diff --git a/CURRENT_TASK.md b/CURRENT_TASK.md
index 980aac6d..58617b24 100644
--- a/CURRENT_TASK.md
+++ b/CURRENT_TASK.md
@@ -1,8 +1,135 @@
-# Current Task: Phase 4 - Tiny Front Optimization
+# Current Task: Phase 5 - Mid/Large Allocation Optimization
+
+**Date**: 2025-11-29
+**Goal**: Mid/Large allocation gap elimination + Config Box application
+**Strategy**: Fix allocation gap (1KB-8KB) + Compile-time config + Mid MT optimization
+**Expected Gain**: +10-26% (57.2M → 63-72M ops/s)
+
+---
+
+## Phase 5 Overview: 5-Step Approach
+
+### Step 1: Mid MT Verification (Pending)
+- **Duration**: 2 days
+- **Risk**: Low
+- **Goal**: Verify Mid MT allocator handles 1KB-8KB range efficiently
+
+**Deliverables**:
+1. Benchmark Mid MT performance for 1KB-8KB sizes
+2. Identify any gaps or inefficiencies
+3. Document current Mid MT behavior
+
+---
+
+### Step 2: Allocation Gap Elimination (Pending)
+- **Duration**: 3 days
+- **Risk**: Medium
+- **Target**: +5-15% improvement
+- **Goal**: Route 1KB-8KB allocations through Mid MT instead of mmap fallback
+
+**Critical Issue**:
+- **File**: `core/box/hak_alloc_api.inc.h:171-216`
+- **Problem**: When ACE disabled, 1KB-8KB falls through to mmap()
+- **Impact**: 1000-5000x slower than O(1) allocation
+
+**Deliverables**:
+1. Fix routing logic in `hak_alloc_api.inc.h`
+2. Route all >1KB allocations through Mid MT
+3. Benchmark improvement
+4. Completion report
+
+---
+
+### Step 3: Mid/Large Config Box (Pending)
+- **Duration**: 3 days
+- **Risk**: Low
+- **Target**: +2-4% improvement
+- **Goal**: Apply Phase 4 Config Box pattern to Mid/Large feature gates
+
+**Runtime ENV Checks to Eliminate**:
+- `HAKMEM_SMALLMID_ENABLE` (SmallMid allocator gate)
+- `HAKMEM_POOL_TLS` (Pool allocator gate)
+- `HAKMEM_BIGCACHE` (BigCache gate)
+- `HAKMEM_ACE` (ACE allocator gate)
+- 4+ other feature checks in hot path
+
+**Deliverables**:
+1. `core/box/mid_large_config_box.h` - Reuse Phase 4 pattern
+2. Replace 5-8 runtime checks with compile-time macros
+3. Build flag: `HAKMEM_MID_LARGE_PGO=1`
+4. Benchmark improvement
+5. Completion report
+
+---
+
+### Step 4: Mid Registry Pre-allocation (Pending)
+- **Duration**: 2 days
+- **Risk**: Low
+- **Target**: Eliminate lock contention in MT workloads
+- **Goal**: Pre-allocate Mid MT registry at init instead of lazy allocation
+
+**Deliverables**:
+1. Modify `hakmem_mid_mt.c` init to pre-allocate registry
+2. Remove registry lock from hot path
+3. Benchmark MT workload improvement
+4. Completion report
+
+---
+
+### Step 5: Documentation & Final Benchmark (Pending)
+- **Duration**: 2 days
+- **Risk**: Low
+- **Goal**: Document Phase 5 results, prepare for Phase 6
+
+**Deliverables**:
+1. Phase 5 completion report
+2. Full benchmark suite comparison
+3. Update CURRENT_TASK.md for Phase 6
+4. Git commit & documentation
+
+---
+
+## Phase 5 Success Criteria
+
+**bench_random_mixed (ws=256)**:
+- Phase 4 result: 57.2M ops/s (Hot/Cold Box, no PGO)
+- Phase 5.1 (Gap fix): 60-65M ops/s (+5-15%)
+- Phase 5.2 (Config Box): 62-68M ops/s (+2-4% cumulative)
+- Phase 5.3 (Registry): 63-70M ops/s (MT improvement)
+- **Phase 5 target**: **63-72M ops/s** ✓ (+10-26% cumulative)
+
+**Allocation Gap Impact**:
+- 1KB-8KB allocations: mmap() → Mid MT (1000-5000x faster)
+
+---
+
+## Current Status: Phase 5 Ready to Start
+
+**Phase 4 Complete** ✅:
+- Step 1: PGO Workflow Box (+6.25%)
+- Step 2: Hot/Cold Path Box (+7.3%)
+- Step 3: Front Config Box (+2.7-4.9%)
+- **Result**: 53.3M → 57.2M ops/s (+7.3%, without PGO)
+
+**Phase 5 Next Actions**:
+1. **Step 1**: Verify Mid MT for 1KB range (2 days)
+2. **Step 2**: Eliminate allocation gap (3 days)
+3. **Step 3**: Apply Config Box pattern (3 days)
+4. **Step 4**: Pre-allocate Mid registry (2 days)
+5. **Step 5**: Documentation & benchmarks (2 days)
+
+**Total Duration**: 12 days / 2 weeks
+
+---
+
+---
+
+# Previous: Phase 4 - Tiny Front Optimization ✅ COMPLETE
 
 **Date**: 2025-11-29
 **Goal**: Tiny allocation throughput 2x improvement (56.8M → 110M+ ops/s)
 **Strategy**: Box化 + PGO + Hot/Cold separation
+**Result**: 53.3M → 57.2M ops/s (+7.3%, without PGO)
 
 ---