diff --git a/src/DimensionalAnalysis.jl b/src/DimensionalAnalysis.jl
index 511492f8d..d0e975fbf 100644
--- a/src/DimensionalAnalysis.jl
+++ b/src/DimensionalAnalysis.jl
@@ -194,14 +194,11 @@ function violates_dimensional_constraints(
 end
 function violates_dimensional_constraints(
     tree::AbstractExpressionNode{T},
-    X_units::Union{AbstractVector{<:Quantity},Nothing},
+    X_units::AbstractVector{<:Quantity},
     y_units::Union{Quantity,Nothing},
     x::AbstractVector{T},
     options::Options,
 ) where {T}
-    if X_units === nothing && y_units === nothing
-        return false
-    end
     allow_wildcards = !(options.dimensionless_constants_only)
     dimensional_output = violates_dimensional_constraints_dispatch(
         tree, X_units, x, options.operators, allow_wildcards
@@ -217,5 +214,15 @@ function violates_dimensional_constraints(
     end
     return violates
 end
+function violates_dimensional_constraints(
+    ::AbstractExpressionNode{T}, ::Nothing, ::Quantity, ::AbstractVector{T}, ::Options
+) where {T}
+    return error("This should never happen. Please submit a bug report.")
+end
+function violates_dimensional_constraints(
+    ::AbstractExpressionNode{T}, ::Nothing, ::Nothing, ::AbstractVector{T}, ::Options
+) where {T}
+    return false
+end
 
 end
diff --git a/src/InterfaceDynamicExpressions.jl b/src/InterfaceDynamicExpressions.jl
index 2645f1b66..ebd88c7f2 100644
--- a/src/InterfaceDynamicExpressions.jl
+++ b/src/InterfaceDynamicExpressions.jl
@@ -129,10 +129,10 @@ end
 Evaluate an expression tree in a way that can be auto-differentiated.
 """
 function differentiable_eval_tree_array(
-    tree::AbstractExpressionNode, X::AbstractArray, options::Options; kws...
+    tree::AbstractExpressionNode, X::AbstractArray, options::Options
 )
     A = expected_array_type(X)
-    return differentiable_eval_tree_array(tree, X, options.operators; kws...)::Tuple{A,Bool}
+    return differentiable_eval_tree_array(tree, X, options.operators)::Tuple{A,Bool}
 end
 
 const WILDCARD_UNIT_STRING = "[?]"
diff --git a/src/LossFunctions.jl b/src/LossFunctions.jl
index 7048ced25..ac5493234 100644
--- a/src/LossFunctions.jl
+++ b/src/LossFunctions.jl
@@ -13,7 +13,7 @@ using ..DimensionalAnalysisModule: violates_dimensional_constraints
 function _loss(
     x::AbstractArray{T}, y::AbstractArray{T}, loss::LT
 ) where {T<:DATA_TYPE,LT<:Union{Function,SupervisedLoss}}
-    if LT <: SupervisedLoss
+    if loss isa SupervisedLoss
         return LossFunctions.mean(loss, x, y)
     else
         l(i) = loss(x[i], y[i])
@@ -24,7 +24,7 @@ end
 function _weighted_loss(
     x::AbstractArray{T}, y::AbstractArray{T}, w::AbstractArray{T}, loss::LT
 ) where {T<:DATA_TYPE,LT<:Union{Function,SupervisedLoss}}
-    if LT <: SupervisedLoss
+    if loss isa SupervisedLoss
         return LossFunctions.sum(loss, x, y, w; normalize=true)
     else
         l(i) = loss(x[i], y[i], w[i])
diff --git a/src/Operators.jl b/src/Operators.jl
index 6933d8d40..cc756f0d6 100644
--- a/src/Operators.jl
+++ b/src/Operators.jl
@@ -96,7 +96,7 @@ function logical_and(x, y)
 end
 
 # Deprecated operations:
-@deprecate pow safe_pow
-@deprecate pow_abs safe_pow
+@deprecate pow(x, y) safe_pow(x, y)
+@deprecate pow_abs(x, y) safe_pow(x, y)
 
 end
diff --git a/src/Options.jl b/src/Options.jl
index 8f2c4ba16..4131a40bf 100644
--- a/src/Options.jl
+++ b/src/Options.jl
@@ -549,10 +549,16 @@ $(OPTION_DESCRIPTIONS)
         )
     end
 
+    is_testing = parse(Bool, get(ENV, "SYMBOLIC_REGRESSION_IS_TESTING", "false"))
+
     if output_file === nothing
         # "%Y-%m-%d_%H%M%S.%f"
         date_time_str = Dates.format(Dates.now(), "yyyy-mm-dd_HHMMSS.sss")
         output_file = "hall_of_fame_" * date_time_str * ".csv"
+        if is_testing
+            tmpdir = mktempdir()
+            output_file = joinpath(tmpdir, output_file)
+        end
     end
 
