diff --git a/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/JsonSchemaProducer.java b/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/JsonSchemaProducer.java
index b4b4825..b39b203 100644
--- a/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/JsonSchemaProducer.java
+++ b/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/JsonSchemaProducer.java
@@ -1203,6 +1203,10 @@ public class JsonSchemaProducer {
             for (String value : entry.getValue().value()) {
                 enumValues.add(value);
             }
+            String defaultValue = entry.getValue().defaultValue();
+            if (defaultValue != null && !defaultValue.isBlank()) {
+                propertySchema.put("default", defaultValue);
+            }
         }
     }
 
diff --git a/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/MCPAgentBlockConfiguration.java b/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/MCPAgentBlockConfiguration.java
index dab66b7..ddb14a9 100644
--- a/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/MCPAgentBlockConfiguration.java
+++ b/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/MCPAgentBlockConfiguration.java
@@ -124,7 +124,9 @@ public class MCPAgentBlockConfiguration extends BlockConfiguration<MCPAgentBlock
     @JsonIgnoreProperties(ignoreUnknown = true)
     public record MCPServerBinding(
             @JsonProperty(required = false)
-            @SchemaAllowedValues({ "CATALOG", "CUSTOM" })
+            // The default lives in effectiveSourceType() below; declared here so the editor can say
+            // which one an empty field is using instead of only that it is using one.
+            @SchemaAllowedValues(value = { "CATALOG", "CUSTOM" }, defaultValue = "CATALOG")
             String sourceType,
             @JsonProperty(required = false)
             @UiEnabledWhen(field = "sourceType", equalsAny = { "CATALOG", "" })
diff --git a/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/MCPAgentChatBlockConfiguration.java b/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/MCPAgentChatBlockConfiguration.java
index 0142b92..79f27bc 100644
--- a/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/MCPAgentChatBlockConfiguration.java
+++ b/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/MCPAgentChatBlockConfiguration.java
@@ -137,7 +137,9 @@ public class MCPAgentChatBlockConfiguration extends BlockConfiguration<MCPAgentC
     @JsonIgnoreProperties(ignoreUnknown = true)
     public record MCPServerBinding(
             @JsonProperty(required = false)
-            @SchemaAllowedValues({ "CATALOG", "CUSTOM" })
+            // Same default as effectiveSourceType() below, declared for the editor - and the same
+            // one the agent block declares, so the two dialogs read alike.
+            @SchemaAllowedValues(value = { "CATALOG", "CUSTOM" }, defaultValue = "CATALOG")
             String sourceType,
             @JsonProperty(required = false)
             @UiEnabledWhen(field = "sourceType", equalsAny = { "CATALOG", "" })
diff --git a/src/main/java/it/cnr/isti/workflow/manager/configurations/annotations/SchemaAllowedValues.java b/src/main/java/it/cnr/isti/workflow/manager/configurations/annotations/SchemaAllowedValues.java
index f21f5d7..ae0550a 100644
--- a/src/main/java/it/cnr/isti/workflow/manager/configurations/annotations/SchemaAllowedValues.java
+++ b/src/main/java/it/cnr/isti/workflow/manager/configurations/annotations/SchemaAllowedValues.java
@@ -9,4 +9,15 @@ import java.lang.annotation.Target;
 @Retention(RetentionPolicy.RUNTIME)
 public @interface SchemaAllowedValues {
     String[] value();
+
+    /**
+     * Which of the values applies when the field is left unset, published as the schema's
+     * {@code default}.
+     *
+     * <p>Here rather than in an annotation of its own because it is the same statement: these are
+     * the values, and this is the one that holds when nobody chose. The editor needs it to say what
+     * an empty field is actually doing - without it, it can only say "using the default" and leave
+     * the reader to guess which.
+     */
+    String defaultValue() default "";
 }
diff --git a/src/test/java/it/cnr/isti/workflow/manager/controllers/BlocksControllerTest.java b/src/test/java/it/cnr/isti/workflow/manager/controllers/BlocksControllerTest.java
index f5eaecf..659f337 100644
--- a/src/test/java/it/cnr/isti/workflow/manager/controllers/BlocksControllerTest.java
+++ b/src/test/java/it/cnr/isti/workflow/manager/controllers/BlocksControllerTest.java
@@ -573,6 +573,18 @@ public class BlocksControllerTest {
                 sharedSessionRef.path("x-retriever-url").asText());
         assertEquals(UiContextKeys.FLOW_ID,
                 sharedSessionRef.path("x-retriever-depends-on").get(0).asText());
+
+        // The binding's sourceType has a default in Java (effectiveSourceType(): blank means
+        // CATALOG) and now declares it, so the editor can say which default an empty field is
+        // using instead of only that it is using one.
+        // The binding is a nested record, so it lives in the definitions and mcpServers.items is a
+        // $ref to it - the same shape the other nested-record assertions in this class walk.
+        JsonNode definitions = schema.has("definitions") ? schema.path("definitions") : schema.path("$defs");
+        JsonNode binding = definitions.path("MCPServerBinding");
+        JsonNode sourceType = binding.path("properties").path("sourceType");
+        assertEquals("CATALOG", sourceType.path("default").asText());
+        assertEquals("CATALOG", sourceType.path("enum").get(0).asText());
+        assertEquals("CUSTOM", sourceType.path("enum").get(1).asText());
         assertTrue(sharedSessionRef.path("x-retriever-requires-auth").asBoolean());
     }