diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Extensions/MutableObjectTypeDefinitionExtensions.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Extensions/MutableObjectTypeDefinitionExtensions.cs
index c9a9a3d41bc..d3c5c321c9b 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Extensions/MutableObjectTypeDefinitionExtensions.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Extensions/MutableObjectTypeDefinitionExtensions.cs
@@ -34,6 +34,8 @@ public static IEnumerable<IDirective> GetFusionLookupDirectives(
                 .ToList();
 
         // To use an abstract lookup, the type must exist in the source schema.
+        // TODO: For abstract lookups we also need to validate that the type
+        //       is part of the abstract type's possible types in the specific source schema.
         if (type.ExistsInSchema(schemaName))
         {
             // Interface lookups.
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceInterfaceTypeCollection.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceInterfaceTypeCollection.cs
index 01fc23818e7..34289061a99 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceInterfaceTypeCollection.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceInterfaceTypeCollection.cs
@@ -3,16 +3,11 @@
 
 namespace HotChocolate.Fusion.Types.Collections;
 
-public class SourceInterfaceTypeCollection
-    : SourceMemberCollection<SourceInterfaceType>
+public class SourceInterfaceTypeCollection(IEnumerable<SourceInterfaceType> members)
+    : SourceMemberCollection<SourceInterfaceType>(members)
     , ISourceComplexTypeCollection<SourceInterfaceType>
     , ISourceComplexTypeCollection<ISourceComplexType>
 {
-    public SourceInterfaceTypeCollection(IEnumerable<SourceInterfaceType> members)
-        : base(members)
-    {
-    }
-
     ISourceComplexType ISourceMemberCollection<ISourceComplexType>.this[string schemaName]
         => this[schemaName];
 
@@ -22,12 +17,9 @@ public ImmutableArray<SourceInterfaceType> Types
     ImmutableArray<ISourceComplexType> ISourceComplexTypeCollection<ISourceComplexType>.Types
         => [.. Members];
 
-    public bool TryGetType(string schemaName, [NotNullWhen(true)] out SourceInterfaceType? type)
-        => TryGetMember(schemaName, out type);
-
-    public bool TryGetType(string schemaName, [NotNullWhen(true)] out ISourceComplexType? type)
+    public bool TryGetMember(string schemaName, [NotNullWhen(true)] out ISourceComplexType? type)
     {
-        if (TryGetMember(schemaName, out var member))
+        if (base.TryGetMember(schemaName, out var member))
         {
             type = member;
             return true;
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceMemberCollection.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceMemberCollection.cs
index e4b647c1fdf..9012162981a 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceMemberCollection.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceMemberCollection.cs
@@ -11,7 +11,7 @@ public class SourceMemberCollection<TMember>
 {
     private readonly FrozenDictionary<string, TMember> _members;
 
-    protected SourceMemberCollection(IEnumerable<TMember> members)
+    public SourceMemberCollection(IEnumerable<TMember> members)
     {
         _members = members.ToFrozenDictionary(t => t.SchemaName);
     }
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceObjectTypeCollection.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceObjectTypeCollection.cs
index 964b507c5ab..e39a1a3e087 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceObjectTypeCollection.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceObjectTypeCollection.cs
@@ -17,12 +17,9 @@ public ImmutableArray<SourceObjectType> Types
     ImmutableArray<ISourceComplexType> ISourceComplexTypeCollection<ISourceComplexType>.Types
         => [.. Members];
 
