diff --git a/.codecov.yml b/.codecov.yml
new file mode 100644
index 0000000..a393180
--- /dev/null
+++ b/.codecov.yml
@@ -0,0 +1,17 @@
+coverage:
+  status:
+    project:
+      default:
+        target: 95%
+        threshold: 1%
+    patch:
+      default:
+        target: 95%
+
+ignore:
+  - "ext/**/*"
+
+comment:
+  layout: "reach,diff,flags,files"
+  behavior: default
+  require_changes: true
diff --git a/docs/src/architecture/macro-internals.md b/docs/src/architecture/macro-internals.md
index d8e5f93..4c2c920 100644
--- a/docs/src/architecture/macro-internals.md
+++ b/docs/src/architecture/macro-internals.md
@@ -115,50 +115,56 @@ end
 # If only checkpoint!(pool, Int64), Float64 arrays won't be rewound!
 ```
 
-### The Solution: `_untracked_flags`
+### The Solution: Bitmask-Based Untracked Tracking
 
-Every `acquire!` call (and convenience functions) marks itself as "untracked":
+Every `acquire!` call (and convenience functions) marks itself as "untracked" with type-specific bitmask information:
 
 ```julia
 # Public API (called from user code outside macro)
 @inline function acquire!(pool, ::Type{T}, n::Int) where {T}
-    _mark_untracked!(pool)              # ← Sets flag!
+    _mark_untracked!(pool, T)           # ← Sets type-specific bitmask!
     _acquire_impl!(pool, T, n)
 end
 
 # Macro-transformed calls skip the marking
 # (because macro already knows about them)
-_acquire_impl!(pool, T, n)               # ← No flag
+_acquire_impl!(pool, T, n)               # ← No marking
 ```
 
+Each fixed-slot type maps to a bit in a `UInt16` bitmask via `_fixed_slot_bit(T)`.
+Non-fixed-slot types set a separate `_untracked_has_others` flag.
+
 ### Flow Diagram
 
 ```
-@with_pool pool begin                    State of pool._untracked_flags
-    │                                    ─────────────────────────────────
-    ├─► checkpoint!(pool, Int64)         depth=2, flag[2]=false
+@with_pool pool begin                    Bitmask state at depth 2
+    │                                    ─────────────────────────────
+    ├─► checkpoint!(pool, Int64)         masks[2]=0x0000, others[2]=false
     │
-    │   A = _acquire_impl!(...)          (macro-transformed, no flag set)
+    │   A = _acquire_impl!(...)          (macro-transformed, no mark)
     │   B = helper!(pool)
     │       └─► zeros!(pool, Float64, N)
-    │           └─► _mark_untracked!(pool)  flag[2]=TRUE ←──┐
-    │                                                        │
-    │   ... more code ...                                    │
-    │                                                        │
-    └─► rewind! check:                                       │
-        if pool._untracked_flags[2]  ─────────────────────────┘
-            rewind!(pool)            # Full rewind (safe)
-        else
+    │           └─► _mark_untracked!(pool, Float64)
+    │               masks[2] |= 0x0001 (Float64 bit) ←───┐
+    │                                                      │
+    │   ... more code ...                                  │
+    │                                                      │
+    └─► rewind! check:                                     │
+        tracked_mask = _tracked_mask_for_types(Int64)      │
+        if _can_use_typed_path(pool, tracked_mask) ────────┘
             rewind!(pool, Int64)     # Typed rewind (fast)
+        else                         # Float64 not in {Int64} → full
+            rewind!(pool)            # Full rewind (safe)
         end
 end
 ```
 
 ### Why This Works
 
-1. **Macro-tracked calls**: Transformed to `_acquire_impl!` → no flag → typed rewind
-2. **Untracked calls**: Use public API → sets flag → triggers full rewind
-3. **Result**: Always safe, with optimization when possible
+1. **Macro-tracked calls**: Transformed to `_acquire_impl!` → no bitmask mark → typed path
+2. **Untracked calls**: Use public API → sets type-specific bitmask → subset check at rewind
+3. **Subset optimization**: If untracked types are a subset of tracked types, the typed path is still safe
+4. **Result**: Always safe, with finer-grained optimization than a single boolean flag
 
 ## Nested `@with_pool` Handling
 
@@ -170,13 +176,13 @@ Each `@with_pool` maintains its own checkpoint depth:
     │
     ├─► @with_pool p2 begin              depth: 2 → 3
     │       v2 = acquire!(p2, Int64, 5)
-    │       helper!(p2)                  # sets flag[3]=true
+    │       helper!(p2)                  # marks bitmask at depth 3
     │       sum(v2)
-    │   end                              depth: 3 → 2, flag[3] checked
+    │   end                              depth: 3 → 2, bitmask checked
     │
     │   # v1 still valid here!
     sum(v1)
-end                                      depth: 2 → 1, flag[2] checked
+end                                      depth: 2 → 1, bitmask checked
 ```
 
 ### Depth Tracking Data Structures
@@ -184,13 +190,15 @@ end                                      depth: 2 → 1, flag[2] checked
 ```julia
 struct AdaptiveArrayPool
     # ... type pools ...
-    _current_depth::Int              # Current scope depth (1 = global)
-    _untracked_flags::Vector{Bool}   # Per-depth flag array
+    _current_depth::Int                     # Current scope depth (1 = global)
+    _untracked_fixed_masks::Vector{UInt16}  # Per-depth: which fixed slots untracked
+    _untracked_has_others::Vector{Bool}     # Per-depth: any non-fixed-slot untracked
 end
 
 # Initialized with sentinel:
-_current_depth = 1                   # Global scope
-_untracked_flags = [false]           # Sentinel for depth=1
+_current_depth = 1                          # Global scope
+_untracked_fixed_masks = [UInt16(0)]        # Sentinel for depth=1
+_untracked_has_others = [false]             # Sentinel for depth=1
 ```
 
 ## Performance Impact
@@ -199,9 +207,12 @@ _untracked_flags = [false]           # Sentinel for depth=1
 |----------|-------------------|----------------|
 | 1 type, no untracked | `checkpoint!(pool, T)` | **~77% faster** |
 | Multiple types, no untracked | `checkpoint!(pool, T1, T2, ...)` | **~50% faster** |
-| Any untracked acquire | `checkpoint!(pool)` | Baseline |
+| Untracked subset of tracked | `checkpoint!(pool, T...)` | **~77% faster** |
+| Unknown untracked types | `checkpoint!(pool)` | Baseline |
 
-The optimization matters most in tight loops with many iterations.
+The optimization matters most in tight loops with many iterations. The bitmask subset
+check allows the typed path even when untracked acquires occur, as long as those types
+are already covered by the macro's tracked set.
 
 ## Code Generation Summary
 
@@ -217,11 +228,11 @@ end
 function compute(data)
     pool = get_task_local_pool()
 
-    # Check if parent scope had untracked (for nested pools)
-    if pool._untracked_flags[pool._current_depth]
-        checkpoint!(pool)                    # Full checkpoint
+    # Bitmask subset check: can typed path handle any untracked acquires?
+    if _can_use_typed_path(pool, _tracked_mask_for_types(Float64))
+        checkpoint!(pool, Float64)           # Typed checkpoint (fast)
     else
-        checkpoint!(pool, Float64)           # Typed checkpoint
+        checkpoint!(pool)                    # Full checkpoint (safe)
     end
 
     try
@@ -229,11 +240,10 @@ function compute(data)
         result = helper!(pool, A)
         return result
     finally
-        # Check if untracked acquires occurred in this scope
-        if pool._untracked_flags[pool._current_depth]
-            rewind!(pool)                    # Full rewind
+        if _can_use_typed_path(pool, _tracked_mask_for_types(Float64))
+            rewind!(pool, Float64)           # Typed rewind (fast)
         else
-            rewind!(pool, Float64)           # Typed rewind
+            rewind!(pool)                    # Full rewind (safe)
         end
     end
 end
@@ -246,8 +256,10 @@ end
 | `_extract_acquire_types(expr, pool_name)` | AST walk to find types |
 | `_filter_static_types(types, local_vars)` | Filter out locally-defined types |
 | `_transform_acquire_calls(expr, pool_name)` | Replace `acquire!` → `_acquire_impl!` |
-| `_mark_untracked!(pool)` | Set untracked flag for current depth |
-| `_generate_typed_checkpoint_call(pool, types)` | Generate `checkpoint!(pool, T...)` |
+| `_mark_untracked!(pool, T)` | Set type-specific bitmask for current depth |
+| `_can_use_typed_path(pool, mask)` | Bitmask subset check for typed vs full path |
+| `_tracked_mask_for_types(T...)` | Compile-time bitmask for tracked types |
+| `_generate_typed_checkpoint_call(pool, types)` | Generate bitmask-aware checkpoint |
 
 ## See Also
 
diff --git a/ext/AdaptiveArrayPoolsCUDAExt/state.jl b/ext/AdaptiveArrayPoolsCUDAExt/state.jl
index a7ccd03..e4e6354 100644
--- a/ext/AdaptiveArrayPoolsCUDAExt/state.jl
+++ b/ext/AdaptiveArrayPoolsCUDAExt/state.jl
@@ -31,9 +31,10 @@ end
 # ==============================================================================
 
 function AdaptiveArrayPools.checkpoint!(pool::CuAdaptiveArrayPool)
-    # Increment depth and initialize untracked flag
+    # Increment depth and initialize untracked bitmask state
     pool._current_depth += 1
-    push!(pool._untracked_flags, false)
+    push!(pool._untracked_fixed_masks, UInt16(0))
+    push!(pool._untracked_has_others, false)
     depth = pool._current_depth
 
