Set useResultSchema on mcpToolTrigger bindings for rich return types by ahmedmuhsin · Pull Request #2554 · microsoft/azure-maven-plugins

ahmedmuhsin · 2026-03-16T22:08:01Z

Summary

Adds return-type inspection to McpAnnotationProcessor to automatically set useResultSchema=true on mcpToolTrigger bindings in function.json when the function returns a rich content type. Also bumps azure-functions-maven-plugin version to 1.42.0.

Changes

McpAnnotationProcessor.java

New method setUseResultSchemaIfNeeded(Method, List of Binding) that inspects the function method's return type
Detects rich types by FQCN string matching: McpToolResult, MCP SDK Content types (TextContent, ImageContent, AudioContent, ResourceLink, EmbeddedResource, CallToolResult)
Detects @McpContent-annotated POJOs via annotation FQCN
Handles List of Content via generic type argument inspection
Checks superclass chain and implemented interfaces for subtype detection
Only sets the flag when there are no output bindings (matches C# McpUseResultSchemaTransformer behavior)

AnnotationHandlerImpl.java

One-line addition: calls McpAnnotationProcessor.setUseResultSchemaIfNeeded(method, bindings) after MCP annotation processing in generateConfiguration()

azure-functions-maven-plugin/pom.xml

Version bump from 1.41.0 to 1.42.0

Design

Uses FQCN string comparison to avoid compile-time dependencies on the MCP modules
The useResultSchema flag tells the host extension to use ToolReturnValueBinder (which understands the McpToolResult envelope) instead of SimpleToolReturnValueBinder (which just calls .toString())

Related PRs

Add azure-functions-java-mcp module for rich content and structured content support Azure/azure-functions-java-additions#49 - New azure-functions-java-mcp module with middleware and types
Add structured content sample functions and azure-functions-java-mcp dependency Azure-Samples/remote-mcp-functions-java#17 - Sample functions demonstrating structured content

Add McpAnnotationProcessor.setUseResultSchemaIfNeeded() which inspects the function method's return type and sets useResultSchema=true on the mcpToolTrigger binding when the return type is: - McpToolResult (azure-functions-java-mcp) - MCP SDK Content types (TextContent, ImageContent, ResourceLink, etc.) - @McpContent-annotated POJOs - List<Content> (including generic type inspection) The flag is only set when there are no output bindings, matching the behavior of the C# McpUseResultSchemaTransformer. Type detection uses FQCN string matching and interface/superclass walking to avoid compile-time dependencies on the MCP modules.

Flanker32 · 2026-03-26T01:59:30Z

...in/java/com/microsoft/azure/toolkit/lib/legacy/function/handlers/McpAnnotationProcessor.java

+                if (implementsInterface(elemClass, MCP_CONTENT_INTERFACE_FQCN)) {
+                    return true;
+                }
+                if (isSubclassOfRichType(elemClass)) {


Do we also need to check whether elemClass hasAnnotationByFqcn

Copilot

Pull request overview

This PR enhances MCP (Model Context Protocol) Azure Functions annotation processing so that generated function.json bindings automatically enable useResultSchema=true for MCP tool triggers when a function returns rich MCP content types, aligning return-value binding behavior with the host extension’s expectations. It also updates the azure-functions-maven-plugin version.

Changes:

Add return-type inspection to set useResultSchema=true on McpToolTrigger bindings when appropriate.
Invoke the new inspection logic during configuration generation.
Bump azure-functions-maven-plugin from 1.41.0 to 1.42.0.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
azure-toolkit-libs/azure-toolkit-appservice-lib/src/main/java/com/microsoft/azure/toolkit/lib/legacy/function/handlers/McpAnnotationProcessor.java	Adds return-type detection logic to decide when to set `useResultSchema` for MCP tool triggers.
azure-toolkit-libs/azure-toolkit-appservice-lib/src/main/java/com/microsoft/azure/toolkit/lib/legacy/function/handlers/AnnotationHandlerImpl.java	Calls the new `setUseResultSchemaIfNeeded` hook during configuration generation.
azure-functions-maven-plugin/pom.xml	Updates plugin version to `1.42.0`.

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Copilot · 2026-03-26T02:02:02Z

...in/java/com/microsoft/azure/toolkit/lib/legacy/function/handlers/McpAnnotationProcessor.java

+    private static final Set<String> RICH_RESULT_TYPE_FQCNS = Set.of(
+        "com.microsoft.azure.functions.mcp.McpToolResult",
+        "io.modelcontextprotocol.spec.McpSchema.CallToolResult"
+    );


RICH_RESULT_TYPE_FQCNS is initialized with Set.of(...), which is only available starting in Java 9. This module is configured for Java 8 (maven.compiler.source/target = 1.8), so this will not compile. Use a Java 8 compatible initialization (e.g., Collections.unmodifiableSet(new HashSet<>(Arrays.asList(...)))).

Copilot · 2026-03-26T02:02:02Z

...in/java/com/microsoft/azure/toolkit/lib/legacy/function/handlers/McpAnnotationProcessor.java

+                .filter(b -> b.getBindingEnum() == BindingEnum.McpToolTrigger)
+                .findFirst();
+
+        if (mcpToolTrigger.isEmpty()) {


Optional.isEmpty() is used here, but Optional#isEmpty was added in Java 11. Since this project targets Java 8, this will not compile. Replace with !mcpToolTrigger.isPresent() (or avoid Optional entirely by using findFirst().orElse(null)).