-    public bool TryGetType(string schemaName, [NotNullWhen(true)] out SourceObjectType? type)
-        => TryGetMember(schemaName, out type);
-
-    public bool TryGetType(string schemaName, [NotNullWhen(true)] out ISourceComplexType? type)
+    public bool TryGetMember(string schemaName, [NotNullWhen(true)] out ISourceComplexType? type)
     {
-        if (TryGetMember(schemaName, out var member))
+        if (base.TryGetMember(schemaName, out var member))
         {
             type = member;
             return true;
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceUnionTypeCollection.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceUnionTypeCollection.cs
new file mode 100644
index 00000000000..ce0d4924c4e
--- /dev/null
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Collections/SourceUnionTypeCollection.cs
@@ -0,0 +1,6 @@
+namespace HotChocolate.Fusion.Types.Collections;
+
+public class SourceUnionTypeCollection(IEnumerable<SourceUnionType> members)
+    : SourceMemberCollection<SourceUnionType>(members)
+{
+}
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Completion/CompletionTools.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Completion/CompletionTools.cs
index 96c418abce8..43bd5483625 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Completion/CompletionTools.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Completion/CompletionTools.cs
@@ -137,6 +137,29 @@ public static SourceInterfaceTypeCollection CreateSourceInterfaceTypeCollection(
         return new SourceInterfaceTypeCollection(temp);
     }
 
+    public static SourceUnionTypeCollection CreateSourceUnionTypeCollection(
+        UnionTypeDefinitionNode typeDef,
+        CompositeSchemaBuilderContext context)
+    {
+        var types = TypeDirectiveParser.Parse(typeDef.Directives);
+        var lookupDirectives = LookupDirectiveParser.Parse(typeDef.Directives);
+        var temp = new SourceUnionType[types.Length];
+
+        for (var i = 0; i < types.Length; i++)
+        {
+            var type = types[i];
+            var lookups = GetLookupBySchema(lookupDirectives, type.SchemaName, typeDef.Name.Value);
+            context.RegisterForCompletionRange(lookups);
+
+            temp[i] = new SourceUnionType(
+                typeDef.Name.Value,
+                type.SchemaName,
+                lookups);
+        }
+
+        return new SourceUnionTypeCollection(temp);
+    }
+
     private static ImmutableArray<Lookup> GetLookupBySchema(
         ImmutableArray<LookupDirective> allLookups,
         string schemaName,
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Completion/CompositeSchemaBuilder.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Completion/CompositeSchemaBuilder.cs
index b31c5c2d362..4bba603eba7 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Completion/CompositeSchemaBuilder.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Completion/CompositeSchemaBuilder.cs
@@ -532,7 +532,8 @@ private static void CompleteUnionType(
     {
         var directives = CompletionTools.CreateDirectiveCollection(typeDef.Directives, context);
         var types = CompletionTools.CreateObjectTypeCollection(typeDef.Types, context);
-        type.Complete(new CompositeUnionTypeCompletionContext(types, directives, FeatureCollection.Empty));
+        var sources = CompletionTools.CreateSourceUnionTypeCollection(typeDef, context);
+        type.Complete(new CompositeUnionTypeCompletionContext(types, directives, sources, FeatureCollection.Empty));
     }
 
     private static void CompleteOutputField(
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Completion/CompositeUnionTypeCompletionContext.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Completion/CompositeUnionTypeCompletionContext.cs
index e028aa92d5d..99e9600a028 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Completion/CompositeUnionTypeCompletionContext.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Completion/CompositeUnionTypeCompletionContext.cs
@@ -6,11 +6,14 @@ namespace HotChocolate.Fusion.Types.Completion;
 internal readonly ref struct CompositeUnionTypeCompletionContext(
     FusionObjectTypeDefinitionCollection types,
     FusionDirectiveCollection directives,
+    SourceUnionTypeCollection sources,
     IFeatureCollection features)
 {
     public FusionDirectiveCollection Directives { get; } = directives;
 
     public FusionObjectTypeDefinitionCollection Types { get; } = types;
 