     # Fixed slots - zero allocation via @generated iteration
@@ -52,17 +53,27 @@ end
 # Type-specific checkpoint (single type)
 @inline function AdaptiveArrayPools.checkpoint!(pool::CuAdaptiveArrayPool, ::Type{T}) where {T}
     pool._current_depth += 1
-    push!(pool._untracked_flags, false)
+    push!(pool._untracked_fixed_masks, UInt16(0))
+    push!(pool._untracked_has_others, false)
     _checkpoint_typed_pool!(AdaptiveArrayPools.get_typed_pool!(pool, T), pool._current_depth)
     nothing
 end
 
 # Type-specific checkpoint (multiple types)
 @generated function AdaptiveArrayPools.checkpoint!(pool::CuAdaptiveArrayPool, types::Type...)
-    checkpoint_exprs = [:(_checkpoint_typed_pool!(AdaptiveArrayPools.get_typed_pool!(pool, types[$i]), pool._current_depth)) for i in 1:length(types)]
+    seen = Set{Any}()
+    unique_indices = Int[]
+    for i in eachindex(types)
+        if !(types[i] in seen)
+            push!(seen, types[i])
+            push!(unique_indices, i)
+        end
+    end
+    checkpoint_exprs = [:(_checkpoint_typed_pool!(AdaptiveArrayPools.get_typed_pool!(pool, types[$i]), pool._current_depth)) for i in unique_indices]
     quote
         pool._current_depth += 1
-        push!(pool._untracked_flags, false)
+        push!(pool._untracked_fixed_masks, UInt16(0))
+        push!(pool._untracked_has_others, false)
         $(checkpoint_exprs...)
         nothing
     end
@@ -91,7 +102,8 @@ function AdaptiveArrayPools.rewind!(pool::CuAdaptiveArrayPool)
         _rewind_typed_pool!(tp, cur_depth)
     end
 
-    pop!(pool._untracked_flags)
+    pop!(pool._untracked_fixed_masks)
+    pop!(pool._untracked_has_others)
     pool._current_depth -= 1
 
     return nothing
@@ -104,22 +116,32 @@ end
         return nothing
     end
     _rewind_typed_pool!(AdaptiveArrayPools.get_typed_pool!(pool, T), pool._current_depth)
-    pop!(pool._untracked_flags)
+    pop!(pool._untracked_fixed_masks)
+    pop!(pool._untracked_has_others)
     pool._current_depth -= 1
     nothing
 end
 
 # Type-specific rewind (multiple types)
 @generated function AdaptiveArrayPools.rewind!(pool::CuAdaptiveArrayPool, types::Type...)
-    rewind_exprs = [:(_rewind_typed_pool!(AdaptiveArrayPools.get_typed_pool!(pool, types[$i]), pool._current_depth)) for i in length(types):-1:1]
-    reset_exprs = [:(reset!(AdaptiveArrayPools.get_typed_pool!(pool, types[$i]))) for i in 1:length(types)]
+    seen = Set{Any}()
+    unique_indices = Int[]
+    for i in eachindex(types)
+        if !(types[i] in seen)
+            push!(seen, types[i])
+            push!(unique_indices, i)
+        end
+    end
+    rewind_exprs = [:(_rewind_typed_pool!(AdaptiveArrayPools.get_typed_pool!(pool, types[$i]), pool._current_depth)) for i in reverse(unique_indices)]
+    reset_exprs = [:(reset!(AdaptiveArrayPools.get_typed_pool!(pool, types[$i]))) for i in unique_indices]
     quote
         if pool._current_depth == 1
             $(reset_exprs...)
             return nothing
         end
         $(rewind_exprs...)
-        pop!(pool._untracked_flags)
+        pop!(pool._untracked_fixed_masks)
+        pop!(pool._untracked_has_others)
         pool._current_depth -= 1
         nothing
     end
@@ -140,10 +162,12 @@ function AdaptiveArrayPools.reset!(pool::CuAdaptiveArrayPool)
         reset!(tp)
     end
 
-    # Reset untracked detection state
+    # Reset depth and bitmask sentinel state
     pool._current_depth = 1
-    empty!(pool._untracked_flags)
-    push!(pool._untracked_flags, false)
+    empty!(pool._untracked_fixed_masks)
+    push!(pool._untracked_fixed_masks, UInt16(0))   # Sentinel: no bits set
+    empty!(pool._untracked_has_others)
+    push!(pool._untracked_has_others, false)         # Sentinel: no others
 
     return pool
 end
@@ -197,10 +221,12 @@ function Base.empty!(pool::CuAdaptiveArrayPool)
     end
     empty!(pool.others)
 
-    # Reset state
+    # Reset depth and bitmask sentinel state
     pool._current_depth = 1
-    empty!(pool._untracked_flags)
-    push!(pool._untracked_flags, false)
+    empty!(pool._untracked_fixed_masks)
+    push!(pool._untracked_fixed_masks, UInt16(0))   # Sentinel: no bits set
+    empty!(pool._untracked_has_others)
+    push!(pool._untracked_has_others, false)         # Sentinel: no others
 
     return pool
 end
diff --git a/ext/AdaptiveArrayPoolsCUDAExt/types.jl b/ext/AdaptiveArrayPoolsCUDAExt/types.jl
index 096984b..056bd18 100644
--- a/ext/AdaptiveArrayPoolsCUDAExt/types.jl
+++ b/ext/AdaptiveArrayPoolsCUDAExt/types.jl
@@ -112,7 +112,8 @@ mutable struct CuAdaptiveArrayPool <: AbstractArrayPool
 
     # State management (same as CPU)
     _current_depth::Int
-    _untracked_flags::Vector{Bool}
+    _untracked_fixed_masks::Vector{UInt16}  # Per-depth: which fixed slots had untracked acquires
+    _untracked_has_others::Vector{Bool}     # Per-depth: any non-fixed-slot untracked acquire?
 
     # Device tracking (safety)
     device_id::Int
@@ -131,7 +132,8 @@ function CuAdaptiveArrayPool()
         CuTypedPool{Bool}(),
         IdDict{DataType, Any}(),
         1,              # _current_depth (1 = global scope)
-        [false],        # _untracked_flags sentinel
+        [UInt16(0)],    # _untracked_fixed_masks: sentinel (no bits set)
+        [false],        # _untracked_has_others: sentinel (no others)
         CUDA.deviceid(dev)  # Use public API
     )
 end
diff --git a/src/acquire.jl b/src/acquire.jl
index 6113202..25038cc 100644
--- a/src/acquire.jl
+++ b/src/acquire.jl
@@ -164,17 +164,23 @@ end
 # ==============================================================================
 
 """
-    _mark_untracked!(pool::AbstractArrayPool)
+    _mark_untracked!(pool::AbstractArrayPool, ::Type{T})
 
-Mark that an untracked acquire has occurred at the current checkpoint depth.
+Mark that an untracked acquire of type `T` has occurred at the current checkpoint depth.
 Called by `acquire!` wrapper; macro-transformed calls use `_acquire_impl!` directly.
 
-With 1-indexed _current_depth (starting at 1 for global scope), this always marks
-the current scope's _untracked_flags.
+For fixed-slot types, sets the corresponding bit in `_untracked_fixed_masks`.
+For non-fixed-slot types, sets `_untracked_has_others` flag.
 """
-@inline function _mark_untracked!(pool::AbstractArrayPool)
-    # Always mark (_current_depth >= 1 guaranteed by sentinel)
-    @inbounds pool._untracked_flags[pool._current_depth] = true
+@inline function _mark_untracked!(pool::AbstractArrayPool, ::Type{T}) where {T}
+    depth = pool._current_depth
+    b = _fixed_slot_bit(T)
+    if b == UInt16(0)
+        @inbounds pool._untracked_has_others[depth] = true
+    else
+        @inbounds pool._untracked_fixed_masks[depth] |= b
+    end
+    nothing
 end
 
 # ==============================================================================
@@ -265,19 +271,19 @@ end
 See also: [`unsafe_acquire!`](@ref) for native array access.
 """
 @inline function acquire!(pool::AbstractArrayPool, ::Type{T}, n::Int) where {T}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _acquire_impl!(pool, T, n)
 end
 
 # Multi-dimensional support (zero-allocation with N-D cache)
 @inline function acquire!(pool::AbstractArrayPool, ::Type{T}, dims::Vararg{Int, N}) where {T, N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _acquire_impl!(pool, T, dims...)
 end
 
 # Tuple support: allows acquire!(pool, T, size(A)) where size(A) returns NTuple{N,Int}
 @inline function acquire!(pool::AbstractArrayPool, ::Type{T}, dims::NTuple{N, Int}) where {T, N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _acquire_impl!(pool, T, dims...)
 end
 
