Covariance transactions would allow a transaction to sign or commit to its own data structures—potentially enabling odd feedback loops. Bitcoin’s design forbids transactions from depending on their own IDs in a direct manner. This is primarily to prevent malleability nightmares and infinite references.

Some proposals for advanced contracts or covenants brush against covariance-like logic, but the base protocol resists any form of self-reference that could break deterministic verification. Disallowing such self-dependencies keeps transaction validation straightforward. Developers who want partial transaction commitments have explored other techniques (like partial signatures or ANYPREVOUT), but direct covariance remains taboo in Bitcoin’s consensus rules.