Initially, each session is marked by the creation of a unique identifier (session_id) coupled with a corresponding randomness (rand_root). This initial stage, referred to as Phase 1, sets the groundwork for subsequent operations. However, the moment a signature completes in what is designated as Phase 2, the session_id associated with that operation is immediately invalidated. This invalidation necessitates the initiation of a new session, should there be a requirement to generate a new Partially Signed Bitcoin Transaction (PSBT) bearing the same session_id. The implication here is that a fresh session, replete with new randomness, must be established from the ground up, essentially restarting the process.

The architecture of this system inherently limits the possibility of PSBT malleability to a specific window within the session lifecycle. More precisely, once a session has been activated during Phase 1, and before the commencement of signature generation in Phase 2, the transaction remains susceptible to certain modifications. Beyond this point, the rigidity of the session framework ensures stability and integrity of the transaction data. This delineation underscores a critical aspect of the protocol's design aimed at mitigating unauthorized alterations to transaction information post-session establishment.

There appears to be a recognition of the need for clearer communication regarding the failure conditions of Phase 2, particularly in instances where an active session_id cannot be identified. Such clarity would undoubtedly enhance the understanding of the protocol's operational parameters, ensuring more predictable and secure outcomes.