Q6-A of docs/cultnet-error-contract-cut.md. message_invalid joins the closed
cultnet.error.v0 code enum (both schema copies), with details.field naming the
request field at fault ("message" when the whole frame cannot be read).
C# ForMessageInvalid, Rust CultNetErrorCode::MessageInvalid, TS
cultNetMessageInvalidError and Python message_invalid_error all write the same
captured bytes (error-vectors.json plus the C# and Rust cases). cultmesh-py and
cultmesh-ts refuse malformed puts and deletes with it; the C# subscription
servers use it for the missing subscriptionId.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
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