## Why
World State restores its structured snapshot on resume so unchanged
sections do not have to be rendered again. That is safe only when the
model-visible fragment represented by the snapshot is still present in
retained history.
For selected executor skills, the failing selected-capability scenario
exposed this state:
```text
persisted World State: selected skill catalog is known
retained model history: selected skill catalog message is missing
next diff: unchanged, so emit nothing
```
The model resumes without being told about the selected skill catalog.
## What changed
World State contributions may now optionally describe the concrete
model-visible fragment that must remain in retained history.
When a persisted snapshot is present:
```text
matching retained fragment exists -> trust snapshot, emit nothing
matching retained fragment missing -> treat section as absent, render current state once
```
The skills extension uses this for non-empty selected-environment
catalogs by matching its exact rendered catalog body. Empty or hidden
catalogs do not require a fragment.
## Scope
This does not clear or rebuild the whole World State baseline. It does
not change skill discovery, cache invalidation, environment
availability, or MCP runtime behavior. It only keeps a persisted section
snapshot and its retained model context consistent across resume/history
reconstruction.
## Coverage
A focused World State regression test verifies both sides:
- a missing retained fragment is rendered again
- a matching retained fragment avoids duplicate injection
jif
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