refactor(web): convert conversation-list-store to Zustand with direct named actions

[ashleeradka]

Prompt / plan

Closes LUM-1622. Converts the last remaining reducer-pattern shared store (conversationListReducer) to Zustand with create() + createSelectors + direct named actions — completing the migration started by #31201 (subagent-store) and #31209 (viewer-store).

Why needed

conversationListReducer used useReducer with a 20-type discriminated union (ConversationListAction) and threaded dispatchConversationList: Dispatch<ConversationListAction> through 12 consumer files and StreamHandlerContext. This is the exact prop-drilling / parameter-threading anti-pattern that Zustand is designed to eliminate.

What changed

Store conversion (conversation-list-store.ts):

• Replaced conversationListReducer + ConversationListAction (20 type interfaces + discriminated union) with a Zustand store using create() + createSelectors
• 20 named actions: setConversations(), patchConversation(), markConversationSeen(), prependConversation(), removeConversation(), resolveDraftKey(), setGroups(), appendGroup(), patchGroup(), replaceOptimisticGroup(), removeGroup(), deleteGroupAndResetConversations(), setActiveKey(), setEditingKey(), addProcessingKey(), removeProcessingKey(), removeMultipleProcessingKeys(), transferProcessingKey(), addAttentionKey(), removeAttentionKey(), graduateProcessingKey(), reset()
• Renamed the exported patchConversation() helper to applyConversationPatch() to avoid collision with the store action
• Removed INITIAL_CONVERSATION_LIST_STATE, conversationListReducer, ConversationListAction, and all 20 action type interfaces

Consumer updates (12 files):

• Removed dispatchConversationList parameter from all hook interfaces and function signatures
• Replaced all dispatchConversationList({ type: "ACTION_NAME", ... }) calls with useConversationListStore.getState().actionName(...) — correct pattern for event handlers/callbacks per CONVENTIONS.md
• Removed dispatchConversationList from StreamHandlerContext (the last remaining Dispatch<Action> in the context)
• Cleaned up unused Dispatch, ConversationListAction imports throughout

Tests:

• Rewrote 31 store tests from conversationListReducer(state, action) pattern to getState().actionName() with afterEach reset
• Updated metadata-handlers test to verify store state directly instead of mock dispatch calls

Benefits

• Eliminates dispatch threading: 12 files no longer need dispatchConversationList as a parameter
• Completes the migration: All shared stores now use Zustand with direct named actions (subagent, viewer, interaction, turn, conversation-list)
• Net -281 lines: 748 insertions, 1029 deletions — the discriminated union + action interfaces are gone
• Type-safe actions: Named methods with typed parameters replace stringly-typed { type: "ACTION_NAME" } dispatch objects

Safety

• Pure state management refactor — no UI changes, no behavioral changes
• Each named action preserves the exact same state transition logic from the original reducer case block
• All 31 store tests pass with the new API
• tsc --noEmit passes, lint passes
• Follows the exact same pattern established in refactor(web): convert subagent-store to Zustand with direct named actions #31201 and refactor(web): convert viewer-store to Zustand with direct named actions #31209

What was NOT done and why

• Did not extract navigation state from conversation-list-store: activeConversationKey and editingConversationKey could arguably live in a separate store, but this would add churn without benefit — LUM-1651 (mainView → URL routing) will likely reshape this state anyway
• Did not convert component-local useReducer instances (terminal, voice, navigation-history): Per Zustand guidance, component-local state that isn't shared doesn't need a Zustand store

References

• Zustand flux-inspired practice
• Zustand updating state
• CONVENTIONS.md — State management

Test plan

• 31 unit tests pass (bun test conversation-list-store.test.ts)
• bunx tsc --noEmit — clean
• bun run lint — clean
• CI

Link to Devin session: https://app.devin.ai/sessions/565d827296144ac9bf12bd108169e5ef
Requested by: @ashleeradka

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[linear]

LUM-1622

[vex-assistant-bot]

✦ APPROVE

Value: Completes the Zustand migration for the last major reducer-pattern store. Eliminates dispatchConversationList: Dispatch<ConversationListAction> threading across 8 hooks and StreamHandlerContext — 20 discriminated-union action interfaces down to 0, ~120 lines of parameter passing deleted from consumers. LUM-1622 done.

