DAG-JSON doesn't reject non-string map keys — violates DAG-JSON spec

[sumanjeet0012]

The DAG-JSON specification requires all map keys to be strings. _prepare_for_json() passes through non-string keys without validation.

Problem

In dag/codecs/dag_json.py:

def _prepare_for_json(node: Any) -> Any:
    if is_cid(node):
        return {_LINK_KEY: str(node)}

    if isinstance(node, dict):
        result = {}
        for k, v in node.items():
            result[k] = _prepare_for_json(v)
            # ← Non-string keys are passed through without validation
        return result
    # ...

The DAG-JSON spec states:

"Map keys MUST be strings."

Additionally, json.dumps() will raise TypeError for non-string keys, but the error message won't mention DAG-JSON spec compliance.

Proposed Solution

Add validation in _prepare_for_json():

def _prepare_for_json(node: Any) -> Any:
    if is_cid(node):
        return {_LINK_KEY: str(node)}

    if isinstance(node, dict):
        result = {}
        for k, v in node.items():
            if not isinstance(k, str):
                raise ValueError(
                    f"DAG-JSON map keys must be strings, got {type(k).__name__}: {k!r}"
                )
            result[k] = _prepare_for_json(v)
        return result
    # ...

Related

• DAG-JSON spec: https://ipld.io/specs/codecs/dag-json/spec/
• File: dag/codecs/dag_json.py