+    public SourceUnionTypeCollection Sources { get; } = sources;
+
     public IFeatureCollection Features { get; } = features;
 }
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Contracts/ISourceComplexTypeCollection.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Contracts/ISourceComplexTypeCollection.cs
index 58df65ac957..ca0f884f47a 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Contracts/ISourceComplexTypeCollection.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Contracts/ISourceComplexTypeCollection.cs
@@ -7,7 +7,5 @@ public interface ISourceComplexTypeCollection<TType>
     : ISourceMemberCollection<TType>
     where TType : ISourceComplexType
 {
-    bool TryGetType(string schemaName, [NotNullWhen(true)] out TType? type);
-
     ImmutableArray<TType> Types { get; }
 }
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Contracts/ISourceMemberCollection.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Contracts/ISourceMemberCollection.cs
index c35792c1ee6..edffc2f372f 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Contracts/ISourceMemberCollection.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Contracts/ISourceMemberCollection.cs
@@ -1,8 +1,9 @@
 using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
 
 namespace HotChocolate.Fusion.Types;
 
-public interface ISourceMemberCollection<out TMember>
+public interface ISourceMemberCollection<TMember>
     : IEnumerable<TMember>
     where TMember : ISourceMember
 {
@@ -13,4 +14,6 @@ public interface ISourceMemberCollection<out TMember>
     bool ContainsSchema(string schemaName);
 
     ImmutableArray<string> Schemas { get; }
+
+    bool TryGetMember(string schemaName, [NotNullWhen(true)] out TMember? type);
 }
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/FusionUnionTypeDefinition.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/FusionUnionTypeDefinition.cs
index 290723d3e96..05023c6d38a 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/FusionUnionTypeDefinition.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/FusionUnionTypeDefinition.cs
@@ -32,6 +32,23 @@ public FusionUnionTypeDefinition(string name, string? description)
 
     public SchemaCoordinate Coordinate => new(Name, ofDirective: false);
 
+    /// <summary>
+    /// Gets the source type definition of this type.
+    /// </summary>
+    /// <value>
+    /// The source type definition of this type.
+    /// </value>
+    public SourceUnionTypeCollection Sources
+    {
+        get;
+        private set
+        {
+            EnsureNotSealed(_completed);
+
+            field = value;
+        }
+    } = null!;
+
     public FusionObjectTypeDefinitionCollection Types
     {
         get;
@@ -73,6 +90,7 @@ internal void Complete(CompositeUnionTypeCompletionContext context)
 
         Directives = context.Directives;
         Types = context.Types;
+        Sources = context.Sources;
         Features = context.Features;
 
         _completed = true;
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Metadata/Lookup.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Metadata/Lookup.cs
index eee393dd04f..0acb0261f47 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Metadata/Lookup.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Metadata/Lookup.cs
@@ -17,7 +17,7 @@ public sealed class Lookup : INeedsCompletion
     /// </summary>
     /// <param name="schemaName">The name of the source schema.</param>
     /// <param name="declaringTypeName">The name of the type that declares the field.</param>
-    /// <param name="name">The name of the lookup field.</param>
+    /// <param name="fieldName">The name of the lookup field.</param>
     /// <param name="arguments">The arguments that represent field requirements.</param>
     /// <param name="fields">The paths to the field that are required.</param>
     /// <exception cref="ArgumentException">
@@ -30,13 +30,13 @@ public sealed class Lookup : INeedsCompletion
     public Lookup(
         string schemaName,
         string declaringTypeName,
-        string name,
+        string fieldName,
         ImmutableArray<LookupArgument> arguments,
         ImmutableArray<IValueSelectionNode> fields)
     {
         ArgumentException.ThrowIfNullOrEmpty(schemaName);
         ArgumentException.ThrowIfNullOrEmpty(declaringTypeName);
-        ArgumentException.ThrowIfNullOrEmpty(name);
+        ArgumentException.ThrowIfNullOrEmpty(fieldName);
 
         if (arguments.Length == 0)
         {
@@ -50,7 +50,7 @@ public Lookup(
 
         _declaringTypeName = declaringTypeName;
         SchemaName = schemaName;
-        Name = name;
+        FieldName = fieldName;
         Arguments = arguments;
         Fields = fields;
     }
@@ -63,7 +63,12 @@ public Lookup(
     /// <summary>
     /// Gets the name of the lookup field.
     /// </summary>
-    public string Name { get; }
+    public string FieldName { get; }
+
+    /// <summary>
+    /// Get the name of lookup field type.
+    /// </summary>
+    public string FieldType => _declaringTypeName;
 
