diff --git a/src/error.rs b/src/error.rs
index e11aef3..0f00064 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -1,6 +1,6 @@
 //! Error handling.
 
-use std::{borrow::Cow, error::Error as ErrorTrait, fmt, io, result, str, string};
+use std::{error::Error as ErrorTrait, fmt, io, result, str, string};
 
 use crate::protocol::{frame::coding::Data, Message};
 use http::Response;
@@ -46,7 +46,7 @@ pub enum Error {
     /// - When reading: buffer capacity exhausted.
     /// - When writing: your message is bigger than the configured max message size
     ///   (64MB by default).
-    Capacity(Cow<'static, str>),
+    Capacity(CapacityErrorType),
     /// Protocol violation.
     Protocol(ProtocolErrorType),
     /// Message send queue full.
@@ -146,38 +146,39 @@ impl From<tls::Error> for Error {
 impl From<httparse::Error> for Error {
     fn from(err: httparse::Error) -> Self {
         match err {
-            httparse::Error::TooManyHeaders => Error::Capacity("Too many headers".into()),
+            httparse::Error::TooManyHeaders => Error::Capacity(CapacityErrorType::TooManyHeaders),
             e => Error::Protocol(ProtocolErrorType::HttparseError(e)),
         }
     }
 }
 
-/// Indicates the specific type/cause of URL error.
-#[derive(Debug, PartialEq, Eq)]
-pub enum UrlErrorType {
-    /// TLS is used despite not being compiled with the TLS feature enabled.
-    TlsFeatureNotEnabled,
-    /// The URL does not include a host name.
-    NoHostName,
-    /// Failed to connect with this URL.
-    UnableToConnect(String),
-    /// Unsupported URL scheme used (only `ws://` or `wss://` may be used).
-    UnsupportedUrlScheme,
-    /// The URL host name, though included, is empty.
-    EmptyHostName,
-    /// The URL does not include a path/query.
-    NoPathOrQuery,
+/// Indicates the specific type/cause of a capacity error.
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub enum CapacityErrorType {
+    /// Too many headers provided (see [`httparse::Error::TooManyHeaders`]).
+    TooManyHeaders,
+    /// Received header is too long.
+    HeaderTooLong,
+    /// Message is bigger than the maximum allowed size.
+    MessageTooLong {
+        /// The size of the message.
+        size: usize,
+        /// The maximum allowed message size.
+        max_size: usize,
+    },
+    /// TCP buffer is full.
+    TcpBufferFull,
 }
 
-impl fmt::Display for UrlErrorType {
+impl fmt::Display for CapacityErrorType {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self {
-            UrlErrorType::TlsFeatureNotEnabled => write!(f, "TLS support not compiled in"),
-            UrlErrorType::NoHostName => write!(f, "No host name in the URL"),
-            UrlErrorType::UnableToConnect(uri) => write!(f, "Unable to connect to {}", uri),
-            UrlErrorType::UnsupportedUrlScheme => write!(f, "URL scheme not supported"),
-            UrlErrorType::EmptyHostName => write!(f, "URL contains empty host name"),
-            UrlErrorType::NoPathOrQuery => write!(f, "No path/query in URL"),
+            CapacityErrorType::TooManyHeaders => write!(f, "Too many headers"),
+            CapacityErrorType::HeaderTooLong => write!(f, "Header too long"),
+            CapacityErrorType::MessageTooLong { size, max_size } => {
+                write!(f, "Message too long: {} > {}", size, max_size)
+            }
+            CapacityErrorType::TcpBufferFull => write!(f, "Incoming TCP buffer is full"),
         }
     }
 }
@@ -302,3 +303,33 @@ impl fmt::Display for ProtocolErrorType {
         }
     }
 }
+
+/// Indicates the specific type/cause of URL error.
+#[derive(Debug, PartialEq, Eq)]
+pub enum UrlErrorType {
+    /// TLS is used despite not being compiled with the TLS feature enabled.
+    TlsFeatureNotEnabled,
+    /// The URL does not include a host name.
+    NoHostName,
+    /// Failed to connect with this URL.
+    UnableToConnect(String),
+    /// Unsupported URL scheme used (only `ws://` or `wss://` may be used).
+    UnsupportedUrlScheme,
+    /// The URL host name, though included, is empty.
+    EmptyHostName,
+    /// The URL does not include a path/query.
+    NoPathOrQuery,
+}
+
+impl fmt::Display for UrlErrorType {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            UrlErrorType::TlsFeatureNotEnabled => write!(f, "TLS support not compiled in"),
+            UrlErrorType::NoHostName => write!(f, "No host name in the URL"),
+            UrlErrorType::UnableToConnect(uri) => write!(f, "Unable to connect to {}", uri),
+            UrlErrorType::UnsupportedUrlScheme => write!(f, "URL scheme not supported"),
+            UrlErrorType::EmptyHostName => write!(f, "URL contains empty host name"),
+            UrlErrorType::NoPathOrQuery => write!(f, "No path/query in URL"),
+        }
+    }
+}
diff --git a/src/handshake/machine.rs b/src/handshake/machine.rs
index e23fb15..05d3a6d 100644
--- a/src/handshake/machine.rs
+++ b/src/handshake/machine.rs
@@ -3,7 +3,7 @@ use log::*;
 use std::io::{Cursor, Read, Write};
 
 use crate::{
-    error::{Error, ProtocolErrorType, Result},
+    error::{CapacityErrorType, Error, ProtocolErrorType, Result},
     util::NonBlockingResult,
 };
 use input_buffer::{InputBuffer, MIN_READ};
@@ -46,7 +46,7 @@ impl<Stream: Read + Write> HandshakeMachine<Stream> {
                 let read = buf
                     .prepare_reserve(MIN_READ)
                     .with_limit(usize::max_value()) // TODO limit size
-                    .map_err(|_| Error::Capacity("Header too long".into()))?
+                    .map_err(|_| Error::Capacity(CapacityErrorType::HeaderTooLong))?
                     .read_from(&mut self.stream)
                     .no_block()?;
                 match read {
diff --git a/src/protocol/frame/mod.rs b/src/protocol/frame/mod.rs
index dfd0bd5..c435762 100644
--- a/src/protocol/frame/mod.rs
+++ b/src/protocol/frame/mod.rs
@@ -8,7 +8,7 @@ mod mask;
 
 pub use self::frame::{CloseFrame, Frame, FrameHeader};
 