@@ -297,7 +303,7 @@ end
 ```
 """
 @inline function acquire!(pool::AbstractArrayPool, x::AbstractArray)
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, eltype(x))
     _acquire_impl!(pool, eltype(x), size(x))
 end
 
@@ -352,18 +358,18 @@ end
 See also: [`acquire!`](@ref) for view-based access.
 """
 @inline function unsafe_acquire!(pool::AbstractArrayPool, ::Type{T}, n::Int) where {T}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _unsafe_acquire_impl!(pool, T, n)
 end
 
 @inline function unsafe_acquire!(pool::AbstractArrayPool, ::Type{T}, dims::Vararg{Int, N}) where {T, N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _unsafe_acquire_impl!(pool, T, dims...)
 end
 
 # Tuple support
 @inline function unsafe_acquire!(pool::AbstractArrayPool, ::Type{T}, dims::NTuple{N, Int}) where {T, N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _unsafe_acquire_impl!(pool, T, dims)
 end
 
@@ -383,7 +389,7 @@ end
 ```
 """
 @inline function unsafe_acquire!(pool::AbstractArrayPool, x::AbstractArray)
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, eltype(x))
     _unsafe_acquire_impl!(pool, eltype(x), size(x))
 end
 
diff --git a/src/convenience.jl b/src/convenience.jl
index 4d5d8f7..62d919f 100644
--- a/src/convenience.jl
+++ b/src/convenience.jl
@@ -43,22 +43,22 @@ end
 See also: [`ones!`](@ref), [`similar!`](@ref), [`acquire!`](@ref)
 """
 @inline function zeros!(pool::AbstractArrayPool, ::Type{T}, dims::Vararg{Int,N}) where {T,N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _zeros_impl!(pool, T, dims...)
 end
 
 @inline function zeros!(pool::AbstractArrayPool, dims::Vararg{Int,N}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, default_eltype(pool))
     _zeros_impl!(pool, default_eltype(pool), dims...)
 end
 
 @inline function zeros!(pool::AbstractArrayPool, ::Type{T}, dims::NTuple{N,Int}) where {T,N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _zeros_impl!(pool, T, dims...)
 end
 
 @inline function zeros!(pool::AbstractArrayPool, dims::NTuple{N,Int}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, default_eltype(pool))
     _zeros_impl!(pool, default_eltype(pool), dims...)
 end
 
@@ -116,22 +116,22 @@ end
 See also: [`zeros!`](@ref), [`similar!`](@ref), [`acquire!`](@ref)
 """
 @inline function ones!(pool::AbstractArrayPool, ::Type{T}, dims::Vararg{Int,N}) where {T,N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _ones_impl!(pool, T, dims...)
 end
 
 @inline function ones!(pool::AbstractArrayPool, dims::Vararg{Int,N}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, default_eltype(pool))
     _ones_impl!(pool, default_eltype(pool), dims...)
 end
 
 @inline function ones!(pool::AbstractArrayPool, ::Type{T}, dims::NTuple{N,Int}) where {T,N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _ones_impl!(pool, T, dims...)
 end
 
 @inline function ones!(pool::AbstractArrayPool, dims::NTuple{N,Int}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, default_eltype(pool))
     _ones_impl!(pool, default_eltype(pool), dims...)
 end
 
@@ -186,11 +186,11 @@ end
 See also: [`falses!`](@ref), [`ones!`](@ref), [`acquire!`](@ref)
 """
 @inline function trues!(pool::AbstractArrayPool, dims::Vararg{Int,N}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, Bit)
     _trues_impl!(pool, dims...)
 end
 @inline function trues!(pool::AbstractArrayPool, dims::NTuple{N,Int}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, Bit)
     _trues_impl!(pool, dims...)
 end
 
@@ -226,11 +226,11 @@ end
 See also: [`trues!`](@ref), [`zeros!`](@ref), [`acquire!`](@ref)
 """
 @inline function falses!(pool::AbstractArrayPool, dims::Vararg{Int,N}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, Bit)
     _falses_impl!(pool, dims...)
 end
 @inline function falses!(pool::AbstractArrayPool, dims::NTuple{N,Int}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, Bit)
     _falses_impl!(pool, dims...)
 end
 
@@ -273,22 +273,22 @@ end
 See also: [`zeros!`](@ref), [`ones!`](@ref), [`acquire!`](@ref)
 """
 @inline function similar!(pool::AbstractArrayPool, x::AbstractArray)
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, eltype(x))
     _similar_impl!(pool, x)
 end
 
 @inline function similar!(pool::AbstractArrayPool, x::AbstractArray, ::Type{T}) where {T}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _similar_impl!(pool, x, T)
 end
 
 @inline function similar!(pool::AbstractArrayPool, x::AbstractArray, dims::Vararg{Int,N}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, eltype(x))
     _similar_impl!(pool, x, dims...)
 end
 
 @inline function similar!(pool::AbstractArrayPool, x::AbstractArray, ::Type{T}, dims::Vararg{Int,N}) where {T,N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _similar_impl!(pool, x, T, dims...)
 end
 
@@ -336,22 +336,22 @@ end
 See also: [`unsafe_ones!`](@ref), [`zeros!`](@ref), [`unsafe_acquire!`](@ref)
 """
 @inline function unsafe_zeros!(pool::AbstractArrayPool, ::Type{T}, dims::Vararg{Int,N}) where {T,N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _unsafe_zeros_impl!(pool, T, dims...)
 end
 
 @inline function unsafe_zeros!(pool::AbstractArrayPool, dims::Vararg{Int,N}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, default_eltype(pool))
     _unsafe_zeros_impl!(pool, default_eltype(pool), dims...)
 end
 
 @inline function unsafe_zeros!(pool::AbstractArrayPool, ::Type{T}, dims::NTuple{N,Int}) where {T,N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _unsafe_zeros_impl!(pool, T, dims...)
 end
 
 @inline function unsafe_zeros!(pool::AbstractArrayPool, dims::NTuple{N,Int}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, default_eltype(pool))
     _unsafe_zeros_impl!(pool, default_eltype(pool), dims...)
 end
 
@@ -403,22 +403,22 @@ end
 See also: [`unsafe_zeros!`](@ref), [`ones!`](@ref), [`unsafe_acquire!`](@ref)
 """
 @inline function unsafe_ones!(pool::AbstractArrayPool, ::Type{T}, dims::Vararg{Int,N}) where {T,N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _unsafe_ones_impl!(pool, T, dims...)
 end
 
 @inline function unsafe_ones!(pool::AbstractArrayPool, dims::Vararg{Int,N}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, default_eltype(pool))
     _unsafe_ones_impl!(pool, default_eltype(pool), dims...)
 end
 
 @inline function unsafe_ones!(pool::AbstractArrayPool, ::Type{T}, dims::NTuple{N,Int}) where {T,N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _unsafe_ones_impl!(pool, T, dims...)
 end
 
 @inline function unsafe_ones!(pool::AbstractArrayPool, dims::NTuple{N,Int}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, default_eltype(pool))
     _unsafe_ones_impl!(pool, default_eltype(pool), dims...)
 end
 
@@ -473,22 +473,22 @@ end
 See also: [`similar!`](@ref), [`unsafe_acquire!`](@ref)
 """
 @inline function unsafe_similar!(pool::AbstractArrayPool, x::AbstractArray)
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, eltype(x))
     _unsafe_similar_impl!(pool, x)
 end
 
 @inline function unsafe_similar!(pool::AbstractArrayPool, x::AbstractArray, ::Type{T}) where {T}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _unsafe_similar_impl!(pool, x, T)
 end
 
 @inline function unsafe_similar!(pool::AbstractArrayPool, x::AbstractArray, dims::Vararg{Int,N}) where {N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, eltype(x))
     _unsafe_similar_impl!(pool, x, dims...)
 end
 
 @inline function unsafe_similar!(pool::AbstractArrayPool, x::AbstractArray, ::Type{T}, dims::Vararg{Int,N}) where {T,N}
-    _mark_untracked!(pool)
+    _mark_untracked!(pool, T)
     _unsafe_similar_impl!(pool, x, T, dims...)
 end
 
diff --git a/src/macros.jl b/src/macros.jl
index d79a65e..c0011f6 100644
--- a/src/macros.jl
+++ b/src/macros.jl
@@ -340,31 +340,14 @@ function _generate_pool_code(pool_name, expr, force_enable; source::Union{LineNu
     # Transform acquire! calls to _acquire_impl! (bypasses untracked marking)
     transformed_expr = _transform_acquire_calls(expr, pool_name)
 
-    # For typed checkpoint, add _untracked_flags check for fallback to full checkpoint
-    # This protects parent scope arrays when entering nested @with_pool
     if use_typed
-        typed_checkpoint_call = _generate_typed_checkpoint_call(esc(pool_name), static_types)
-        checkpoint_call = quote
-            if @inbounds $(esc(pool_name))._untracked_flags[$(esc(pool_name))._current_depth]
-                $checkpoint!($(esc(pool_name)))  # Full checkpoint (parent had untracked)
-            else
-                $typed_checkpoint_call  # Fast typed checkpoint
-            end
-        end
+        checkpoint_call = _generate_typed_checkpoint_call(esc(pool_name), static_types)
     else
         checkpoint_call = :($checkpoint!($(esc(pool_name))))
     end
 
