fix(server-utils): Match google-genai per-message config replacement on chat spans

@google/genai resolves each request's config as `params.config ?? chat.config`, so a
per-message config given to chat.sendMessage() or chat.sendMessageStream() replaces the
create-time config wholesale rather than merging with it. The previous shallow merge welded
create-time fields such as systemInstruction, tools and sampling settings onto message spans
that did not actually send them, over-reporting what the request carried.

Fall back to the create-time config only when the message carries no config of its own. Lock
the corrected attributes in the per-message config test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: zkasuran

packages/server-utils/src/ai/google-genai/index.ts

@@ -257,9 +257,10 @@ function asConfigObject(config: unknown): Record<string, unknown> | undefined {
 /**
  * Merge the parameters captured at `chats.create()` time onto a `chat.sendMessage()` /
  * `chat.sendMessageStream()` call, so the config that is defined once on the chat lands on every
- * message span. The create-time config is the default and a per-message config overrides it key by
- * key. Only `model` and `config` are inherited; the create `history` is intentionally left off the
- * message spans. See issue #20086.
+ * message span. The SDK resolves the request config as `params.config ?? chat.config`, so a
+ * per-message config replaces the create-time config wholesale and the create-time config only
+ * applies when the message omits one. Only `model` and `config` are inherited; the create `history`
+ * is intentionally left off the message spans. See issue #20086.
  */
 function mergeChatCreateParams(
   chatCreateParams: Record<string, unknown> | undefined,
@@ -275,10 +276,15 @@ function mergeChatCreateParams(
     merged.model = chatCreateParams.model;
   }
 
-  const createConfig = asConfigObject(chatCreateParams.config);
+  // @google/genai sends `params.config ?? chat.config`, so a per-message config replaces the
+  // create-time config wholesale rather than merging into it. Fall back to the create-time config
+  // only when the message did not carry one, otherwise the span reports fields that were not sent.
   const callConfig = asConfigObject(callParams?.config);
-  if (createConfig || callConfig) {
-    merged.config = { ...createConfig, ...callConfig };
+  if (!callConfig) {
+    const createConfig = asConfigObject(chatCreateParams.config);
+    if (createConfig) {
+      merged.config = createConfig;
+    }
   }
 
   return merged;

packages/server-utils/test/ai/lib/tracing/google-genai.test.ts

@@ -139,7 +139,7 @@ describe('instrumentGoogleGenAIClient chat config propagation (#20086)', () => {
     expect(data[GEN_AI_SYSTEM_INSTRUCTIONS]).toBe('[{"type":"text","content":"You are a friendly robot."}]');
   });
 
-  it('lets a per-message config override the config from chats.create()', async () => {
+  it('replaces the chats.create() config when a message provides its own', async () => {
     const endedSpans = setupClient();
     const instrumented = instrumentGoogleGenAIClient(createFakeClient());
 
@@ -149,10 +149,14 @@ describe('instrumentGoogleGenAIClient chat config propagation (#20086)', () => {
     await chat.sendMessage({ message: 'Tell me a joke', config: { temperature: 0.1 } });
 
     const data = spanToJSON(endedSpans[0]!).data;
-    // Per-call temperature wins; the untouched create-time values still come through.
+    // @google/genai resolves the request config as `params.config ?? chat.config`, so the
+    // per-message config is sent on its own. The create-time fields it omits are not part of the
+    // request, so they must not appear on the span.
     expect(data[GEN_AI_REQUEST_TEMPERATURE]).toBe(0.1);
-    expect(data[GEN_AI_REQUEST_TOP_P]).toBe(0.9);
-    expect(data[GEN_AI_REQUEST_MAX_TOKENS]).toBe(150);
+    expect(data[GEN_AI_REQUEST_TOP_P]).toBeUndefined();
+    expect(data[GEN_AI_REQUEST_MAX_TOKENS]).toBeUndefined();
+    expect(data[GEN_AI_TOOL_DEFINITIONS]).toBeUndefined();
+    expect(data[GEN_AI_SYSTEM_INSTRUCTIONS]).toBeUndefined();
   });
 
   it('does not leak chat config onto models.generateContent spans', async () => {