diff --git a/xtraplatform-cql/src/main/java/de/ii/xtraplatform/cql/domain/CqlVisitorCopy.java b/xtraplatform-cql/src/main/java/de/ii/xtraplatform/cql/domain/CqlVisitorCopy.java
index 4c27154bd..3a5aa5d75 100644
--- a/xtraplatform-cql/src/main/java/de/ii/xtraplatform/cql/domain/CqlVisitorCopy.java
+++ b/xtraplatform-cql/src/main/java/de/ii/xtraplatform/cql/domain/CqlVisitorCopy.java
@@ -225,7 +225,9 @@ public CqlNode visit(Bbox bbox, List<CqlNode> children) {
 
   @Override
   public CqlNode visit(Function function, List<CqlNode> children) {
-    return function;
+    return Function.of(
+        function.getName(),
+        children.stream().map(Operand.class::cast).collect(Collectors.toList()));
   }
 
   @Override
diff --git a/xtraplatform-cql/src/main/java/de/ii/xtraplatform/cql/domain/Function.java b/xtraplatform-cql/src/main/java/de/ii/xtraplatform/cql/domain/Function.java
index 81a43ac79..dbf38d6e1 100644
--- a/xtraplatform-cql/src/main/java/de/ii/xtraplatform/cql/domain/Function.java
+++ b/xtraplatform-cql/src/main/java/de/ii/xtraplatform/cql/domain/Function.java
@@ -61,11 +61,7 @@ public void serialize(Function value, JsonGenerator gen, SerializerProvider seri
       gen.writeFieldName("args");
       gen.writeStartArray();
       for (Operand operand : value.getArgs()) {
-        if (operand instanceof ScalarLiteral) {
-          gen.writeString(String.format("%s", ((ScalarLiteral) operand).getValue().toString()));
-        } else {
-          gen.writeObject(operand);
-        }
+        gen.writeObject(operand);
       }
       gen.writeEndArray();
       gen.writeEndObject();
diff --git a/xtraplatform-cql/src/test/groovy/de/ii/xtraplatform/cql/app/CqlFunctionArgumentsSpec.groovy b/xtraplatform-cql/src/test/groovy/de/ii/xtraplatform/cql/app/CqlFunctionArgumentsSpec.groovy
new file mode 100644
index 000000000..d82d5d6bc
--- /dev/null
+++ b/xtraplatform-cql/src/test/groovy/de/ii/xtraplatform/cql/app/CqlFunctionArgumentsSpec.groovy
@@ -0,0 +1,91 @@
+/*
+ * Copyright 2026 interactive instruments GmbH
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+package de.ii.xtraplatform.cql.app
+
+import de.ii.xtraplatform.cql.domain.Cql
+import de.ii.xtraplatform.cql.domain.Cql2Expression
+import de.ii.xtraplatform.cql.domain.CqlNode
+import de.ii.xtraplatform.cql.domain.CqlVisitorCopy
+import de.ii.xtraplatform.cql.domain.Eq
+import de.ii.xtraplatform.cql.domain.Function
+import de.ii.xtraplatform.cql.domain.Parameter
+import de.ii.xtraplatform.cql.domain.Property
+import de.ii.xtraplatform.cql.domain.ScalarLiteral
+import de.ii.xtraplatform.jsonschema.domain.ImmutableJsonSchemaNumber
+import spock.lang.Shared
+import spock.lang.Specification
+
+class CqlFunctionArgumentsSpec extends Specification {
+
+    @Shared
+    Cql cql
+
+    def setupSpec() {
+        cql = new CqlImpl()
+    }
+
+    def 'numeric arguments of a function in CQL2 JSON are numbers'() {
+
+        given:
+        String cqlJson = """
+            {
+                "op": "=",
+                "args": [
+                    {"function": {"name": "dwithin", "args": [{"property": "geometrie"}, ${literal}]}},
+                    true
+                ]
+            }
+        """
+
+        when:
+        Cql2Expression actual = cql.read(cqlJson, Cql.Format.JSON)
+        def argument = ((Function) ((Eq) actual).getArgs().get(0)).getArgs().get(1)
+
+        then:
+        argument == expected
+
+        where:
+        literal || expected
+        "300"   || ScalarLiteral.of(300)
+        "300.5" || ScalarLiteral.of(300.5d)
+    }
+
+    def 'literal arguments of a function keep their type when written to and read from CQL2 JSON'() {
+
+        given:
+        def filter = Eq.ofFunction(Function.of("dwithin", [Property.of("geometrie"), literal]), ScalarLiteral.of(true))
+
+        when:
+        Cql2Expression actual = cql.read(cql.write(filter, Cql.Format.JSON), Cql.Format.JSON)
+
+        then:
+        actual == filter
+
+        where:
+        literal << [ScalarLiteral.of(300), ScalarLiteral.of(300.5d), ScalarLiteral.of("300")]
+    }
+
+    def 'a copying visitor replaces parameters in the arguments of a function'() {
+
+        given:
+        def parameter = Parameter.of("afstand", new ImmutableJsonSchemaNumber.Builder().build())
+        def filter = Eq.ofFunction(Function.of("dwithin", [Property.of("geometrie"), parameter]), ScalarLiteral.of(true))
+        def resolver = new CqlVisitorCopy() {
+            @Override
+            CqlNode visit(Parameter p, List<CqlNode> children) {
+                return ScalarLiteral.of(300)
+            }
+        }
+
+        when:
+        def actual = filter.accept(resolver)
+
+        then:
+        actual == Eq.ofFunction(Function.of("dwithin", [Property.of("geometrie"), ScalarLiteral.of(300)]), ScalarLiteral.of(true))
+    }
+}