self.abort();
+            return Err(error);
+        }
+        self.ready = true;
+        Ok(version.unwrap().into())
+    }
+    pub fn list_tools(&mut self, cursor: Option<&str>, cancel: &AtomicBool) -> Result<Value> {
+        self.require_tools()?;
+        if cursor.is_some_and(|s| s.is_empty() || s.len() > 1024) {
+            return Err(invalid());
+        }
+        let value = self.request(
+            "tools/list",
+            cursor.map_or_else(|| json!({}), |c| json!({"cursor":c})),
+            Duration::from_secs(10),
+            cancel,
+        )?;
+        if !value["tools"]
+            .as_array()
+            .is_some_and(|tools| tools.len() <= 500)
+            || value
+                .get("nextCursor")
+                .is_some_and(|v| !v.as_str().is_some_and(|s| !s.is_empty() && s.len() <= 1024))
+        {
+            self.abort();
+            return Err(invalid());
+        }
+        Ok(value)
+    }
+    pub fn call_tool(
+        &mut self,
+        name: &str,
+        arguments: Value,
+        cancel: &AtomicBool,
+    ) -> Result<Value> {
+        self.require_tools()?;
+        if name.is_empty()
+            || name.len() > 256
+            || name.chars().any(char::is_control)
+            || !arguments.is_object()
+        {
+            return Err(invalid());
+        }
+        let value = self.request(
+            "tools/call",
+            json!({"name":name,"arguments":arguments}),
+            Duration::from_secs(60),
+            cancel,
+        )?;
+        if !value["content"].is_array()
+            || value.get("isError").is_some_and(|v| !v.is_boolean())
+            || serde_json::to_vec(&value).map_err(|_| invalid())?.len() > 256 * 1024
+        {
+            self.abort();
+            return Err(HostError::new("EXTENSION_MCP_TOOL_RESULT_INVALID"));
+        }
+        Ok(value)
+    }
+    fn require_tools(&self) -> Result<()> {
+        if self.failed {
+            return Err(HostError::new("EXTENSION_MCP_SESSION_FAILED"));
+        }
+        if !self.ready {
+            return Err(HostError::new("EXTENSION_MCP_NOT_INITIALIZED"));
+        }
+        if !self.tools {
+            return Err(HostError::new("EXTENSION_MCP_TOOLS_UNAVAILABLE"));
+        }
+        Ok(())
+    }
+    fn send(&self, value: Value) -> Result<()> {
+        self.pump.send_wait(
+            serde_json::to_vec(&value).map_err(|_| invalid())?,
+            Duration::from_millis(50),
+        )
+    }
+    fn abort(&mut self) {
+        self.failed = true;
+        let _ = self.process.terminate();
+    }
+    fn request(
+        &mut self,
+        method: &str,
+        params: Value,
+        budget: Duration,
+        cancel: &AtomicBool,
+    ) -> Result<Value> {
+        if self.failed {
+            return Err(HostError::new("EXTENSION_MCP_SESSION_FAILED"));
+        }
+        if cancel.load(Ordering::Acquire) {
+            return Err(HostError::new("EXTENSION_MCP_CANCELLED"));
+        }
+        let id = format!("opennexus.{}", self.next);
+        self.next = self.next.checked_add(1).ok_or_else(invalid)?;
+        let deadline = self.process.start_tool_call()?;
+        let started = Instant::now();
+        let result = (|| {
+            self.send(json!({"jsonrpc":"2.0","id":id,"method":method,"params":params}))?;
+            loop {
+                self.process.check_authorization()?;
+                deadline.check()?;
+                if cancel.load(Ordering::Acquire) || started.elapsed() >= budget {
+                    // initialize cannot be cancelled at the protocol level.
+                    if method != "initialize" {
+                        let _ = self.send(json!({"jsonrpc":"2.0","method":"notifications/cancelled","params":{"requestId":id}}));
+                    }
+                    return Err(HostError::new(if cancel.load(Ordering::Acquire) {
+                        "EXTENSION_MCP_CANCELLED"
+                    } else {
+                        "EXTENSION_MCP_TIMEOUT"
+                    }));
+                }
+                let received = self.pump.receive(Duration::from_millis(20));
+                self.process.check_authorization()?;
+                deadline.check()?;
+                let bytes = match received {
+                    Ok(Event::Frame(bytes)) => bytes,
+                    Ok(Event::Closed) => {
+                        return Err(HostError::new("EXTENSION_MCP_CONNECTION_CLOSED"))
+                    }
+                    Err(error) if error.code == "EXTENSION_IO_TIMEOUT" => continue,
+                    Err(error) => return Err(error),
+                };
+                let message = decode(&bytes)?;
+                if let Some(method) = message.method {
+                    if let Some(id) = message.id {
+                        // No sampling/roots/elicitation capability was advertised.