     nuna = length(unary_operators)
diff --git a/src/SearchUtils.jl b/src/SearchUtils.jl
index 4079a977d..fb2da4876 100644
--- a/src/SearchUtils.jl
+++ b/src/SearchUtils.jl
@@ -37,6 +37,9 @@ Base.@kwdef struct RuntimeOptions{PARALLELISM,DIM_OUT,RETURN_STATE}
     runtests::Bool
     verbosity::Int64
     progress::Bool
+    parallelism::Val{PARALLELISM}
+    dim_out::Val{DIM_OUT}
+    return_state::Val{RETURN_STATE}
 end
 @unstable @inline function Base.getproperty(
     roptions::RuntimeOptions{P,D,R}, name::Symbol
@@ -108,8 +111,8 @@ macro sr_spawner(expr, kws...)
     @assert all(ex -> ex.head == :(=), kws)
     @assert any(ex -> ex.args[1] == :parallelism, kws)
     @assert any(ex -> ex.args[1] == :worker_idx, kws)
-    parallelism = kws[findfirst(ex -> ex.args[1] == :parallelism, kws)].args[2]
-    worker_idx = kws[findfirst(ex -> ex.args[1] == :worker_idx, kws)].args[2]
+    parallelism = kws[findfirst(ex -> ex.args[1] == :parallelism, kws)::Int].args[2]
+    worker_idx = kws[findfirst(ex -> ex.args[1] == :worker_idx, kws)::Int].args[2]
     return quote
         if $(parallelism) == :serial
             $(expr)
diff --git a/src/SymbolicRegression.jl b/src/SymbolicRegression.jl
index 0ea9e5599..01632afb3 100644
--- a/src/SymbolicRegression.jl
+++ b/src/SymbolicRegression.jl
@@ -570,7 +570,7 @@ function equation_search(
     # Underscores here mean that we have mutated the variable
     return _equation_search(
         datasets,
-        RuntimeOptions{concurrency,dim_out,_return_state}(;
+        RuntimeOptions(;
             niterations=niterations,
             total_cycles=options.populations * niterations,
             numprocs=_numprocs,
@@ -580,6 +580,9 @@ function equation_search(
             runtests=runtests,
             verbosity=_verbosity,
             progress=_progress,
+            parallelism=Val(concurrency),
+            dim_out=Val(dim_out),
+            return_state=Val(_return_state),
         ),
         options,
         saved_state,
diff --git a/src/Utils.jl b/src/Utils.jl
index 08cccceba..9636252e6 100644
--- a/src/Utils.jl
+++ b/src/Utils.jl
@@ -6,7 +6,7 @@ using MacroTools: splitdef, combinedef
 
 const pseudo_time = Ref(0)
 
-function get_birth_order(; deterministic=false)::Int
+function get_birth_order(; deterministic::Bool=false)::Int
     """deterministic gives a birth time with perfect resolution, but is not thread safe."""
     if deterministic
         global pseudo_time
diff --git a/src/deprecates.jl b/src/deprecates.jl
index 374756d0a..54816a408 100644
--- a/src/deprecates.jl
+++ b/src/deprecates.jl
@@ -5,11 +5,11 @@ import .MutationFunctionsModule: gen_random_tree, gen_random_tree_fixed_size
 
 @deprecate(
     gen_random_tree(length::Int, options::Options, nfeatures::Int, t::Type),
-    gen_random_tree(length, options, nfeatures, t, Node)
+    gen_random_tree(length, options, nfeatures, t)
 )
 @deprecate(
     gen_random_tree_fixed_size(node_count::Int, options::Options, nfeatures::Int, t::Type),
-    gen_random_tree_fixed_size(node_count, options, nfeatures, t, Node)
+    gen_random_tree_fixed_size(node_count, options, nfeatures, t)
 )
 
 @deprecate(
diff --git a/src/precompile.jl b/src/precompile.jl
index 313db452f..87442695d 100644
--- a/src/precompile.jl
+++ b/src/precompile.jl
@@ -85,7 +85,7 @@ function do_precompilation(::Val{mode}) where {mode}
                     return_state=false,
                     verbosity=0,
                 )
-                nout == 1 && calculate_pareto_frontier(hof)
+                nout == 1 && calculate_pareto_frontier(hof::HallOfFame)
             end
         end
     end
diff --git a/test/Project.toml b/test/Project.toml
index 422b648aa..b6f96c2d9 100644
--- a/test/Project.toml
+++ b/test/Project.toml
@@ -6,6 +6,7 @@ Distributed = "8ba89e20-285c-5b6f-9357-94700520ee1b"
 DynamicExpressions = "a40a106e-89c9-4ca8-8020-a735e8728b6b"
 DynamicQuantities = "06fc5a27-2a28-4c7c-a15d-362465fb6821"
 ForwardDiff = "f6369f11-7733-5829-9624-2563aa707210"
+JET = "c3a54625-cd67-489e-a8e7-0a5a0ff4e31b"
 JSON3 = "0f8b85d8-7281-11e9-16c2-39a750bddbf1"
 LineSearches = "d3d80556-e9d4-5f37-9878-2ab0fcc64255"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
diff --git a/test/full.jl b/test/full.jl
deleted file mode 100644
index f5ecb3e2a..000000000
--- a/test/full.jl
+++ /dev/null
@@ -1,53 +0,0 @@
-using SymbolicRegression
-using Test
-using SafeTestsets
-using SymbolicRegression: string_tree
-using Random
-
-@safetestset "Test mixed settings." begin
-    include("test_mixed.jl")
-end
-
-@safetestset "Testing fast-cycle and custom variable names, with mutations" begin
-    include("test_fast_cycle.jl")
-end
-
-@safetestset "Testing whether we can stop based on clock time." begin
-    include("test_stop_on_clock.jl")
-end
-
-@safetestset "Running README example." begin
-    include("../example.jl")
-end
-
-@safetestset "Testing whether the recorder works." begin
-    include("test_recorder.jl")
-end
-
-@safetestset "Testing whether deterministic mode works." begin
-    include("test_deterministic.jl")
-end
-
-@safetestset "Testing whether early stop criteria works." begin
-    include("test_early_stop.jl")
-end
-
-@safetestset "Test MLJ integration" begin
-    include("test_mlj.jl")
-end
-
-@testset "Testing whether we can move operators to workers." begin
-    include("test_custom_operators_multiprocessing.jl")
-end
-
-@testset "Test whether the precompilation script works." begin
-    include("test_precompilation.jl")
-end
-
-@testset "Test whether custom objectives work." begin
-    include("test_custom_objectives.jl")
-end
-
-@testset "Test abstract numbers" begin
-    include("test_abstract_numbers.jl")
-end
diff --git a/test/runtests.jl b/test/runtests.jl
index edfa4eb86..dd2209874 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -1,26 +1,168 @@
-using SafeTestsets
-using Test
+using TestItems: @testitem
+using TestItemRunner: @run_package_tests
 
