diff --git a/crates/mcp/src/backend.rs b/crates/mcp/src/backend.rs
index 78cec12b2..a07c89391 100644
--- a/crates/mcp/src/backend.rs
+++ b/crates/mcp/src/backend.rs
@@ -1,12 +1,17 @@
 //! Where MCP tool calls end up: a control-channel method call.
 
-use std::io::{BufRead, BufReader, Write};
+use std::io::{BufReader, Write};
 use std::net::{IpAddr, SocketAddr, TcpStream, ToSocketAddrs};
 use std::path::Path;
 use std::time::Duration;
 
 use serde_json::{Value, json};
 
+use crate::server::{BoundedLine, read_bounded_line};
+
+/// Longest reply line accepted from the app (screenshots and `document.inspect` are large).
+const MAX_REPLY_LINE: usize = 256 << 20;
+
 /// Something that answers control-channel methods (`engine.execute`, `document.inspect`,
 /// `ui.pointer`, `ui.render`, …). See `wordcraft_ui_egui::control` for the full list.
 pub trait Backend {
@@ -161,11 +166,11 @@ impl Remote {
         writer.write_all(line.as_bytes())?;
         writer.write_all(b"\n")?;
         writer.flush()?;
-        let mut reply = String::new();
-        if reader.read_line(&mut reply)? == 0 {
-            return Err(std::io::Error::new(std::io::ErrorKind::UnexpectedEof, "control channel closed"));
+        match read_bounded_line(reader, MAX_REPLY_LINE)? {
+            BoundedLine::Line(reply) => Ok(reply),
+            BoundedLine::Eof => Err(std::io::Error::new(std::io::ErrorKind::UnexpectedEof, "control channel closed")),
+            BoundedLine::TooLong => Err(std::io::Error::new(std::io::ErrorKind::InvalidData, "reply from app too long")),
         }
-        Ok(reply)
     }
 
     /// One request and its reply, with one retry on a new connection (the app may have
@@ -189,6 +194,11 @@ impl Remote {
                 Ok(r) => break r,
                 Err(e) => {
                     self.conn = None;
+                    // A reply that arrived but can't be read (too long, not UTF-8): the app has
+                    // run the request, so sending it again could run a state-changing one twice.
+                    if e.kind() == std::io::ErrorKind::InvalidData {
+                        return Err(format!("bad reply from app: {e}"));
+                    }
                     if !first {
                         return Err(unreachable(&self.addr, e, self.connected));
                     }
@@ -246,6 +256,7 @@ impl Backend for Remote {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::io::BufRead;
     use std::net::{SocketAddr, TcpListener};
     use std::sync::Arc;
     use std::sync::atomic::{AtomicUsize, Ordering};
@@ -335,6 +346,37 @@ mod tests {
         (port, rx)
     }
 
+    #[test]
+    fn an_unreadable_reply_is_not_retried() {
+        // The app answered something that can't be read (here not UTF-8; an over-long reply
+        // takes the same path): the request ran, so it must not be sent again.
+        let listener = TcpListener::bind(("127.0.0.1", 0)).unwrap();
+        let port = listener.local_addr().unwrap().port();
+        let seen = Arc::new(AtomicUsize::new(0));
+        let n = seen.clone();
+        std::thread::spawn(move || {
+            for stream in listener.incoming().flatten() {
+                let n = n.clone();
+                std::thread::spawn(move || {
+                    let mut out = stream.try_clone().unwrap();
+                    for line in BufReader::new(stream).lines() {
+                        if line.is_err() {
+                            return;
+                        }
+                        n.fetch_add(1, Ordering::SeqCst);
+                        if out.write_all(b"\xff\xfe\n").is_err() {
+                            return;
+                        }
+                    }
+                });
+            }
+        });
+        let mut r = Remote::connect_with(&format!("127.0.0.1:{port}"), fixed_key("k1")).unwrap();
+        let e = r.call("engine.execute", json!({"command": "text.insert", "params": {"text": "x"}})).unwrap_err();
+        assert!(e.contains("bad reply"), "{e}");
+        assert_eq!(seen.load(Ordering::SeqCst), 1, "the request was sent once");
+    }
+
     fn fixed_key(key: &'static str) -> KeyFinder {
         Box::new(move |_| Ok(key.to_string()))
     }
diff --git a/crates/mcp/src/server.rs b/crates/mcp/src/server.rs
index 63f6764c6..0bca29e64 100644
--- a/crates/mcp/src/server.rs
+++ b/crates/mcp/src/server.rs
@@ -1,6 +1,6 @@
 //! JSON-RPC 2.0 framing and the MCP lifecycle / tools / resources methods.
 