-    # For typed checkpoint, add _untracked_flags check for fallback to full rewind
     if use_typed
-        typed_rewind_call = _generate_typed_rewind_call(esc(pool_name), static_types)
-        rewind_call = quote
-            if @inbounds $(esc(pool_name))._untracked_flags[$(esc(pool_name))._current_depth]
-                $rewind!($(esc(pool_name)))  # Full rewind (untracked detected)
-            else
-                $typed_rewind_call  # Fast typed rewind
-            end
-        end
+        rewind_call = _generate_typed_rewind_call(esc(pool_name), static_types)
     else
         rewind_call = :($rewind!($(esc(pool_name))))
     end
@@ -449,22 +432,8 @@ function _generate_pool_code_with_backend(backend::Symbol, pool_name, expr, forc
         pool_getter = :($_get_pool_for_backend($(Val{backend}())))
 
         if use_typed
-            typed_checkpoint_call = _generate_typed_checkpoint_call(esc(pool_name), static_types)
-            checkpoint_call = quote
-                if @inbounds $(esc(pool_name))._untracked_flags[$(esc(pool_name))._current_depth]
-                    $checkpoint!($(esc(pool_name)))
-                else
-                    $typed_checkpoint_call
-                end
-            end
-            typed_rewind_call = _generate_typed_rewind_call(esc(pool_name), static_types)
-            rewind_call = quote
-                if @inbounds $(esc(pool_name))._untracked_flags[$(esc(pool_name))._current_depth]
-                    $rewind!($(esc(pool_name)))
-                else
-                    $typed_rewind_call
-                end
-            end
+            checkpoint_call = _generate_typed_checkpoint_call(esc(pool_name), static_types)
+            rewind_call = _generate_typed_rewind_call(esc(pool_name), static_types)
         else
             checkpoint_call = :($checkpoint!($(esc(pool_name))))
             rewind_call = :($rewind!($(esc(pool_name))))
@@ -509,30 +478,14 @@ function _generate_pool_code_with_backend(backend::Symbol, pool_name, expr, forc
     # Use Val{backend}() for compile-time dispatch - fully inlinable
     pool_getter = :($_get_pool_for_backend($(Val{backend}())))
 
-    # Generate checkpoint call (typed or full)
     if use_typed
-        typed_checkpoint_call = _generate_typed_checkpoint_call(esc(pool_name), static_types)
-        checkpoint_call = quote
-            if @inbounds $(esc(pool_name))._untracked_flags[$(esc(pool_name))._current_depth]
-                $checkpoint!($(esc(pool_name)))  # Full checkpoint (parent had untracked)
-            else
-                $typed_checkpoint_call  # Fast typed checkpoint
-            end
-        end
+        checkpoint_call = _generate_typed_checkpoint_call(esc(pool_name), static_types)
     else
         checkpoint_call = :($checkpoint!($(esc(pool_name))))
     end
 
-    # Generate rewind call (typed or full)
     if use_typed
-        typed_rewind_call = _generate_typed_rewind_call(esc(pool_name), static_types)
-        rewind_call = quote
-            if @inbounds $(esc(pool_name))._untracked_flags[$(esc(pool_name))._current_depth]
-                $rewind!($(esc(pool_name)))  # Full rewind (untracked detected)
-            else
-                $typed_rewind_call  # Fast typed rewind
-            end
-        end
+        rewind_call = _generate_typed_rewind_call(esc(pool_name), static_types)
     else
         rewind_call = :($rewind!($(esc(pool_name))))
     end
@@ -586,30 +539,14 @@ function _generate_function_pool_code_with_backend(backend::Symbol, pool_name, f
     # Use Val{backend}() for compile-time dispatch
     pool_getter = :($_get_pool_for_backend($(Val{backend}())))
 
-    # Generate checkpoint call (typed or full)
     if use_typed
-        typed_checkpoint_call = _generate_typed_checkpoint_call(esc(pool_name), static_types)
-        checkpoint_call = quote
-            if @inbounds $(esc(pool_name))._untracked_flags[$(esc(pool_name))._current_depth]
-                $checkpoint!($(esc(pool_name)))
-            else
-                $typed_checkpoint_call
-            end
-        end
+        checkpoint_call = _generate_typed_checkpoint_call(esc(pool_name), static_types)
     else
         checkpoint_call = :($checkpoint!($(esc(pool_name))))
     end
 
-    # Generate rewind call (typed or full)
     if use_typed
-        typed_rewind_call = _generate_typed_rewind_call(esc(pool_name), static_types)
-        rewind_call = quote
-            if @inbounds $(esc(pool_name))._untracked_flags[$(esc(pool_name))._current_depth]
-                $rewind!($(esc(pool_name)))
-            else
-                $typed_rewind_call
-            end
-        end
+        rewind_call = _generate_typed_rewind_call(esc(pool_name), static_types)
     else
         rewind_call = :($rewind!($(esc(pool_name))))
     end
@@ -655,31 +592,14 @@ function _generate_function_pool_code(pool_name, func_def, force_enable, disable
     # Transform acquire! calls to _acquire_impl! (bypasses untracked marking)
     transformed_body = _transform_acquire_calls(body, pool_name)
 
-    # For typed checkpoint, add _untracked_flags check for fallback to full checkpoint
-    # This protects parent scope arrays when entering nested @with_pool
     if use_typed
-        typed_checkpoint_call = _generate_typed_checkpoint_call(esc(pool_name), static_types)
-        checkpoint_call = quote
-            if @inbounds $(esc(pool_name))._untracked_flags[$(esc(pool_name))._current_depth]
-                $checkpoint!($(esc(pool_name)))  # Full checkpoint (parent had untracked)
-            else
-                $typed_checkpoint_call  # Fast typed checkpoint
-            end
-        end
+        checkpoint_call = _generate_typed_checkpoint_call(esc(pool_name), static_types)
     else
         checkpoint_call = :($checkpoint!($(esc(pool_name))))
     end
 
-    # For typed checkpoint, add _untracked_flags check for fallback to full rewind
     if use_typed
-        typed_rewind_call = _generate_typed_rewind_call(esc(pool_name), static_types)
-        rewind_call = quote
-            if @inbounds $(esc(pool_name))._untracked_flags[$(esc(pool_name))._current_depth]
-                $rewind!($(esc(pool_name)))  # Full rewind (untracked detected)
-            else
-                $typed_rewind_call  # Fast typed rewind
-            end
-        end
+        rewind_call = _generate_typed_rewind_call(esc(pool_name), static_types)
     else
         rewind_call = :($rewind!($(esc(pool_name))))
     end
@@ -982,30 +902,48 @@ end
 """
     _generate_typed_checkpoint_call(pool_expr, types)
 
-Generate checkpoint!(pool, T1, T2, ...) call expression.
+Generate bitmask-aware checkpoint call. When types are known at compile time,
+emits a conditional: if untracked types ⊆ tracked types → typed checkpoint,
+otherwise → full checkpoint.
 """
 function _generate_typed_checkpoint_call(pool_expr, types)
     if isempty(types)
         return :($checkpoint!($pool_expr))
     else
-        # esc types so they resolve in caller's namespace (Float64, not AdaptiveArrayPools.Float64)
         escaped_types = [esc(t) for t in types]
-        return :($checkpoint!($pool_expr, $(escaped_types...)))
+        typed_call = :($checkpoint!($pool_expr, $(escaped_types...)))
+        full_call = :($checkpoint!($pool_expr))
+        return quote
+            if $_can_use_typed_path($pool_expr, $_tracked_mask_for_types($(escaped_types...)))
+                $typed_call
+            else
+                $full_call
+            end
+        end
     end
 end
 
 """
     _generate_typed_rewind_call(pool_expr, types)
 
-Generate rewind!(pool, T1, T2, ...) call expression.
-Types are passed in original order; rewind! handles reversal internally.
+Generate bitmask-aware rewind call. When types are known at compile time,
+emits a conditional: if untracked types ⊆ tracked types → typed rewind,
+otherwise → full rewind.
 """
 function _generate_typed_rewind_call(pool_expr, types)
     if isempty(types)
         return :($rewind!($pool_expr))
     else
         escaped_types = [esc(t) for t in types]
-        return :($rewind!($pool_expr, $(escaped_types...)))
+        typed_call = :($rewind!($pool_expr, $(escaped_types...)))
+        full_call = :($rewind!($pool_expr))
+        return quote
+            if $_can_use_typed_path($pool_expr, $_tracked_mask_for_types($(escaped_types...)))
+                $typed_call
+            else
+                $full_call
+            end
+        end
     end
 end
 
diff --git a/src/state.jl b/src/state.jl
index c9b2a66..9a831d5 100644
--- a/src/state.jl
+++ b/src/state.jl
@@ -13,9 +13,10 @@ After warmup, this function has **zero allocation**.
 See also: [`rewind!`](@ref), [`@with_pool`](@ref)
 """
 function checkpoint!(pool::AdaptiveArrayPool)
-    # Increment depth and initialize untracked flag
+    # Increment depth and initialize untracked bitmask state
     pool._current_depth += 1
-    push!(pool._untracked_flags, false)
+    push!(pool._untracked_fixed_masks, UInt16(0))
+    push!(pool._untracked_has_others, false)
     depth = pool._current_depth
 
     # Fixed slots - zero allocation via @generated iteration
@@ -37,13 +38,14 @@ end
 Save state for a specific type only. Used by optimized macros that know
 which types will be used at compile time.
 
-Also updates _current_depth and _untracked_flags for untracked acquire detection.
+Also updates _current_depth and bitmask state for untracked acquire detection.
 
