deepseek-harness/docs/adr/0012-dev-invariants-over-deep-readonly.md

ADR 0012: Dev-mode invariants over compile-time deep-readonly

Status: accepted (2026-06-13)

Context

The session log is append-only by contract, but the types don't enforce it: session.events returns readonly SessionEvent[] whose elements are mutable, and deriveMessages() handed the logged content arrays/blocks out by reference. The loop then passes those derived messages into the agent/request waterfall and on to adapters, where mutating the request is sanctioned — so a request middleware could reach back and rewrite history, silently breaking replay equivalence and the derived-history guarantee. Separately, the event taxonomy (turn/step nesting, seq monotonicity, tool-call/result pairing, legal status transitions) was asserted only where individual tests happened to look.

Two ways to defend the log: make immutability part of the type (DeepReadonly<SessionEvent> on the way out), or catch corruption at runtime in dev. The RFC (005) proposed the runtime route; RFC 008 proposed the type route.

Decision

Reject the pervasive DeepReadonly<T> type flip. Instead:

1. Always-on: deriveMessages() deep-clones the content it emits (one structuredClone per derived message). In-flight mutation of a request can no longer reach the log — this is the real fix, and it costs nothing meaningful next to a model call.
2. Dev-mode: a new dsh-invariants plugin (pure listeners, off in production, on in tests and demos) asserts the event contract and Object.freezes logged event data so any other code that mutates a logged event throws instead of corrupting silently. Seeded sessions are frozen and checked on session/created (the constructor copies the seed without emitting session/event).

The invariants encode the real contract, not an idealized one: a tool/call may have no tool/result (a thrown tools/execute waterfall ends the step), and both idle→disposed and running→disposed are legal.

DeepReadonly was rejected because it is compile-time only (a plugin casts straight through it), high type-noise across every log/message consumer and adapter, and would force readonly types through code where mutation is the sanctioned API. The clone draws the mutable/immutable boundary exactly at "logged vs in-flight" without any of that noise.

Consequences

• History corruption is caught loudly in tests and demos, at zero production cost and zero type noise. The trade-off is that the guarantee is dynamic (a dev-mode tripwire) rather than static.
• The invariants plugin doubles as executable documentation of the event taxonomy — the assertions are the contract.
• Session.events keeps its readonly SessionEvent[] type; no consumer churn.
• This folds in RFC 008 — there is no separate deep-readonly ADR; this records the decision to not pursue that approach. InvariantError is a plain Error with a code for now; a later taxonomy change can promote it.