diff --git a/.planning/phases/16-store-modularization/16-02-PLAN.md b/.planning/phases/16-store-modularization/16-02-PLAN.md
index 707a6482..fd6c54ff 100644
--- a/.planning/phases/16-store-modularization/16-02-PLAN.md
+++ b/.planning/phases/16-store-modularization/16-02-PLAN.md
@@ -142,16 +142,50 @@ The goal is to reduce executeWorkflow from ~800 lines to ~200 lines of orchestra
   <done>executeWorkflow refactored to ~200 lines, uses extracted modules, all tests pass</done>
 </task>
 
+<task type="auto">
+  <name>Task 4: Add unit tests for execution modules</name>
+  <files>src/store/execution/__tests__/utils.test.ts, src/store/execution/__tests__/nodeExecutors.test.ts</files>
+  <action>
+Create comprehensive tests for the extracted execution modules:
+
+1. Create src/store/execution/__tests__/utils.test.ts:
+   - Test topologicalSort with simple linear graph (A -> B -> C)
+   - Test topologicalSort with branching graph (A -> B, A -> C, B -> D, C -> D)
+   - Test topologicalSort throws on cycle detection (A -> B -> C -> A)
+   - Test topologicalSort with disconnected nodes
+   - Test buildProviderHeaders returns correct headers for each provider type
+   - Test buildProviderHeaders with missing API keys returns base headers only
+   - Test getSourceOutput for each node type returns correct type/value
+
+2. Create src/store/execution/__tests__/nodeExecutors.test.ts:
+   - Test executeImageInput returns { continue: true } (no-op)
+   - Test executePrompt returns { continue: true } (no-op)
+   - Test executeAnnotation updates outputImage when sourceImage provided
+   - Test executeOutput updates node data with connected content
+   - Test executeNanaBanana handles missing text input (returns error)
+   - Test executeLlmGenerate handles missing text input (returns error)
+   - Mock fetch for API call tests
+   - Mock updateNodeData to verify state updates
+
+Focus on testing the pure logic and error cases. API success paths can use mocked fetch responses.
+  </action>
+  <verify>npm test passes with new tests, coverage report shows execution modules covered</verify>
+  <done>Unit tests added for execution utilities and node executors, all tests pass</done>
+</task>
+
 </tasks>
 
 <verification>
 Before declaring plan complete:
 - [ ] `npm run build` succeeds without errors
-- [ ] `npm test` passes all tests
+- [ ] `npm test` passes all tests (existing + new execution module tests)
 - [ ] executeWorkflow function reduced from ~800 to ~200 lines
 - [ ] Workflow execution still works (test with a simple prompt -> generate -> output flow)
 - [ ] regenerateNode still works
 - [ ] Error handling preserved
+- [ ] topologicalSort has test coverage including cycle detection
+- [ ] buildProviderHeaders has test coverage for all provider types
+- [ ] Node executors have test coverage for error cases
 </verification>
 
 <success_criteria>
@@ -160,6 +194,7 @@ Before declaring plan complete:
 - All verification checks pass
 - executeWorkflow is significantly shorter and more readable
 - Node execution logic is isolated and testable
+- Execution modules have comprehensive test coverage
 - No regression in workflow execution behavior
 </success_criteria>
 
diff --git a/.planning/phases/16-store-modularization/16-03-PLAN.md b/.planning/phases/16-store-modularization/16-03-PLAN.md
index 3ec5e805..e6df2702 100644
--- a/.planning/phases/16-store-modularization/16-03-PLAN.md
+++ b/.planning/phases/16-store-modularization/16-03-PLAN.md
@@ -184,17 +184,62 @@ Target: Main store file reduced from ~2800 to ~1500 lines.
   <done>Store composed from slices, main file significantly reduced, all functionality preserved</done>
 </task>
 
+<task type="auto">
+  <name>Task 4: Add unit tests for slices</name>
+  <files>src/store/slices/__tests__/groupSlice.test.ts, src/store/slices/__tests__/persistenceSlice.test.ts</files>
+  <action>
+Create comprehensive tests for the extracted slices:
+
+1. Create src/store/slices/__tests__/groupSlice.test.ts:
+   - Test createGroup calculates correct bounding box from node positions
+   - Test createGroup assigns unique colors to groups
+   - Test createGroup sets groupId on all provided nodes
+   - Test deleteGroup removes group and clears groupId from nodes
+   - Test addNodesToGroup updates nodes' groupId
+   - Test removeNodesFromGroup clears groupId
+   - Test toggleGroupLock toggles the locked state
+   - Test moveGroupNodes updates position of all nodes in group
+   - Use Zustand's testing pattern: create isolated store instance per test
+
+2. Create src/store/slices/__tests__/persistenceSlice.test.ts:
+   - Test setWorkflowMetadata sets all metadata fields
+   - Test setWorkflowName marks as unsaved
+   - Test markAsUnsaved sets hasUnsavedChanges to true
+   - Test saveToFile calls fetch with correct payload (mock fetch)
+   - Test saveToFile updates lastSavedAt and hasUnsavedChanges on success
+   - Test loadWorkflow hydrates nodes/edges and resets execution state
+   - Test loadWorkflow migrates legacy nanoBanana nodes
+   - Test clearWorkflow resets all state
+   - Test initializeAutoSave/cleanupAutoSave manage interval
+   - Mock fetch, localStorage, and imageStorage utilities
+
+Testing pattern for Zustand slices:
+```typescript
+const createTestStore = () => create<WorkflowStore>()((...args) => ({
+  ...createGroupSlice(...args),
+  // minimal mock of other state needed
+  nodes: [],
+  edges: [],
+}));
+```
+  </action>
+  <verify>npm test passes with new slice tests, coverage for group and persistence slices</verify>
+  <done>Unit tests added for both slices, all tests pass, slice functionality verified</done>
+</task>
+
 </tasks>
 
 <verification>
 Before declaring plan complete:
 - [ ] `npm run build` succeeds without errors
-- [ ] `npm test` passes all tests
+- [ ] `npm test` passes all tests (existing + new slice tests)
 - [ ] workflowStore.ts reduced from ~2800 to ~1500 lines
 - [ ] Group operations work (create, lock, delete)
 - [ ] Save/load workflow works
 - [ ] Auto-save works
 - [ ] All exports maintained for backward compatibility
+- [ ] groupSlice has test coverage for all operations
+- [ ] persistenceSlice has test coverage for save/load/auto-save
 </verification>
 
 <success_criteria>
@@ -203,6 +248,7 @@ Before declaring plan complete:
 - All verification checks pass
 - Store properly composed from slices
 - Each slice is focused and cohesive
+- All slices have comprehensive test coverage
 - No regression in any store functionality
 - Phase 16 complete: Store Modularization achieved
 </success_criteria>