fix(mcp): emit valid JSON Schema when converting ADK tools to MCP tools

PiperOrigin-RevId: 999650939

Author: kvmilos

core/src/main/java/com/google/adk/models/chat/ChatCompletionsRequest.java

@@ -783,7 +783,8 @@ private static Map<String, Object> toMap(Object schema) {
 
   /**
    * Lowercases the {@code type} names in a raw schema, which {@link #schemaNormalizerModule} only
-   * does for typed schemas. ADK's own MCP export sends genai's uppercase names, such as "OBJECT".
+   * does for typed schemas. MCP servers built with older ADK Java releases send genai's uppercase
+   * names, such as "OBJECT".
    *
    * @param node The schema node to adjust in place.
    */

core/src/main/java/com/google/adk/tools/mcp/ConversionUtils.java

@@ -16,19 +16,27 @@
 
 package com.google.adk.tools.mcp;
 
+import com.fasterxml.jackson.core.type.TypeReference;
+import com.google.adk.JsonBaseModel;
 import com.google.adk.tools.BaseTool;
+import com.google.common.base.Ascii;
+import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.genai.types.FunctionDeclaration;
 import com.google.genai.types.Schema;
-import io.modelcontextprotocol.json.McpJsonDefaults;
-import io.modelcontextprotocol.json.McpJsonMapper;
+import com.google.genai.types.Type;
 import io.modelcontextprotocol.spec.McpSchema;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
 import java.util.Optional;
+import org.jspecify.annotations.Nullable;
 
-/** Utility class for converting between different representations of MCP tools. */
+/** Static helpers for converting ADK tools to MCP tools. */
 public final class ConversionUtils {
 
-  private static final McpJsonMapper jsonMapper = McpJsonDefaults.getMapper();
+  private static final TypeReference<Map<String, Object>> MAP_TYPE = new TypeReference<>() {};
 
   /**
    * Input schema for a tool with no parameters. This is the schema the deprecated {@code
@@ -38,16 +46,101 @@ public final class ConversionUtils {
   private static final ImmutableMap<String, Object> NO_PARAMETERS_SCHEMA =
       ImmutableMap.of("type", "object");
 
+  private static final ImmutableMap<String, Object> NULL_SCHEMA = ImmutableMap.of("type", "null");
+
+  /**
+   * Converts an ADK tool to an MCP tool. Uses the declaration's {@code parametersJsonSchema} when
+   * present and non-empty, falls back to {@code parameters} converted to JSON Schema, and defaults
+   * to {@code {"type": "object"}}.
+   */
   public static McpSchema.Tool adkToMcpToolType(BaseTool tool) {
-    Optional<Schema> parameters = tool.declaration().flatMap(FunctionDeclaration::parameters);
-    if (parameters.isEmpty()) {
-      return McpSchema.Tool.builder(tool.name(), NO_PARAMETERS_SCHEMA)
-          .description(tool.description())
-          .build();
-    }
-    return McpSchema.Tool.builder(tool.name(), jsonMapper, parameters.get().toJson())
-        .description(tool.description())
-        .build();
+    Optional<FunctionDeclaration> declaration = tool.declaration();
+    Map<String, Object> inputSchema =
+        declaration
+            .flatMap(FunctionDeclaration::parametersJsonSchema)
+            // ADK's Jackson 2 mapper: a Jackson 3 MCP SDK mapper would read a JsonNode as a bean.
+            .map(schema -> JsonBaseModel.getMapper().convertValue(schema, MAP_TYPE))
+            .filter(schema -> !schema.isEmpty())
+            .or(
+                () ->
+                    declaration
+                        .flatMap(FunctionDeclaration::parameters)
+                        .map(ConversionUtils::toJsonSchema))
+            .orElse(NO_PARAMETERS_SCHEMA);
+    return McpSchema.Tool.builder(tool.name(), inputSchema).description(tool.description()).build();
+  }
+
+  /** Converts a genai {@link Schema}, which spells types in uppercase, to JSON Schema. */
+  private static Map<String, Object> toJsonSchema(Schema schema) {
+    Map<String, Object> json = JsonBaseModel.getMapper().convertValue(schema, MAP_TYPE);
+    // MCP requires the input schema itself to have "type": "object", so it cannot be nullable.
+    json.remove("nullable");
+    normalizeSchema(json);
+    return json;
+  }
+
+  /**
+   * Normalizes {@code node} and its nested schemas in place: lowercases {@code type}, drops a type
+   * with no JSON Schema name such as {@code TYPE_UNSPECIFIED} so the schema accepts any value, and
+   * replaces genai's {@code nullable}, which JSON Schema does not have.
+   */
+  private static void normalizeSchema(Object node) {
+    if (!(node instanceof Map<?, ?> rawSchema)) {
+      return;
+    }
+    // Safe: convertValue built the whole tree from Map<String, Object> nodes.
+    @SuppressWarnings("unchecked")
+    Map<String, Object> schema = (Map<String, Object>) rawSchema;
+    normalizeType(schema);
+    normalizeSubschemas(schema);
+    if (Objects.equals(schema.remove("nullable"), true)) {
+      allowNull(schema);
+    }
+  }
+
+  private static void normalizeType(Map<String, Object> schema) {
+    if (!(schema.get("type") instanceof String type)) {
+      return;
+    }
+    // Type maps a name it does not know to TYPE_UNSPECIFIED; dropping it keeps the schema valid.
+    Type.Known known = new Type(type).knownEnum();
+    if (known == Type.Known.TYPE_UNSPECIFIED) {
+      schema.remove("type");
+    } else {
+      schema.put("type", Ascii.toLowerCase(known.name()));
+    }
+  }
+
+  private static void normalizeSubschemas(Map<String, Object> schema) {
+    if (schema.get("properties") instanceof Map<?, ?> properties) {
+      properties.values().forEach(ConversionUtils::normalizeSchema);
+    }
+    normalizeSchema(schema.get("items"));
+    if (schema.get("anyOf") instanceof List<?> anyOf) {
+      anyOf.forEach(ConversionUtils::normalizeSchema);
+    }
+  }
+
+  /**
+   * Allows null in whichever of {@code type}, {@code enum} and {@code anyOf} {@code schema} has,
+   * since JSON Schema has no {@code nullable} keyword.
+   */
+  private static void allowNull(Map<String, Object> schema) {
+    if (schema.get("type") instanceof String type && !type.equals("null")) {
+      schema.put("type", ImmutableList.of(type, "null"));
+    }
+    if (schema.get("enum") instanceof List<?> values && !values.contains(null)) {
+      schema.put("enum", append(values, null));
+    }
+    if (schema.get("anyOf") instanceof List<?> anyOf && !anyOf.contains(NULL_SCHEMA)) {
+      schema.put("anyOf", append(anyOf, NULL_SCHEMA));
+    }
+  }
+
+  private static List<Object> append(List<?> list, @Nullable Object element) {
+    List<Object> result = new ArrayList<>(list);
+    result.add(element);
+    return result;
   }
 
   private ConversionUtils() {}