 ENV["SYMBOLIC_REGRESSION_TEST"] = "true"
-TEST_SUITE = get(ENV, "SYMBOLIC_REGRESSION_TEST_SUITE", "all")
+tags_to_run = let t = get(ENV, "SYMBOLIC_REGRESSION_TEST_SUITE", "unit,integration")
+    t = split(t, ",")
+    t = map(Symbol, t)
+    t
+end
+
+@eval @run_package_tests filter = ti -> !isdisjoint(ti.tags, $tags_to_run)
+
+@testitem "JET tests" tags = [:integration, :jet] begin
+    test_jet_file = joinpath((@__DIR__), "test_jet.jl")
+    run(`$(Base.julia_cmd()) --startup-file=no $test_jet_file`)
+end
+
+@testitem "Test custom operators and additional types" tags = [:unit] begin
+    include("test_operators.jl")
+end
+
+@testitem "Test tree construction and scoring" tags = [:unit] begin
+    include("test_tree_construction.jl")
+end
+
+@testitem "Test SymbolicUtils interface" tags = [:unit] begin
+    include("test_symbolic_utils.jl")
+end
 
-if TEST_SUITE in ("all", "integration")
-    @safetestset "Aqua tests" begin
-        include("test_aqua.jl")
-    end
+@testitem "Test constraints interface" tags = [:unit] begin
+    include("test_constraints.jl")
 end
 
-# Trigger extensions:
-using LoopVectorization, Bumper, Zygote
+@testitem "Test custom losses" tags = [:unit] begin
+    include("test_losses.jl")
+end
+
+@testitem "Test derivatives" tags = [:unit] begin
+    include("test_derivatives.jl")
+end
+
+@testitem "Test simplification" tags = [:unit] begin
+    include("test_simplification.jl")
+end
+
+@testitem "Test printing" tags = [:unit] begin
+    include("test_print.jl")
+end
+
+@testitem "Test validity of expression evaluation" tags = [:unit] begin
+    include("test_evaluation.jl")
+end
+
+@testitem "Test turbo mode with NaN" tags = [:unit] begin
+    include("test_turbo_nan.jl")
+end
+
+@testitem "Test validity of integer expression evaluation" tags = [:unit] begin
+    include("test_integer_evaluation.jl")
+end
+
+@testitem "Test tournament selection" tags = [:unit] begin
+    include("test_prob_pick_first.jl")
+end
+
+@testitem "Test crossover mutation" tags = [:unit] begin
+    include("test_crossover.jl")
+end
+
+@testitem "Test NaN detection in evaluator" tags = [:unit] begin
+    include("test_nan_detection.jl")
+end
+
+@testitem "Test nested constraint checking" tags = [:unit] begin
+    include("test_nested_constraints.jl")
+end
+
+@testitem "Test complexity evaluation" tags = [:unit] begin
+    include("test_complexity.jl")
+end
+
+@testitem "Test options" tags = [:unit] begin
+    include("test_options.jl")
+end
+
+@testitem "Test hash of tree" tags = [:unit] begin
+    include("test_hash.jl")
+end
+
+@testitem "Test migration" tags = [:unit] begin
+    include("test_migration.jl")
+end
+
+@testitem "Test deprecated options" tags = [:unit] begin
+    include("test_deprecation.jl")
+end
+
+@testitem "Test optimization mutation" tags = [:unit] begin
+    include("test_optimizer_mutation.jl")
+end
+
+@testitem "Test RunningSearchStatistics" tags = [:unit] begin
+    include("test_search_statistics.jl")
+end
+
+@testitem "Test utils" tags = [:unit] begin
+    include("test_utils.jl")
+end
+
+@testitem "Test units" tags = [:integration] begin
+    include("test_units.jl")
+end
+
+@testitem "Dataset" tags = [:unit] begin
+    include("test_dataset.jl")
+end
+
+@testitem "Test mixed settings." tags = [:integration] begin
+    include("test_mixed.jl")
+end
+
+@testitem "Testing fast-cycle and custom variable names" tags = [:integration] begin
+    include("test_fast_cycle.jl")
+end
+
+@testitem "Testing whether we can stop based on clock time." tags = [:integration] begin
+    include("test_stop_on_clock.jl")
+end
+
+@testitem "Running README example." tags = [:integration] begin
+    include("../example.jl")
+end
+
+@testitem "Testing whether the recorder works." tags = [:integration] begin
+    include("test_recorder.jl")
+end
+
+@testitem "Testing whether deterministic mode works." tags = [:integration] begin
+    include("test_deterministic.jl")
+end
+
+@testitem "Testing whether early stop criteria works." tags = [:integration] begin
+    include("test_early_stop.jl")
+end
+
+@testitem "Test MLJ integration" tags = [:integration] begin
+    include("test_mlj.jl")
+end
+
+@testitem "Testing whether we can move operators to workers." tags = [:integration] begin
+    include("test_custom_operators_multiprocessing.jl")
+end
+
+@testitem "Test whether the precompilation script works." tags = [:integration] begin
+    include("test_precompilation.jl")
+end
+
+@testitem "Test whether custom objectives work." tags = [:integration] begin
+    include("test_custom_objectives.jl")
+end
 