+                        self.send(if method == "ping" { json!({"jsonrpc":"2.0","id":id,"result":{}}) }
+                            else { json!({"jsonrpc":"2.0","id":id,"error":{"code":-32601,"message":"Method not supported"}}) })?;
+                    } else if method == "notifications/tools/list_changed" {
+                        self.tools_changed = true;
+                    }
+                    continue;
+                }
+                if message.id != Some(json!(id)) {
+                    return Err(HostError::new("EXTENSION_MCP_RESPONSE_ID_MISMATCH"));
+                }
+                return Ok(message.result);
+            }
+        })();
+        match result {
+            Ok(value) => {
+                if let Err(error) = deadline.finish() {
+                    self.abort();
+                    return Err(error);
+                }
+                value.ok_or_else(|| HostError::new("EXTENSION_MCP_REMOTE_ERROR"))
+            }
+            Err(error) => {
+                self.abort();
+                drop(deadline);
+                Err(error)
+            }
+        }
+    }
+    pub fn take_tools_changed(&mut self) -> bool {
+        std::mem::take(&mut self.tools_changed)
+    }
+    pub fn shutdown(mut self) -> Result<()> {
+        self.pump.close_input();
+        let _ = self.process.wait(Duration::from_millis(200));
+        self.pump.shutdown()
+    }
+}
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn malformed_and_ambiguous_envelopes_are_rejected() {
+        for input in [
+            r#"[]"#,
+            r#"{"jsonrpc":"2.0","id":null,"method":"ping"}"#,
+            r#"{"jsonrpc":"2.0","method":"ping","params":null}"#,
+            r#"{"jsonrpc":"2.0","id":1,"id":2,"result":{}}"#,
+            r#"{"jsonrpc":"1.0","id":1,"result":{}}"#,
+            r#"{"jsonrpc":"2.0","id":null,"result":{}}"#,
+            r#"{"jsonrpc":"2.0","id":1,"result":{},"error":{"code":1,"message":"bad"}}"#,
+            r#"{"jsonrpc":"2.0","id":1,"method":"ping","result":{}}"#,
+        ] {
+            assert!(decode(input.as_bytes()).is_err(), "{input}");
+        }
+        assert!(decode(br#"{"jsonrpc":"2.0","id":"request","result":{}}"#).is_ok());
+        assert!(decode(br#"{"jsonrpc":"2.0","id":1,"method":"ping"}"#).is_ok());
+        assert!(
+            decode(br#"{"jsonrpc":"2.0","method":"notifications/tools/list_changed"}"#).is_ok()
+        );
+    }
+}
@@ -84,3 +84,6 @@ pub mod extension_stdio;

 #[cfg(all(windows, feature = "desktop"))]
 pub mod extension_io;
+
+#[cfg(all(windows, feature = "desktop"))]
+pub mod extension_mcp;
@@ -3,6 +3,42 @@ use std::net::{SocketAddr, TcpStream, UdpSocket};
 use std::time::Duration;
 fn main() {
    let args: Vec<_> = std::env::args().collect();
+    if args.get(1).is_some_and(|s| s.starts_with("mcp")) {
+        use std::io::{BufRead, Write};
+        fn read(reader: &mut impl BufRead) -> String { let mut line = String::new(); reader.read_line(&mut line).unwrap(); line }
+        fn id(line: &str) -> &str { line.split("\"id\":\"").nth(1).unwrap().split('"').next().unwrap() }
+        fn reply(id: &str, result: &str) { println!("{{\"jsonrpc\":\"2.0\",\"id\":\"{id}\",\"result\":{result}}}"); std::io::stdout().flush().unwrap(); }
+        let mut input = std::io::stdin().lock();
+        let request = read(&mut input);
+        assert!(request.contains("\"method\":\"initialize\""));
+        reply(id(&request), if args[1] == "mcp_bad_version" { r#"{"protocolVersion":"unknown","capabilities":{},"serverInfo":{"name":"fixture","version":"1"}}"# } else { r#"{"protocolVersion":"2025-11-25","capabilities":{"tools":{}},"serverInfo":{"name":"fixture","version":"1"}}"# });
+        assert!(read(&mut input).contains("notifications/initialized"));
+        let request = read(&mut input);
+        assert!(request.contains("tools/list"));
+        reply(id(&request), r#"{"tools":[{"name":"echo","inputSchema":{"type":"object"}}]}"#);
+        let mut request = read(&mut input);
+        assert!(request.contains("tools/call"));
+        if args[1] == "mcp_cancel" || args[1] == "mcp_deadline" {
+            let _child = std::process::Command::new(std::env::current_exe().unwrap()).arg("wait").spawn().unwrap();
+            std::thread::sleep(Duration::from_secs(120));
+            return;
+        }
+        if args[1] == "mcp_remote_error" {
+            println!("{{\"jsonrpc\":\"2.0\",\"id\":\"{}\",\"error\":{{\"code\":-32602,\"message\":\"fixture error\"}}}}", id(&request));
+            std::io::stdout().flush().unwrap();
+            request = read(&mut input);
+            assert!(request.contains("tools/call"));
+        }
+        println!(r#"{{"jsonrpc":"2.0","id":"server-ping","method":"ping"}}"#);
+        std::io::stdout().flush().unwrap();
+        let ping = read(&mut input);
+        assert!(ping.contains("server-ping") && ping.contains("result"));
+        println!(r#"{{"jsonrpc":"2.0","method":"notifications/tools/list_changed"}}"#);
+        reply(if args[1] == "mcp_wrong_id" { "wrong-request" } else { id(&request) }, r#"{"content":[{"type":"text","text":"native MCP success"}]}"#);
+        // MCP server remains alive between calls until Host closes stdin.
+        while !read(&mut input).is_empty() {}
+        return;
+    }
    if args.get(1).is_some_and(|s| s == "stderr_flood" || s == "stdout_flood") {
        use std::io::Write;
        let _child = std::process::Command::new(std::env::current_exe().unwrap()).arg("wait").spawn().unwrap();