diff --git a/src/Model.jl b/src/Model.jl
index 25b9380..06d53b6 100644
--- a/src/Model.jl
+++ b/src/Model.jl
@@ -78,16 +78,10 @@ end
 @gen function covariance_prior(idx::Int, config::GPConfig)
     node_dist = get_node_dist(idx, config)
     node_type = {(idx, :node_type)} ~ categorical(node_dist)
+    NodeType = config.index_to_node[node_type]
 
-    # LeafNode
-    if node_type in [
-            config.Constant,
-            config.Linear,
-            config.SquaredExponential,
-            config.GammaExponential,
-            config.Periodic,
-            ]
-        NodeType = config.index_to_node[node_type]
+    # LeafNode — any registered LeafNode subtype, not a fixed code list
+    if NodeType <: GP.LeafNode
         params = []
         for field in fieldnames(NodeType)
             log_param  = {(idx, field)} ~ normal(0, 1)
@@ -103,7 +97,6 @@ end
         config = GPConfig(config; changepoints=false)
         left_node = {*} ~ covariance_prior(idx_l, config)
         right_node = {*} ~ covariance_prior(idx_r, config)
-        NodeType = config.index_to_node[node_type]
         node = NodeType(left_node, right_node)
 
     # ChangePoint
diff --git a/src/inference_rejuv_tree_da.jl b/src/inference_rejuv_tree_da.jl
index 272d884..7eca361 100644
--- a/src/inference_rejuv_tree_da.jl
+++ b/src/inference_rejuv_tree_da.jl
@@ -53,16 +53,10 @@ end
     node_dist = get_node_dist_attach_detach(idx, path_to_hole, force_cp, config)
     isnothing(node_dist) && return
     node_type = {(idx, :node_type)} ~ categorical(node_dist)
+    NodeType = config.index_to_node[node_type]
 
-    # LeafNode
-    if node_type in [
-            config.Constant,
-            config.Linear,
-            config.SquaredExponential,
-            config.GammaExponential,
-            config.Periodic,
-            ]
-        NodeType = config.index_to_node[node_type]
+    # LeafNode — any registered LeafNode subtype, not a fixed code list
+    if NodeType <: GP.LeafNode
         for field in fieldnames(NodeType)
             {(idx, field)} ~ normal(0, 1)
         end
diff --git a/src/inference_utils.jl b/src/inference_utils.jl
index df0e74a..6c437b1 100644
--- a/src/inference_utils.jl
+++ b/src/inference_utils.jl
@@ -197,9 +197,11 @@ end
 
 """Convert Node to the integer code."""
 function node_to_integer(node::Node, config::GPConfig)
-    nodename = split(string(typeof(node)), ".")[end]
-    field = Symbol(nodename)
-    return getfield(config, field)
+    T = typeof(node)
+    for (code, NodeType) in config.index_to_node
+        NodeType === T && return code
+    end
+    error("Node type $T is not registered in config.index_to_node")
 end
 
 """Convert Node to Gen.ChoiceMap."""
diff --git a/test/runtests.jl b/test/runtests.jl
index 057c94e..89e5ac2 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -17,7 +17,8 @@ using Test
 using AutoGP
 
