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-# Phase 4 Step 2: Research Efficient Sync
-
-## Research Findings
-
-### Todoist Sync API
-
-**Test Results (from `research_test.go`):**
-- Full sync response: **17,502 bytes**
-- Incremental sync response: **179 bytes** (99% reduction!)
-- `sync_token` successfully enables incremental updates
-
-**Response Structure:**
-```json
-{
-  "sync_token": "...",
-  "full_sync": true/false,
-  "items": [...]  // tasks
-}
-```
-
-**Item (Task) Fields Available:**
-- `id`, `content`, `description`, `project_id`
-- `checked` (completion status)
-- `is_deleted` (for handling deletions)
-- `updated_at` (for tracking changes)
-- `due` (with `date`, `is_recurring`, `string`)
-
-**Implementation Approach:**
-1. Store `sync_token` in database (new table or cache_metadata)
-2. On refresh, call Sync API with stored token
-3. Apply incremental changes to local database:
-   - Update modified tasks
-   - Delete tasks where `is_deleted=true`
-   - Mark tasks where `checked=true` as completed
-4. If `full_sync=true` in response, clear and rebuild local state
-
-### Trello API Optimization
-
-**Test Results:**
-- Full boards response: **48,228 bytes** (all fields)
-- Limited fields (id,name): **462 bytes** (99% reduction!)
-- Single board with embedded cards/lists: **10,452 bytes**
-
-**Optimization Strategies:**
-
-1. **Field Filtering** (immediate win):
-   ```
-   /boards?fields=id,name,dateLastActivity
-   /boards/{id}/cards?fields=id,name,idList,due,url
-   ```
-
-2. **Batch Requests** (reduce API calls):
-   ```
-   /boards/{id}?cards=visible&card_fields=id,name,idList,due,url&lists=open&list_fields=id,name
-   ```
-   - Fetches board + cards + lists in single request
-
-3. **`dateLastActivity` Caching**:
-   - Store `dateLastActivity` for each board
-   - On refresh, fetch boards with minimal fields
-   - Only fetch cards for boards where `dateLastActivity` changed
-   - Note: `since` parameter exists but returns full data (not truly incremental)
-
-### Architecture Implications
-
-**Current Architecture:**
-- Cache-aside pattern: Check cache -> Miss -> Fetch API -> Store in cache
-- Problem: Full data fetch on every cache miss
-
-**Proposed Architecture:**
-- **Sync Engine Pattern**:
-  - SQLite is the source of truth
-  - Sync process updates SQLite incrementally
-  - Read operations always hit SQLite (fast)
-
-**Migration Path:**
-1. Phase A: Add field filtering to Trello (immediate benefit, no architecture change)
-2. Phase B: Implement Todoist Sync API with sync_token storage
-3. Phase C: Add dateLastActivity tracking for Trello boards
-
-## Implementation Plan
-
-### Phase A: Trello Field Optimization (Low effort, High impact)
-
-1. Update `GetBoards` to use `fields=id,name,dateLastActivity`
-2. Update `GetCards` to use `fields=id,name,idList,due,url,desc`
-3. Consider batch endpoint for single-board-with-cards
-
-### Phase B: Todoist Incremental Sync (Medium effort, High impact)
-
-1. Add `sync_tokens` table:
-   ```sql
-   CREATE TABLE sync_tokens (
-     service TEXT PRIMARY KEY,
-     token TEXT NOT NULL,
-     updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
-   );
-   ```
-
-2. Create `SyncTasks` method in `TodoistClient`:
-   - Accept previous sync_token
-   - Return new sync_token + changed items
-
-3. Create `ApplyTaskChanges` method in `Store`:
-   - Handle updates, deletions, completions
-   - Update sync_token after successful apply
-
-4. Update `fetchTasks` to use sync when token exists
-
-### Phase C: Trello Smart Refresh (Medium effort, Medium impact)
-
-1. Store `dateLastActivity` per board in database
-2. On refresh:
-   - Fetch boards with minimal fields
-   - Compare `dateLastActivity` with stored values
-   - Only fetch cards for changed boards
-
-## Recommendation
-
-**Start with Phase A** - simple field filtering provides immediate 99% reduction in Trello response size with minimal code changes.
-
-**Then Phase B** - Todoist Sync API provides the best efficiency gains (99% reduction) and is well-documented.
-
-**Phase C is optional** - Trello doesn't have true incremental sync, so the benefit is limited to skipping unchanged boards.