-if TEST_SUITE in ("all", "unit")
-    @safetestset "Unit tests" begin
-        include("unittest.jl")
-    end
+@testitem "Test abstract numbers" tags = [:integration] begin
+    include("test_abstract_numbers.jl")
 end
 
-if TEST_SUITE in ("all", "integration")
-    @eval @testset "End to end test" begin
-        include("full.jl")
-    end
+@testitem "Aqua tests" tags = [:integration, :aqua] begin
+    include("test_aqua.jl")
 end
diff --git a/test/test_abstract_numbers.jl b/test/test_abstract_numbers.jl
index 60155be2c..d1ca4412b 100644
--- a/test/test_abstract_numbers.jl
+++ b/test/test_abstract_numbers.jl
@@ -1,6 +1,6 @@
 using SymbolicRegression
-using Test
 using Random
+include("test_params.jl")
 
 get_base_type(::Type{<:Complex{BT}}) where {BT} = BT
 
diff --git a/test/test_constraints.jl b/test/test_constraints.jl
index a7afb3ad9..0a6306923 100644
--- a/test/test_constraints.jl
+++ b/test/test_constraints.jl
@@ -1,7 +1,6 @@
 using DynamicExpressions: count_depth
 using SymbolicRegression
 using SymbolicRegression: check_constraints
-using Test
 include("test_params.jl")
 
 _inv(x) = 1 / x
diff --git a/test/test_crossover.jl b/test/test_crossover.jl
index 941ef8cfc..c1da48bb0 100644
--- a/test/test_crossover.jl
+++ b/test/test_crossover.jl
@@ -1,6 +1,5 @@
 println("Testing crossover function.")
 using SymbolicRegression
-using Test
 using SymbolicRegression: crossover_trees
 include("test_params.jl")
 
diff --git a/test/test_custom_objectives.jl b/test/test_custom_objectives.jl
index 6b9cb1ff4..3696f1750 100644
--- a/test/test_custom_objectives.jl
+++ b/test/test_custom_objectives.jl
@@ -1,17 +1,26 @@
 using SymbolicRegression
-using Test
 include("test_params.jl")
 
-function my_custom_loss(
-    tree::AbstractExpressionNode{T}, dataset::Dataset{T}, options::Options
-) where {T}
-    # We multiply the tree by 2.0:
-    tree = Node(1, tree, Node(T; val=2.0))
-    out, completed = eval_tree_array(tree, dataset.X, options)
-    if !completed
-        return T(Inf)
+def = quote
+    function my_custom_loss(
+        tree::$(AbstractExpressionNode){T}, dataset::$(Dataset){T}, options::$(Options)
+    ) where {T}
+        # We multiply the tree by 2.0:
+        tree = $(Node)(1, tree, $(Node)(T; val=2.0))
+        out, completed = $(eval_tree_array)(tree, dataset.X, options)
+        if !completed
+            return T(Inf)
+        end
+        return sum(abs, out .- dataset.y)
     end
-    return sum(abs, out .- dataset.y)
+end
+
+# TODO: Required for workers as they assume the function is defined in the Main module
+if (@__MODULE__) != Core.Main
+    Core.eval(Core.Main, def)
+    eval(:(using Main: my_custom_loss))
+else
+    eval(def)
 end
 
 options = Options(;
diff --git a/test/test_custom_operators.jl b/test/test_custom_operators.jl
index 46be30c3e..50e72cd16 100644
--- a/test/test_custom_operators.jl
+++ b/test/test_custom_operators.jl
@@ -1,5 +1,4 @@
 using SymbolicRegression
-using Test
 using Random
 
