fix(mcp): stop stdio cleanly after output failure

Wake the serving loop after protocol output fails and preserve the actual writer error. Use one process-owned stdin worker so the CLI can finish normal backend cleanup without waiting for client EOF, and terminate the output worker even when a progress hook is retained.

Closes #1527

Co-Authored-By: OpenAI Codex <noreply@openai.com>

apps/lightcraft-cli/src/main.rs

@@ -17,7 +17,7 @@
 
 mod alloc_release;
 
-use std::io::{BufReader, Write};
+use std::io::Write;
 use std::path::Path;
 use std::process::ExitCode;
 
@@ -364,8 +364,7 @@ fn mcp(args: &[String]) -> Result<(), String> {
     };
     eprintln!("lightcraft-cli mcp: serving MCP on stdio ({})", backend.describe());
     let mut server = Server::new(backend).with_command_tools(!compact);
-    let stdin = std::io::stdin();
-    server.serve(BufReader::new(stdin), std::io::stdout()).map_err(|e| e.to_string())
+    server.serve_stdio(std::io::stdout()).map_err(|e| e.to_string())
 }
 
 fn merge(args: &[String]) -> Result<(), String> {

apps/lightcraft-cli/tests/mcp_shutdown.rs

@@ -0,0 +1,38 @@
+//! A broken protocol output must not require the client to close its input pipe.
+
+use std::io::{Read, Write};
+use std::process::{Command, Stdio};
+use std::time::{Duration, Instant};
+
+#[test]
+fn broken_stdout_exits_with_stdin_still_open() {
+    let mut child = Command::new(env!("CARGO_BIN_EXE_lightcraft-cli"))
+        .arg("mcp")
+        .stdin(Stdio::piped())
+        .stdout(Stdio::piped())
+        .stderr(Stdio::piped())
+        .spawn()
+        .unwrap();
+    let mut input = child.stdin.take().unwrap();
+    drop(child.stdout.take());
+    input.write_all(b"{\"jsonrpc\":\"2.0\",\"id\":1,\"method\":\"initialize\"}\n{\"jsonrpc\":").unwrap();
+    input.flush().unwrap();
+    let deadline = Instant::now() + Duration::from_secs(10);
+    let status = loop {
+        if let Some(status) = child.try_wait().unwrap() {
+            break status;
+        }
+        if Instant::now() >= deadline {
+            child.kill().unwrap();
+            child.wait().unwrap();
+            panic!("MCP did not exit after stdout failed while stdin stayed open");
+        }
+        std::thread::sleep(Duration::from_millis(10));
+    };
+    let mut stderr = String::new();
+    child.stderr.take().unwrap().read_to_string(&mut stderr).unwrap();
+    assert!(stderr.contains("serving MCP on stdio"), "CLI must reach the MCP transport: {stderr}");
+    assert!(!status.success(), "broken protocol output must be reported: {stderr}");
+    assert!(stderr.contains("lightcraft-cli:"), "CLI must report the transport error: {stderr}");
+    drop(input);
+}