     /// <summary>
     /// Gets the arguments that represent field requirements.
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Metadata/SourceUnionType.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Metadata/SourceUnionType.cs
new file mode 100644
index 00000000000..9bc20835f72
--- /dev/null
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Metadata/SourceUnionType.cs
@@ -0,0 +1,15 @@
+using System.Collections.Immutable;
+
+namespace HotChocolate.Fusion.Types;
+
+public sealed class SourceUnionType(
+    string name,
+    string schemaName,
+    ImmutableArray<Lookup> lookups) : ISourceMember
+{
+    public string Name { get; } = name;
+
+    public string SchemaName { get; } = schemaName;
+
+    public ImmutableArray<Lookup> Lookups { get; } = lookups;
+}
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution/Planning/OperationDefinitionBuilder.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution/Planning/OperationDefinitionBuilder.cs
index 6c3f35f7d28..8222d050ef4 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution/Planning/OperationDefinitionBuilder.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution/Planning/OperationDefinitionBuilder.cs
@@ -12,6 +12,7 @@ internal sealed class OperationDefinitionBuilder
     private string? _name;
     private string? _description;
     private Lookup? _lookup;
+    private string? _lookupTypeRefinement;
     private string? _requirementKey;
     private SelectionSetNode? _selectionSet;
 
@@ -40,9 +41,10 @@ public OperationDefinitionBuilder SetDescription(string? description)
         return this;
     }
 
-    public OperationDefinitionBuilder SetLookup(Lookup? lookup, string? requirementKey)
+    public OperationDefinitionBuilder SetLookup(Lookup? lookup, string? lookupTypeRefinement, string? requirementKey)
     {
         _lookup = lookup;
+        _lookupTypeRefinement = lookupTypeRefinement;
         _requirementKey = requirementKey;
         return this;
     }
@@ -76,7 +78,7 @@ public OperationDefinitionBuilder SetSelectionSet(SelectionSetNode selectionSet)
             }
 
             var lookupField = new FieldNode(
-                new NameNode(_lookup.Name),
+                new NameNode(_lookup.FieldName),
                 null,
                 [],
                 arguments,
@@ -87,7 +89,13 @@ public OperationDefinitionBuilder SetSelectionSet(SelectionSetNode selectionSet)
             var indexBuilder = index.ToBuilder();
             indexBuilder.Register(selectionSet);
             index = indexBuilder;
-            selectionPath = selectionPath.AppendField(_lookup.Name);
+
+            if (!string.IsNullOrEmpty(_lookupTypeRefinement))
+            {
+                selectionPath = selectionPath.AppendFragment(_lookupTypeRefinement);
+            }
+
+            selectionPath = selectionPath.AppendField(_lookup.FieldName);
         }
 
         var definition = new OperationDefinitionNode(
diff --git a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution/Planning/OperationPlanner.cs b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution/Planning/OperationPlanner.cs
index fd81280d548..1b08659b3b2 100644
--- a/src/HotChocolate/Fusion-vnext/src/Fusion.Execution/Planning/OperationPlanner.cs
+++ b/src/HotChocolate/Fusion-vnext/src/Fusion.Execution/Planning/OperationPlanner.cs
@@ -293,7 +293,32 @@ lookup is null
                 requirements = requirements.Add(argumentRequirementKey, operationRequirement);
             }
 
