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 d11114f..c82476f 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
@@ -18,7 +18,6 @@ import it.cnr.isti.workflow.manager.configurations.annotations.DynamicSchema;
 import it.cnr.isti.workflow.manager.configurations.annotations.AcceptsVariablePlaceholder;
 import it.cnr.isti.workflow.manager.configurations.annotations.ConfigurableAsInput;
 import it.cnr.isti.workflow.manager.configurations.annotations.UiContextKeys;
-import it.cnr.isti.workflow.manager.configurations.annotations.DefaultsWhenEmpty;
 import it.cnr.isti.workflow.manager.configurations.annotations.SchemaAllowedValues;
 import it.cnr.isti.workflow.manager.configurations.annotations.UiEnabledWhen;
 import it.cnr.isti.workflow.manager.configurations.annotations.UiOptionalGroup;
@@ -158,10 +157,9 @@ public class MCPAgentBlockConfiguration extends BlockConfiguration<MCPAgentBlock
     @JsonIgnoreProperties(ignoreUnknown = true)
     public record MCPServerBinding(
             @JsonProperty(required = false)
-            // 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.
+            // A new binding starts on CATALOG, and blank reads as CATALOG - see effectiveSourceType()
+            // below. Not a default to go back to: CATALOG and CUSTOM are alternatives, picked alike.
             @SchemaAllowedValues(value = { "CATALOG", "CUSTOM" }, defaultValue = "CATALOG")
-            @DefaultsWhenEmpty
             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 bee4091..2dbcd93 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
@@ -21,7 +21,6 @@ import it.cnr.isti.workflow.manager.configurations.annotations.ConfigurableAsInp
 import it.cnr.isti.workflow.manager.configurations.annotations.DynamicSchema;
 import it.cnr.isti.workflow.manager.configurations.annotations.FieldRetriever;
 import it.cnr.isti.workflow.manager.configurations.annotations.LongText;
-import it.cnr.isti.workflow.manager.configurations.annotations.DefaultsWhenEmpty;
 import it.cnr.isti.workflow.manager.configurations.annotations.SchemaAllowedValues;
 import it.cnr.isti.workflow.manager.configurations.annotations.Structural;
 import it.cnr.isti.workflow.manager.configurations.annotations.UiEnabledWhen;
@@ -142,10 +141,9 @@ public class MCPAgentChatBlockConfiguration extends BlockConfiguration<MCPAgentC
     @JsonIgnoreProperties(ignoreUnknown = true)
     public record MCPServerBinding(
             @JsonProperty(required = false)
-            // Same default as effectiveSourceType() below, declared for the editor - and the same
-            // one the agent block declares, so the two dialogs read alike.
+            // Starts on CATALOG like the agent block's, so the two dialogs read alike; an
+            // alternative to pick, not a default to go back to.
             @SchemaAllowedValues(value = { "CATALOG", "CUSTOM" }, defaultValue = "CATALOG")
-            @DefaultsWhenEmpty
             String sourceType,
             @JsonProperty(required = false)
             @UiEnabledWhen(field = "sourceType", equalsAny = { "CATALOG", "" })
diff --git a/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/UploadInput.java b/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/UploadInput.java
index 7a9305c..1116e2f 100644
--- a/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/UploadInput.java
+++ b/src/main/java/it/cnr/isti/workflow/manager/blocks/configurations/UploadInput.java
@@ -9,7 +9,6 @@ import com.fasterxml.jackson.annotation.JsonIgnore;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
 import it.cnr.isti.workflow.manager.configurations.annotations.FieldRetriever;
-import it.cnr.isti.workflow.manager.configurations.annotations.DefaultsWhenEmpty;
 import it.cnr.isti.workflow.manager.configurations.annotations.SchemaAllowedValues;
 import it.cnr.isti.workflow.manager.configurations.annotations.UiContextKeys;
 import it.cnr.isti.workflow.manager.configurations.annotations.UiDescription;
@@ -29,12 +28,11 @@ import jakarta.validation.constraints.Size;
 public record UploadInput(
 
         /**
-         * Which of the two ways this file arrives. The default lives in {@link #effectiveSource()}
-         * and is declared here too, so the editor can say which one an untouched row is using rather
-         * than only that it is using one.
+         * Which of the two ways this file arrives. A new row starts on INPUT; blank, as an older row
+         * may be, reads as INPUT too - see {@link #effectiveSource()}. Not a default to go back to:
+         * the two are alternatives, and INPUT is chosen from the list like GLOBAL is.
          */
         @SchemaAllowedValues(value = { SOURCE_INPUT, SOURCE_GLOBAL }, defaultValue = SOURCE_INPUT)
-        @DefaultsWhenEmpty
         @UiLabel("File comes from")
         @UiDescription("Uploaded to an input of this step, or taken from a global input of the flow.")
         @JsonProperty(required = false) String source,
diff --git a/src/test/java/it/cnr/isti/workflow/manager/blocks/configurations/LLMBlockSchemaTest.java b/src/test/java/it/cnr/isti/workflow/manager/blocks/configurations/LLMBlockSchemaTest.java
index 898a51f..08d6158 100644
--- a/src/test/java/it/cnr/isti/workflow/manager/blocks/configurations/LLMBlockSchemaTest.java
+++ b/src/test/java/it/cnr/isti/workflow/manager/blocks/configurations/LLMBlockSchemaTest.java
@@ -32,4 +32,14 @@ class LLMBlockSchemaTest {
         assertEquals(java.util.List.of("name", "llmDescriptor", "prompt", "uploadInputs", "skills", "mcpServers",
                 "outputs"), order.stream().filter(name -> !name.equals("type")).toList());
     }
+
+    @Test
+    void whereAFileComesFromIsAChoiceNotADefaultToGoBackTo() {
+        JsonNode schema = schemaProducer.generateSchemaNode(LLMBlockConfiguration.class);
+
+        JsonNode source = schema.get("definitions").get("UploadInput").get("properties").get("source");
+
+        assertEquals("INPUT", source.get("default").asText(), "a new row starts on INPUT");
+        assertEquals(false, source.has("x-ui-defaults-when-empty"), "no Use default beside a choice of two");
+    }
 }
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 aade55e..6a7f083 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
@@ -611,9 +611,9 @@ public class BlocksControllerTest {
         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.
+        // A new binding starts on CATALOG (effectiveSourceType(): blank means CATALOG too). It is
+        // one of two alternatives, not a default to go back to, so the editor offers no "Use
+        // default" beside it.
         // 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");
@@ -622,6 +622,7 @@ public class BlocksControllerTest {
         assertEquals("CATALOG", sourceType.path("default").asText());
         assertEquals("CATALOG", sourceType.path("enum").get(0).asText());
         assertEquals("CUSTOM", sourceType.path("enum").get(1).asText());
+        assertFalse(sourceType.has("x-ui-defaults-when-empty"));
         assertTrue(sharedSessionRef.path("x-retriever-requires-auth").asBoolean());
     }