diff --git a/src/MathOptLazy.jl b/src/MathOptLazy.jl
index ca03e1e..8964b4d 100644
--- a/src/MathOptLazy.jl
+++ b/src/MathOptLazy.jl
@@ -631,6 +631,30 @@ MOI.get(model::Optimizer, ::MOI.SimplexIterations) = model.simplex_iterations
 
 MOI.get(model::Optimizer, ::MOI.SolveTimeSec) = model.solve_time_sec
 
+### MathOptLazy.NumberOfConstraintsActive
+
+"""
+    NumberOfConstraintsActive{F,LazyScalarSet{S}}() where {
+        F<:MOI.AbstractScalarFunction,
+        S<:MOI.AbstractScalarSet,
+    }
+
+Return a count of the number of constraints that are active in the subproblem.
+"""
+struct NumberOfConstraintsActive{
+    F<:MOI.AbstractScalarFunction,
+    S<:LazyScalarSet{<:MOI.AbstractScalarSet},
+} <: MOI.AbstractModelAttribute end
+
+MOI.is_set_by_optimize(::NumberOfConstraintsActive) = true
+
+function MOI.get(
+    model::Optimizer,
+    ::NumberOfConstraintsActive{F,LazyScalarSet{S}},
+) where {F<:MOI.AbstractScalarFunction,S<:MOI.AbstractScalarSet}
+    return count(==(_kLAZY_CONSTRAINT_ACTIVE), _data(model, F, S).status)
+end
+
 ### MOI.optimize!
 
 function _try_get(model::Optimizer, attr::MOI.AbstractModelAttribute, default)
diff --git a/test/runtests.jl b/test/runtests.jl
index 6e45b69..5dc46ff 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -43,6 +43,9 @@ function test_jump_cached_knapsack()
     @test termination_status(model) == OPTIMAL
     @test primal_status(model) == FEASIBLE_POINT
     @test all(<=(1 + 1e-6), value(x))
+    F = MOI.ScalarAffineFunction{Float64}
+    S = MathOptLazy.LazyScalarSet{MOI.LessThan{Float64}}
+    @test get_attribute(model, MathOptLazy.NumberOfConstraintsActive{F,S}()) > 0
     return
 end
 
@@ -54,10 +57,15 @@ function test_jump_direct_knapsack()
     @constraint(model, [i in 1:N], x[i] <= 1, MathOptLazy.Lazy())
     @constraint(model, sum(abs(cos(i)) * x[i] for i in 1:N) <= 0.1 * N)
     @objective(model, Max, sum(abs(sin(i)) * x[i] for i in 1:N))
+    F = MOI.ScalarAffineFunction{Float64}
+    S = MathOptLazy.LazyScalarSet{MOI.LessThan{Float64}}
+    attr = MathOptLazy.NumberOfConstraintsActive{F,S}()
+    @test MOI.get(backend(model), attr) == 0
     optimize!(model)
     @test termination_status(model) == OPTIMAL
     @test primal_status(model) == FEASIBLE_POINT
     @test all(<=(1 + 1e-6), value(x))
+    @test get_attribute(model, attr) > 0
     return
 end