-            operationBuilder.SetLookup(lookup, requirementKey);
+            string? lookupTypeRefinement = null;
+            if (_schema.Types.TryGetType(lookup.FieldType, out var lookupFieldType)
+                && lookupFieldType != workItem.SelectionSet.Type
+                && !resolvable.Selections.All(s => s is InlineFragmentNode inlineFragment
+                    && inlineFragment.TypeCondition?.Name.Value == workItem.SelectionSet.Type.Name))
+            {
+                var typeRefinement =
+                    new InlineFragmentNode(
+                        null,
+                        new NamedTypeNode(workItem.SelectionSet.Type.Name),
+                        [],
+                        resolvable);
+                var selectionSetWithTypeRefinement = new SelectionSetNode(null, [typeRefinement]);
+
+                var indexBuilder = index.ToBuilder();
+
+                indexBuilder.Register(resolvable, selectionSetWithTypeRefinement);
+
+                index = indexBuilder;
+
+                operationBuilder.SetSelectionSet(selectionSetWithTypeRefinement);
+
+                lookupTypeRefinement = workItem.SelectionSet.Type.Name;
+            }
+
+            operationBuilder.SetLookup(lookup, lookupTypeRefinement, requirementKey);
         }
 
         (var definition, index, var source) = operationBuilder.Build(index);
@@ -664,7 +689,32 @@ private void PlanFieldWithRequirement(
             requirements = requirements.Add(argumentRequirementKey, operationRequirement);
         }
 
-        operationBuilder.SetLookup(workItem.Lookup, requirementKey);
+        string? lookupTypeRefinement = null;
+        if (_schema.Types.TryGetType(lookup.FieldType, out var lookupFieldType)
+            && lookupFieldType != selectionSetStub.Type
+            && !selectionSetNode.Selections.All(s => s is InlineFragmentNode inlineFragment
+                && inlineFragment.TypeCondition?.Name.Value == selectionSetStub.Type.Name))
+        {
+            var typeRefinement =
+                new InlineFragmentNode(
+                    null,
+                    new NamedTypeNode(selectionSetStub.Type.Name),
+                    [],
+                    selectionSetNode);
+            var selectionSetWithTypeRefinement = new SelectionSetNode(null, [typeRefinement]);
+
+            var indexBuilder = index.ToBuilder();
+
+            indexBuilder.Register(selectionSetNode, selectionSetWithTypeRefinement);
+
+            index = indexBuilder;
+
+            operationBuilder.SetSelectionSet(selectionSetWithTypeRefinement);
+
+            lookupTypeRefinement = selectionSetStub.Type.Name;
+        }
+
+        operationBuilder.SetLookup(workItem.Lookup, lookupTypeRefinement, requirementKey);
 
         var (definition, _, source) = operationBuilder.Build(index);
 
@@ -1046,7 +1096,7 @@ public static void Enqueue(
                 break;
 
             case FieldRequirementWorkItem wi:
-                possiblePlans.EnqueueRequirePlanNodes(planNodeTemplate, wi);
+                possiblePlans.EnqueueRequirePlanNodes(planNodeTemplate, wi, compositeSchema);
                 break;
 
             default:
@@ -1083,7 +1133,7 @@ private static void EnqueueLookupPlanNodes(
 
         foreach (var (schemaName, resolutionCost) in compositeSchema.GetPossibleSchemas(workItem.SelectionSet))
         {
-            foreach (var lookup in workItem.SelectionSet.Type.GetPossibleLookups(schemaName))
+            foreach (var lookup in workItem.SelectionSet.Type.GetPossibleLookups(schemaName, compositeSchema))
             {
                 possiblePlans.Enqueue(
                     planNodeTemplate with
@@ -1100,7 +1150,8 @@ planNodeTemplate with
     private static void EnqueueRequirePlanNodes(
         this PriorityQueue<PlanNode, double> possiblePlans,
         PlanNode planNodeTemplate,
-        FieldRequirementWorkItem workItem)
+        FieldRequirementWorkItem workItem,
+        FusionSchemaDefinition compositeSchema)
     {
         var backlog = planNodeTemplate.Backlog.Pop();
 
@@ -1116,7 +1167,7 @@ planNodeTemplate with
                         Backlog = backlog.Push(workItem)
                     });
 
