diff --git a/Cargo.lock b/Cargo.lock
index ce97b77..e0b0a71 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1317,7 +1317,7 @@ checksum = "42f5e15c9953c5e4ccceeb2e7382a716482c34515315f7b03532b8b4e8393d2d"
 
 [[package]]
 name = "openai-harmony"
-version = "0.0.3"
+version = "0.0.4"
 dependencies = [
  "anyhow",
  "base64",
diff --git a/Cargo.toml b/Cargo.toml
index 25d070c..23fa1ac 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "openai-harmony"
-version = "0.0.3"
+version = "0.0.4"
 edition = "2021"
 license = "Apache-2.0"
 repository = "https://github.com/openai/harmony"
diff --git a/src/encoding.rs b/src/encoding.rs
index d57f8ec..6a9305b 100644
--- a/src/encoding.rs
+++ b/src/encoding.rs
@@ -836,12 +836,13 @@ impl Render<Message> for HarmonyEncoding {
         // finally content type
         if let Some(content_type) = &message.content_type {
             // <|constrain|> is a unique case which needs to be tokenized as a special token
-            if let Some(constrain_marker) = self.mapped_format_token(FormattingToken::ConstrainedFormat) {
-                if content_type.starts_with(constrain_marker) {
+            if let Some(constrain_marker) =
+                self.mapped_format_token(FormattingToken::ConstrainedFormat)
+            {
+                if let Some(rest) = content_type.strip_prefix(constrain_marker) {
                     // Render the space, then the constrain marker as a special token, then the rest as text (if any)
                     self.render_text_into(" ", into)?;
                     self.render_formatting_token_into(FormattingToken::ConstrainedFormat, into)?;
-                    let rest = &content_type[constrain_marker.len()..];
                     if !rest.is_empty() {
                         self.render_text_into(rest, into)?;
                     }