 ~77% faster than full checkpoint! when only one type is used.
 """
 @inline function checkpoint!(pool::AdaptiveArrayPool, ::Type{T}) where T
     pool._current_depth += 1
-    push!(pool._untracked_flags, false)
+    push!(pool._untracked_fixed_masks, UInt16(0))
+    push!(pool._untracked_has_others, false)
     _checkpoint_typed_pool!(get_typed_pool!(pool, T), pool._current_depth)
     nothing
 end
@@ -67,7 +69,8 @@ compile-time unrolling. Increments _current_depth once for all types.
     checkpoint_exprs = [:(_checkpoint_typed_pool!(get_typed_pool!(pool, types[$i]), pool._current_depth)) for i in unique_indices]
     quote
         pool._current_depth += 1
-        push!(pool._untracked_flags, false)
+        push!(pool._untracked_fixed_masks, UInt16(0))
+        push!(pool._untracked_has_others, false)
         $(checkpoint_exprs...)
         nothing
     end
@@ -118,7 +121,8 @@ function rewind!(pool::AdaptiveArrayPool)
         _rewind_typed_pool!(tp, cur_depth)
     end
 
-    pop!(pool._untracked_flags)
+    pop!(pool._untracked_fixed_masks)
+    pop!(pool._untracked_has_others)
     pool._current_depth -= 1
 
     return nothing
@@ -128,7 +132,7 @@ end
     rewind!(pool::AdaptiveArrayPool, ::Type{T})
 
 Restore state for a specific type only.
-Also updates _current_depth and _untracked_flags.
+Also updates _current_depth and bitmask state.
 """
 @inline function rewind!(pool::AdaptiveArrayPool, ::Type{T}) where T
     # Safety guard: at global scope (depth=1), delegate to reset!
@@ -137,7 +141,8 @@ Also updates _current_depth and _untracked_flags.
         return nothing
     end
     _rewind_typed_pool!(get_typed_pool!(pool, T), pool._current_depth)
-    pop!(pool._untracked_flags)
+    pop!(pool._untracked_fixed_masks)
+    pop!(pool._untracked_has_others)
     pool._current_depth -= 1
     nothing
 end
@@ -167,7 +172,8 @@ Decrements _current_depth once after all types are rewound.
             return nothing
         end
         $(rewind_exprs...)
-        pop!(pool._untracked_flags)
+        pop!(pool._untracked_fixed_masks)
+        pop!(pool._untracked_has_others)
         pool._current_depth -= 1
         nothing
     end
@@ -284,8 +290,10 @@ function Base.empty!(pool::AdaptiveArrayPool)
 
     # Reset untracked detection state (1-based sentinel pattern)
     pool._current_depth = 1                   # 1 = global scope (sentinel)
-    empty!(pool._untracked_flags)
-    push!(pool._untracked_flags, false)       # Sentinel: global scope starts with false
+    empty!(pool._untracked_fixed_masks)
+    push!(pool._untracked_fixed_masks, UInt16(0))   # Sentinel: no bits set
+    empty!(pool._untracked_has_others)
+    push!(pool._untracked_has_others, false)         # Sentinel: no others
 
     return pool
 end
@@ -318,7 +326,7 @@ Reset pool state without clearing allocated storage.
 This function:
 - Resets all `n_active` counters to 0
 - Restores all checkpoint stacks to sentinel state
-- Resets `_current_depth` and `_untracked_flags`
+- Resets `_current_depth` and untracked bitmask state
 
 Unlike `empty!`, this **preserves** all allocated vectors, views, and N-D arrays
 for reuse, avoiding reallocation costs.
@@ -363,8 +371,10 @@ function reset!(pool::AdaptiveArrayPool)
 
     # Reset untracked detection state (1-based sentinel pattern)
     pool._current_depth = 1                   # 1 = global scope (sentinel)
-    empty!(pool._untracked_flags)
-    push!(pool._untracked_flags, false)       # Sentinel: global scope starts with false
+    empty!(pool._untracked_fixed_masks)
+    push!(pool._untracked_fixed_masks, UInt16(0))   # Sentinel: no bits set
+    empty!(pool._untracked_has_others)
+    push!(pool._untracked_has_others, false)         # Sentinel: no others
 
     return pool
 end
@@ -398,6 +408,46 @@ See also: [`reset!(::AdaptiveArrayPool)`](@ref), [`rewind!`](@ref)
     end
 end
 
+# ==============================================================================
+# Bitmask Helpers for Typed Path Decisions
+# ==============================================================================
+
+"""
+    _tracked_mask_for_types(types::Type...) -> UInt16
+
+Compute compile-time bitmask for the types tracked by a typed checkpoint/rewind.
+Uses `@generated` for zero-overhead constant folding.
+
+Returns `UInt16(0)` when called with no arguments.
+Non-fixed-slot types contribute `UInt16(0)` (their bit is 0).
+"""
+@generated function _tracked_mask_for_types(types::Type...)
+    mask = UInt16(0)
+    for i in 1:length(types)
+        T = types[i].parameters[1]
+        mask |= _fixed_slot_bit(T)
+    end
+    return :(UInt16($mask))
+end
+
+"""
+    _can_use_typed_path(pool::AbstractArrayPool, tracked_mask::UInt16) -> Bool
+
+Check if the typed (fast) checkpoint/rewind path is safe to use.
+
+Returns `true` when all untracked acquires at the current depth are a subset
+of the tracked types (bitmask subset check) AND no non-fixed-slot types were used.
+
+The subset check: `(untracked_mask & ~tracked_mask) == 0` means every bit set
+in `untracked_mask` is also set in `tracked_mask`.
+"""
+@inline function _can_use_typed_path(pool::AbstractArrayPool, tracked_mask::UInt16)
+    depth = pool._current_depth
+    untracked_mask = @inbounds pool._untracked_fixed_masks[depth]
+    has_others = @inbounds pool._untracked_has_others[depth]
+    return (untracked_mask & ~tracked_mask) == UInt16(0) && !has_others
+end
+
 # ==============================================================================
 # DisabledPool State Management (no-ops)
 # ==============================================================================
diff --git a/src/types.jl b/src/types.jl
index 0b6f62f..e6adb4c 100644
--- a/src/types.jl
+++ b/src/types.jl
@@ -366,6 +366,22 @@ Tests verify synchronization automatically.
 """
 const FIXED_SLOT_FIELDS = (:float64, :float32, :int64, :int32, :complexf64, :complexf32, :bool, :bits)
 
+# ==============================================================================
+# Fixed-Slot Bit Mapping (for typed untracked tracking)
+# ==============================================================================
+# Maps each fixed-slot type to a unique bit in a UInt16 bitmask.
+# Bit ordering matches FIXED_SLOT_FIELDS. Non-fixed types return UInt16(0).
+
+@inline _fixed_slot_bit(::Type{Float64})    = UInt16(1) << 0
+@inline _fixed_slot_bit(::Type{Float32})    = UInt16(1) << 1
+@inline _fixed_slot_bit(::Type{Int64})      = UInt16(1) << 2
+@inline _fixed_slot_bit(::Type{Int32})      = UInt16(1) << 3
+@inline _fixed_slot_bit(::Type{ComplexF64}) = UInt16(1) << 4
+@inline _fixed_slot_bit(::Type{ComplexF32}) = UInt16(1) << 5
+@inline _fixed_slot_bit(::Type{Bool})       = UInt16(1) << 6
+@inline _fixed_slot_bit(::Type{Bit})        = UInt16(1) << 7
+@inline _fixed_slot_bit(::Type)             = UInt16(0)  # non-fixed-slot → triggers has_others
+
 # ==============================================================================
 # AdaptiveArrayPool
 # ==============================================================================
@@ -392,7 +408,8 @@ mutable struct AdaptiveArrayPool <: AbstractArrayPool
 
     # Untracked acquire detection (1-based sentinel pattern)
     _current_depth::Int             # Current scope depth (1 = global scope)
-    _untracked_flags::Vector{Bool}  # Per-depth flag: true if untracked acquire occurred
+    _untracked_fixed_masks::Vector{UInt16}  # Per-depth: which fixed slots had untracked acquires
+    _untracked_has_others::Vector{Bool}     # Per-depth: any non-fixed-slot untracked acquire?
 end
 
 function AdaptiveArrayPool()
@@ -407,7 +424,8 @@ function AdaptiveArrayPool()
         BitTypedPool(),
         IdDict{DataType, Any}(),
         1,              # _current_depth: 1 = global scope (sentinel)
-        [false]         # _untracked_flags: sentinel for global scope
+        [UInt16(0)],    # _untracked_fixed_masks: sentinel (no bits set)
+        [false]         # _untracked_has_others: sentinel (no others)
     )
 end
 
diff --git a/test/test_macro_expansion.jl b/test/test_macro_expansion.jl
index 76638d6..67c0dae 100644
--- a/test/test_macro_expansion.jl
+++ b/test/test_macro_expansion.jl
@@ -708,3 +708,76 @@ end
     end
 
