diff --git a/rust/core/src/clients/mod.rs b/rust/core/src/clients/mod.rs
new file mode 100644
index 0000000..d811535
--- /dev/null
+++ b/rust/core/src/clients/mod.rs
@@ -0,0 +1,26 @@
+//! Protocol clients. Conventions, shared by every client so the session manager treats them alike:
+//! - `connect()` blocks the calling thread (the manager calls it from a worker thread, as Go
+//!   called it from a goroutine) and returns once the connection is usable.
+//! - After connecting, the client owns its own reader thread(s) and reports through callbacks:
+//!   data, a fatal error, and close (exactly once, whether remote or local).
+//! - `write`/`resize`/`close` are callable from any thread; `close` is idempotent and must not
+//!   fire `on_close` re-entrantly while holding locks.
+
+use std::sync::Arc;
+
+pub type DataFn = Arc<dyn Fn(&[u8]) + Send + Sync>;
+pub type ErrorFn = Arc<dyn Fn(String) + Send + Sync>;
+pub type CloseFn = Arc<dyn Fn() + Send + Sync>;
+
+#[derive(Clone)]
+pub struct Callbacks {
+    pub on_data: DataFn,
+    pub on_error: ErrorFn,
+    pub on_close: CloseFn,
+}
+
+impl Callbacks {
+    pub fn noop() -> Callbacks {
+        Callbacks { on_data: Arc::new(|_| {}), on_error: Arc::new(|_| {}), on_close: Arc::new(|| {}) }
+    }
+}
diff --git a/rust/core/src/lib.rs b/rust/core/src/lib.rs
index e17c290..5eaec28 100644
--- a/rust/core/src/lib.rs
+++ b/rust/core/src/lib.rs
@@ -1,6 +1,7 @@
 //! nexus-core: everything in Nexus Terminal that isn't drawing — storage, secrets, protocol
 //! clients and the session manager. The egui app is one client of this crate.
 
+pub mod clients;
 pub mod config;
 pub mod exportimport;
 pub mod models;