fix: handle malformed messages missing <|message|> token

docs/format.md

@@ -107,8 +107,12 @@ convo = Conversation.from_messages(
 tokens = encoding.render_conversation_for_completion(convo, Role.ASSISTANT)
 
 # After receiving a token response
-# Do not pass in the stop token
-parsed_response = encoding.parse_messages_from_completion_tokens(new_tokens, Role.ASSISTANT)
+# Do not pass in the stop token. Set strict=False to tolerate malformed headers.
+parsed_response = encoding.parse_messages_from_completion_tokens(
+    new_tokens,
+    Role.ASSISTANT,
+    strict=True,
+)
 ```
 
 Additionally the openai_harmony library also includes a StreamableParser for parsing and decoding as the model is generating new tokens. This can be helpful for example to stream output and handle unicode characters during decoding.
@@ -269,7 +273,7 @@ If you are not using function tool calling your developer message would just loo
 
 Where `{instructions}` is replaced with your “system prompt”.
 
-For defining function calling tools, [check out the dedicated section](#function-calling).  
+For defining function calling tools, [check out the dedicated section](#function-calling).
 For defining an output format to be used in structured outputs, [check out this section of the guide](#structured-output).
 
 ### Reasoning
@@ -301,7 +305,7 @@ And the actual answer is:
 2 + 2 = 4
 ```
 
-**Important:**  
+**Important:**
 The model has not been trained to the same safety standards in the chain-of-thought as it has for final output. We recommend not to show the chain-of-thought to your users as they might contain harmful content. [Learn more in the model card](https://openai.com/index/gpt-oss-model-card/).
 
 #### Handling reasoning output in subsequent sampling

docs/python.md

@@ -107,12 +107,14 @@ Methods:
 - `render_conversation_for_training(conversation, config=None)` – render a conversation for training.
 - `render_conversation(conversation, config=None)` – render a conversation without appending a new role.
 - `render(message)` – render a single message into tokens.
-- `parse_messages_from_completion_tokens(tokens, role=None)` – parse tokens back into `Message` objects.
+- `parse_messages_from_completion_tokens(tokens, role=None, strict=True)` – parse tokens back into `Message` objects (set `strict=False` to enable permissive parsing).
 - `decode_utf8(tokens)` – decode tokens with the underlying tokenizer.
 - `stop_tokens()` / `stop_tokens_for_assistant_actions()` – lists of stop tokens.
 
+Use `strict=False` when you need the parser to recover from malformed model output that omits markers such as `<|message|>`.
+
 ### `StreamableParser`
-Incremental parser built on top of an encoding. Construct with `StreamableParser(encoding, role)` and feed tokens via `process(token)`.  Inspect state via properties like `current_content`, `current_role`, `tokens` and `state`.
+Incremental parser built on top of an encoding. Construct with `StreamableParser(encoding, role)` and feed tokens via `process(token)`.  Inspect state via properties like `current_content`, `current_role`, `tokens` and `state`. Pass `strict=False` to enable permissive parsing (mirrors `ParseOptions { strict: false }` on the Rust side).
 
 ### `load_harmony_encoding(name)`
 Return a `HarmonyEncoding` by name.  Accepts either the string name or a value from the `HarmonyEncodingName` enum (`HARMONY_GPT_OSS`).

docs/rust.md

@@ -88,12 +88,15 @@ Important methods:
 - `render_conversation_for_training(conversation, config)` – render a conversation for training data.
 - `render_conversation(conversation, config)` – render a conversation without appending a new role.
 - `render(message)` – render a single message into tokens.
-- `parse_messages_from_completion_tokens(tokens, role)` – parse a list of tokens back into messages.
+- `parse_messages_from_completion_tokens(tokens, role)` – parse a list of tokens back into messages using strict validation.
+- `parse_messages_from_completion_tokens_with_options(tokens, role, options)` – parse tokens with custom `ParseOptions` (e.g. to disable strict validation).
 - `stop_tokens()` and `stop_tokens_for_assistant_actions()` – sets of stop tokens for sampling.
 
+`ParseOptions` currently exposes a single field, `strict`, which defaults to `true`. Set it to `false` when you need to recover from malformed model output in downstream systems.
+
 ### `StreamableParser`
 
-Incremental parser that consumes tokens one by one. Create with `StreamableParser::new(encoding, role)` and feed tokens via `process`. Access information via getters like `current_content`, `current_role`, `messages`, `tokens` and `state_json`.
+Incremental parser that consumes tokens one by one. Create with `StreamableParser::new(encoding, role)` and feed tokens via `process`. Access information via getters like `current_content`, `current_role`, `messages`, `tokens` and `state_json`. Use `StreamableParser::new_with_options(encoding, role, options)` when you need to override defaults such as `ParseOptions { strict: false }`.
 
