Learn / Who changes state

The two write paths

Ordinary application changes travel Message → update → modifyFields:

user / agent / behavior / external fact
        │
        ▼
     Message
        │
        ▼
      update
        │
        ▼
   modifyFields
        │
        ▼
    next Model

Infrastructure installs already-derived values through structural seams:

checkpoint / URL / cache result / reconciliation
        │
        ▼
 ModelRef / WritableProjection
        │
   set / modify
        │
        ▼
    next Model

ModelRef.modify is focused structural replacement — set of a function applied to get — not a transition mechanism. It delegates through the ref's own set, so container-aware .at/.index insertion keeps working.

The matrix

Situation	Owner	Change mechanism
Local UI/domain state	application	Message → update → `modifyFields`
Submodel state	Submodel	child Message → child update → `modifyFields`
Server fact	server	Remote Message → `Data.reduce`
Replicated user operation	application/durable history	Message → application update → `modifyFields`
Sync checkpoint	authoritative replica	`WritableProjection.set`
URL/KV restoration	local application	Mirror → `WritableProjection.set`
Agent action	application	Agent → Message → update
UI behavior	application	Behavior → Message → update

The matrix#

The matrix