fix: TryAddPath exact-match check for duplicate bound operation paths

When a duplicate path is detected in TryAddPath, the existing heuristic
checks if the new operation's binding type has derived types and, if so,
keeps the first entry assuming it was more specific. This is incorrect
when the first entry came from a base type (e.g., directoryObject) and
the new entry is the exact match for the entity set (e.g., servicePrincipal).

Add an exact-match check: if the operation's binding type matches the
entity set type, always replace the existing entry regardless of derived
types. This ensures paths like /servicePrincipals/delta() return the
correct type (servicePrincipal) instead of the base type (directoryObject).

Fixes #808

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: gavinbarron

src/Microsoft.OpenApi.OData.Reader/Common/Utils.cs

@@ -372,6 +372,13 @@ internal static bool TryAddPath(this IDictionary<string, OpenApiPathItem> pathIt
 
                 if (derivedTypes?.Any() ?? false)
                 {
+                    if (boundEntityType != null && boundEntityType == operationEntityType)
+                    {
+                        // The operation's binding type exactly matches the entity set's type,
+                        // so this is a more specific overload than whatever was added first.
+                        pathItems[pathName] = pathItem;
+                        return true;
+                    }
                     if (boundEntityType != null && !derivedTypes.Contains(boundEntityType))
                     {
                         Debug.WriteLine($"Duplicate paths present but entity type of binding parameter '{operationEntityType}' " +

test/Microsoft.OpenAPI.OData.Reader.Tests/Edm/ODataPathProviderTests.cs

@@ -12,6 +12,7 @@
 using Microsoft.OData.Edm;
 using Microsoft.OData.Edm.Csdl;
 using Microsoft.OData.Edm.Validation;
+using Microsoft.OpenApi.Models;
 using Microsoft.OpenApi.OData.Tests;
 using Xunit;
 
@@ -921,6 +922,76 @@ private static IEdmModel GetNavPropModel(string annotation)
             return GetEdmModel(template);
         }
 
+        [Fact]
+        public void GetPathsForDerivedTypeDeltaFunctionUsesCorrectReturnType()
+        {
+            // Arrange – mirrors the Graph scenario:
+            //   directoryObject (base) has delta with RequiresExplicitBinding
+            //   servicePrincipal (derived) has its own delta
+            //   agentIdentity (derived from servicePrincipal) causes servicePrincipal to have derived types
+            // Bug: TryAddPath kept the base-type delta for /servicePrincipals/delta() because
+            //      servicePrincipal has derived types.
+            string csdl = @"<edmx:Edmx Version=""4.0"" xmlns:edmx=""http://docs.oasis-open.org/odata/ns/edmx"">
+  <edmx:DataServices>
+    <Schema Namespace=""NS"" xmlns=""http://docs.oasis-open.org/odata/ns/edm"">
+      <EntityType Name=""directoryObject"">
+        <Key>
+          <PropertyRef Name=""id"" />
+        </Key>
+        <Property Name=""id"" Type=""Edm.String"" Nullable=""false"" />
+      </EntityType>
+      <EntityType Name=""servicePrincipal"" BaseType=""NS.directoryObject"">
+        <Property Name=""appId"" Type=""Edm.String"" />
+      </EntityType>
+      <EntityType Name=""agentIdentity"" BaseType=""NS.servicePrincipal"">
+        <Property Name=""blueprintId"" Type=""Edm.String"" />
+      </EntityType>
+      <Function Name=""delta"" IsBound=""true"">
+        <Parameter Name=""bindingParameter"" Type=""Collection(NS.directoryObject)"" />
+        <ReturnType Type=""Collection(NS.directoryObject)"" />
+        <Annotation Term=""Org.OData.Core.V1.RequiresExplicitBinding"" />
+      </Function>
+      <Function Name=""delta"" IsBound=""true"">
+        <Parameter Name=""bindingParameter"" Type=""Collection(NS.servicePrincipal)"" />
+        <ReturnType Type=""Collection(NS.servicePrincipal)"" />
+      </Function>
+      <EntityContainer Name=""Default"">
+        <EntitySet Name=""directoryObjects"" EntityType=""NS.directoryObject"" />
+        <EntitySet Name=""servicePrincipals"" EntityType=""NS.servicePrincipal"" />
+      </EntityContainer>
+      <Annotations Target=""NS.directoryObject"">
+        <Annotation Term=""Org.OData.Core.V1.ExplicitOperationBindings"">
+          <Collection>
+            <String>NS.delta</String>
+          </Collection>
+        </Annotation>
+      </Annotations>
+    </Schema>
+  </edmx:DataServices>
+</edmx:Edmx>";
+
+            bool result = CsdlReader.TryParse(XElement.Parse(csdl).CreateReader(), out IEdmModel model, out _);
+            Assert.True(result);
+
+            var settings = new OpenApiConvertSettings();
+            OpenApiDocument doc = model.ConvertToOpenApi(settings);
+
+            // Assert – the /servicePrincipals/NS.delta() path should exist and its response
+            // should reference NS.servicePrincipal, not NS.directoryObject
+            Assert.True(doc.Paths.ContainsKey("/servicePrincipals/NS.delta()"),
+                $"Expected path not found. Available: {string.Join(", ", doc.Paths.Keys)}");
+
+            var pathItem = doc.Paths["/servicePrincipals/NS.delta()"];
+            var getOp = pathItem.Operations[OperationType.Get];
+            Assert.NotNull(getOp);
+
+            // The response schema title should reference servicePrincipal, not directoryObject
+            var responseKey = getOp.Responses.Keys.First();
+            var response = getOp.Responses[responseKey];
+            var schema = response.Content["application/json"].Schema;
+            Assert.Contains("servicePrincipal", schema.Title);
+        }
+
         private static IEdmModel GetEdmModel(string schema)
         {
             bool parsed = SchemaReader.TryParse(new XmlReader[] { XmlReader.Create(new StringReader(schema)) }, out IEdmModel parsedModel, out IEnumerable<EdmError> errors);