-                foreach (var lookup in workItem.Selection.Field.DeclaringType.GetPossibleLookups(schemaName))
+                foreach (var lookup in workItem.Selection.Field.DeclaringType.GetPossibleLookups(schemaName, compositeSchema))
                 {
                     possiblePlans.Enqueue(
                         planNodeTemplate with
@@ -1130,7 +1181,7 @@ planNodeTemplate with
             }
             else
             {
-                foreach (var lookup in workItem.Selection.Field.DeclaringType.GetPossibleLookups(schemaName))
+                foreach (var lookup in workItem.Selection.Field.DeclaringType.GetPossibleLookups(schemaName, compositeSchema))
                 {
                     possiblePlans.Enqueue(
                         planNodeTemplate with
@@ -1215,12 +1266,42 @@ static void TrackSchema(Dictionary<string, int> possibleSchemas, string schemaNa
         }
     }
 
-    private static IEnumerable<Lookup> GetPossibleLookups(this ITypeDefinition type, string schemaName)
+    private static IEnumerable<Lookup> GetPossibleLookups(
+        this ITypeDefinition type,
+        string schemaName,
+        FusionSchemaDefinition compositeSchema)
     {
+        // TODO: Currently we just check that the type exists in the given source schema
+        //       and that there are lookups for itself and / or the abstract types
+        //       it's a part of. However, we don't check that the type is part of the
+        //       abstract type in the given source schema.
         if (type is FusionComplexTypeDefinition complexType
-            && complexType.Sources.TryGetType(schemaName, out var source))
+            && complexType.Sources.TryGetMember(schemaName, out var source))
         {
-            return source.Lookups;
+            var lookups = new List<Lookup>(source.Lookups);
+
+            foreach (var interfaceType in complexType.Implements)
+            {
+                if (interfaceType.Sources.TryGetMember(schemaName, out var interfaceSource))
+                {
+                    lookups.AddRange(interfaceSource.Lookups);
+                }
+            }
+
+            var unionTypes = compositeSchema.Types
+                .OfType<FusionUnionTypeDefinition>()
+                .Where(u => u.Types.Contains(type))
+                .ToArray();
+
+            foreach (var unionType in unionTypes)
+            {
+                if (unionType.Sources.TryGetMember(schemaName, out var unionSource))
+                {
+                    lookups.AddRange(unionSource.Lookups);
+                }
+            }
+
+            return lookups;
         }
 
         return [];
diff --git a/src/HotChocolate/Fusion-vnext/test/Fusion.Execution.Tests/Planning/OperationPlannerTests.cs b/src/HotChocolate/Fusion-vnext/test/Fusion.Execution.Tests/Planning/OperationPlannerTests.cs
index 85fb44d2f74..b2c5899a61a 100644
--- a/src/HotChocolate/Fusion-vnext/test/Fusion.Execution.Tests/Planning/OperationPlannerTests.cs
+++ b/src/HotChocolate/Fusion-vnext/test/Fusion.Execution.Tests/Planning/OperationPlannerTests.cs
@@ -148,6 +148,116 @@ query GetTopProducts {
         MatchSnapshot(plan);
     }
 