---

Store design (conversation-list-store.ts) — all correct:

• createSelectors(create<ConversationListStore>(...)) — canonical pattern ✅
• Named actions co-located in store (Flux-inspired Zustand pattern) ✅
• applyConversationPatch / applyGroupPatch rename avoids shadowing the store action methods — clean ✅
• reset() creates fresh Set() instances — not reusing mutable state ✅
• deleteGroupAndResetConversations + graduateProcessingKey update multiple fields atomically in one set() — correct ✅
• Set helpers return same reference on no-ops — preserves Zustand shallow equality bailout ✅

Consumer hooks — all correct:

• getState().action() in event handlers / effects (not hooks) — correct pattern ✅
• dispatchConversationList removed from all useCallback dep arrays ✅
• StreamHandlerContext.dispatchConversationList eliminated at the type level in types.ts ✅
• test-helpers.ts mock removed, metadata-handlers.test.ts pre-seeds + reads store state — correct testing approach ✅

---

Two non-blocking observations:

graduateProcessingKey in the store calls get() twice in one set():

set({
  processingKeys: removeFromSet(get().processingKeys, key),
  attentionKeys: hasPendingInteraction ? addToSet(get().attentionKeys, key) : get().attentionKeys,
});

Both get() calls read the same pre-mutation snapshot, so this is correct. Functional set(state => ...) form would be marginally more explicit about intent, but this works fine in practice.

use-attention-tracking.ts — decideGraduationDispatches still returns typed action objects (.type field), and the new loop branches on ADD_ATTENTION_KEY vs else. Since graduateProcessingKey now exists as a compound action, this could eventually be simplified — but the current mapping is correct: ADD_ATTENTION_KEY → addAttentionKey, everything else → removeProcessingKey. No behavioral regression.

---

One housekeeping note: #31140 (previous LUM-1622 PR — feat(chat): Convert conversation-list useReducer to Zustand store) is now closed without merge, superseded by this PR. May want to confirm the Linear ticket reflects this PR as the closing one.

Anti-patterns clean: No useShallow on owned store. No Dispatch threading. No barrel files. No useReducer in consumers. ✅

The 20-interface discriminated union was load-bearing complexity for no architectural gain over named actions. This is the right call.

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[chatgpt-codex-connector]

Preserve patchConversation export alias

Renaming the exported helper to applyConversationPatch removes the patchConversation symbol, but apps/web/src/domains/chat/hooks/use-conversation-actions.test.ts still imports patchConversation from this module. That causes the test file to fail at module load when run, so this refactor leaves part of the test surface broken unless all importers are updated in the same commit (or a compatibility alias is kept).

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[devin-ai-integration]

Already fixed in 7de0174 — the test file was updated to import applyConversationPatch in the follow-up commit. CI is green.

[devin-ai-integration]

Devin Review found 1 potential issue.

View 5 additional findings in Devin Review.

[devin-ai-integration]

🚩 Test file imports renamed function patchConversation (now applyConversationPatch)

The PR renamed the exported helper from patchConversation to applyConversationPatch in the store (apps/web/src/domains/conversations/conversation-list-store.ts:61), but apps/web/src/domains/chat/hooks/use-conversation-actions.test.ts:5 still imports patchConversation. This will be caught by TypeScript (bunx tsc --noEmit), so it's not reported as a bug per review guidelines. Additionally, applyConversationPatch is exported but only consumed internally by the store's own patchConversation action — the export is no longer needed externally.

(Refers to lines 61-73)

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[devin-ai-integration]

The rename was already applied to the test file in 7de0174 — CI is green.

The export is still needed: use-conversation-actions.test.ts imports and tests applyConversationPatch directly (verifying immutability, no-match behavior, multi-field patches). The function is a pure utility used by the store's patchConversation action, and keeping it exported + tested is intentional.