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RFC 008: Deep-readonly public surfaces
Status: proposed
Problem
The session log is append-only by contract, but session.events returns readonly SessionEvent[] whose elements are mutable: a plugin can reach in and rewrite history (events[0].data.content.push(...)), silently breaking replay equivalence and the derived-history guarantee. The same applies to derived messages and prompt assemblies passed through waterfalls — mutation is sometimes the intended idiom (waterfall middleware mutates the request) and sometimes corruption (mutating a logged event), and the types don't distinguish.
Proposal
Make immutability part of the type where mutation is corruption:
SessionEventdata becomesDeepReadonlyon the way OUT of a session (events,session/eventlisteners);append()keeps taking plain mutable input. ADeepReadonly<T>utility type lands in dsh-llm next to the brand/never helpers.deriveMessages()returns deep-readonly messages; the loop clones before handing a mutable request to theagent/requestwaterfall (mutation there is sanctioned — the clone makes the boundary explicit and cheap, once per step).PromptAssemblystays mutable through its waterfall (sanctioned) but the registry's internal section list is cloned per assembly (already true).- Optionally, dev-mode
Object.freezeof event data behind the RFC 005 invariants flag, so sanctioned-mutation violations throw in tests rather than corrupting silently.
Plan
Introduce DeepReadonly, flip the session read paths, fix resulting compile errors in consumers (expected: a handful in tests), add the freeze-in-dev option alongside RFC 005's invariants plugin.
Risks
DeepReadonly types can produce noisy errors at waterfall boundaries where mutation IS the API — keep the mutable/readonly boundary exactly at "logged vs in-flight" and document it in the session README.