What causes "`default` should not be passed to dumps" in langchain-ai/langchain?

Remove the default kwarg and rely on LangChain's built-in serialization

langchain-ai/langchain · error · ValueError

`default` should not be passed to dumps

Error message

`default` should not be passed to dumps

What it means

Raised by langchain_core.load.dumps when the caller passes json.dumps' `default` kwarg. LangChain installs its own serializer (_serialize_value) as the default handler, and a user-supplied `default` would silently bypass LangChain's escaping of user data (including 'lc'-keyed dicts and secret handling), so it is explicitly rejected.

Solutions

Remove the default kwarg and rely on LangChain's built-in serialization
Pre-convert custom types to dicts/primitives before calling dumps
Wrap custom classes as Serializable subclasses so dumps handles them natively

Example fix

# before
dumps(obj, default=lambda o: o.__dict__)
# after
dumps(obj)  # pre-convert custom types to plain dicts yourself

Defensive patterns

Strategy: validation

Validate before calling

kwargs.pop('default', None)  # never forward `default` to langchain's dumps
from langchain_core.load import dumps
dumps(obj, **kwargs)

Try / catch

try:
    dumps(obj, **json_kwargs)
except ValueError as e:
    if 'default' in str(e):
        json_kwargs.pop('default', None)
        dumps(obj, **json_kwargs)
    else:
        raise

Prevention

Do not reuse json.dumps kwargs with langchain_core.load.dumps
Pre-convert custom types to plain data instead of relying on a default hook

When it happens

Trigger: dumps(obj, default=my_encoder); forwarding **json_kwargs collected from elsewhere that happen to include 'default'. Any kwargs are otherwise passed through to json.dumps, but 'default' is reserved.

Common situations: Reusing a json.dumps(...) call's kwargs verbatim when switching to langchain dumps; attempting to support custom types via a default hook instead of Serializable.

Related errors

AI-assisted analysis of langchain-ai/langchain@e32fa9a52e (2026-08-14). Data as JSON: /api/errors/f5e81a1256b42fb9. Report an issue: GitHub.

Appendix: source

Thrown at libs/core/langchain_core/load/dump.py:94

        deserialization.

    Args:
        obj: The object to dump.
        pretty: Whether to pretty print the json.

            If `True`, the json will be indented by either 2 spaces or the amount
            provided in the `indent` kwarg.
        **kwargs: Additional arguments to pass to `json.dumps`

    Returns:
        A JSON string representation of the object.

    Raises:
        ValueError: If `default` is passed as a kwarg.
    """
    if "default" in kwargs:
        msg = "`default` should not be passed to dumps"
        raise ValueError(msg)

    obj = _dump_pydantic_models(obj)
    serialized = _serialize_value(obj)

    if pretty:
        indent = kwargs.pop("indent", 2)
        return json.dumps(serialized, indent=indent, **kwargs)
    return json.dumps(serialized, **kwargs)


def dumpd(obj: Any) -> Any:
    """Return a dict representation of an object.

    Note:
        Plain dicts containing an `'lc'` key are automatically escaped to prevent
        confusion with LC serialization format. The escape marker is removed during
        deserialization.

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