fix(mcp): propagate internal serialization errors instead of silently swallowing them to prevent LLM hallucination

server/src/handlers/graph.rs

@@ -151,8 +151,14 @@ impl McpTool for CreateRelationsHandler {
             Ok(r) => r,
             Err(e) => {
                 let err_msg = e.to_string();
-                if err_msg.contains("missing field `from`") || err_msg.contains("missing field `to`") || err_msg.contains("missing field `relation_type`") {
-                    return Err(format!("Schema error: {}. Note that the relation schema strictly uses 'from', 'to', and 'relation_type' (not 'source', 'target', or 'relationType'). Please correct your tool call arguments.", err_msg));
+                if err_msg.contains("missing field `from`")
+                    || err_msg.contains("missing field `to`")
+                    || err_msg.contains("missing field `relation_type`")
+                {
+                    return Err(format!(
+                        "Schema error: {}. Note that the relation schema strictly uses 'from', 'to', and 'relation_type' (not 'source', 'target', or 'relationType'). Please correct your tool call arguments.",
+                        err_msg
+                    ));
                 }
                 return Err(err_msg);
             }
@@ -166,15 +172,22 @@ impl McpTool for CreateRelationsHandler {
                     if from_exists && to_exists {
                         g.relations.push(relation);
                     } else {
-                        if !from_exists { missing_nodes.insert(relation.from); }
-                        if !to_exists { missing_nodes.insert(relation.to); }
+                        if !from_exists {
+                            missing_nodes.insert(relation.from);
+                        }
+                        if !to_exists {
+                            missing_nodes.insert(relation.to);
+                        }
                     }
                 }
             }
         });
         if !missing_nodes.is_empty() {
             let missing: Vec<_> = missing_nodes.into_iter().collect();
-            return Err(format!("Error: Relations dropped due to missing entities: {}", missing.join(", ")));
+            return Err(format!(
+                "Error: Relations dropped due to missing entities: {}",
+                missing.join(", ")
+            ));
         }
         Ok("Relations created".to_string())
     }
@@ -205,7 +218,10 @@ impl McpTool for AddObservationsHandler {
             }
         });
         if !missing_entities.is_empty() {
-            return Err(format!("Error: Observations dropped for missing entities: {}", missing_entities.join(", ")));
+            return Err(format!(
+                "Error: Observations dropped for missing entities: {}",
+                missing_entities.join(", ")
+            ));
         }
         Ok("Observations added".to_string())
     }
@@ -239,7 +255,10 @@ impl McpTool for DeleteEntitiesHandler {
         });
 
         if !missing.is_empty() {
-            return Err(format!("Error: Entities not found: {}. Please use the search_nodes or read_graph tools to verify the exact entity names.", missing.join(", ")));
+            return Err(format!(
+                "Error: Entities not found: {}. Please use the search_nodes or read_graph tools to verify the exact entity names.",
+                missing.join(", ")
+            ));
         }
 
         let idx = state.get_search_index();
@@ -280,7 +299,10 @@ impl McpTool for DeleteObservationsHandler {
             }
         });
         if !missing.is_empty() {
-            return Err(format!("Error: Entities not found: {}. Please use the search_nodes or read_graph tools to verify the exact entity names.", missing.join(", ")));
+            return Err(format!(
+                "Error: Entities not found: {}. Please use the search_nodes or read_graph tools to verify the exact entity names.",
+                missing.join(", ")
+            ));
         }
         Ok("Observations deleted".to_string())
     }
@@ -308,7 +330,10 @@ impl McpTool for DeleteRelationsHandler {
             missing_count = to_rem.len() - (initial_len - master.relations.len());
         });
         if missing_count > 0 {
-            return Err(format!("Error: {} relation(s) not found in graph. Please verify exact relation properties using read_graph.", missing_count));
+            return Err(format!(
+                "Error: {} relation(s) not found in graph. Please verify exact relation properties using read_graph.",
+                missing_count
+            ));
         }
         Ok("Relations deleted".to_string())
     }
@@ -536,7 +561,10 @@ impl McpTool for CondenseEntityHandler {
             }
         });
         if missing {
-            return Err(format!("Error: Entity '{}' not found. Please verify the exact entity name using search_nodes.", req.entity_name));
+            return Err(format!(
+                "Error: Entity '{}' not found. Please verify the exact entity name using search_nodes.",
+                req.entity_name
+            ));
         }
         Ok("Entity condensed".to_string())
     }
@@ -588,7 +616,10 @@ impl McpTool for MergeEntitiesHandler {
             });
         });
         if missing {
-            return Err(format!("Error: Source entity '{}' not found. Please verify the exact entity name using search_nodes.", req.source_entity));
+            return Err(format!(
+                "Error: Source entity '{}' not found. Please verify the exact entity name using search_nodes.",
+                req.source_entity
+            ));
         }
         Ok("Entities merged".to_string())
     }
@@ -691,7 +722,7 @@ mod tests {
         });
         let res = handler.execute(args, state.clone()).await.unwrap();
         assert_eq!(res, "Relations created");
-        
+
         // Test semantic LLM schema feedback (User request)
         let bad_args = json!({
             "relations": [