 ## registry module
 

python/openai_harmony/__init__.py

@@ -520,10 +520,14 @@ def render(
     # -- Parsing -------------------------------------------------------
 
     def parse_messages_from_completion_tokens(
-        self, tokens: Sequence[int], role: Optional[Role] | None = None
+        self,
+        tokens: Sequence[int],
+        role: Optional[Role] | None = None,
+        *,
+        strict: bool = True,
     ) -> List[Message]:
         raw_json: str = self._inner.parse_messages_from_completion_tokens(
-            list(tokens), None if role is None else str(role.value)
+            list(tokens), None if role is None else str(role.value), strict
         )
         return [Message.from_dict(m) for m in json.loads(raw_json)]
 
@@ -619,9 +623,15 @@ class StreamState(Enum):
 class StreamableParser:
     """Incremental parser over completion tokens."""
 
-    def __init__(self, encoding: HarmonyEncoding, role: Role | None):
+    def __init__(
+        self,
+        encoding: HarmonyEncoding,
+        role: Role | None,
+        *,
+        strict: bool = True,
+    ) -> None:
         role_str = str(role.value) if role is not None else None
-        self._inner = _PyStreamableParser(encoding._inner, role_str)
+        self._inner = _PyStreamableParser(encoding._inner, role_str, strict)
 
     def process(self, token: int) -> "StreamableParser":
         self._inner.process(token)

src/encoding.rs

@@ -369,22 +369,38 @@ impl HarmonyEncoding {
         Ok(())
     }
 
-    pub fn parse_messages_from_completion_tokens<I>(
+    pub fn parse_messages_from_completion_tokens_with_options<I>(
         &self,
         tokens: I,
         role: Option<Role>,
+        options: ParseOptions,
     ) -> anyhow::Result<Vec<Message>>
     where
         I: IntoIterator<Item = Rank>,
     {
-        let mut parser = StreamableParser::new(self.clone(), role)?;
+        let mut parser = StreamableParser::new_with_options(self.clone(), role, options)?;
         for token in tokens {
             parser.process(token)?;
         }
         parser.process_eos()?;
         Ok(parser.into_messages())
     }
 
+    pub fn parse_messages_from_completion_tokens<I>(
+        &self,
+        tokens: I,
+        role: Option<Role>,
+    ) -> anyhow::Result<Vec<Message>>
+    where
+        I: IntoIterator<Item = Rank>,
+    {
+        self.parse_messages_from_completion_tokens_with_options(
+            tokens,
+            role,
+            ParseOptions::default(),
+        )
+    }
+
     /// Helper to convert a JSON schema (OpenAPI style) to a TypeScript type definition.
     fn json_schema_to_typescript(schema: &serde_json::Value, indent: &str) -> String {
         // Helper to check if this schema is an enum
@@ -1019,6 +1035,17 @@ impl Render<crate::chat::DeveloperContent> for HarmonyEncoding {
     }
 }
 
+#[derive(Clone, Copy, Debug)]
+pub struct ParseOptions {
+    pub strict: bool,
+}
+
+impl Default for ParseOptions {
+    fn default() -> Self {
+        Self { strict: true }
+    }
+}
+
 /// Incremental parser that can consume tokens one by one.
 ///
 /// It keeps track of all tokens seen so far, exposes all fully parsed messages
@@ -1032,6 +1059,7 @@ pub struct StreamableParser {
     stop_tokens: HashSet<Rank>,
     last_content_delta: Option<String>,
     undecoded_tokens: Vec<Rank>,
+    options: ParseOptions,
 }
 
 #[derive(Clone, Debug, serde::Serialize, serde::Deserialize)]
@@ -1049,6 +1077,15 @@ pub enum StreamState {
 impl StreamableParser {
     /// Create a new streaming parser starting with the given role.
     pub fn new(encoding: HarmonyEncoding, role: Option<Role>) -> anyhow::Result<Self> {
+        Self::new_with_options(encoding, role, ParseOptions::default())
+    }
+
+    /// Create a new streaming parser with explicit options.
+    pub fn new_with_options(
+        encoding: HarmonyEncoding,
+        role: Option<Role>,
+        options: ParseOptions,
+    ) -> anyhow::Result<Self> {
         let stop_tokens = encoding.stop_tokens()?;
         let (state, next_role) = match role {
             Some(role) => (
@@ -1068,6 +1105,7 @@ impl StreamableParser {
             stop_tokens,
             last_content_delta: None,
             undecoded_tokens: Vec::new(),
+            options,
         })
     }
 
