Fix duplicate inherited discriminators in Java models (#11824)

## Summary

- merge redundant leaf discriminator declarations with propagated parent
discriminators
- validate that duplicate declarations have matching constant values and
Java types
- emit fixed propagated discriminator fields as `final` without changing
shared serialization behavior
- regenerate Java clientcore and generator-test golden models

Fixes #11802

## Testing

- Java generator build
- Java emitter build
- generator core tests (53 passed)
- full `http-client-generator-clientcore-test/Generate.ps1` regeneration
- full `http-client-generator-test/Generate.ps1` regeneration
- generated the Foundry Agents service spec from #11802, including a
partial-update pass, and compiled all 496 Java sources with Java 17
- Java formatting and lint checks

---------

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Copilot-Session: 54b93dc2-48ce-4250-a6ea-5bd9b9a7ab2b
Copilot-Session: 00e7fd30-d342-4b5a-ad7e-f16198c7dbde
Copilot-Session: e6933b7d-0cee-4855-a585-2e15b24c70c5
Copilot-Session: d4e8259f-d938-4448-9321-8eaf17f8b53d

Author: 3 people

.chronus/changes/fix-java-duplicate-discriminators-2026-09-02.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-java"
+---
+
+Prevent duplicate Java discriminator members while preserving inherited discriminators in stream-style XML serialization.

packages/http-client-java/generator/http-client-generator-clientcore-test/src/main/java/type/model/inheritance/nesteddiscriminator/GoblinShark.java

@@ -16,7 +16,7 @@ public final class GoblinShark extends Shark {
      * Discriminator property for Fish.
      */
     @Metadata(properties = { MetadataProperties.GENERATED })
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

packages/http-client-java/generator/http-client-generator-clientcore-test/src/main/java/type/model/inheritance/nesteddiscriminator/SawShark.java

@@ -16,7 +16,7 @@ public final class SawShark extends Shark {
      * Discriminator property for Fish.
      */
     @Metadata(properties = { MetadataProperties.GENERATED })
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

packages/http-client-java/generator/http-client-generator-clientcore-test/src/main/java/type/model/inheritance/nesteddiscriminator/Shark.java

@@ -16,7 +16,7 @@ public class Shark extends Fish {
      * Discriminator property for Fish.
      */
     @Metadata(properties = { MetadataProperties.GENERATED })
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.

packages/http-client-java/generator/http-client-generator-core/src/main/java/com/microsoft/typespec/http/client/generator/core/implementation/ClientModelPropertiesManager.java

@@ -22,6 +22,7 @@
 import java.util.function.BiConsumer;
 import java.util.function.Consumer;
 import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 /**
  * Manages metadata about properties in a {@link ClientModel} and how they correlate with model class generation.
@@ -124,10 +125,11 @@ public ClientModelPropertiesManager(ClientModel model, JavaSettings settings) {
             xmlRootElementNamespace = model.getXmlNamespace();
         }
 
-        Set<String> thisModelPropertySerializeNames = model.getProperties()
-            .stream()
+        Set<String> thisModelPropertySerializeNames = Stream.concat(
             // discriminator property is known to be redefined in subclass
-            .filter(property -> !property.isPolymorphicDiscriminator())
+            model.getProperties().stream().filter(property -> !property.isPolymorphicDiscriminator()),
+            // Canonicalized parent discriminators mask inherited ordinary properties with the same wire name.
+            model.getParentPolymorphicDiscriminators().stream())
             .map(ClientModelProperty::getSerializedName)
             .filter(name -> Objects.nonNull(name) && !name.isEmpty())
             .collect(Collectors.toSet());

packages/http-client-java/generator/http-client-generator-core/src/main/java/com/microsoft/typespec/http/client/generator/core/implementation/PolymorphicDiscriminatorHandler.java

@@ -12,6 +12,7 @@
 import com.microsoft.typespec.http.client.generator.core.model.javamodel.JavaFile;
 import com.microsoft.typespec.http.client.generator.core.model.javamodel.JavaVisibility;
 import com.microsoft.typespec.http.client.generator.core.util.ClientModelUtil;
+import java.util.Objects;
 import java.util.function.Consumer;
 import java.util.function.Function;
 