 @testset "AutoGP" begin
-    @testset "test_GP.jl"        begin include("test_GP.jl") end
-    @testset "test_api.jl"       begin include("test_api.jl") end
-    @testset "test_serialize.jl" begin include("test_serialize.jl") end
+    @testset "test_GP.jl"                 begin include("test_GP.jl") end
+    @testset "test_api.jl"                begin include("test_api.jl") end
+    @testset "test_serialize.jl"          begin include("test_serialize.jl") end
+    @testset "test_extensible_config.jl"  begin include("test_extensible_config.jl") end
 end
diff --git a/test/test_extensible_config.jl b/test/test_extensible_config.jl
new file mode 100644
index 0000000..2b28223
--- /dev/null
+++ b/test/test_extensible_config.jl
@@ -0,0 +1,154 @@
+# Copyright 2025 CMU Probabilistic Computing Systems Lab
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+using Test
+using Dates
+
+using AutoGP
+using AutoGP: GP
+import AutoGP: Model
+import AutoGP: Inference
+
+using Gen
+
+# ---------------------------------------------------------------------------
+# Test-only primitive kernel fixture (NOT a shipped kernel). Behaves like a
+# SquaredExponential so it is a well-behaved, positive-definite covariance.
+# ---------------------------------------------------------------------------
+struct DummyLeaf <: GP.LeafNode
+    lengthscale::Real
+    amplitude::Real
+    DummyLeaf(lengthscale::Real, amplitude::Real=1) = new(lengthscale, amplitude)
+end
+
+GP.eval_cov(node::DummyLeaf, t1, t2) =
+    node.amplitude * exp(-0.5 * (t1 - t2)^2 / node.lengthscale^2)
+
+function GP.eval_cov(node::DummyLeaf, ts::Vector{Float64})
+    dx = ts .- ts'
+    return node.amplitude .* exp.(-0.5 .* dx .* dx ./ node.lengthscale^2)
+end
+
+GP.reparameterize(node::DummyLeaf, t::GP.LinearTransform) =
+    DummyLeaf(node.lengthscale / abs(t.slope), node.amplitude)
+
+GP.rescale(node::DummyLeaf, t::GP.LinearTransform) =
+    DummyLeaf(node.lengthscale, t.slope^2 * node.amplitude)
+
+# Extended config registering DummyLeaf at code 6 and shifting the operators
+# to codes 7/8/9. Leaf mass is placed entirely on DummyLeaf so that fitted
+# trees are guaranteed to contain it.
+function extended_config(; changepoints=true)
+    norm(v) = v ./ sum(v)
+    return GP.GPConfig(
+        index_to_node = Dict{Integer,Type{<:GP.Node}}(
+            1 => GP.Constant,
+            2 => GP.Linear,
+            3 => GP.SquaredExponential,
+            4 => GP.GammaExponential,
+            5 => GP.Periodic,
+            6 => DummyLeaf,
+            7 => GP.Plus,
+            8 => GP.Times,
+            9 => GP.ChangePoint),
+        Plus        = 7,
+        Times       = 8,
+        ChangePoint = 9,
+        node_dist_leaf = norm([0., 0, 0, 0, 0, 1]),
+        node_dist_nocp = norm([0., 0, 0, 0, 0, 8, 1, 1]),
+        node_dist_cp   = norm([0., 0, 0, 0, 0, 8, 1, 1, 1]),
+        changepoints   = changepoints,
+    )
+end
+
+@testset "test_extensible_config" begin
+
+    # -----------------------------------------------------------------------
+    # Behaviour identity: the type-based leaf gate must route exactly the
+    # same codes as the legacy hardcoded code list for the default config.
+    # -----------------------------------------------------------------------
+    @testset "leaf-gate routing unchanged for default config" begin
+        config = GP.GPConfig()
+        legacy_leaf_codes = [
+            config.Constant,
+            config.Linear,
+            config.SquaredExponential,
+            config.GammaExponential,
+            config.Periodic]
+        for (code, NodeType) in config.index_to_node
+            @test (NodeType <: GP.LeafNode) == (code in legacy_leaf_codes)
+        end
+    end
+
+    # node_to_integer (Edit 3) must return the same codes as before for every
+    # built-in node type.
+    @testset "node_to_integer round-trips built-in nodes" begin
+        config = GP.GPConfig()
+        @test Inference.node_to_integer(GP.Constant(0.5), config) == config.Constant
+        @test Inference.node_to_integer(GP.Linear(0.1), config) == config.Linear
+        @test Inference.node_to_integer(GP.SquaredExponential(0.5), config) == config.SquaredExponential
+        @test Inference.node_to_integer(GP.GammaExponential(0.5, 1.0), config) == config.GammaExponential
+        @test Inference.node_to_integer(GP.Periodic(0.5, 1.0), config) == config.Periodic
+        @test Inference.node_to_integer(GP.Plus(GP.Constant(1.0), GP.Constant(1.0)), config) == config.Plus
+        @test Inference.node_to_integer(GP.Times(GP.Constant(1.0), GP.Constant(1.0)), config) == config.Times
+        @test Inference.node_to_integer(GP.ChangePoint(GP.Constant(1.0), GP.Constant(1.0), 0.5, 0.01), config) == config.ChangePoint
+    end
+
+    # The grammar still samples deterministically under a fixed seed.
+    @testset "covariance_prior deterministic under fixed seed" begin
+        config = GP.GPConfig()
+        AutoGP.seed!(42)
+        t1 = Gen.simulate(Model.covariance_prior, (1, config))
+        AutoGP.seed!(42)
+        t2 = Gen.simulate(Model.covariance_prior, (1, config))
+        @test Gen.get_retval(t1) ≈ Gen.get_retval(t2)
+        @test Gen.get_score(t1) == Gen.get_score(t2)
+    end
+
+    # -----------------------------------------------------------------------
+    # Extensibility regression: a custom LeafNode can be proposed and used.
+    # -----------------------------------------------------------------------
+    @testset "fit_smc! proposes a custom LeafNode" begin
+        config = extended_config()
+        # Greedy's length-based leaf classification recognizes the new primitive.
+        leaf_types = [config.index_to_node[c] for c in AutoGP.Greedy.get_leaf_node_types(config)]
+        @test DummyLeaf in leaf_types
+
+        ds = [Date(2025, 1, i) for i in 1:6]
+        y = [1.0, 2.0, 1.5, 2.5, 1.2, 2.2]
+        AutoGP.seed!(7)
+        model = AutoGP.GPModel(ds, y; n_particles=4, config=config)
+        AutoGP.fit_smc!(model; n_mcmc=2, n_hmc=2, schedule=[3, 6])
+        kernels = AutoGP.covariance_kernels(model)
+        @test any(GP.has_leaf(k, DummyLeaf) for k in kernels)
+    end
+
+    @testset "decompose / predict_sum handle a custom LeafNode" begin
+        config = extended_config(; changepoints=false)
+        ds = [Date(2025, 1, i) for i in 1:6]
+        y = [1.0, 2.0, 1.5, 2.5, 1.2, 2.2]
+        AutoGP.seed!(11)
+        model = AutoGP.GPModel(ds, y; n_particles=2, config=config)
+        AutoGP.fit_smc!(model; n_mcmc=2, n_hmc=2, schedule=[3, 6])
+
+        # Both paths route through node_to_integer on a custom primitive (Edit 3).
+        models = AutoGP.decompose(model)
+        @test length(models) == AutoGP.num_particles(model)
+
+        ds_future = [Date(2025, 1, i) for i in 7:9]
+        predictions = AutoGP.predict_sum(model, ds_future, DummyLeaf)
+        @test size(predictions, 1) > 0
+    end
+
+end