# RFC: Deep-readonly public surfaces

Status: rejected — the pervasive `DeepReadonly<T>` type flip was rejected in favor of an always-on `deriveMessages` clone plus dev-mode `Object.freeze` + invariants. The immutability *goal* shipped via that alternative; see [dev-mode invariants](../../implemented/architecture/2026-06-11-dev-invariants-over-deep-readonly.md).

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## 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

> **Implemented differently — see the Status line and [dev-mode invariants](../../implemented/architecture/2026-06-11-dev-invariants-over-deep-readonly.md).** The `DeepReadonly<T>` design below was rejected as written (compile-only, high type-noise, castable). What shipped: an always-on deep clone in `deriveMessages` (closing the request/adapter aliasing path) plus a dev-mode `Object.freeze` + invariants plugin. The proposal text is kept for the record.

Make immutability part of the type where mutation is corruption:

- `SessionEvent` data becomes `DeepReadonly` on the way OUT of a session (`events`, `session/event` listeners); `append()` keeps taking plain mutable input. A `DeepReadonly<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 the `agent/request` waterfall (mutation there is sanctioned — the clone makes the boundary explicit and cheap, once per step).
- `PromptAssembly` stays mutable through its waterfall (sanctioned) but the registry's internal section list is cloned per assembly (already true).
- Optionally, dev-mode `Object.freeze` of event data behind [the dev-mode invariants](../../implemented/architecture/2026-06-11-dev-invariants-over-deep-readonly.md) 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 [the dev-mode invariants](../../implemented/architecture/2026-06-11-dev-invariants-over-deep-readonly.md) 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.
