langchain-ai/langgraph · error · TypeError

Cannot JSON-serialize type object: {obj!r}. Did you mean to

Error message

Cannot JSON-serialize type object: {obj!r}. Did you mean to pass an instance of the object instead?
Received type: {obj!r}

What it means

Error "Cannot JSON-serialize type object: {obj!r}. Did you mean to pass an instance of the object instead? Received type: {obj!r}" thrown in langchain-ai/langgraph.

Source

Thrown at libs/sdk-py/langgraph_sdk/_shared/utilities.py:76

        if header in custom_headers:
            raise ValueError(f"Cannot set reserved header '{header}'")

    headers = {
        "User-Agent": f"langgraph-sdk-py/{langgraph_sdk.__version__}",
        **custom_headers,
    }
    resolved_api_key = _get_api_key(api_key)
    if resolved_api_key:
        headers["x-api-key"] = resolved_api_key

    return headers


def _orjson_default(obj: Any) -> Any:
    is_class = isinstance(obj, type)
    if hasattr(obj, "model_dump") and callable(obj.model_dump):
        if is_class:
            raise TypeError(
                f"Cannot JSON-serialize type object: {obj!r}. Did you mean to pass an instance of the object instead?"
                f"\nReceived type: {obj!r}"
            )
        return obj.model_dump()
    elif hasattr(obj, "dict") and callable(obj.dict):
        if is_class:
            raise TypeError(
                f"Cannot JSON-serialize type object: {obj!r}. Did you mean to pass an instance of the object instead?"
                f"\nReceived type: {obj!r}"
            )
        return obj.dict()
    elif isinstance(obj, (set, frozenset)):
        return list(obj)
    else:
        raise TypeError(f"Object of type {type(obj)} is not JSON serializable")


# Compiled regex pattern for extracting run metadata from Content-Location header

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/sdk-py/langgraph_sdk/_shared/utilities.py:76 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of langchain-ai/langgraph@38031739e5 (2026-08-26). Data as JSON: /api/errors/ec890414f79dbed7. Report an issue: GitHub.