@@ -116,7 +117,13 @@ private static void declareFieldInternal(ClientModelProperty discriminator, Clie
                 && settings.isShareJsonSerializableCode()) {
                 classBlock.memberVariable(JavaVisibility.PackagePrivate, fieldSignature);
             } else if (!allPolymorphicModelsInSamePackage || !settings.isShareJsonSerializableCode()) {
-                classBlock.privateMemberVariable(fieldSignature);
+                // Fixed inherited parent discriminators are final; active discriminators remain mutable for fallback.
+                if (discriminator.isConstant()
+                    && !Objects.equals(discriminator.getSerializedName(), model.getPolymorphicDiscriminatorName())) {
+                    classBlock.privateFinalMemberVariable(fieldSignature);
+                } else {
+                    classBlock.privateMemberVariable(fieldSignature);
+                }
             }
         }
     }

packages/http-client-java/generator/http-client-generator-core/src/main/java/com/microsoft/typespec/http/client/generator/core/mapper/ModelMapper.java

@@ -28,6 +28,7 @@
 import java.util.Collection;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.ListIterator;
 import java.util.Objects;
 import java.util.Set;
 import java.util.function.Function;
@@ -355,19 +356,15 @@ public ClientModel map(ObjectSchema compositeType) {
             result = builder.build();
 
             if (isPolymorphic && !CoreUtils.isNullOrEmpty(derivedTypes)) {
-                // Walk the polymorphic hierarchy finding places where the parent model and child model have different
-                // polymorphic discriminators. When this case is found add the parent polymorphic discriminator as a
-                // parent
-                // polymorphic discriminator to the child model. This is necessary to ensure that the child model
-                // generates
-                // the correct serialization in multi-level polymorphic structures.
+                // Preserve the fixed outer discriminator when a child starts a nested discriminator hierarchy.
                 for (ClientModel derivedType : derivedTypes) {
                     if (!Objects.equals(polymorphicDiscriminator, derivedType.getPolymorphicDiscriminatorName())) {
                         ClientModelProperty parentDiscriminator = result.getPolymorphicDiscriminator()
                             .newBuilder()
                             .defaultValue(result.getPolymorphicDiscriminator()
                                 .getClientType()
                                 .defaultValueExpression(derivedType.getSerializedName()))
+                            .constant(true)
                             .build();
 
                         passPolymorphicDiscriminatorToChildren(parentDiscriminator, derivedType);
@@ -381,14 +378,60 @@ public ClientModel map(ObjectSchema compositeType) {
         return result;
     }
 