-use std::io::{BufRead, Write};
+use std::io::{BufRead, Read, Write};
 
 use serde_json::{Value, json};
 
@@ -28,6 +28,49 @@ execute format.bold → inspect_document to verify → render_page to look at th
 pub const DOC_URI: &str = "wordcraft://document";
 pub const COMMANDS_URI: &str = "wordcraft://commands";
 
+/// Longest request line accepted on stdin.
+pub const MAX_REQUEST_LINE: usize = 1 << 20;
+
+/// One bounded line read.
+pub(crate) enum BoundedLine {
+    Eof,
+    Line(String),
+    /// The line exceeded the limit; the rest of it has been discarded.
+    TooLong,
+}
+
+/// Read one line of at most `max` bytes (excluding the newline) without buffering more than
+/// `max + 1` bytes; an over-long line is skipped up to its newline.
+pub(crate) fn read_bounded_line(reader: &mut impl BufRead, max: usize) -> std::io::Result<BoundedLine> {
+    let mut buf = Vec::new();
+    reader.by_ref().take(max as u64 + 1).read_until(b'\n', &mut buf)?;
+    if buf.is_empty() {
+        return Ok(BoundedLine::Eof);
+    }
+    if buf.last() != Some(&b'\n') && buf.len() > max {
+        loop {
+            let chunk = reader.fill_buf()?;
+            if chunk.is_empty() {
+                break;
+            }
+            let found = chunk.iter().position(|&b| b == b'\n');
+            let used = found.map_or(chunk.len(), |i| i + 1);
+            reader.consume(used);
+            if found.is_some() {
+                break;
+            }
+        }
+        return Ok(BoundedLine::TooLong);
+    }
+    if buf.last() == Some(&b'\n') {
+        buf.pop();
+        if buf.last() == Some(&b'\r') {
+            buf.pop();
+        }
+    }
+    String::from_utf8(buf).map(BoundedLine::Line).map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e))
+}
+
 /// An MCP server bound to one backend.
 pub struct Server {
     backend: Box<dyn Backend>,
@@ -58,10 +101,14 @@ impl Server {
 
     /// Serve newline-delimited JSON-RPC until `input` closes. Logs go to stderr only (stdout is
     /// the protocol stream).
-    pub fn serve(&mut self, input: impl BufRead, mut output: impl Write) -> std::io::Result<()> {
-        for line in input.lines() {
-            let line = line?;
-            if let Some(reply) = self.handle_line(&line) {
+    pub fn serve(&mut self, mut input: impl BufRead, mut output: impl Write) -> std::io::Result<()> {
+        loop {
+            let reply = match read_bounded_line(&mut input, MAX_REQUEST_LINE)? {
+                BoundedLine::Eof => break,
+                BoundedLine::Line(line) => self.handle_line(&line),
+                BoundedLine::TooLong => Some(error(Value::Null, INVALID_REQUEST, "request line too long").to_string()),
+            };
+            if let Some(reply) = reply {
                 output.write_all(reply.as_bytes())?;
                 output.write_all(b"\n")?;
                 output.flush()?;
diff --git a/crates/mcp/src/tests.rs b/crates/mcp/src/tests.rs
index 12ea973f1..f23f72f75 100644
--- a/crates/mcp/src/tests.rs
+++ b/crates/mcp/src/tests.rs
@@ -228,3 +228,20 @@ fn type_text_paragraphs_reports_a_refusal_and_types_nothing_after_it() {
     assert_eq!(calls[0].0, "text.insert");
     assert_eq!(calls[0].1["text"], "one\rtwo", "the line break is a paragraph break");
 }
+
+/// #3685: an over-long request line gets an error reply and the server keeps serving.
+#[test]
+fn over_long_request_line_is_rejected_and_serving_continues() {
+    let mut s = Server::new(Box::new(Headless::default()));
+    let mut input = vec![b'a'; (1 << 20) + 10];
+    input.push(b'\n');
+    input.extend_from_slice(br#"{"jsonrpc":"2.0","id":5,"method":"ping"}"#);
+    input.push(b'\n');
+    let mut out = Vec::new();
+    s.serve(std::io::Cursor::new(input), &mut out).unwrap();
+    let text = String::from_utf8(out).unwrap();
+    let lines: Vec<&str> = text.lines().collect();
+    assert_eq!(lines.len(), 2, "{text}");
+    assert!(lines[0].contains("too long"), "{}", lines[0]);
+    assert!(lines[1].contains(r#""id":5"#), "{}", lines[1]);
+}