 # Test that we can work with custom operators:
diff --git a/test/test_custom_operators_multiprocessing.jl b/test/test_custom_operators_multiprocessing.jl
index 164278bae..2fca2298e 100644
--- a/test/test_custom_operators_multiprocessing.jl
+++ b/test/test_custom_operators_multiprocessing.jl
@@ -1,18 +1,26 @@
 using SymbolicRegression
-using Test
 
-_plus(x, y) = x + y
-_mult(x, y) = x * y
-_div(x, y) = x / y
-_min(x, y) = x - y
-_cos(x) = cos(x)
-_exp(x) = exp(x)
+defs = quote
+    _plus(x, y) = x + y
+    _mult(x, y) = x * y
+    _div(x, y) = x / y
+    _min(x, y) = x - y
+    _cos(x) = cos(x)
+    _exp(x) = exp(x)
+    early_stop(loss, c) = ((loss <= 1e-10) && (c <= 10))
+    my_loss(x, y, w) = abs(x - y)^2 * w
+end
+
+# This is needed as workers are initialized in `Core.Main`!
+if (@__MODULE__) != Core.Main
+    Core.eval(Core.Main, defs)
+    eval(:(using Main: _plus, _mult, _div, _min, _cos, _exp, early_stop, my_loss))
+else
+    eval(defs)
+end
 
 X = randn(Float32, 5, 100)
 y = _mult.(2, _cos.(X[4, :])) + _mult.(X[1, :], X[1, :])
-early_stop(loss, c) = ((loss <= 1e-10) && (c <= 10))
-
-my_loss(x, y, w) = abs(x - y)^2 * w
 
 options = SymbolicRegression.Options(;
     binary_operators=(_plus, _mult, _div, _min),
diff --git a/test/test_dataset.jl b/test/test_dataset.jl
index fbff37af3..9fdbcfd74 100644
--- a/test/test_dataset.jl
+++ b/test/test_dataset.jl
@@ -1,5 +1,4 @@
 using SymbolicRegression
-using Test
 using DispatchDoctor: allow_unstable
 
 @testset "Dataset construction" begin
diff --git a/test/test_deprecation.jl b/test/test_deprecation.jl
index 2553b6b8c..dda563741 100644
--- a/test/test_deprecation.jl
+++ b/test/test_deprecation.jl
@@ -1,5 +1,4 @@
 using SymbolicRegression
-using Test
 
 # Deprecated kwargs should still work:
 options = Options(;
diff --git a/test/test_derivatives.jl b/test/test_derivatives.jl
index ce444ee17..ee2d4ddfd 100644
--- a/test/test_derivatives.jl
+++ b/test/test_derivatives.jl
@@ -1,4 +1,3 @@
-using Test
 using SymbolicRegression
 using SymbolicRegression: eval_diff_tree_array, eval_grad_tree_array
 using Random
diff --git a/test/test_deterministic.jl b/test/test_deterministic.jl
index 8fa1d32c8..d541eea70 100644
--- a/test/test_deterministic.jl
+++ b/test/test_deterministic.jl
@@ -1,5 +1,4 @@
 using SymbolicRegression
-using Test
 using Random
 
 macro maybe_inferred(ex)
diff --git a/test/test_early_stop.jl b/test/test_early_stop.jl
index a50c47522..3ba36e555 100644
--- a/test/test_early_stop.jl
+++ b/test/test_early_stop.jl
@@ -1,5 +1,4 @@
 using SymbolicRegression
-using Test
 
 X = randn(Float32, 5, 100)
 y = 2 * cos.(X[4, :]) + X[1, :] .^ 2
diff --git a/test/test_evaluation.jl b/test/test_evaluation.jl
index 21625c73c..a3c05dd0c 100644
--- a/test/test_evaluation.jl
+++ b/test/test_evaluation.jl
@@ -1,6 +1,5 @@
 using SymbolicRegression
 using Random
-using Test
 include("test_params.jl")
 
 # Test simple evaluations:
diff --git a/test/test_fast_cycle.jl b/test/test_fast_cycle.jl
index 1d186faf6..e7d2de845 100644
--- a/test/test_fast_cycle.jl
+++ b/test/test_fast_cycle.jl
@@ -1,5 +1,4 @@
 using SymbolicRegression
-using Test
 using Random
 include("test_params.jl")
 
diff --git a/test/test_graph_nodes.jl b/test/test_graph_nodes.jl
index be0d38a20..639546dc0 100644
--- a/test/test_graph_nodes.jl
+++ b/test/test_graph_nodes.jl
@@ -1,5 +1,4 @@
 using SymbolicRegression
-using Test
 
 options = Options(; binary_operators=[+, -, *, /], unary_operators=[cos, sin], maxsize=30)
 
diff --git a/test/test_hash.jl b/test/test_hash.jl
index e8a84c7ca..5013efea9 100644
--- a/test/test_hash.jl
+++ b/test/test_hash.jl
@@ -1,5 +1,4 @@
 using SymbolicRegression
-using Test
 
 options = Options(; binary_operators=(+, *, ^, /, greater), unary_operators=(cos,))
 @extend_operators options
diff --git a/test/test_integer_evaluation.jl b/test/test_integer_evaluation.jl
index 4abc3ec68..4b84f8be7 100644
--- a/test/test_integer_evaluation.jl
+++ b/test/test_integer_evaluation.jl
@@ -1,6 +1,5 @@
 using SymbolicRegression
 using Random
-using Test
 include("test_params.jl")
 