 end # Source Location Preservation
+
+# ==============================================================================
+# Phase 3: Bitmask-aware checkpoint/rewind in macro expansion
+# ==============================================================================
+
+@testset "Bitmask-aware typed path in expansion" begin
+    @testset "@with_pool typed expansion uses _can_use_typed_path" begin
+        expr = @macroexpand @with_pool pool begin
+            v = acquire!(pool, Float64, 10)
+            sum(v)
+        end
+
+        expr_str = string(expr)
+
+        # Should use _can_use_typed_path instead of _untracked_flags
+        @test occursin("_can_use_typed_path", expr_str)
+        @test !occursin("_untracked_flags", expr_str)
+
+        # Should use _tracked_mask_for_types
+        @test occursin("_tracked_mask_for_types", expr_str)
+    end
+
+    @testset "@with_pool full path (dynamic types) has no bitmask" begin
+        expr = @macroexpand @with_pool pool begin
+            local_arr = rand(10)
+            v = acquire!(pool, local_arr)
+            sum(v)
+        end
+
+        expr_str = string(expr)
+
+        # Full path (dynamic type) → no _can_use_typed_path
+        @test !occursin("_can_use_typed_path", expr_str)
+        @test !occursin("_tracked_mask_for_types", expr_str)
+    end
+
+    @testset "@maybe_with_pool typed expansion uses _can_use_typed_path" begin
+        expr = @macroexpand @maybe_with_pool pool begin
+            v = acquire!(pool, Float64, 10)
+            sum(v)
+        end
+
+        expr_str = string(expr)
+
+        @test occursin("_can_use_typed_path", expr_str)
+        @test !occursin("_untracked_flags", expr_str)
+    end
+
+    @testset "@with_pool :cpu backend uses _can_use_typed_path" begin
+        expr = @macroexpand @with_pool :cpu pool begin
+            v = acquire!(pool, Float64, 10)
+            sum(v)
+        end
+
+        expr_str = string(expr)
+
+        @test occursin("_can_use_typed_path", expr_str)
+        @test !occursin("_untracked_flags", expr_str)
+    end
+
+    @testset "@with_pool function def uses _can_use_typed_path" begin
+        expr = @macroexpand @with_pool pool function test_fn(x)
+            v = acquire!(pool, Float64, length(x))
+            v .= x
+            sum(v)
+        end
+
+        expr_str = string(expr)
+
+        @test occursin("_can_use_typed_path", expr_str)
+        @test !occursin("_untracked_flags", expr_str)
+    end
+end
diff --git a/test/test_state.jl b/test/test_state.jl
index 9881f41..52d25bd 100644
--- a/test/test_state.jl
+++ b/test/test_state.jl
@@ -306,7 +306,8 @@
             reset!(pool)
 
             @test pool._current_depth == 1
-            @test pool._untracked_flags == [false]
+            @test pool._untracked_fixed_masks == [UInt16(0)]
+            @test pool._untracked_has_others == [false]
             @test pool.float64._checkpoint_n_active == [0]  # Sentinel only
             @test pool.float64._checkpoint_depths == [0]    # Sentinel only
         end
@@ -493,7 +494,7 @@
             rewind!(pool)
             @test pool.float64.n_active == 0
             @test pool._current_depth == 1
-            @test pool._untracked_flags == [false]
+            @test pool._untracked_fixed_masks == [UInt16(0)]
         end
 
         @testset "rewind! after reset!" begin
@@ -734,25 +735,24 @@
 
     @testset "Parent scope protection via full checkpoint" begin
         # Test: Parent scope arrays are protected by automatic full checkpoint
-        # when entering @with_pool with _untracked_flags[_current_depth] = true
+        # when untracked acquire occurred at current depth
 
         # Helper function that acquires Int64 (called from inside @with_pool)
         # Since it's defined outside the macro, acquire! won't be transformed
         function untracked_helper(p)
-            acquire!(p, Int64, 5)  # This will mark _untracked_flags = true
+            acquire!(p, Int64, 5)  # This will mark untracked bitmask
         end
 
         pool = get_task_local_pool()
         empty!(pool)  # Start fresh
 
-        # Acquire Int64 array OUTSIDE @with_pool - marks global _untracked_flags
+        # Acquire Int64 array OUTSIDE @with_pool - marks untracked bitmask
         v_parent = acquire!(pool, Int64, 10)
         v_parent .= 42  # Initialize
         @test pool.int64.n_active == 1
-        @test pool._untracked_flags[1] == true  # Global scope marked
+        @test pool._untracked_fixed_masks[1] == AdaptiveArrayPools._fixed_slot_bit(Int64)
 
-        # Enter @with_pool - should do FULL checkpoint (because _untracked_flags[1] = true)
-        # This protects the parent's Int64 arrays
+        # Enter @with_pool - full checkpoint protects parent's Int64 arrays
         @with_pool pool begin
             v_float = acquire!(pool, Float64, 100)  # Tracked
             untracked_helper(pool)                   # Untracked Int64 acquire!
@@ -776,7 +776,7 @@
         v_parent = acquire!(pool, Int32, 7)
         v_parent .= Int32(123)
         @test pool.int32.n_active == 1
-        @test pool._untracked_flags[1] == true
+        @test pool._untracked_fixed_masks[1] == AdaptiveArrayPools._fixed_slot_bit(Int32)
 
         # Helper for Int32
         function int32_helper(p)
@@ -827,7 +827,7 @@
         pool = AdaptiveArrayPool()
 
         # No global untracked acquire
-        @test pool._untracked_flags[1] == false
+        @test pool._untracked_fixed_masks[1] == UInt16(0)
 
         # Checkpoint/rewind with typed - should work normally
         checkpoint!(pool)
@@ -1002,7 +1002,7 @@
         @test pool.bool.n_active == 0
         @test pool.complexf64.n_active == 0
         @test pool._current_depth == 1
-        @test pool._untracked_flags == [false]
+        @test pool._untracked_fixed_masks == [UInt16(0)]
 
