Compatibility work often asks extension authors to meet a new contract and leaves everyone else to discover what broke. Tau's PR #375 takes the harder public position: the runtime can change its message, event, tool and extension shapes, but persisted Tau-v1 session history gets an explicit migration path rather than a quiet incompatibility.
The old session has a designated door
The change is visible in the JSONL reader. Before the new models validate a stored entry, it maps former custom messages, assistant tool calls and old tool-result fields into the new canonical shape. That is deliberately less glamorous than a new event stream, but it makes a session file an upgrade concern rather than an archaeological surprise.
Following a public grammar without claiming a treaty
Tau's extension guide says the Python design follows Pi's extension system, and Pi's own documentation shows why that grammar matters: extensions can add tools, commands, event handlers and UI. The evidence does not show Pi endorsing Tau's change or any extension already working unchanged in both. The signal to watch is whether Tau publishes a compatibility fixture or an independent extension author puts this boundary under real use.