 # Test evaluation on integer-based trees.
diff --git a/test/test_jet.jl b/test/test_jet.jl
new file mode 100644
index 000000000..28476febb
--- /dev/null
+++ b/test/test_jet.jl
@@ -0,0 +1,27 @@
+if VERSION < v"1.10.0"
+    exit(0)
+end
+
+dir = mktempdir()
+
+@info "Starting test_jet.jl" dir
+
+using Pkg
+@info "Creating environment..."
+Pkg.activate(dir; io=devnull)
+Pkg.develop(; path=dirname(@__DIR__), io=devnull)
+Pkg.add(["JET", "Preferences"]; io=devnull)
+@info "Done!"
+
+using Preferences
+cd(dir)
+Preferences.set_preferences!("SymbolicRegression", "instability_check" => "disable")
+
+using SymbolicRegression
+using JET
+
+@info "Running tests..."
+JET.test_package(SymbolicRegression; target_defined_modules=true)
+@info "Done!"
+
+@info "test_jet.jl finished"
diff --git a/test/test_losses.jl b/test/test_losses.jl
index e5e450772..9277ed23c 100644
--- a/test/test_losses.jl
+++ b/test/test_losses.jl
@@ -1,7 +1,6 @@
 using SymbolicRegression
 using SymbolicRegression: eval_loss
 using Random
-using Test
 include("test_params.jl")
 
 _loss = SymbolicRegression.LossFunctionsModule._loss
diff --git a/test/test_migration.jl b/test/test_migration.jl
index fd1d546a7..13c69753e 100644
--- a/test/test_migration.jl
+++ b/test/test_migration.jl
@@ -1,5 +1,4 @@
 using SymbolicRegression
-using Test
 using Random: seed!
 
 seed!(0)
diff --git a/test/test_mixed.jl b/test/test_mixed.jl
index bff1693d9..1da2b1a3e 100644
--- a/test/test_mixed.jl
+++ b/test/test_mixed.jl
@@ -1,7 +1,6 @@
 using SymbolicRegression
-using Test
 using SymbolicRegression: string_tree
-using Random
+using Random, Bumper, LoopVectorization
 include("test_params.jl")
 
 for i in 0:5
diff --git a/test/test_mlj.jl b/test/test_mlj.jl
index 2b0cdc7de..ca0416209 100644
--- a/test/test_mlj.jl
+++ b/test/test_mlj.jl
@@ -3,7 +3,6 @@ using SymbolicRegression:
     Node, SRRegressor, MultitargetSRRegressor, node_to_symbolic, symbolic_to_node
 using MLJTestInterface: MLJTestInterface as MTI
 using MLJBase: machine, fit!, report, predict
-using Test
 using SymbolicUtils: SymbolicUtils
 using Suppressor: @capture_err
 
diff --git a/test/test_nan_detection.jl b/test/test_nan_detection.jl
index dce861579..fb2a21d1f 100644
--- a/test/test_nan_detection.jl
+++ b/test/test_nan_detection.jl
@@ -1,6 +1,5 @@
 println("Testing NaN detection.")
 using SymbolicRegression
-using Test
 
 for T in [Float16, Float32, Float64], turbo in [true, false]
     T == Float16 && turbo && continue
diff --git a/test/test_nested_constraints.jl b/test/test_nested_constraints.jl
index f117e7f94..8d3932c01 100644
--- a/test/test_nested_constraints.jl
+++ b/test/test_nested_constraints.jl
@@ -1,6 +1,5 @@
 println("Test operator nesting and flagging.")
 using SymbolicRegression
-using Test
 
 function create_options(nested_constraints)
     return Options(;
diff --git a/test/test_operators.jl b/test/test_operators.jl
index 21d519dba..47b83418a 100644
--- a/test/test_operators.jl
+++ b/test/test_operators.jl
@@ -18,7 +18,6 @@ using SymbolicRegression:
     logical_or,
     logical_and,
     gamma
-using Test
 using Random: MersenneTwister
 using Suppressor: @capture_err
 using LoopVectorization
diff --git a/test/test_optimizer_mutation.jl b/test/test_optimizer_mutation.jl
index abaf44bde..41f3bd67d 100644
--- a/test/test_optimizer_mutation.jl
+++ b/test/test_optimizer_mutation.jl
@@ -4,7 +4,6 @@ using SymbolicRegression: Dataset, RunningSearchStatistics, RecordType
 using Optim: Optim
 using SymbolicRegression.MutateModule: next_generation
 using DynamicExpressions: get_constants
-using Test
 
 mutation_weights = (; optimize=1e30)  # We also test whether a named tuple works.
 options = Options(;
diff --git a/test/test_options.jl b/test/test_options.jl
index 1a6a8b7f0..9c7bc0d99 100644
--- a/test/test_options.jl
+++ b/test/test_options.jl
@@ -1,4 +1,3 @@
-using Test
 using SymbolicRegression
 using Optim: Optim
 
diff --git a/test/test_params.jl b/test/test_params.jl
index 75f78ab49..7aafd7f48 100644
--- a/test/test_params.jl
+++ b/test/test_params.jl
@@ -3,10 +3,14 @@ using Optim: Optim
 using LineSearches: LineSearches
 using Test: Test
 
