feat: Add ChatCompletionsResponse to LlmResponse conversion

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

@@ -0,0 +1,169 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.adk.models.chat;
+
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
+import com.fasterxml.jackson.annotation.JsonInclude;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.core.type.TypeReference;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.genai.types.FunctionCall;
+import com.google.genai.types.Part;
+import java.util.Base64;
+import java.util.Map;
+import org.jspecify.annotations.Nullable;
+
+/** Shared models for Chat Completions Request and Response. */
+@JsonIgnoreProperties(ignoreUnknown = true)
+@JsonInclude(JsonInclude.Include.NON_NULL)
+final class ChatCompletionsCommon {
+
+  private ChatCompletionsCommon() {}
+
+  private static final ObjectMapper objectMapper = new ObjectMapper();
+
+  public static final String ROLE_ASSISTANT = "assistant";
+  public static final String ROLE_MODEL = "model";
+
+  public static final String METADATA_KEY_ID = "id";
+  public static final String METADATA_KEY_CREATED = "created";
+  public static final String METADATA_KEY_OBJECT = "object";
+  public static final String METADATA_KEY_SYSTEM_FINGERPRINT = "system_fingerprint";
+  public static final String METADATA_KEY_SERVICE_TIER = "service_tier";
+
+  /**
+   * See
+   * https://developers.openai.com/api/reference/resources/chat#(resource)%20chat.completions%20%3E%20(model)%20chat_completion_message_tool_call%20%3E%20(schema)
+   */
+  @JsonIgnoreProperties(ignoreUnknown = true)
+  @JsonInclude(JsonInclude.Include.NON_NULL)
+  static class ToolCall {
+    /** See class definition for more details. */
+    public Integer index;
+
+    /** See class definition for more details. */
+    public String id;
+
+    /** See class definition for more details. */
+    public String type;
+
+    /** See class definition for more details. */
+    public Function function;
+
+    /** See class definition for more details. */
+    public Custom custom;
+
+    /**
+     * Used to supply additional parameters for specific models, for example:
+     * https://ai.google.dev/gemini-api/docs/openai#thinking
+     */
+    @JsonProperty("extra_content")
+    public Map<String, Object> extraContent;
+
+    /**
+     * Converts the tool call to a {@link Part}.
+     *
+     * @return a {@link Part} containing the function call, or {@code null} if this tool call does
+     *     not contain a function call.
+     */
+    public @Nullable Part toPart() {
+      if (function != null) {
+        FunctionCall fc = function.toFunctionCall(id);
+        Part part = Part.builder().functionCall(fc).build();
+        return applyThoughtSignature(part);
+      }
+      return null;
+    }
+
+    /**
+     * Applies the thought signature from {@code extraContent} to the given {@link Part} if present.
+     * This is used to support the Google Gemini/Vertex AI implementation of the chat/completions
+     * API.
+     *
+     * @param part the {@link Part} to modify.
+     * @return a new {@link Part} with the thought signature applied, or the original {@link Part}
+     *     if no thought signature is found.
+     */
+    public Part applyThoughtSignature(Part part) {
+      if (extraContent != null && extraContent.containsKey("google")) {
+        Object googleObj = extraContent.get("google");
+        if (googleObj instanceof Map<?, ?> googleMap) {
+          Object sigObj = googleMap.get("thought_signature");
+          if (sigObj instanceof String sig) {
+            return part.toBuilder().thoughtSignature(Base64.getDecoder().decode(sig)).build();
+          }
+        }
+      }
+      return part;
+    }
+  }
+
+  /**
+   * See
+   * https://developers.openai.com/api/reference/resources/chat#(resource)%20chat.completions%20%3E%20(model)%20chat_completion_message_function_tool_call%20%3E%20(schema)
+   */
+  @JsonIgnoreProperties(ignoreUnknown = true)
+  @JsonInclude(JsonInclude.Include.NON_NULL)
+  static class Function {
+    /** See class definition for more details. */
+    public String name;
+
+    /** See class definition for more details. */
+    public String arguments; // JSON string
+
+    /**
+     * Converts this function to a {@link FunctionCall}.
+     *
+     * @param toolCallId the ID of the tool call, or {@code null} if not applicable.
+     * @return the {@link FunctionCall} object.
+     */
+    public FunctionCall toFunctionCall(@Nullable String toolCallId) {
+      FunctionCall.Builder fcBuilder = FunctionCall.builder();
+      if (name != null) {
+        fcBuilder.name(name);
+      }
+      if (arguments != null) {
+        try {
+          Map<String, Object> args =
+              objectMapper.readValue(arguments, new TypeReference<Map<String, Object>>() {});
+          fcBuilder.args(args);
+        } catch (Exception e) {
+          throw new IllegalArgumentException(
+              "Failed to parse function arguments JSON: " + arguments, e);
+        }
+      }
+      if (toolCallId != null) {
+        fcBuilder.id(toolCallId);
+      }
+      return fcBuilder.build();
+    }
+  }
+
+  /**
+   * See
+   * https://developers.openai.com/api/reference/resources/chat#(resource)%20chat.completions%20%3E%20(model)%20chat_completion_custom_tool%20%3E%20(schema)
+   */
+  @JsonIgnoreProperties(ignoreUnknown = true)
+  @JsonInclude(JsonInclude.Include.NON_NULL)
+  static class Custom {
+    /** See class definition for more details. */
+    public String input;
+
+    /** See class definition for more details. */
+    public String name;
+  }
+}