+    /**
+     * Propagates a fixed discriminator from an outer hierarchy through a nested discriminator hierarchy.
+     * <p>
+     * The {@code parentDiscriminator} is the fixed outer selection, while {@code child}'s discriminator remains the
+     * active discriminator for its own descendants. For example, given an outer {@code type} discriminator, a
+     * {@code type="message"} child that introduces {@code role}, and a {@code role="assistant"} grandchild, both nested
+     * models retain the canonical {@code type="message"} value while {@code role} controls nested dispatch.
+     * <p>
+     * A child can also declare an ordinary fixed property with the same wire name as the propagated discriminator.
+     * Keeping both representations would generate duplicate fields and accessors. Such a property must match
+     * {@code parentDiscriminator}'s Java name, wire type, client type, and fixed value, and must be constant; otherwise
+     * mapping fails. A valid property is removed from {@link ClientModel#getProperties()}, leaving
+     * {@code parentDiscriminator} as the canonical entry in
+     * {@link ClientModel#getParentPolymorphicDiscriminators()}.
+     * <p>
+     * Parent models map after their children, so the canonical entry is inserted at index zero to retain
+     * outer-to-inner discriminator order. The fixed discriminator is then recursively propagated to every descendant.
+     * For current stream-style JSON, same-package hierarchies may serialize this metadata through shared
+     * {@code toJsonShared} code. When hierarchy models are in different packages, or sharing is disabled, each model
+     * serializes inherited discriminator metadata through {@code serializeParentJsonProperties}, which consumes
+     * {@link ClientModel#getParentPolymorphicDiscriminators()}.
+     *
+     * @param parentDiscriminator the fixed discriminator selected by the outer hierarchy
+     * @param child the nested-hierarchy model that receives the fixed discriminator
+     * @throws IllegalStateException if the child declares the same wire name without matching constant status, Java
+     * name, wire type, client type, and fixed value
+     */
     private static void passPolymorphicDiscriminatorToChildren(ClientModelProperty parentDiscriminator,
         ClientModel child) {
-        // Due to the execution order of ModelMapper, where children models complete mapping before the parent model,
-        // the parent polymorphic discriminator needs to be added at index 0. Reason, given an example where there are
-        // three models, where model #1 is the root parent with discriminator type, model #2 is a child of model #2 with
-        // discriminator kind, and model #3 is a child of model #3 with discriminator form. The order if this running
-        // will have model #2 add its discriminator to model #3 before model #1 runs adding its discriminator to #2 and
-        // #3. We want #3 to have the ordering of [type, kind], to represent the ordering of the parent models.
+        ListIterator<ClientModelProperty> iterator = child.getProperties().listIterator();
+        while (iterator.hasNext()) {
+            ClientModelProperty childProperty = iterator.next();
+            if (!Objects.equals(parentDiscriminator.getSerializedName(), childProperty.getSerializedName())) {
+                continue;
+            }
+
+            if (!childProperty.isConstant()
+                || !Objects.equals(parentDiscriminator.getName(), childProperty.getName())
+                || !Objects.equals(parentDiscriminator.getWireType(), childProperty.getWireType())
+                || !Objects.equals(parentDiscriminator.getClientType(), childProperty.getClientType())
+                || !Objects.equals(parentDiscriminator.getDefaultValue(), childProperty.getDefaultValue())) {
+                throw new IllegalStateException("Property '" + childProperty.getSerializedName() + "' on model '"
+                    + child.getName() + "' does not match its inherited polymorphic discriminator. Expected (name="
+                    + parentDiscriminator.getName() + ", constant=true, type=" + parentDiscriminator.getClientType()
+                    + ", value=" + String.valueOf(parentDiscriminator.getDefaultValue()) + "), but found (name="
+                    + childProperty.getName() + ", constant=" + childProperty.isConstant() + ", type="
+                    + childProperty.getClientType() + ", value=" + String.valueOf(childProperty.getDefaultValue())
+                    + ").");
+            }
+
+            iterator.remove();
+            break;
+        }
+
         child.getParentPolymorphicDiscriminators().add(0, parentDiscriminator);
 
         for (ClientModel derived : child.getDerivedModels()) {

packages/http-client-java/generator/http-client-generator-core/src/main/java/com/microsoft/typespec/http/client/generator/core/template/ModelTemplate.java

@@ -875,31 +875,19 @@ private void addModelConstructor(ClientModel model, ClientModelPropertiesManager
 
                         superProperties.append(property.getName());
                     } else {
-                        /*
-                         * here because the property in superclass constructor is overwritten in this model
-                         * one example is
-                         *
-                         * model ParentModel {
-                         * property: string;
-                         * }
-                         * model Model extends ParentModel {
-                         * property: "constant";
-                         * }
-                         *
-                         * we use the property in this model to initiate the superclass
-                         */
-                        ClientModelProperty propertyInThisModel = model.getProperties()
-                            .stream()
+                        // Canonicalized discriminators can supply a fixed inherited constructor argument.
+                        ClientModelProperty overridingProperty = Stream
+                            .concat(model.getProperties().stream(), model.getParentPolymorphicDiscriminators().stream())
                             .filter(p -> Objects.equals(p.getSerializedName(), property.getSerializedName()))
                             .findFirst()
                             .orElse(null);
-                        if (propertyInThisModel != null) {
-                            if (propertyInThisModel.isConstant() && !property.isConstant()) {
+                        if (overridingProperty != null) {
+                            if (overridingProperty.isConstant() && !property.isConstant()) {
                                 // property changed to constant in this model, use constant value to initiate super
                                 // class
-                                superProperties.append(propertyInThisModel.getDefaultValue());
+                                superProperties.append(overridingProperty.getDefaultValue());
                             } else {
-                                superProperties.append(propertyInThisModel.getName());
+                                superProperties.append(overridingProperty.getName());
                             }
                         } else {
                             // this should not happen

packages/http-client-java/generator/http-client-generator-core/src/main/java/com/microsoft/typespec/http/client/generator/core/template/StreamSerializationModelTemplate.java