         empty!(pool)
     end
@@ -1318,4 +1318,613 @@
         empty!(pool)
     end
 
+    # ==========================================================================
+    # Typed-Aware Untracked Tracking — Phase 1: Bitmask Metadata Lifecycle
+    # ==========================================================================
+
+    @testset "Bitmask metadata: constructor sentinel" begin
+        pool = AdaptiveArrayPool()
+
+        # New fields exist
+        @test hasfield(AdaptiveArrayPool, :_untracked_fixed_masks)
+        @test hasfield(AdaptiveArrayPool, :_untracked_has_others)
+
+        # Sentinel values at depth=1 (global scope)
+        @test pool._untracked_fixed_masks == [UInt16(0)]
+        @test pool._untracked_has_others == [false]
+        @test length(pool._untracked_fixed_masks) == 1
+        @test length(pool._untracked_has_others) == 1
+    end
+
+    @testset "Bitmask metadata: checkpoint! pushes sentinels" begin
+        pool = AdaptiveArrayPool()
+
+        # Full checkpoint
+        checkpoint!(pool)
+        @test length(pool._untracked_fixed_masks) == 2
+        @test length(pool._untracked_has_others) == 2
+        @test pool._untracked_fixed_masks[2] == UInt16(0)
+        @test pool._untracked_has_others[2] == false
+
+        # Another checkpoint
+        checkpoint!(pool)
+        @test length(pool._untracked_fixed_masks) == 3
+        @test length(pool._untracked_has_others) == 3
+
+        # Cleanup
+        rewind!(pool)
+        rewind!(pool)
+    end
+
+    @testset "Bitmask metadata: typed checkpoint! pushes sentinels" begin
+        pool = AdaptiveArrayPool()
+
+        # Single-type checkpoint
+        checkpoint!(pool, Float64)
+        @test length(pool._untracked_fixed_masks) == 2
+        @test length(pool._untracked_has_others) == 2
+        @test pool._untracked_fixed_masks[2] == UInt16(0)
+        @test pool._untracked_has_others[2] == false
+        rewind!(pool, Float64)
+
+        # Multi-type checkpoint
+        checkpoint!(pool, Float64, Float32)
+        @test length(pool._untracked_fixed_masks) == 2
+        @test length(pool._untracked_has_others) == 2
+        @test pool._untracked_fixed_masks[2] == UInt16(0)
+        @test pool._untracked_has_others[2] == false
+        rewind!(pool, Float64, Float32)
+    end
+
+    @testset "Bitmask metadata: rewind! pops" begin
+        pool = AdaptiveArrayPool()
+
+        checkpoint!(pool)
+        @test length(pool._untracked_fixed_masks) == 2
+        @test length(pool._untracked_has_others) == 2
+
+        rewind!(pool)
+        @test length(pool._untracked_fixed_masks) == 1
+        @test length(pool._untracked_has_others) == 1
+        # Sentinel preserved
+        @test pool._untracked_fixed_masks[1] == UInt16(0)
+        @test pool._untracked_has_others[1] == false
+    end
+
+    @testset "Bitmask metadata: typed rewind! pops" begin
+        pool = AdaptiveArrayPool()
+
+        checkpoint!(pool, Float64)
+        @test length(pool._untracked_fixed_masks) == 2
+
+        rewind!(pool, Float64)
+        @test length(pool._untracked_fixed_masks) == 1
+        @test length(pool._untracked_has_others) == 1
+
+        # Multi-type
+        checkpoint!(pool, Float64, Int64)
+        @test length(pool._untracked_fixed_masks) == 2
+
+        rewind!(pool, Float64, Int64)
+        @test length(pool._untracked_fixed_masks) == 1
+    end
+
+    @testset "Bitmask metadata: reset! restores sentinel" begin
+        pool = AdaptiveArrayPool()
+
+        # Build up state
+        checkpoint!(pool)
+        checkpoint!(pool)
+        @test length(pool._untracked_fixed_masks) == 3
+
+        reset!(pool)
+        @test pool._untracked_fixed_masks == [UInt16(0)]
+        @test pool._untracked_has_others == [false]
+        @test pool._current_depth == 1
+    end
+
+    @testset "Bitmask metadata: empty! restores sentinel" begin
+        pool = AdaptiveArrayPool()
+
+        # Build up state
+        checkpoint!(pool)
+        acquire!(pool, Float64, 10)
+        checkpoint!(pool)
+        @test length(pool._untracked_fixed_masks) == 3
+
+        empty!(pool)
+        @test pool._untracked_fixed_masks == [UInt16(0)]
+        @test pool._untracked_has_others == [false]
+        @test pool._current_depth == 1
+    end
+
+    @testset "Bitmask metadata: multiple checkpoint/rewind cycles" begin
+        pool = AdaptiveArrayPool()
+
+        for _ in 1:5
+            checkpoint!(pool)
+            rewind!(pool)
+        end
+
+        # No stack leaks — should be back to sentinel only
+        @test length(pool._untracked_fixed_masks) == 1
+        @test length(pool._untracked_has_others) == 1
+        @test pool._current_depth == 1
+    end
+
+    @testset "Bitmask metadata: nested depth tracking" begin
+        pool = AdaptiveArrayPool()
+
+        # Depth 2
+        checkpoint!(pool)
+        @test length(pool._untracked_fixed_masks) == 2
+
+        # Depth 3
+        checkpoint!(pool)
+        @test length(pool._untracked_fixed_masks) == 3
+
+        # Depth 4
+        checkpoint!(pool)
+        @test length(pool._untracked_fixed_masks) == 4
+
+        # Pop back
+        rewind!(pool)
+        @test length(pool._untracked_fixed_masks) == 3
+
+        rewind!(pool)
+        @test length(pool._untracked_fixed_masks) == 2
+
+        rewind!(pool)
+        @test length(pool._untracked_fixed_masks) == 1
+    end
+
+    # ==========================================================================
+    # Typed-Aware Untracked Tracking — Phase 2: _fixed_slot_bit + typed marking
+    # ==========================================================================
+
+    @testset "_fixed_slot_bit dispatch" begin
+        using AdaptiveArrayPools: _fixed_slot_bit, Bit
+
+        # Each fixed slot returns a unique nonzero bit
+        @test _fixed_slot_bit(Float64)    == UInt16(1) << 0
+        @test _fixed_slot_bit(Float32)    == UInt16(1) << 1
+        @test _fixed_slot_bit(Int64)      == UInt16(1) << 2
+        @test _fixed_slot_bit(Int32)      == UInt16(1) << 3
+        @test _fixed_slot_bit(ComplexF64) == UInt16(1) << 4
+        @test _fixed_slot_bit(ComplexF32) == UInt16(1) << 5
+        @test _fixed_slot_bit(Bool)       == UInt16(1) << 6
+        @test _fixed_slot_bit(Bit)        == UInt16(1) << 7
+
+        # Non-fixed-slot types return 0
+        @test _fixed_slot_bit(UInt8)    == UInt16(0)
+        @test _fixed_slot_bit(UInt16)   == UInt16(0)
+        @test _fixed_slot_bit(Float16)  == UInt16(0)
+        @test _fixed_slot_bit(String)   == UInt16(0)
+
+        # All 8 bits are unique (no collisions)
+        bits = [_fixed_slot_bit(T) for T in (Float64, Float32, Int64, Int32, ComplexF64, ComplexF32, Bool, Bit)]
+        @test length(unique(bits)) == 8
+        @test all(b -> b != UInt16(0), bits)
+    end
+
+    @testset "Typed _mark_untracked!: fixed-slot types set mask bits" begin
+        using AdaptiveArrayPools: _mark_untracked!, _fixed_slot_bit
+
+        pool = AdaptiveArrayPool()
+        checkpoint!(pool)  # depth=2
+
+        # Mark Float64 untracked
+        _mark_untracked!(pool, Float64)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Float64)
+        @test pool._untracked_has_others[2] == false
+
+        # Mark Float32 additionally — bits accumulate
+        _mark_untracked!(pool, Float32)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Float64) | _fixed_slot_bit(Float32)
+        @test pool._untracked_has_others[2] == false
+
+        # Mark Float64 again — idempotent
+        _mark_untracked!(pool, Float64)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Float64) | _fixed_slot_bit(Float32)
+
+        rewind!(pool)
+    end
+
+    @testset "Typed _mark_untracked!: non-fixed-slot types set has_others" begin
+        using AdaptiveArrayPools: _mark_untracked!, _fixed_slot_bit
+
+        pool = AdaptiveArrayPool()
+        checkpoint!(pool)  # depth=2
+
+        # Mark UInt8 (not a fixed slot)
+        _mark_untracked!(pool, UInt8)
+        @test pool._untracked_fixed_masks[2] == UInt16(0)  # mask unchanged
+        @test pool._untracked_has_others[2] == true
+
+        rewind!(pool)
+    end
+
+    @testset "Typed _mark_untracked!: mixed fixed + others" begin
+        using AdaptiveArrayPools: _mark_untracked!, _fixed_slot_bit
+
+        pool = AdaptiveArrayPool()
+        checkpoint!(pool)
+
+        _mark_untracked!(pool, Float64)
+        _mark_untracked!(pool, UInt8)  # others
+        _mark_untracked!(pool, Int64)
+
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Float64) | _fixed_slot_bit(Int64)
+        @test pool._untracked_has_others[2] == true
+
+        rewind!(pool)
+    end
+
+    @testset "Typed _mark_untracked!: nested depth isolation" begin
+        using AdaptiveArrayPools: _mark_untracked!, _fixed_slot_bit
+
+        pool = AdaptiveArrayPool()
+
+        # Depth 2
+        checkpoint!(pool)
+        _mark_untracked!(pool, Float64)
+
+        # Depth 3
+        checkpoint!(pool)
+        _mark_untracked!(pool, Int32)
+
+        # Depth 3 has only Int32
+        @test pool._untracked_fixed_masks[3] == _fixed_slot_bit(Int32)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Float64)
+
+        # Depth 1 (sentinel) untouched
+        @test pool._untracked_fixed_masks[1] == UInt16(0)
+
+        rewind!(pool)
+        rewind!(pool)
+    end
+
+    @testset "Public acquire! sets typed bitmask" begin
+        using AdaptiveArrayPools: _fixed_slot_bit
+
+        pool = AdaptiveArrayPool()
+        checkpoint!(pool)  # depth=2
+
+        # acquire! outside @with_pool calls _mark_untracked!(pool, T)
+        acquire!(pool, Float64, 10)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Float64)
+
+        acquire!(pool, Int64, 5)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Float64) | _fixed_slot_bit(Int64)
+
+        rewind!(pool)
+    end
+
+    @testset "Public unsafe_acquire! sets typed bitmask" begin
+        using AdaptiveArrayPools: _fixed_slot_bit
+
+        pool = AdaptiveArrayPool()
+        checkpoint!(pool)
+
+        unsafe_acquire!(pool, Float32, 10)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Float32)
+
+        rewind!(pool)
+    end
+
+    @testset "Convenience functions set typed bitmask" begin
+        using AdaptiveArrayPools: _fixed_slot_bit, Bit
+
+        pool = AdaptiveArrayPool()
+
+        # zeros! with explicit type
+        checkpoint!(pool)
+        zeros!(pool, Float64, 10)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Float64)
+        rewind!(pool)
+
+        # zeros! without type → default_eltype → Float64
+        checkpoint!(pool)
+        zeros!(pool, 10)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Float64)
+        rewind!(pool)
+
+        # ones! with type
+        checkpoint!(pool)
+        ones!(pool, Int32, 10)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Int32)
+        rewind!(pool)
+
+        # trues! → Bit type
+        checkpoint!(pool)
+        trues!(pool, 10)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Bit)
+        rewind!(pool)
+
+        # falses! → Bit type
+        checkpoint!(pool)
+        falses!(pool, 10)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Bit)
+        rewind!(pool)
+
