fix(google-vertex): don't send streamFunctionCallArguments for vertex doGenerate

.changeset/six-pumas-refuse.md

@@ -0,0 +1,5 @@
+---
+"@ai-sdk/google": patch
+---
+
+fix(google-vertex): don't send streamFunctionCallArguments for unary API calls and change default to false

content/providers/01-ai-sdk-providers/16-google-vertex.mdx

@@ -350,7 +350,7 @@ The following optional provider options are available for Google Vertex models:
   Optional. When set to true, function call arguments will be streamed
   incrementally in streaming responses. This enables `tool-input-delta` events
   to arrive as the model generates function call arguments, reducing perceived
-  latency for tool calls. Defaults to `true` for Vertex AI providers. Only supported on the Vertex AI API (not the Gemini API).
+  latency for tool calls. Defaults to `false`. Only supported on the Vertex AI API (not the Gemini API) with Gemini 3+ models.
 
   Consult [Google's Documentation](https://docs.cloud.google.com/vertex-ai/generative-ai/docs/multimodal/function-calling#streaming-fc) for details.
 
@@ -469,8 +469,7 @@ For Gemini 3 Pro and later models on Vertex AI, you can stream function call
 arguments as they are generated by setting `streamFunctionCallArguments` to
 `true`. This reduces perceived latency when functions need to be called, as
 `tool-input-delta` events arrive incrementally instead of waiting for the
-complete arguments. This option is `true` by default and you can opt out by 
-setting it to false.
+complete arguments. This option defaults to `false`.
 
 ```ts
 import { vertex } from '@ai-sdk/google-vertex';
@@ -491,7 +490,7 @@ const result = streamText({
   },
   providerOptions: {
     vertex: {
-      streamFunctionCallArguments: false,
+      streamFunctionCallArguments: true,
     } satisfies GoogleLanguageModelOptions,
   },
 });

examples/ai-functions/src/stream-text/google/vertex-stream-function-call-args.ts

@@ -16,6 +16,11 @@ run(async () => {
         }),
       },
     },
+    providerOptions: {
+      vertex: {
+        streamFunctionCallArguments: true,
+      },
+    },
     includeRawChunks: true,
   });
 

packages/google/src/google-generative-ai-language-model.test.ts

@@ -5713,7 +5713,7 @@ describe('doStream', () => {
     ).toMatchInlineSnapshot(`undefined`);
   });
 
-  it('should default streamFunctionCallArguments to true for Vertex provider even without provider option', async () => {
+  it('should default streamFunctionCallArguments to false for Vertex provider without provider option', async () => {
     server.urls[TEST_URL_GEMINI_PRO].response = {
       type: 'stream-chunks',
       chunks: [
@@ -5759,14 +5759,10 @@ describe('doStream', () => {
       ],
     });
 
-    expect((await server.calls[0].requestBodyJson).toolConfig)
-      .toMatchInlineSnapshot(`
-      {
-        "functionCallingConfig": {
-          "streamFunctionCallArguments": true,
-        },
-      }
-    `);
+    expect(
+      (await server.calls[0].requestBodyJson).toolConfig?.functionCallingConfig
+        ?.streamFunctionCallArguments,
+    ).toMatchInlineSnapshot(`undefined`);
   });
 
   it('should allow Vertex provider to opt out of streamFunctionCallArguments by setting it to false', async () => {
@@ -5826,6 +5822,139 @@ describe('doStream', () => {
     ).toBeUndefined();
   });
 
+  it('should not send streamFunctionCallArguments for Vertex provider doGenerate (unary API)', async () => {
+    server.urls[TEST_URL_GEMINI_PRO].response = {
+      type: 'json-value',
+      body: {
+        candidates: [
+          {
+            content: { parts: [{ text: 'Hello' }], role: 'model' },
+            finishReason: 'STOP',
+            safetyRatings: SAFETY_RATINGS,
+          },
+        ],
+        usageMetadata: {
+          promptTokenCount: 1,
+          candidatesTokenCount: 1,
+          totalTokenCount: 2,
+        },
+      },
+    };
+
+    const vertexModel = new GoogleGenerativeAILanguageModel('gemini-pro', {
+      provider: 'google.vertex.chat',
+      baseURL: 'https://generativelanguage.googleapis.com/v1beta',
+      headers: { 'x-goog-api-key': 'test-api-key' },
+      generateId: () => 'test-id',
+    });
+
+    await vertexModel.doGenerate({
+      prompt: TEST_PROMPT,
+      tools: [
+        {
+          type: 'function',
+          name: 'test-tool',
+          inputSchema: {
+            type: 'object',
+            properties: { value: { type: 'string' } },
+            required: ['value'],
+            additionalProperties: false,
+            $schema: 'http://json-schema.org/draft-07/schema#',
+          },
+        },
+      ],
+    });
+
+    expect(await server.calls[0].requestBodyJson).toMatchInlineSnapshot(`
+      {
+        "contents": [
+          {
+            "parts": [
+              {
+                "text": "Hello",
+              },
+            ],
+            "role": "user",