@@ -1123,6 +1161,34 @@ impl StreamableParser {
                             content_tokens: Vec::new(),
                         };
                     }
+                    Some(token) if !self.options.strict && self.stop_tokens.contains(&token) => {
+                        // Encountered a stop token while in Header state. This means we have
+                        // accumulated header tokens but never saw a <|message|> token, so the
+                        // message is malformed. If we have a role, parse header metadata and
+                        // treat remaining tokens as content.
+                        if let Some(role) = next_role_clone {
+                            if !header_tokens.is_empty() {
+                                let decoded =
+                                    self.encoding.tokenizer().decode_utf8(header_tokens)?;
+                                let (header, remaining_content) =
+                                    self.parse_header_from_string(decoded, Some(role), false)?;
+
+                                // Use remaining content if present, otherwise empty string
+                                let text = remaining_content.unwrap_or_default();
+                                let message = Message {
+                                    author: header.author.clone(),
+                                    recipient: header.recipient.clone(),
+                                    channel: header.channel.clone(),
+                                    content_type: header.content_type.clone(),
+                                    content: vec![Content::Text(TextContent { text })],
+                                };
+                                self.messages.push(message);
+                            }
+                        }
+                        // Transition to ExpectStart to wait for the next message
+                        self.state = StreamState::ExpectStart;
+                        self.next_role = None;
+                    }
                     Some(token) => {
                         header_tokens.push(token);
                     }
@@ -1194,17 +1260,18 @@ impl StreamableParser {
         Ok(self)
     }
 
-    fn parse_header_from_tokens(
+    /// Helper to parse header metadata from a decoded string.
+    /// Returns the parsed header and any remaining content after extracting header parts.
+    ///
+    /// If `parse_recipient_and_type` is true, tries to parse recipient and content_type from
+    /// whitespace-separated tokens (normal header parsing). If false, treats all remaining
+    /// text after extracting channel as content (for malformed messages).
+    fn parse_header_from_string(
         &self,
-        header_tokens: &[Rank],
+        mut header_string: String,
         role: Option<Role>,
-    ) -> anyhow::Result<ParsedHeader> {
-        let mut header_string = self
-            .encoding
-            .tokenizer()
-            .decode_utf8(header_tokens)
-            .context("could not decode header")?;
-
+        parse_recipient_and_type: bool,
+    ) -> anyhow::Result<(ParsedHeader, Option<String>)> {
         let mut channel: Option<String> = None;
         if let Some(channel_marker) = self.encoding.mapped_format_token(FormattingToken::Channel) {
             if let Some(idx) = header_string.find(channel_marker) {
@@ -1280,10 +1347,9 @@ impl StreamableParser {
 
         let mut recipient: Option<String> = None;
         let mut content_type: Option<String> = None;
+        let remaining_content: Option<String>;
 
-        if !parts.is_empty() {
-            // Determine whether the last token is a content-type or part of the
-            // recipient specification.
+        if parse_recipient_and_type && !parts.is_empty() {
             let num_parts = parts.len();
             // SAFETY: we know that there is at least one part remaining, because of is_empty check above
             let last_part = parts.pop().unwrap();
@@ -1308,25 +1374,61 @@ impl StreamableParser {
                     };
                 }
             }
+
+            // Any remaining parts are content (not header metadata)
+            remaining_content = if !parts.is_empty() {
+                Some(parts.join(" "))
+            } else {
+                None
+            };
+        } else {
+            // Treat all remaining parts as content when not parsing recipient and content type
+            remaining_content = if !parts.is_empty() {
+                Some(parts.join(" "))
+            } else {
+                None
+            };
         }
-        anyhow::ensure!(
-            parts.is_empty(),
-            "unexpected tokens remaining in message header: {:?}",
-            parts
-        );
 
         let author = if role == Role::Tool {
             let name = role_str_opt;
             Author { role, name }
         } else {
             Author { role, name: None }
         };
-        Ok(ParsedHeader {
-            author,
-            recipient,
-            channel,
-            content_type,
-        })
+        Ok((
+            ParsedHeader {
+                author,
+                recipient,
+                channel,
+                content_type,
+            },
+            remaining_content,
+        ))
+    }
+
+    fn parse_header_from_tokens(
+        &self,
+        header_tokens: &[Rank],
+        role: Option<Role>,
+    ) -> anyhow::Result<ParsedHeader> {
+        let header_string = self
+            .encoding
+            .tokenizer()
+            .decode_utf8(header_tokens)
+            .context("could not decode header")?;
+
+        let (header, remaining_content) =
+            self.parse_header_from_string(header_string, role, true)?;
+
+        if remaining_content.is_some() {
+            anyhow::bail!(
+                "unexpected tokens remaining in message header: {:?}",
+                remaining_content
+            );
+        }
+
+        Ok(header)
     }
 
     /// Return the textual content of the current message so far.

src/lib.rs

@@ -6,7 +6,7 @@ mod registry;
 mod tiktoken;
 pub mod tiktoken_ext;
 
-pub use encoding::{HarmonyEncoding, StreamableParser};
+pub use encoding::{HarmonyEncoding, ParseOptions, StreamableParser};
 pub use registry::load_harmony_encoding;
 pub use registry::HarmonyEncodingName;