+        # similar! with template array
+        checkpoint!(pool)
+        similar!(pool, rand(Float32, 5))
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Float32)
+        rewind!(pool)
+    end
+
+    @testset "Convenience functions: non-fixed-slot type sets has_others" begin
+        pool = AdaptiveArrayPool()
+
+        checkpoint!(pool)
+        zeros!(pool, UInt8, 10)
+        @test pool._untracked_has_others[2] == true
+        @test pool._untracked_fixed_masks[2] == UInt16(0)
+        rewind!(pool)
+    end
+
+    # ==================================================================
+    # Phase 3: _tracked_mask_for_types and _can_use_typed_path
+    # ==================================================================
+
+    @testset "_tracked_mask_for_types: computes correct mask" begin
+        using AdaptiveArrayPools: _tracked_mask_for_types
+
+        # No args → zero mask
+        @test _tracked_mask_for_types() == UInt16(0)
+
+        # Single types
+        @test _tracked_mask_for_types(Float64) == _fixed_slot_bit(Float64)
+        @test _tracked_mask_for_types(Float32) == _fixed_slot_bit(Float32)
+        @test _tracked_mask_for_types(Bit) == _fixed_slot_bit(Bit)
+
+        # Multiple types → OR combination
+        @test _tracked_mask_for_types(Float64, Float32) == (_fixed_slot_bit(Float64) | _fixed_slot_bit(Float32))
+        @test _tracked_mask_for_types(Float64, Int32) == (_fixed_slot_bit(Float64) | _fixed_slot_bit(Int32))
+
+        # All 8 fixed types
+        all_mask = _tracked_mask_for_types(Float64, Float32, Int64, Int32, ComplexF64, ComplexF32, Bool, Bit)
+        expected = UInt16(0)
+        for T in (Float64, Float32, Int64, Int32, ComplexF64, ComplexF32, Bool, Bit)
+            expected |= _fixed_slot_bit(T)
+        end
+        @test all_mask == expected
+
+        # Non-fixed-slot types contribute UInt16(0)
+        @test _tracked_mask_for_types(UInt8) == UInt16(0)
+        @test _tracked_mask_for_types(Float64, UInt8) == _fixed_slot_bit(Float64)
+
+        # Duplicate types → idempotent
+        @test _tracked_mask_for_types(Float64, Float64) == _fixed_slot_bit(Float64)
+    end
+
+    @testset "_can_use_typed_path: truth table" begin
+        using AdaptiveArrayPools: _can_use_typed_path, _tracked_mask_for_types
+
+        pool = AdaptiveArrayPool()
+        checkpoint!(pool)  # depth = 2
+
+        # Case 1: no untracked at all → typed path OK
+        @test _can_use_typed_path(pool, _tracked_mask_for_types(Float64)) == true
+
+        # Case 2: untracked Float64, tracked includes Float64 → subset → OK
+        pool._untracked_fixed_masks[2] = _fixed_slot_bit(Float64)
+        @test _can_use_typed_path(pool, _tracked_mask_for_types(Float64)) == true
+
+        # Case 3: untracked Float64, tracked is Float32 only → NOT subset → full
+        @test _can_use_typed_path(pool, _tracked_mask_for_types(Float32)) == false
+
+        # Case 4: untracked Float64|Float32, tracked Float64 only → partial → full
+        pool._untracked_fixed_masks[2] = _fixed_slot_bit(Float64) | _fixed_slot_bit(Float32)
+        @test _can_use_typed_path(pool, _tracked_mask_for_types(Float64)) == false
+
+        # Case 5: untracked Float64|Float32, tracked Float64|Float32 → exact match → OK
+        @test _can_use_typed_path(pool, _tracked_mask_for_types(Float64, Float32)) == true
+
+        # Case 6: untracked Float64 + has_others → always full
+        pool._untracked_fixed_masks[2] = _fixed_slot_bit(Float64)
+        pool._untracked_has_others[2] = true
+        @test _can_use_typed_path(pool, _tracked_mask_for_types(Float64)) == false
+
+        # Case 7: no fixed untracked but has_others → always full
+        pool._untracked_fixed_masks[2] = UInt16(0)
+        pool._untracked_has_others[2] = true
+        @test _can_use_typed_path(pool, _tracked_mask_for_types(Float64)) == false
+
+        rewind!(pool)
+    end
+
+    # ==================================================================
+    # Phase 3: End-to-end runtime scenarios
+    # ==================================================================
+
+    @testset "Scenario A: typed rewind preserved when untracked ⊆ tracked" begin
+        # Helper function acquires Float64 OUTSIDE @with_pool → untracked
+        function _scenario_a_helper!(pool)
+            acquire!(pool, Float64, 5)
+        end
+
+        pool = AdaptiveArrayPool()
+        checkpoint!(pool)  # outer scope
+
+        # Macro scope uses Float64 → tracked set = {Float64}
+        @with_pool pool begin
+            v = acquire!(pool, Float64, 10)
+            v .= 1.0
+            _scenario_a_helper!(pool)  # untracked Float64 → subset of tracked
+        end
+
+        # Pool state should be correct after rewind
+        @test pool.float64.n_active == 0  # all rewound (outer had no acquires)
+        rewind!(pool)
+    end
+
+    @testset "Scenario B: full rewind when untracked NOT ⊆ tracked" begin
+        # Helper acquires Float32 while @with_pool only tracks Float64
+        function _scenario_b_helper!(pool)
+            acquire!(pool, Float32, 5)
+        end
+
+        pool = AdaptiveArrayPool()
+        checkpoint!(pool)
+
+        @with_pool pool begin
+            v = acquire!(pool, Float64, 10)
+            v .= 1.0
+            _scenario_b_helper!(pool)  # untracked Float32 → NOT subset of {Float64}
+        end
+
+        # Both types should be correctly rewound
+        @test pool.float64.n_active == 0
+        @test pool.float32.n_active == 0
+        rewind!(pool)
+    end
+
+    @testset "Scenario C: others triggers full rewind" begin
+        # Helper acquires UInt8 (non-fixed-slot) → has_others = true
+        function _scenario_c_helper!(pool)
+            acquire!(pool, UInt8, 5)
+        end
+
+        pool = AdaptiveArrayPool()
+        checkpoint!(pool)
+
+        @with_pool pool begin
+            v = acquire!(pool, Float64, 10)
+            v .= 1.0
+            _scenario_c_helper!(pool)  # untracked UInt8 → has_others → full
+        end
+
+        @test pool.float64.n_active == 0
+        @test get_typed_pool!(pool, UInt8).n_active == 0
+        rewind!(pool)
+    end
+
+    @testset "Scenario D: nested checkpoint with parent untracked ⊆ child tracked" begin
+        # Helper acquires Float64 OUTSIDE @with_pool → untracked
+        function _scenario_d_helper!(pool)
+            acquire!(pool, Float64, 3)
+        end
+
+        # Inner scope as function to avoid Julia scoping conflict:
+        # nested `local pool` in same try-block soft scope causes UndefVarError
+        function _scenario_d_inner!()
+            @with_pool pool begin
+                v2 = acquire!(pool, Float64, 5)
+                v2 .= 2.0
+                @test pool.float64.n_active >= 3  # v1 + helper + v2
+            end
+        end
+
+        pool = AdaptiveArrayPool()
+
+        # Parent @with_pool uses Float64
+        @with_pool pool begin
+            v1 = acquire!(pool, Float64, 10)
+            v1 .= 1.0
+            _scenario_d_helper!(pool)  # marks untracked Float64 in parent scope
+
+            # Nested @with_pool also uses Float64 → can use typed checkpoint
+            _scenario_d_inner!()
+
+            # After nested rewind, v1 and helper should still be active
+            @test v1[1] == 1.0  # v1 still valid
+        end
+
+        @test pool.float64.n_active == 0
+    end
+
+    @testset "Scenario E: nested checkpoint with parent untracked has_others → full" begin
+        # Helper acquires UInt8 (non-fixed-slot) → has_others = true
+        function _scenario_e_helper!(pool)
+            acquire!(pool, UInt8, 3)
+        end
+
+        # Inner scope as function (avoids Julia scoping conflict with same-name local)
+        function _scenario_e_inner!()
+            @with_pool pool begin
+                v2 = acquire!(pool, Float64, 5)
+                v2 .= 2.0
+            end
+        end
+
+        pool = AdaptiveArrayPool()
+
+        @with_pool pool begin
+            v1 = acquire!(pool, Float64, 10)
+            v1 .= 1.0
+            _scenario_e_helper!(pool)  # marks has_others in parent scope
+
+            # Nested @with_pool: parent had has_others → must use full checkpoint
+            _scenario_e_inner!()
+
+            @test v1[1] == 1.0  # v1 still valid after nested rewind
+        end
+
+        @test pool.float64.n_active == 0
+        @test get_typed_pool!(pool, UInt8).n_active == 0
+    end
+
+    # ==================================================================
+    # Phase 4: Legacy _untracked_flags removal verification
+    # ==================================================================
+    @testset "Phase 4: _untracked_flags field removed from AdaptiveArrayPool" begin
+        # The legacy boolean _untracked_flags field has been replaced by
+        # bitmask-based tracking (_untracked_fixed_masks + _untracked_has_others).
+        # Verify it no longer exists as a struct field.
+        @test !(:_untracked_flags in fieldnames(AdaptiveArrayPool))
+
+        # Verify the bitmask fields ARE present (they are the replacement)
+        @test :_untracked_fixed_masks in fieldnames(AdaptiveArrayPool)
+        @test :_untracked_has_others in fieldnames(AdaptiveArrayPool)
+    end
+
+    @testset "Phase 4: bitmask stacks have no stale state after lifecycle ops" begin
+        pool = AdaptiveArrayPool()
+
+        # Initial sentinel state
+        @test pool._untracked_fixed_masks == [UInt16(0)]
+        @test pool._untracked_has_others == [false]
+
+        # Checkpoint → mark → rewind cycle leaves no stale bits
+        checkpoint!(pool)
+        _mark_untracked!(pool, Float64)
+        @test pool._untracked_fixed_masks[2] == _fixed_slot_bit(Float64)
+        rewind!(pool)
+        @test pool._untracked_fixed_masks == [UInt16(0)]  # back to sentinel
+        @test pool._untracked_has_others == [false]
+
+        # Nested checkpoint → mark others → rewind cleans up
+        checkpoint!(pool)  # depth 2
+        checkpoint!(pool)  # depth 3
+        _mark_untracked!(pool, UInt8)  # others at depth 3
+        @test pool._untracked_has_others[3] == true
+        rewind!(pool)  # back to depth 2
+        @test length(pool._untracked_has_others) == 2
+        @test pool._untracked_has_others[2] == false  # depth 2 clean
+        rewind!(pool)  # back to depth 1
+        @test pool._untracked_fixed_masks == [UInt16(0)]
+        @test pool._untracked_has_others == [false]
+
+        # reset! restores sentinel state after deep nesting
+        checkpoint!(pool)
+        checkpoint!(pool)
+        _mark_untracked!(pool, Float32)
+        _mark_untracked!(pool, Int64)
+        reset!(pool)
+        @test pool._untracked_fixed_masks == [UInt16(0)]
+        @test pool._untracked_has_others == [false]
+        @test pool._current_depth == 1
+
+        # empty! also restores sentinel state
+        checkpoint!(pool)
+        _mark_untracked!(pool, ComplexF64)
+        _mark_untracked!(pool, UInt16)
+        empty!(pool)
+        @test pool._untracked_fixed_masks == [UInt16(0)]
+        @test pool._untracked_has_others == [false]
+        @test pool._current_depth == 1
+    end
+
 end # State Management
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