+          },
+        ],
+        "generationConfig": {},
+        "tools": [
+          {
+            "functionDeclarations": [
+              {
+                "description": "",
+                "name": "test-tool",
+                "parameters": {
+                  "properties": {
+                    "value": {
+                      "type": "string",
+                    },
+                  },
+                  "required": [
+                    "value",
+                  ],
+                  "type": "object",
+                },
+              },
+            ],
+          },
+        ],
+      }
+    `);
+  });
+
+  it('should not send streamFunctionCallArguments for Vertex provider doGenerate even when explicitly set', async () => {
+    server.urls[TEST_URL_GEMINI_PRO].response = {
+      type: 'json-value',
+      body: {
+        candidates: [
+          {
+            content: { parts: [{ text: 'Hello' }], role: 'model' },
+            finishReason: 'STOP',
+            safetyRatings: SAFETY_RATINGS,
+          },
+        ],
+        usageMetadata: {
+          promptTokenCount: 1,
+          candidatesTokenCount: 1,
+          totalTokenCount: 2,
+        },
+      },
+    };
+
+    const vertexModel = new GoogleGenerativeAILanguageModel('gemini-pro', {
+      provider: 'google.vertex.chat',
+      baseURL: 'https://generativelanguage.googleapis.com/v1beta',
+      headers: { 'x-goog-api-key': 'test-api-key' },
+      generateId: () => 'test-id',
+    });
+
+    await vertexModel.doGenerate({
+      prompt: TEST_PROMPT,
+      providerOptions: {
+        vertex: {
+          streamFunctionCallArguments: true,
+        },
+      },
+    });
+
+    expect(await server.calls[0].requestBodyJson).toMatchInlineSnapshot(`
+      {
+        "contents": [
+          {
+            "parts": [
+              {
+                "text": "Hello",
+              },
+            ],
+            "role": "user",
+          },
+        ],
+        "generationConfig": {},
+      }
+    `);
+  });
+
   it('should only pass valid provider options', async () => {
     prepareChunksFixtureResponse('google-text');
 

packages/google/src/google-generative-ai-language-model.ts

@@ -88,22 +88,25 @@ export class GoogleGenerativeAILanguageModel implements LanguageModelV4 {
     return this.config.supportedUrls?.() ?? {};
   }
 
-  private async getArgs({
-    prompt,
-    maxOutputTokens,
-    temperature,
-    topP,
-    topK,
-    frequencyPenalty,
-    presencePenalty,
-    stopSequences,
-    responseFormat,
-    seed,
-    tools,
-    toolChoice,
-    reasoning,
-    providerOptions,
-  }: LanguageModelV4CallOptions) {
+  private async getArgs(
+    {
+      prompt,
+      maxOutputTokens,
+      temperature,
+      topP,
+      topK,
+      frequencyPenalty,
+      presencePenalty,
+      stopSequences,
+      responseFormat,
+      seed,
+      tools,
+      toolChoice,
+      reasoning,
+      providerOptions,
+    }: LanguageModelV4CallOptions,
+    { isStreaming = false }: { isStreaming?: boolean } = {},
+  ) {
     const warnings: SharedV4Warning[] = [];
 
     const providerOptionsName = this.config.provider.includes('vertex')
@@ -184,9 +187,10 @@ export class GoogleGenerativeAILanguageModel implements LanguageModelV4 {
         ? { ...resolvedThinking, ...googleOptions?.thinkingConfig }
         : undefined;
 
-    const streamFunctionCallArguments = isVertexProvider
-      ? (googleOptions?.streamFunctionCallArguments ?? true)
-      : undefined;
+    const streamFunctionCallArguments =
+      isStreaming && isVertexProvider
+        ? (googleOptions?.streamFunctionCallArguments ?? false)
+        : undefined;
 
     const toolConfig =
       googleToolConfig ||
@@ -478,7 +482,10 @@ export class GoogleGenerativeAILanguageModel implements LanguageModelV4 {
   async doStream(
     options: LanguageModelV4CallOptions,
   ): Promise<LanguageModelV4StreamResult> {
-    const { args, warnings, providerOptionsName } = await this.getArgs(options);
+    const { args, warnings, providerOptionsName } = await this.getArgs(
+      options,
+      { isStreaming: true },
+    );
 
     const headers = combineHeaders(
       await resolve(this.config.headers),

packages/google/src/google-generative-ai-options.ts

@@ -192,9 +192,10 @@ export const googleLanguageModelOptions = lazySchema(() =>
       /**
        * Optional. When set to true, function call arguments will be streamed
        * incrementally via partialArgs in streaming responses. Only supported
-       * on the Vertex AI API (not the Gemini API).
+       * on the Vertex AI API (not the Gemini API) and only for Gemini 3+
+       * models.
        *
-       * @default true
+       * @default false
        *
        * https://docs.cloud.google.com/vertex-ai/generative-ai/docs/multimodal/function-calling#streaming-fc
        */