+    [Fact]
+    public void Plan_Simple_Interface_Lookup()
+    {
+        // arrange
+        var schema = ComposeSchema(
+            """
+            schema @schemaName(value: "A") {
+              query: Query
+            }
+
+            type Query {
+              topProduct: Product
+            }
+
+            type Product {
+              id: ID!
+              name: String!
+            }
+            """,
+            """
+            schema @schemaName(value: "B") {
+              query: Query
+            }
+
+            type Query {
+              node(id: ID!): Node @lookup @internal
+            }
+
+            interface Node {
+              id: ID!
+            }
+
+            type Product implements Node {
+              id: ID!
+              price: Float!
+            }
+            """);
+
+        // act
+        var plan = PlanOperation(
+            schema,
+            """
+            query GetTopProducts {
+              topProduct {
+                id
+                name
+                price
+              }
+            }
+            """);
+
+        // assert
+        MatchSnapshot(plan);
+    }
+
+    [Fact]
+    public void Plan_Simple_Union_Lookup()
+    {
+        // arrange
+        var schema = ComposeSchema(
+            """
+            schema @schemaName(value: "A") {
+              query: Query
+            }
+
+            type Query {
+              topProduct: Product
+              # Just here to satisfy satisfiability as I can't make the union lookup internal...
+              productById(id: ID!): Product @lookup @internal
+            }
+
+            type Product {
+              id: ID!
+              name: String!
+            }
+            """,
+            """
+            schema @schemaName(value: "B") {
+              query: Query
+            }
+
+            type Query {
+              lookupUnionById(id: ID!): SomeUnion @lookup
+            }
+
+            union SomeUnion = Product
+
+            type Product {
+              id: ID!
+              price: Float!
+            }
+            """);
+
+        // act
+        var plan = PlanOperation(
+            schema,
+            """
+            query GetTopProducts {
+              topProduct {
+                id
+                name
+                price
+              }
+            }
+            """);
+
+        // assert
+        MatchSnapshot(plan);
+    }
+
     [Fact]
     public void Plan_Simple_Requirement()
     {
diff --git a/src/HotChocolate/Fusion-vnext/test/Fusion.Execution.Tests/Planning/__snapshots__/OperationPlannerTests.Plan_Simple_Interface_Lookup.yaml b/src/HotChocolate/Fusion-vnext/test/Fusion.Execution.Tests/Planning/__snapshots__/OperationPlannerTests.Plan_Simple_Interface_Lookup.yaml
new file mode 100644
index 00000000000..a6080f2fdcc
--- /dev/null
+++ b/src/HotChocolate/Fusion-vnext/test/Fusion.Execution.Tests/Planning/__snapshots__/OperationPlannerTests.Plan_Simple_Interface_Lookup.yaml
@@ -0,0 +1,41 @@
+operation:
+  - document: >-
+      query GetTopProducts {
+        topProduct {
+          id
+          name
+          price
+          id @fusion__requirement
+        }
+      }
+    name: GetTopProducts
+    hash: 123
+nodes:
+  - id: 1
+    schema: A
+    operation: >-
+      query GetTopProducts_123_1 {
+        topProduct {
+          id
+          name
+        }
+      }
+  - id: 2
+    schema: B
+    operation: >-
+      query GetTopProducts_123_2(
+        $__fusion_1_id: ID!
+      ) {
+        node(id: $__fusion_1_id) {
+          ... on Product {
+            price
+          }
+        }
+      }
+    source: $.node.<Product>
+    target: $.topProduct
+    requirements:
+      - name: __fusion_1_id
+        selectionMap: id
+    dependencies:
+      - id: 1
diff --git a/src/HotChocolate/Fusion-vnext/test/Fusion.Execution.Tests/Planning/__snapshots__/OperationPlannerTests.Plan_Simple_Union_Lookup.yaml b/src/HotChocolate/Fusion-vnext/test/Fusion.Execution.Tests/Planning/__snapshots__/OperationPlannerTests.Plan_Simple_Union_Lookup.yaml
new file mode 100644
index 00000000000..052aeaaaf0e
--- /dev/null
+++ b/src/HotChocolate/Fusion-vnext/test/Fusion.Execution.Tests/Planning/__snapshots__/OperationPlannerTests.Plan_Simple_Union_Lookup.yaml
@@ -0,0 +1,41 @@
+operation:
+  - document: >-
+      query GetTopProducts {
+        topProduct {
+          id
+          name
+          price
+          id @fusion__requirement
+        }
+      }
+    name: GetTopProducts
+    hash: 123
+nodes:
+  - id: 1
+    schema: A
+    operation: >-
+      query GetTopProducts_123_1 {
+        topProduct {
+          id
+          name
+        }
+      }
+  - id: 2
+    schema: B
+    operation: >-
+      query GetTopProducts_123_2(
+        $__fusion_1_id: ID!
+      ) {
+        lookupUnionById(id: $__fusion_1_id) {
+          ... on Product {
+            price
+          }
+        }
+      }
+    source: $.lookupUnionById.<Product>
+    target: $.topProduct
+    requirements:
+      - name: __fusion_1_id
+        selectionMap: id
+    dependencies:
+      - id: 1