Make amendments following feedback.

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Logging/LogEntryHelper.cs

@@ -76,15 +76,17 @@ public static LogEntry DisallowedInaccessibleDirectiveArgument(
             new SchemaCoordinate(directiveName, argumentName: argument.Name, ofDirective: true),
             schema: schema);
 
-    public static LogEntry ExternalArgumentDefaultMismatch(string fieldName, string typeName)
-        => new(
+    public static LogEntry ExternalArgumentDefaultMismatch(string fieldName, string typeName, string argumentName)
+    {
+        var coordinate = new SchemaCoordinate(typeName, fieldName, argumentName);
+        return new LogEntry(
             string.Format(
                 LogEntryHelper_ExternalArgumentDefaultMismatch,
-                fieldName,
-                typeName),
+                coordinate),
             LogEntryCodes.ExternalArgumentDefaultMismatch,
             LogSeverity.Error,
-            new SchemaCoordinate(typeName, fieldName));
+            coordinate);
+    }
 
     public static LogEntry ExternalMissingOnBase(string fieldName, string typeName)
         => new(

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/PreMergeValidation/Rules/ExternalArgumentDefaultMismatchRule.cs

@@ -0,0 +1,56 @@
+using System.Collections.Immutable;
+using HotChocolate.Fusion.Events;
+using HotChocolate.Skimmed;
+using static HotChocolate.Fusion.Logging.LogEntryHelper;
+
+namespace HotChocolate.Fusion.PreMergeValidation.Rules;
+
+/// <summary>
+/// This rule ensures that arguments on fields marked as <c>@external</c> have default values
+/// compatible with the corresponding arguments on fields from other source schemas where the field
+/// is defined (non-<c>@external</c>).
+/// </summary>
+/// <seealso href="https://graphql.github.io/composite-schemas-spec/draft/#sec-External-Argument-Default-Mismatch">
+/// Specification
+/// </seealso>
+internal sealed class ExternalArgumentDefaultMismatchRule : IEventHandler<OutputFieldGroupEvent>
+{
+    public void Handle(OutputFieldGroupEvent @event, CompositionContext context)
+    {
+        var (fieldName, fieldGroup, typeName) = @event;
+
+        IReadOnlyCollection<OutputFieldInfo> externalFields =
+            [..fieldGroup.Where(i => ValidationHelper.IsExternal(i.Field))];
+        if (externalFields.Count == 0)
+        {
+            return;
+        }
+
+        var argumentNames = fieldGroup
+            .SelectMany(i => i.Field.Arguments, (_, arg) => arg.Name)
+            .ToHashSet();
+
+        foreach (var argumentName in argumentNames)
+        {
+            IReadOnlyList<InputFieldDefinition> arguments = [..fieldGroup.Select(i => i.Field.Arguments[argumentName])];
+            var defaultValue = arguments[0].DefaultValue;
+
+            foreach (var argument in arguments)
+            {
+                var currentDefaultValue = argument.DefaultValue;
+                var match = (currentDefaultValue, defaultValue) switch
+                {
+                    (null, null) => true,
+                    (not null, null) => false,
+                    (null, not null) => false,
+                    _ => currentDefaultValue.Value!.Equals(defaultValue.Value)
+                };
+
+                if (!match)
+                {
+                    context.Log.Write(ExternalArgumentDefaultMismatch(fieldName, typeName, argumentName));
+                }
+            }
+        }
+    }
+}

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Properties/CompositionResources.resx

@@ -1,7 +1,8 @@
 <?xml version="1.0" encoding="utf-8"?>
 
 <root>
-  <xsd:schema id="root" xmlns="" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
+  <xsd:schema id="root" xmlns="" xmlns:xsd="http://www.w3.org/2001/XMLSchema"
+              xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
     <xsd:element name="root" msdata:IsDataSet="true">
 
     </xsd:element>
@@ -13,10 +14,14 @@
     <value>1.3</value>
   </resheader>
   <resheader name="reader">
-    <value>System.Resources.ResXResourceReader, System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+    <value>System.Resources.ResXResourceReader, System.Windows.Forms, Version=2.0.0.0, Culture=neutral,
+      PublicKeyToken=b77a5c561934e089
+    </value>
   </resheader>
   <resheader name="writer">
-    <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+    <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=2.0.0.0, Culture=neutral,
+      PublicKeyToken=b77a5c561934e089
+    </value>
   </resheader>
   <data name="ErrorHelper_PreMergeValidationFailed" xml:space="preserve">
     <value>Pre-merge validation failed. View the composition log for details.</value>
@@ -36,13 +41,13 @@
   <data name="LogEntryHelper_DisallowedInaccessibleDirectiveArgument" xml:space="preserve">
     <value>The argument '{0}' on built-in directive type '{1}' is not accessible.</value>
   </data>
+  <data name="LogEntryHelper_ExternalArgumentDefaultMismatch" xml:space="preserve">
+    <value>The argument with schema coordinate '{0}' has inconsistent default values.</value>
+  </data>
   <data name="LogEntryHelper_ExternalMissingOnBase" xml:space="preserve">
     <value>Field '{0}' on type '{1}' is only declared as external.</value>
   </data>
   <data name="LogEntryHelper_OutputFieldTypesNotMergeable" xml:space="preserve">
     <value>Field '{0}' on type '{1}' is not mergeable.</value>
   </data>
-  <data name="LogEntryHelper_ExternalArgumentDefaultMismatch" xml:space="preserve">
-    <value>Field '{0}' on type '{1}' must have consistent default values.</value>
-  </data>
 </root>