diff --git a/src/app/builder_protocol.rs b/src/app/builder_protocol.rs
index 3e3296a0..19132f20 100644
--- a/src/app/builder_protocol.rs
+++ b/src/app/builder_protocol.rs
@@ -22,6 +22,11 @@ where
     /// The protocol defines hooks for connection setup, frame modification, and
     /// command completion. It is wrapped in an [`Arc`] and stored for later use
     /// by the connection actor.
+    ///
+    /// At present, the protocol must use `ProtocolError = ()`. This keeps the
+    /// protocol object safe for dynamic dispatch, maintains a uniform
+    /// interface across connections, and avoids leaking application-specific
+    /// error types into the builder API.
     #[must_use]
     pub fn with_protocol<P>(self, protocol: P) -> Self
     where
diff --git a/src/fragment/tests.rs b/src/fragment/tests.rs
index c0283c47..1338aff1 100644
--- a/src/fragment/tests.rs
+++ b/src/fragment/tests.rs
@@ -1,3 +1,9 @@
+//! Unit tests for the fragmentation and reassembly subsystem.
+//!
+//! Covers `FragmentHeader` field access, `FragmentSeries` ordering and
+//! validation, `Fragmenter` splitting and message ID management, and
+//! `Reassembler` assembly with size limits and expiry handling.
+
 use std::{
     num::NonZeroUsize,
     time::{Duration, Instant},
diff --git a/src/hooks.rs b/src/hooks.rs
index 86581544..4b7f1381 100644
--- a/src/hooks.rs
+++ b/src/hooks.rs
@@ -19,10 +19,19 @@ use crate::{
 pub struct ConnectionContext;
 
 /// Trait encapsulating protocol-specific logic and callbacks.
+///
+/// `WireframeProtocol` allows a custom `ProtocolError` type, but
+/// [`crate::app::WireframeApp::with_protocol`] currently requires
+/// `ProtocolError = ()` so the protocol can be stored behind dynamic dispatch
+/// with a uniform interface. This constraint may be relaxed in a future
+/// release.
 pub trait WireframeProtocol: Send + Sync + 'static {
     /// Frame type written to the socket.
     type Frame: FrameLike;
     /// Custom error type for protocol operations.
+    ///
+    /// When installed via [`crate::app::WireframeApp::with_protocol`], this
+    /// must currently be `()`.
     type ProtocolError;
 
     /// Called once when a new connection is established. The provided
@@ -45,10 +54,10 @@ pub trait WireframeProtocol: Send + Sync + 'static {
     ///
     /// impl WireframeProtocol for MyProtocol {
     ///     type Frame = Vec<u8>;
-    ///     type ProtocolError = String;
+    ///     type ProtocolError = ();
     ///
-    ///     fn handle_error(&self, error: Self::ProtocolError, _ctx: &mut ConnectionContext) {
-    ///         tracing::error!(error = %error, "protocol error");
+    ///     fn handle_error(&self, _error: Self::ProtocolError, _ctx: &mut ConnectionContext) {
+    ///         tracing::error!("protocol error");
     ///         // Custom handling here
     ///     }
     /// }
@@ -81,7 +90,7 @@ pub trait WireframeProtocol: Send + Sync + 'static {
     ///
     /// impl WireframeProtocol for MyProtocol {
     ///     type Frame = Vec<u8>;
-    ///     type ProtocolError = String;
+    ///     type ProtocolError = ();
     ///
     ///     fn on_eof(&self, error: &EofError, partial_data: &[u8], _ctx: &mut ConnectionContext) {
     ///         match error {