(bug) Jotai "store mutation during atom read" warning in the agent playground

[mmabrouk]

The browser console persistently logs Detected store mutation during atom read. This is not supported. in the agent playground. This is a state-management anti-pattern that can cause intermittent UI state corruption.

Context

Found while root-causing the Advanced-drawer bug during the v0.105.0 release-night QA (2026-07-14). The drawer bug itself turned out to be a separate webpack IgnorePlugin issue, fixed in #5323. This Jotai warning is unrelated and remains after that fix.

Steps to reproduce

1. Open the agent playground.
2. Open the browser console.
3. Interact with the playground (drives atom reads/writes).
4. Observe repeated Detected store mutation during atom read. This is not supported. warnings.

Evidence

docs/design/agent-workflows/projects/qa/ui-exploration-20260714.md on branch qa-prerelease-agent-runtime (console captures).

[linear-code]

AGE-3958

[dosubot]

I dug into the codebase and can pinpoint a few concrete sources of this warning:

1. Side-effecting property getters in playgroundUIMiddleware.ts — Properties like displayedVariants and viewType call addToValueReferences() during property access, which mutates a reference array while Jotai is reading atoms [1].

2. Direct atomStore.set() inside SWR mutate callbacks — For example, toggleVariantDisplay calls atomStore.set(allRevisionsAtom, ...) inside swr.mutate() [2], and appSchemaMiddleware does the same during background loading [3].

3. queueMicrotask()-scheduled mutations — Background loading in appSchemaMiddleware uses queueMicrotask() to schedule atom writes [4], which can execute during or overlapping with atom reads.

4. Direct store access outside React — The playground extensively uses atomStore.get()/atomStore.set() outside React components [5], bypassing Jotai's reactivity tracking. These get called from middleware comparison functions while mutations are in flight.

To fix these, the main moves would be:

• Strip side effects out of the property getters (move addToValueReferences tracking to a non-atom mechanism)
• Replace direct atomStore.set() calls with useSetAtom or proper atom write functions where possible
• Avoid scheduling mutations via queueMicrotask() that can race with reads — use Jotai's async atom patterns instead

PR #5323 already flagged this as a separate follow-up item [6].

To reply, just mention @dosu.

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