@@ -2059,8 +2059,16 @@ private void writeToXml(JavaClass classBlock) {
                             + propertiesManager.getXmlNamespaceConstant(namespace) + ");"));
 
                     // Assumption for XML is polymorphic discriminators are attributes.
-                    if (propertiesManager.getDiscriminatorProperty() != null) {
-                        serializeXml(methodBlock, propertiesManager.getDiscriminatorProperty().getProperty(), false);
+                    ClientModelPropertyWithMetadata discriminatorProperty
+                        = propertiesManager.getDiscriminatorProperty();
+                    model.getParentPolymorphicDiscriminators()
+                        .stream()
+                        .filter(discriminator -> discriminatorProperty == null
+                            || !Objects.equals(discriminator.getSerializedName(),
+                                discriminatorProperty.getProperty().getSerializedName()))
+                        .forEach(discriminator -> serializeXml(methodBlock, discriminator, false));
+                    if (discriminatorProperty != null) {
+                        serializeXml(methodBlock, discriminatorProperty.getProperty(), false);
                     }
 
                     propertiesManager.forEachSuperXmlAttribute(property -> serializeXml(methodBlock, property, true));
@@ -2214,7 +2222,7 @@ private void writeSuperTypeFromXml(JavaClass classBlock) {
                         + propertiesManager.getXmlNamespaceConstant(discriminatorProperty.getXmlNamespace()) + ", "
                         + "\"" + discriminatorProperty.getSerializedName() + "\");");
                 } else {
-                    methodBlock.line("String discriminatorValue = reader.getStringAttribute(" + "\""
+                    methodBlock.line("String discriminatorValue = reader.getStringAttribute(null, " + "\""
                         + discriminatorProperty.getSerializedName() + "\");");
                 }
 
@@ -2226,12 +2234,18 @@ private void writeSuperTypeFromXml(JavaClass classBlock) {
                 // Add deserialization for all child types.
                 List<ClientModel> childTypes = getAllChildTypes(model, new ArrayList<>());
                 for (ClientModel childType : childTypes) {
+                    boolean sameDiscriminator = Objects.equals(childType.getPolymorphicDiscriminatorName(),
+                        model.getPolymorphicDiscriminatorName());
+                    if (!sameDiscriminator && !Objects.equals(childType.getParentModelName(), model.getName())) {
+                        continue;
+                    }
+
+                    String deserializationMethod = (isSuperTypeWithDiscriminator(childType) && sameDiscriminator)
+                        ? ".fromXmlInternal(reader, finalRootElementName)"
+                        : ".fromXml(reader, finalRootElementName)";
                     ifBlock = ifOrElseIf(methodBlock, ifBlock,
                         "\"" + childType.getSerializedName() + "\".equals(discriminatorValue)",
-                        ifStatement -> ifStatement
-                            .methodReturn(childType.getName() + (isSuperTypeWithDiscriminator(childType)
-                                ? ".fromXmlInternal(reader, finalRootElementName)"
-                                : ".fromXml(reader, finalRootElementName)")));
+                        ifStatement -> ifStatement.methodReturn(childType.getName() + deserializationMethod));
                 }
 
                 if (ifBlock == null) {
@@ -2439,6 +2453,10 @@ private void writeFromXmlDeserialization(JavaBlock methodBlock) {
         }
 
         private void deserializeXmlAttribute(JavaBlock methodBlock, ClientModelProperty attribute, boolean fromSuper) {
+            if (attribute.isRequired() && attribute.isConstant() && !attribute.isPolymorphicDiscriminator()) {
+                return;
+            }
+
             String xmlAttributeDeserialization = getSimpleXmlDeserialization(attribute.getWireType(), null,
                 attribute.getXmlName(), propertiesManager.getXmlNamespaceConstant(attribute.getXmlNamespace()), true);
 

packages/http-client-java/generator/http-client-generator-test/src/main/java/tsptest/armstreamstyleserialization/models/GoblinShark.java

@@ -21,7 +21,7 @@ public final class GoblinShark extends Shark {
     /*
      * Discriminator property for Fish.
      */
-    private String kind = "shark";
+    private final String kind = "shark";
 
     /*
      * The sharktype property.