-use crate::error::{Error, Result};
+use crate::error::{CapacityErrorType, Error, Result};
 use input_buffer::{InputBuffer, MIN_READ};
 use log::*;
 use std::io::{Error as IoError, ErrorKind as IoErrorKind, Read, Write};
@@ -133,9 +133,10 @@ impl FrameCodec {
                     // Enforce frame size limit early and make sure `length`
                     // is not too big (fits into `usize`).
                     if length > max_size as u64 {
-                        return Err(Error::Capacity(
-                            format!("Message length too big: {} > {}", length, max_size).into(),
-                        ));
+                        return Err(Error::Capacity(CapacityErrorType::MessageTooLong {
+                            size: length as usize,
+                            max_size,
+                        }));
                     }
 
                     let input_size = cursor.get_ref().len() as u64 - cursor.position();
@@ -155,7 +156,7 @@ impl FrameCodec {
                 .in_buffer
                 .prepare_reserve(MIN_READ)
                 .with_limit(usize::max_value())
-                .map_err(|_| Error::Capacity("Incoming TCP buffer is full".into()))?
+                .map_err(|_| Error::Capacity(CapacityErrorType::TcpBufferFull))?
                 .read_from(stream)?;
             if size == 0 {
                 trace!("no frame received");
@@ -206,6 +207,8 @@ impl FrameCodec {
 #[cfg(test)]
 mod tests {
 
+    use crate::error::{CapacityErrorType, Error};
+
     use super::{Frame, FrameSocket};
 
     use std::io::Cursor;
@@ -266,9 +269,9 @@ mod tests {
     fn size_limit_hit() {
         let raw = Cursor::new(vec![0x82, 0x07, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07]);
         let mut sock = FrameSocket::new(raw);
-        assert_eq!(
-            sock.read_frame(Some(5)).unwrap_err().to_string(),
-            "Space limit exceeded: Message length too big: 7 > 5"
-        );
+        match sock.read_frame(Some(5)) {
+            Err(Error::Capacity(CapacityErrorType::MessageTooLong { size: 7, max_size: 5 })) => {}
+            _ => panic!(),
+        }
     }
 }
diff --git a/src/protocol/message.rs b/src/protocol/message.rs
index f799dbf..1e9ce42 100644
--- a/src/protocol/message.rs
+++ b/src/protocol/message.rs
@@ -6,7 +6,7 @@ use std::{
 };
 
 use super::frame::CloseFrame;
-use crate::error::{Error, Result};
+use crate::error::{CapacityErrorType, Error, Result};
 
 mod string_collect {
     use utf8::DecodeError;
@@ -122,9 +122,10 @@ impl IncompleteMessage {
         let portion_size = tail.as_ref().len();
         // Be careful about integer overflows here.
         if my_size > max_size || portion_size > max_size - my_size {
-            return Err(Error::Capacity(
-                format!("Message too big: {} + {} > {}", my_size, portion_size, max_size).into(),
-            ));
+            return Err(Error::Capacity(CapacityErrorType::MessageTooLong {
+                size: my_size + portion_size,
+                max_size,
+            }));
         }
 
         match self.collector {
diff --git a/src/protocol/mod.rs b/src/protocol/mod.rs
index be002e8..21465b5 100644
--- a/src/protocol/mod.rs
+++ b/src/protocol/mod.rs
@@ -669,6 +669,7 @@ impl<T> CheckConnectionReset for Result<T> {
 #[cfg(test)]
 mod tests {
     use super::{Message, Role, WebSocket, WebSocketConfig};
+    use crate::error::{CapacityErrorType, Error};
 
     use std::{io, io::Cursor};
 
@@ -711,10 +712,11 @@ mod tests {
         ]);
         let limit = WebSocketConfig { max_message_size: Some(10), ..WebSocketConfig::default() };
         let mut socket = WebSocket::from_raw_socket(WriteMoc(incoming), Role::Client, Some(limit));
-        assert_eq!(
-            socket.read_message().unwrap_err().to_string(),
-            "Space limit exceeded: Message too big: 7 + 6 > 10"
-        );
+
+        match socket.read_message() {
+            Err(Error::Capacity(CapacityErrorType::MessageTooLong { size: 13, max_size: 10 })) => {}
+            _ => panic!(),
+        }
     }
 
     #[test]
@@ -722,9 +724,10 @@ mod tests {
         let incoming = Cursor::new(vec![0x82, 0x03, 0x01, 0x02, 0x03]);
         let limit = WebSocketConfig { max_message_size: Some(2), ..WebSocketConfig::default() };
         let mut socket = WebSocket::from_raw_socket(WriteMoc(incoming), Role::Client, Some(limit));
-        assert_eq!(
-            socket.read_message().unwrap_err().to_string(),
-            "Space limit exceeded: Message too big: 0 + 3 > 2"
-        );
+
+        match socket.read_message() {
+            Err(Error::Capacity(CapacityErrorType::MessageTooLong { size: 3, max_size: 2 })) => {}
+            _ => panic!(),
+        }
     }
 }