Always set new node bias to zero to preserve the original NEAT paper concept of keeping the change almost neutral.

Author: CodeReclaimers

neat/genome.py

@@ -386,6 +386,13 @@ def mutate_add_node(self, config):
         conn_to_split = choice(list(self.connections.values()))
         new_node_id = config.get_new_node_key(self.nodes)
         ng = self.create_node(config, new_node_id)
+
+        # Make the new node as neutral as possible with respect to the
+        # existing connection: start with zero bias regardless of the
+        # global bias initialization distribution.
+        if hasattr(ng, "bias"):
+            ng.bias = 0.0
+
         self.nodes[new_node_id] = ng
 
         # Disable this connection and create two new connections joining its nodes via

tests/test_genome_mutations.py

@@ -246,6 +246,30 @@ def test_add_node_preserves_weight_path(self):
         self.assertEqual(incoming_conn.weight, 1.0,
                         "Incoming connection should have weight 1.0")
 
+    def test_add_node_initial_bias_zero(self):
+        """Test that newly added nodes start with zero bias."""
+        genome = self.create_minimal_genome()
+
+        if not genome.connections:
+            genome.mutate_add_connection(self.config.genome_config)
+
+        # Record nodes before mutation
+        nodes_before = set(genome.nodes.keys())
+
+        genome.mutate_add_node(self.config.genome_config)
+
+        # Identify the newly added node
+        nodes_after = set(genome.nodes.keys())
+        new_nodes = nodes_after - nodes_before
+
+        self.assertEqual(len(new_nodes), 1, "Should add exactly one new node")
+        new_node_id = next(iter(new_nodes))
+
+        new_node = genome.nodes[new_node_id]
+        self.assertTrue(hasattr(new_node, "bias"))
+        self.assertEqual(new_node.bias, 0.0,
+                         "Newly added node bias should be initialized to 0.0")
+    
     # ========== Add Connection Mutation Tests ==========
 
     def test_add_connection_creates_new_connection(self):