-maximum_residual = 1e-2
-custom_cos(x) = cos(x)
+ENV["SYMBOLIC_REGRESSION_IS_TESTING"] = "true"
 
-default_params = (
+const maximum_residual = 1e-2
+if !@isdefined(custom_cos) || !hasmethod(custom_cos, (String,))
+    @eval custom_cos(x) = cos(x)
+end
+
+const default_params = (
     binary_operators=(/, +, *),
     unary_operators=(exp, custom_cos),
     constraints=nothing,
diff --git a/test/test_print.jl b/test/test_print.jl
index 99052851a..fff027504 100644
--- a/test/test_print.jl
+++ b/test/test_print.jl
@@ -1,4 +1,3 @@
-using Test
 using SymbolicRegression
 using SymbolicRegression.UtilsModule: split_string
 
diff --git a/test/test_prob_pick_first.jl b/test/test_prob_pick_first.jl
index 84ec44357..8967f0e04 100644
--- a/test/test_prob_pick_first.jl
+++ b/test/test_prob_pick_first.jl
@@ -1,7 +1,6 @@
 println("Testing whether tournament_selection_p works.")
 using SymbolicRegression
 using DynamicExpressions.EquationModule: with_type_parameters
-using Test
 include("test_params.jl")
 
 n = 10
diff --git a/test/test_recorder.jl b/test/test_recorder.jl
index 065877884..95d7b7348 100644
--- a/test/test_recorder.jl
+++ b/test/test_recorder.jl
@@ -1,8 +1,10 @@
 using SymbolicRegression
 using SymbolicRegression.UtilsModule: recursive_merge
-using Test
 using JSON3
+include("test_params.jl")
 
+base_dir = mktempdir()
+recorder_file = joinpath(base_dir, "pysr_recorder.json")
 X = 2 .* randn(Float32, 2, 1000)
 y = 3 * cos.(X[2, :]) + X[1, :] .^ 2 .- 2
 
@@ -10,7 +12,7 @@ options = SymbolicRegression.Options(;
     binary_operators=(+, *, /, -),
     unary_operators=(cos,),
     use_recorder=true,
-    recorder_file="pysr_recorder.json",
+    recorder_file=recorder_file,
     crossover_probability=0.0,  # required for recording, as not set up to track crossovers.
     populations=2,
     population_size=100,
diff --git a/test/test_search_statistics.jl b/test/test_search_statistics.jl
index 7247aae2a..35a9b0175 100644
--- a/test/test_search_statistics.jl
+++ b/test/test_search_statistics.jl
@@ -1,7 +1,6 @@
 using SymbolicRegression
 using SymbolicRegression.AdaptiveParsimonyModule:
     RunningSearchStatistics, update_frequencies!, move_window!, normalize_frequencies!
-using Test
 using Random
 
 options = Options()
diff --git a/test/test_simplification.jl b/test/test_simplification.jl
index 0518c77c8..ad6c0a562 100644
--- a/test/test_simplification.jl
+++ b/test/test_simplification.jl
@@ -47,20 +47,20 @@ tree_copy = convert(Node, eqn, options)
 # with custom operators, and unary operators:
 x1, x2, x3 = Node("x1"), Node("x2"), Node("x3")
 pow_abs2(x, y) = abs(x)^y
-custom_cos(x) = cos(x)^2
+custom_cos2(x) = cos(x)^2
 
 options = Options(;
-    binary_operators=(+, *, -, /, pow_abs2), unary_operators=(custom_cos, exp, sin)
+    binary_operators=(+, *, -, /, pow_abs2), unary_operators=(custom_cos2, exp, sin)
 )
 @extend_operators options
 tree = (
     ((x2 + x2) * ((-0.5982493 / pow_abs2(x1, x2)) / -0.54734415)) + (
         sin(
-            custom_cos(
+            custom_cos2(
                 sin(1.2926733 - 1.6606787) /
                 sin(((0.14577048 * x1) + ((0.111149654 + x1) - -0.8298334)) - -1.2071426),
-            ) * (custom_cos(x3 - 2.3201916) + ((x1 - (x1 * x2)) / x2)),
-        ) / (0.14854191 - ((custom_cos(x2) * -1.6047639) - 0.023943262))
+            ) * (custom_cos2(x3 - 2.3201916) + ((x1 - (x1 * x2)) / x2)),
+        ) / (0.14854191 - ((custom_cos2(x2) * -1.6047639) - 0.023943262))
     )
 )
 # We use `index_functions` to avoid converting the custom operators into the primitives.
diff --git a/test/test_stop_on_clock.jl b/test/test_stop_on_clock.jl
index f6c40dc3a..295586e57 100644
--- a/test/test_stop_on_clock.jl
+++ b/test/test_stop_on_clock.jl
@@ -1,5 +1,4 @@
 using SymbolicRegression
-using Test
 using Random
 include("test_params.jl")
 
diff --git a/test/test_symbolic_utils.jl b/test/test_symbolic_utils.jl
index a5c040cfc..3e074abaa 100644
--- a/test/test_symbolic_utils.jl
+++ b/test/test_symbolic_utils.jl
@@ -1,6 +1,5 @@
 using SymbolicUtils
 using SymbolicRegression
-using Test
 include("test_params.jl")
 
