@@ -58,9 +58,10 @@ we port only the shared core to Rust, never the whole product.
 - Step 2: adapter ruled out. Per architecture decision §21, the shared core
   is being ported to Rust instead: `apps/headless-rs` now carries the full
   protocol layer (39 commands, strict validation, navigation boundary,
-  artifact rules, 1 MiB codec) with 10 tests mirroring the Swift suite's
+  artifact rules, 1 MiB codec) plus the platform-neutral control-transport
+  seam and secure Unix backend from #140. The Rust suite mirrors the Swift
   security-critical cases, and CI builds it natively on Linux, macOS, and
-  Windows. Next increments: transport, CLI parser, Chromium CDP host.
+  Windows. Next increments: Windows named pipes, CLI parser, Chromium CDP host.
 - Step 3: WSL2 install documented in README; macOS/Linux packaging already
   shipped in the E-series work. Once the Rust core gains a host, Windows
   gets a real native story (winget/MSI) instead of WSL2.
 
@@ -9,6 +9,7 @@
 pub mod error;
 pub mod json;
 pub mod protocol;
+pub mod transport;
 pub mod url;
 pub mod validate;
 
 
@@ -0,0 +1,180 @@
+//! Local control transport primitives.
+//!
+//! Framing and request correlation are platform-independent. Platform
+//! backends provide authenticated local connections without adding a network
+//! listener. The Unix backend is implemented first; Windows named pipes plug
+//! into the same traits in a later increment.
+
+use std::fmt;
+use std::io::{self, Read, Write};
+
+use crate::error::ValidationError;
+use crate::protocol::{codec, CommandRequest, CommandResponse, UNKNOWN_REQUEST_IDENTIFIER};
+use crate::HEADLESS_MAXIMUM_MESSAGE_BYTES;
+
+#[cfg(unix)]
+pub mod unix;
+
+#[derive(Debug)]
+pub enum TransportError {
+    TimedOut,
+    ConnectionFailed,
+    ConnectionClosed,
+    MessageTooLarge,
+    InvalidRuntimeDirectory,
+    EndpointOutsideRuntimeDirectory,
+    AlreadyRunning,
+    PeerDenied,
+    MismatchedResponse,
+    Protocol(ValidationError),
+    Io {
+        operation: &'static str,
+        source: io::Error,
+    },
+}
+
+impl TransportError {
+    fn from_io(operation: &'static str, error: io::Error) -> Self {
+        match error.kind() {
+            io::ErrorKind::TimedOut | io::ErrorKind::WouldBlock => Self::TimedOut,
+            io::ErrorKind::BrokenPipe
+            | io::ErrorKind::ConnectionAborted
+            | io::ErrorKind::ConnectionReset
+            | io::ErrorKind::NotConnected
+            | io::ErrorKind::UnexpectedEof
+            | io::ErrorKind::WriteZero => Self::ConnectionClosed,
+            _ => Self::Io {
+                operation,
+                source: error,
+            },
+        }
+    }
+}
+
+impl fmt::Display for TransportError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::TimedOut => write!(f, "Headless host did not respond before the deadline"),
+            Self::ConnectionFailed => write!(f, "Headless host is not running"),
+            Self::ConnectionClosed => write!(f, "Headless host closed the connection"),
+            Self::MessageTooLarge => write!(f, "Headless host message exceeded the size limit"),
+            Self::InvalidRuntimeDirectory => {
+                write!(
+                    f,
+                    "Local runtime directory is not private to the current user"
+                )
+            }
+            Self::EndpointOutsideRuntimeDirectory => {
+                write!(
+                    f,
+                    "Local endpoint must be inside the private runtime directory"
+                )
+            }
+            Self::AlreadyRunning => write!(
+                f,
+                "Another Headless host is already using the local endpoint"
+            ),
+            Self::PeerDenied => write!(f, "Local transport peer is not authorized"),
+            Self::MismatchedResponse => write!(f, "Headless host replied to a different request"),
+            Self::Protocol(error) => write!(f, "{error}"),
+            Self::Io { operation, source } => {
+                write!(f, "Local transport {operation} failed: {source}")
+            }
+        }
+    }
+}
+
+impl std::error::Error for TransportError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            Self::Protocol(error) => Some(error),
+            Self::Io { source, .. } => Some(source),
+            _ => None,
+        }
+    }
+}
+
+impl From<ValidationError> for TransportError {
+    fn from(value: ValidationError) -> Self {
+        Self::Protocol(value)
+    }
+}
+
+pub trait ControlConnection: Read + Write + Send {}
+
+impl<T: Read + Write + Send> ControlConnection for T {}
+
+pub trait ControlListener {
+    type Connection: ControlConnection;
+
+    fn accept(&self) -> Result<Self::Connection, TransportError>;
+}
+
+/// Read exactly one newline-delimited frame without allowing its buffer to
+/// grow beyond the wire cap. Bytes after the first newline are irrelevant to
+/// the one-request-per-connection protocol and are intentionally ignored.
+pub fn read_frame<R: Read>(reader: &mut R) -> Result<Vec<u8>, TransportError> {
+    let mut frame = Vec::with_capacity(8_192);
+    let mut chunk = [0_u8; 8_192];
+
+    loop {
+        let count = loop {
+            match reader.read(&mut chunk) {
+                Ok(count) => break count,
+                Err(error) if error.kind() == io::ErrorKind::Interrupted => continue,
+                Err(error) => return Err(TransportError::from_io("read", error)),
+            }
+        };
+        if count == 0 {
+            return Err(TransportError::ConnectionClosed);
+        }
+
+        if let Some(newline) = chunk[..count].iter().position(|byte| *byte == b'\n') {
+            let frame_end = newline + 1;
+            if frame.len() + frame_end > HEADLESS_MAXIMUM_MESSAGE_BYTES {
+                return Err(TransportError::MessageTooLarge);
+            }
+            frame.extend_from_slice(&chunk[..frame_end]);
+            return Ok(frame);
+        }
+
+        if frame.len() + count >= HEADLESS_MAXIMUM_MESSAGE_BYTES {
+            return Err(TransportError::MessageTooLarge);
+        }
+        frame.extend_from_slice(&chunk[..count]);
+    }
+}
+
+pub fn write_frame<W: Write>(writer: &mut W, frame: &[u8]) -> Result<(), TransportError> {
+    if frame.len() > HEADLESS_MAXIMUM_MESSAGE_BYTES {
+        return Err(TransportError::MessageTooLarge);
+    }
+    writer
+        .write_all(frame)
+        .map_err(|error| TransportError::from_io("write", error))
+}
+
+/// Exchange one validated request and correlated response on an authenticated
+/// local connection.
+pub fn exchange<C: ControlConnection>(
+    connection: &mut C,
+    request: &CommandRequest,
+) -> Result<CommandResponse, TransportError> {
+    request.validate()?;
+    exchange_validated(connection, request)
+}
+
+pub(crate) fn exchange_validated<C: ControlConnection>(
+    connection: &mut C,
+    request: &CommandRequest,
+) -> Result<CommandResponse, TransportError> {
+    let request_frame = codec::encode_line(request)?;
+    write_frame(connection, &request_frame)?;
+
+    let response_frame = read_frame(connection)?;
+    let response: CommandResponse = codec::decode_line(&response_frame)?;
+    if response.id != request.id && response.id != UNKNOWN_REQUEST_IDENTIFIER {
+        return Err(TransportError::MismatchedResponse);
+    }
+    Ok(response)
+}