Why astream_events not support error event

[deershark]

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Example Code

async def astream_events(
        self,
        input: Any,
        config: Optional[RunnableConfig] = None,
        *,
        version: Literal["v1", "v2"] = "v2",
        include_names: Optional[Sequence[str]] = None,
        include_types: Optional[Sequence[str]] = None,
        include_tags: Optional[Sequence[str]] = None,
        exclude_names: Optional[Sequence[str]] = None,
        exclude_types: Optional[Sequence[str]] = None,
        exclude_tags: Optional[Sequence[str]] = None,
        **kwargs: Any,
    ) -> AsyncIterator[StreamEvent]:
        """Generate a stream of events.

        Use to create an iterator over StreamEvents that provide real-time information
        about the progress of the Runnable, including StreamEvents from intermediate
        results.

        A StreamEvent is a dictionary with the following schema:

        - ``event``: **str** - Event names are of the
            format: on_[runnable_type]_(start|stream|end).
        - ``name``: **str** - The name of the Runnable that generated the event.
        - ``run_id``: **str** - randomly generated ID associated with the given execution of
            the Runnable that emitted the event.
            A child Runnable that gets invoked as part of the execution of a
            parent Runnable is assigned its own unique ID.
        - ``parent_ids``: **List[str]** - The IDs of the parent runnables that
            generated the event. The root Runnable will have an empty list.
            The order of the parent IDs is from the root to the immediate parent.
            Only available for v2 version of the API. The v1 version of the API
            will return an empty list.
        - ``tags``: **Optional[List[str]]** - The tags of the Runnable that generated
            the event.
        - ``metadata``: **Optional[Dict[str, Any]]** - The metadata of the Runnable
            that generated the event.
        - ``data``: **Dict[str, Any]**


        Below is a table that illustrates some events that might be emitted by various
        chains. Metadata fields have been omitted from the table for brevity.
        Chain definitions have been included after the table.

        **ATTENTION** This reference table is for the V2 version of the schema.

        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | event                | name             | chunk                           | input                                         | output                                          |
        +======================+==================+=================================+===============================================+=================================================+
        | on_chat_model_start  | [model name]     |                                 | {"messages": [[SystemMessage, HumanMessage]]} |                                                 |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_chat_model_stream | [model name]     | AIMessageChunk(content="hello") |                                               |                                                 |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_chat_model_end    | [model name]     |                                 | {"messages": [[SystemMessage, HumanMessage]]} | AIMessageChunk(content="hello world")           |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_llm_start         | [model name]     |                                 | {'input': 'hello'}                            |                                                 |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_llm_stream        | [model name]     | 'Hello'                         |                                               |                                                 |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_llm_end           | [model name]     |                                 | 'Hello human!'                                |                                                 |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_chain_start       | format_docs      |                                 |                                               |                                                 |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_chain_stream      | format_docs      | "hello world!, goodbye world!"  |                                               |                                                 |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_chain_end         | format_docs      |                                 | [Document(...)]                               | "hello world!, goodbye world!"                  |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_tool_start        | some_tool        |                                 | {"x": 1, "y": "2"}                            |                                                 |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_tool_end          | some_tool        |                                 |                                               | {"x": 1, "y": "2"}                              |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_retriever_start   | [retriever name] |                                 | {"query": "hello"}                            |                                                 |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_retriever_end     | [retriever name] |                                 | {"query": "hello"}                            | [Document(...), ..]                             |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_prompt_start      | [template_name]  |                                 | {"question": "hello"}                         |                                                 |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+
        | on_prompt_end        | [template_name]  |                                 | {"question": "hello"}                         | ChatPromptValue(messages: [SystemMessage, ...]) |
        +----------------------+------------------+---------------------------------+-----------------------------------------------+-------------------------------------------------+

Description

I am trying to use astream_events to get the event return of Runnable, but after reading the documentation and testing, I learned that it is impossible to get error type events (such as: on_tool_error). Will this be supported in subsequent versions?

System Info

langchain==0.3.20
langchain-community==0.3.19
langchain-core==0.3.45
langchain-deepseek==0.1.2
langchain-mcp-adapters==0.0.5
langchain-openai==0.3.9
langchain-postgres==0.0.13
langchain-text-splitters==0.3.6

[deershark]

It is replyed in this issue: #30708