feat: Add thought signature support for chat completions

- https://ai.google.dev/gemini-api/docs/thought-signatures
- Also adds logging that's similar to the GenAI flow
- Also fixes some issues that appeared when testing streaming
   - Final tool call no longer returns the accumulated text
   - Add thought signature support

PiperOrigin-RevId: 919115462

Author: google-genai-bot

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

@@ -190,14 +190,21 @@ private static Duration resolveCallTimeout(HttpOptions httpOptions) {
   public Flowable<LlmResponse> complete(LlmRequest llmRequest, boolean stream) {
     return Flowable.defer(
         () -> {
+          String effectiveModelName = llmRequest.model().orElse("?");
+          logger.trace("Chat Completion Request Contents: {}", llmRequest.contents());
+          llmRequest.config().ifPresent(c -> logger.trace("Chat Completion Request Config: {}", c));
+
           ChatCompletionsRequest dtoRequest =
               ChatCompletionsRequest.fromLlmRequest(llmRequest, stream);
           String jsonPayload = objectMapper.writeValueAsString(dtoRequest);
-          logger.trace(
-              "Chat Completion Request: model={}, stream={}, messagesCount={}",
-              dtoRequest.model,
-              dtoRequest.stream,
-              dtoRequest.messages != null ? dtoRequest.messages.size() : 0);
+          logger.trace("Chat Completion Request JSON: {}", jsonPayload);
+
+          if (stream) {
+            logger.debug(
+                "Sending streaming chat-completion request to model {}", effectiveModelName);
+          } else {
+            logger.debug("Sending chat-completion request to model {}", effectiveModelName);
+          }
 
           Request.Builder requestBuilder =
               new Request.Builder().url(completionsUrl).post(RequestBody.create(jsonPayload, JSON));
@@ -209,11 +216,7 @@ public Flowable<LlmResponse> complete(LlmRequest llmRequest, boolean stream) {
           requestBuilder.header("Content-Type", JSON.toString());
 
           Request request = requestBuilder.build();
-          if (stream) {
-            return createStreamingFlowable(request);
-          } else {
-            return createNonStreamingFlowable(request);
-          }
+          return stream ? createStreamingFlowable(request) : createNonStreamingFlowable(request);
         });
   }
 
@@ -274,10 +277,14 @@ public void onResponse(Call call, Response response) {
                       // A single malformed chunk must not abort the entire stream. Log a
                       // warning and continue.
                       try {
+                        logger.trace("Raw streaming chat-completion chunk: {}", data);
                         ChatCompletionsResponse.ChatCompletionChunk chunk =
                             objectMapper.readValue(
                                 data, ChatCompletionsResponse.ChatCompletionChunk.class);
                         ImmutableList<LlmResponse> responses = collection.processChunk(chunk);
+                        if (!responses.isEmpty()) {
+                          logger.debug("Responses to emit: {}", responses);
+                        }
                         for (LlmResponse resp : responses) {
                           emitter.onNext(resp);
                         }
@@ -341,9 +348,12 @@ public void onResponse(Call call, Response response) {
         }
 
         String jsonResponse = body.string();
+        logger.trace("Raw non-streaming chat-completion response: {}", jsonResponse);
         ChatCompletionsResponse.ChatCompletion completion =
             objectMapper.readValue(jsonResponse, ChatCompletionsResponse.ChatCompletion.class);
-        emitter.onNext(completion.toLlmResponse());
+        LlmResponse llmResponse = completion.toLlmResponse();
+        logger.debug("Response to emit: {}", llmResponse);
+        emitter.onNext(llmResponse);
         emitter.onComplete();
       } catch (Exception e) {
         emitter.tryOnError(e);

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

@@ -26,6 +26,7 @@
 import com.google.adk.JsonBaseModel;
 import com.google.adk.models.LlmRequest;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.genai.types.Content;
 import com.google.genai.types.FunctionDeclaration;
 import com.google.genai.types.FunctionResponse;
@@ -351,6 +352,9 @@ private static List<Message> processContent(Content content) {
     List<ChatCompletionsCommon.ToolCall> toolCalls = new ArrayList<>();
     List<Message> toolResponses = new ArrayList<>();
     List<String> refusals = new ArrayList<>();
+    // Capture a message-level thought_signature from the first text Part that carries one.
+    // This signature must be echoed back on subsequent turns to ensure proper round-tripping.
+    byte[][] textThoughtSignature = new byte[][] {null};
 
     content
         .parts()
@@ -370,6 +374,9 @@ private static List<Message> processContent(Content content) {
                   if (split.refusal() != null) {
                     refusals.add(split.refusal());
                   }
+                  if (textThoughtSignature[0] == null && part.thoughtSignature().isPresent()) {
+                    textThoughtSignature[0] = part.thoughtSignature().get();
+                  }
                 } else if (part.inlineData().isPresent()) {
                   contentParts.add(processInlineDataPart(part));
                 } else if (part.fileData().isPresent()) {
@@ -403,6 +410,15 @@ private static List<Message> processContent(Content content) {
           msg.content = new MessageContent(ImmutableList.copyOf(contentParts));
         }
       }
+      // Round-trip the message-level thought_signature for assistant text responses.
+      if (textThoughtSignature[0] != null) {
+        msg.extraContent =
+            ImmutableMap.of(
+                "google",
+                ImmutableMap.of(
+                    "thought_signature",
+                    Base64.getEncoder().encodeToString(textThoughtSignature[0])));
+      }
       List<Message> messages = new ArrayList<>();
       messages.add(msg);
       return messages;
@@ -446,6 +462,10 @@ private static ContentPart processFileDataPart(Part part) {
   /**
    * Processes a function call part and returns a mapped ToolCall.
    *
+   * <p>If the source {@link Part} carries a {@code thoughtSignature}, it is round-tripped back out
+   * as a base64-encoded string in {@code extra_content.google.thought_signature} to satisfy
+   * endpoint requirements.
+   *
    * @param part The input part containing a requested function call or invocation.
    * @return The mapped function call tool call.
    */
@@ -464,6 +484,13 @@ private static ChatCompletionsCommon.ToolCall processFunctionCallPart(Part part)
       }
     }
     toolCall.function = function;
+    part.thoughtSignature()
+        .ifPresent(
+            sigBytes -> {
+              String sig = Base64.getEncoder().encodeToString(sigBytes);
+              toolCall.extraContent =
+                  ImmutableMap.of("google", ImmutableMap.of("thought_signature", sig));
+            });
     return toolCall;
   }
 
@@ -616,6 +643,13 @@ static class Message {
 
     /** See class definition for more details. */
     public String refusal;
+
+    /**
+     * Message-level additional parameters used by some providers. Used for round-tripping data like
+     * {@code extra_content.google.thought_signature}.
+     */
+    @JsonProperty("extra_content")
+    public Map<String, Object> extraContent;
   }
 
   /**