 _inv(x) = 1 / x
diff --git a/test/test_tree_construction.jl b/test/test_tree_construction.jl
index f055ea566..f233bf118 100644
--- a/test/test_tree_construction.jl
+++ b/test/test_tree_construction.jl
@@ -2,7 +2,6 @@ using SymbolicRegression
 using Random
 using SymbolicRegression: eval_loss, score_func, Dataset
 using ForwardDiff
-using Test
 include("test_params.jl")
 
 x1 = 2.0
diff --git a/test/test_turbo_nan.jl b/test/test_turbo_nan.jl
index 9165c82c9..2447b8253 100644
--- a/test/test_turbo_nan.jl
+++ b/test/test_turbo_nan.jl
@@ -1,5 +1,4 @@
 using SymbolicRegression
-using Test
 
 bad_op(x::T) where {T} = (x >= 0) ? x : T(0)
 
diff --git a/test/test_units.jl b/test/test_units.jl
index ae0855a38..0e58173e1 100644
--- a/test/test_units.jl
+++ b/test/test_units.jl
@@ -30,7 +30,6 @@ using DynamicQuantities:
     sym_uparse,
     ustrip,
     dimension
-using Test
 using MLJBase: MLJBase as MLJ
 using MLJModelInterface: MLJModelInterface as MMI
 include("utils.jl")
diff --git a/test/test_utils.jl b/test/test_utils.jl
index e13f6d34e..67ceb0dcc 100644
--- a/test/test_utils.jl
+++ b/test/test_utils.jl
@@ -2,7 +2,6 @@ using SymbolicRegression
 using SymbolicRegression.UtilsModule:
     findmin_fast, argmin_fast, bottomk_fast, is_anonymous_function
 using Random
-using Test
 
 function simple_bottomk(x, k)
     idx = sortperm(x)[1:k]
diff --git a/test/unittest.jl b/test/unittest.jl
deleted file mode 100644
index f4cd99cba..000000000
--- a/test/unittest.jl
+++ /dev/null
@@ -1,104 +0,0 @@
-using SafeTestsets
-
-# Trigger extensions:
-using LoopVectorization
-
-@safetestset "Test custom operators and additional types" begin
-    include("test_operators.jl")
-end
-
-@safetestset "Test tree construction and scoring" begin
-    include("test_tree_construction.jl")
-end
-
-@safetestset "Test SymbolicUtils interface" begin
-    include("test_symbolic_utils.jl")
-end
-
-@safetestset "Test constraints interface" begin
-    include("test_constraints.jl")
-end
-
-@safetestset "Test custom losses" begin
-    include("test_losses.jl")
-end
-
-@safetestset "Test derivatives" begin
-    include("test_derivatives.jl")
-end
-
-@safetestset "Test simplification" begin
-    include("test_simplification.jl")
-end
-
-@safetestset "Test printing" begin
-    include("test_print.jl")
-end
-
-@safetestset "Test validity of expression evaluation" begin
-    include("test_evaluation.jl")
-end
-
-@safetestset "Test turbo mode with NaN" begin
-    include("test_turbo_nan.jl")
-end
-
-@safetestset "Test validity of integer expression evaluation" begin
-    include("test_integer_evaluation.jl")
-end
-
-@safetestset "Test tournament selection" begin
-    include("test_prob_pick_first.jl")
-end
-
-@safetestset "Test crossover mutation" begin
-    include("test_crossover.jl")
-end
-
-@safetestset "Test NaN detection in evaluator" begin
-    include("test_nan_detection.jl")
-end
-
-@safetestset "Test nested constraint checking" begin
-    include("test_nested_constraints.jl")
-end
-
-@safetestset "Test complexity evaluation" begin
-    include("test_complexity.jl")
-end
-
-@safetestset "Test options" begin
-    include("test_options.jl")
-end
-
-@safetestset "Test hash of tree" begin
-    include("test_hash.jl")
-end
-
-@safetestset "Test migration" begin
-    include("test_migration.jl")
-end
-
-@safetestset "Test deprecated options" begin
-    include("test_deprecation.jl")
-end
-
-@safetestset "Test optimization mutation" begin
-    include("test_optimizer_mutation.jl")
-end
-
-@safetestset "Test RunningSearchStatistics" begin
-    include("test_search_statistics.jl")
-end
-
-@safetestset "Test utils" begin
-    include("test_utils.jl")
-end
-
-@safetestset "Test units" begin
-    include("test_units.jl")
-end
-
-@safetestset "Dataset" begin
-    include("test_dataset.jl")
-end