Suggested change

if (mcpToolTrigger.isEmpty()) {

if (!mcpToolTrigger.isPresent()) {

Copilot · 2026-03-26T02:02:02Z

...in/java/com/microsoft/azure/toolkit/lib/legacy/function/handlers/McpAnnotationProcessor.java

+        // Check List<Content> or List<? extends Content> via generic return type
+        if (List.class.isAssignableFrom(returnType) && genericReturnType instanceof ParameterizedType) {
+            final ParameterizedType pt = (ParameterizedType) genericReturnType;
+            final Type[] typeArgs = pt.getActualTypeArguments();
+            if (typeArgs.length > 0 && typeArgs[0] instanceof Class<?>) {
+                final Class<?> elemClass = (Class<?>) typeArgs[0];
+                final String elemFqcn = elemClass.getCanonicalName();
+                if (elemFqcn != null && RICH_RESULT_TYPE_FQCNS.contains(elemFqcn)) {
+                    return true;
+                }
+                if (implementsInterface(elemClass, MCP_CONTENT_INTERFACE_FQCN)) {
+                    return true;
+                }
+                if (isSubclassOfRichType(elemClass)) {
+                    return true;
+                }
+            }


The List<Content> or List<? extends Content> check doesn't actually handle wildcards or other non-Class<?> type arguments: typeArgs[0] will be a WildcardType for List<? extends Content>, so this branch won't detect it. To match the comment/PR intent, handle WildcardType (upper bounds) and ParameterizedType when extracting the element type.

Copilot · 2026-03-26T02:02:03Z

...in/java/com/microsoft/azure/toolkit/lib/legacy/function/handlers/McpAnnotationProcessor.java

+     * Checks whether a class implements or extends any known rich result type by walking
+     * both the superclass chain and the implemented interfaces.


The Javadoc says this method walks "both the superclass chain and the implemented interfaces", but the implementation only checks the superclass chain. Either update the comment to match the implementation or extend the method to also inspect implemented interfaces if that's required for correctness.

Suggested change

* Checks whether a class implements or extends any known rich result type by walking

* both the superclass chain and the implemented interfaces.

* Checks whether a class extends any known rich result type by walking

* up the superclass chain.

Copilot · 2026-03-26T02:02:03Z

...in/java/com/microsoft/azure/toolkit/lib/legacy/function/handlers/McpAnnotationProcessor.java

+    public static void setUseResultSchemaIfNeeded(final Method method, final List<Binding> bindings) {
+        // Find the MCP tool trigger binding
+        final Optional<Binding> mcpToolTrigger = bindings.stream()
+                .filter(b -> b.getBindingEnum() == BindingEnum.McpToolTrigger)
+                .findFirst();
+
+        if (mcpToolTrigger.isEmpty()) {
+            return;
+        }
+
+        // Don't set useResultSchema when there are output bindings
+        final boolean hasOutputBindings = bindings.stream()
+                .anyMatch(b -> b.getBindingEnum().getDirection() == BindingEnum.Direction.OUT);
+        if (hasOutputBindings) {
+            return;
+        }
+
+        final Class<?> returnType = method.getReturnType();
+        if (returnType.equals(Void.TYPE)) {
+            return;
+        }
+
+        if (needsResultSchema(returnType, method.getGenericReturnType())) {
+            mcpToolTrigger.get().setAttribute("useResultSchema", true);
+        }
+    }


New return-type inspection logic (setUseResultSchemaIfNeeded / needsResultSchema) is not covered by unit tests. There are existing tests for McpAnnotationProcessor in this module; please add tests for the key scenarios (no output bindings vs. with output bindings; direct rich type; Content implementations; @McpContent POJO; and List<Content> / wildcard generic cases).

This was referenced Mar 16, 2026

Add azure-functions-java-mcp module for rich content and structured content support Azure/azure-functions-java-additions#49

Open

Add structured content sample functions and azure-functions-java-mcp dependency Azure-Samples/remote-mcp-functions-java#17

Open

ahmedmuhsin force-pushed the feature/mcp-structured-content branch from 0bd8840 to 03b6d2b Compare March 17, 2026 18:57

ahmedmuhsin force-pushed the feature/mcp-structured-content branch from 03b6d2b to fa97ae7 Compare March 17, 2026 19:11

Flanker32 requested a review from Copilot March 26, 2026 01:58

Copilot started reviewing on behalf of Flanker32 March 26, 2026 01:59 View session

Flanker32 reviewed Mar 26, 2026

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Set useResultSchema on mcpToolTrigger bindings for rich return types#2554

Set useResultSchema on mcpToolTrigger bindings for rich return types#2554
ahmedmuhsin wants to merge 1 commit intomicrosoft:developfrom
ahmedmuhsin:feature/mcp-structured-content

ahmedmuhsin commented Mar 16, 2026 •

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	if (mcpToolTrigger.isEmpty()) {
	if (!mcpToolTrigger.isPresent()) {

		* Checks whether a class implements or extends any known rich result type by walking
		* both the superclass chain and the implemented interfaces.

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ahmedmuhsin commented Mar 16, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Summary

Changes

McpAnnotationProcessor.java

AnnotationHandlerImpl.java

azure-functions-maven-plugin/pom.xml

Design

Related PRs

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ahmedmuhsin commented Mar 16, 2026 •

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