diff --git a/crates/chanora_protocol/src/adapter.rs b/crates/chanora_protocol/src/adapter.rs
index 7962ca7..d4c9b74 100644
--- a/crates/chanora_protocol/src/adapter.rs
+++ b/crates/chanora_protocol/src/adapter.rs
@@ -582,10 +582,14 @@ impl ProtocolClient {
         message: String,
         target: MessageTarget,
     ) -> Result<(), ProtocolError> {
+        let validated = match target {
+            MessageTarget::Poke(_) => crate::dto::validate_poke_message(&message),
+            _ => crate::dto::validate_message(&message),
+        };
         let (tx, rx) = oneshot::channel();
         self.tx
             .send(Request::SendTextMessage {
-                message,
+                message: validated,
                 target,
                 reply: tx,
             })
diff --git a/crates/chanora_protocol/src/dto.rs b/crates/chanora_protocol/src/dto.rs
index 84452b2..77e6315 100644
--- a/crates/chanora_protocol/src/dto.rs
+++ b/crates/chanora_protocol/src/dto.rs
@@ -5,6 +5,58 @@ use serde::{Deserialize, Serialize};
 
 pub use crate::poke_limiter::PokeStrength;
 
+/// TS3 protocol limits for outbound fields.
+pub const MAX_NICKNAME_LEN: usize = 30;
+pub const MAX_MESSAGE_LEN: usize = 1024;
+pub const MAX_POKE_LEN: usize = 100;
+pub const MAX_CHANNEL_NAME_LEN: usize = 40;
+
+/// Truncate `s` to at most `max_len` UTF-8 characters, splitting at a
+/// char boundary if needed. Returns the (possibly shortened) string.
+pub fn validate_and_truncate(s: &str, max_len: usize) -> String {
+    if s.len() <= max_len {
+        return s.to_string();
+    }
+    // Find the last char boundary at or before max_len.
+    let mut end = max_len;
+    while !s.is_char_boundary(end) {
+        end -= 1;
+    }
+    s[..end].to_string()
+}
+
+/// Validate a nickname: trim whitespace, reject empty, truncate to
+/// [`MAX_NICKNAME_LEN`].
+pub fn validate_nickname(nick: &str) -> Result<String, &'static str> {
+    let trimmed = nick.trim();
+    if trimmed.is_empty() {
+        return Err("nickname must not be empty");
+    }
+    Ok(validate_and_truncate(trimmed, MAX_NICKNAME_LEN))
+}
+
+/// Validate a chat message: truncate to [`MAX_MESSAGE_LEN`]. Empty
+/// messages are allowed (poke-without-message is valid per DEC-037).
+pub fn validate_message(msg: &str) -> String {
+    validate_and_truncate(msg, MAX_MESSAGE_LEN)
+}
+
+/// Validate a poke message: truncate to [`MAX_POKE_LEN`]. Empty
+/// messages are allowed.
+pub fn validate_poke_message(msg: &str) -> String {
+    validate_and_truncate(msg, MAX_POKE_LEN)
+}
+
+/// Validate a channel name: trim whitespace, reject empty, truncate
+/// to [`MAX_CHANNEL_NAME_LEN`].
+pub fn validate_channel_name(name: &str) -> Result<String, &'static str> {
+    let trimmed = name.trim();
+    if trimmed.is_empty() {
+        return Err("channel name must not be empty");
+    }
+    Ok(validate_and_truncate(trimmed, MAX_CHANNEL_NAME_LEN))
+}
+
 /// Opaque server-side channel identifier. Internal representation is
 /// the upstream u64 but callers must treat it as opaque.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
@@ -674,6 +726,120 @@ mod tests {
         roundtrip_json(&PokeStrength::SuppressedOverflow);
     }
 
+    // ── validation tests ───────────────────────────────────────────
+
+    #[test]
+    fn validate_and_truncate_short_string_unchanged() {
+        assert_eq!(validate_and_truncate("hello", 10), "hello");
+    }
+
+    #[test]
+    fn validate_and_truncate_exact_boundary() {
+        assert_eq!(validate_and_truncate("12345", 5), "12345");
+    }
+
+    #[test]
+    fn validate_and_truncate_truncates_ascii() {
+        assert_eq!(validate_and_truncate("hello world", 5), "hello");
+    }
+
+    #[test]
+    fn validate_and_truncate_respects_char_boundary() {
+        // é is 2 bytes; truncating at byte 1 would panic without
+        // char-boundary logic.
+        let s = "aé";
+        // s.len() == 3 (a=1, é=2). max_len=2 → must drop é.
+        assert_eq!(validate_and_truncate(s, 2), "a");
+    }
+
+    #[test]
+    fn validate_and_truncate_emoji_multibyte() {
+        let s = "🎮🎮🎮";
+        // Each emoji is 4 bytes. max_len=5 → only first emoji (4 bytes).
+        assert_eq!(validate_and_truncate(s, 5), "🎮");
+    }
+
+    #[test]
+    fn validate_and_truncate_empty() {
+        assert_eq!(validate_and_truncate("", 10), "");
+    }
+
+    #[test]
+    fn validate_nickname_ok() {
+        assert_eq!(validate_nickname("Alice").unwrap(), "Alice");
+    }
+
+    #[test]
+    fn validate_nickname_trims_whitespace() {
+        assert_eq!(validate_nickname("  Bob  ").unwrap(), "Bob");
+    }
+
+    #[test]
+    fn validate_nickname_rejects_empty() {
+        assert!(validate_nickname("").is_err());
+        assert!(validate_nickname("   ").is_err());
+    }
+
+    #[test]
+    fn validate_nickname_truncates_long() {
+        let long = "A".repeat(100);
+        let result = validate_nickname(&long).unwrap();
+        assert!(result.len() <= MAX_NICKNAME_LEN);
+        assert_eq!(result.len(), MAX_NICKNAME_LEN);
+    }
+
+    #[test]
+    fn validate_message_short_unchanged() {
+        assert_eq!(validate_message("hi"), "hi");
+    }
+
+    #[test]
+    fn validate_message_empty_allowed() {
+        assert_eq!(validate_message(""), "");
+    }
+
+    #[test]
+    fn validate_message_truncates_long() {
+        let long = "x".repeat(2000);
+        let result = validate_message(&long);
+        assert!(result.len() <= MAX_MESSAGE_LEN);
+    }
+
+    #[test]
+    fn validate_poke_message_truncates_to_shorter_limit() {
+        let long = "y".repeat(200);
+        let result = validate_poke_message(&long);
+        assert!(result.len() <= MAX_POKE_LEN);
+    }
+
+    #[test]
+    fn validate_poke_message_empty_allowed() {
+        assert_eq!(validate_poke_message(""), "");
+    }
+
+    #[test]
+    fn validate_channel_name_ok() {
+        assert_eq!(validate_channel_name("General").unwrap(), "General");
+    }
+
+    #[test]
+    fn validate_channel_name_trims_whitespace() {
+        assert_eq!(validate_channel_name("  AFK  ").unwrap(), "AFK");
+    }
+
+    #[test]
+    fn validate_channel_name_rejects_empty() {
+        assert!(validate_channel_name("").is_err());
+        assert!(validate_channel_name("   ").is_err());
+    }
+
+    #[test]
+    fn validate_channel_name_truncates_long() {
+        let long = "C".repeat(100);
+        let result = validate_channel_name(&long).unwrap();
+        assert!(result.len() <= MAX_CHANNEL_NAME_LEN);
+    }
+
     #[test]
     fn client_info_equality_and_clone() {
         let a = ClientId(42);