ARTF payload is not valid JSON
The RTBRequest or RTBResponse could not be parsed as JSON, so no envelope or mutation check can run.
What it means
The ARTF payload handed to the validator is not parseable JSON. Nothing downstream can run: the envelope members, the carried OpenRTB payloads, and the mutation checks all need a parsed document.
Why it matters
ARTF transports these messages as protobuf over gRPC, so JSON usually appears at exactly two points: a hand-written fixture, and a debugging dump. Both are where a truncated or double-encoded document slips in unnoticed.
How to fix it
Serialize with protojson rather than by hand, and check the exact bytes. If the document came out of a log, confirm it was not truncated at a line-length limit.
Example that trips it
{ "id": "ep-1", "mutations": [ Truncated mutation set: the arrays never close.
Check your payload
Paste a bid request or response into the tester to see whether this code fires against it, or gate on the id in CI with the CLI: the id is stable, wording is not.
Back to the full diagnostic code reference.