Backport: feat(google-vertex): add support for streaming tool arguments input

.changeset/itchy-frogs-sparkle.md

@@ -0,0 +1,5 @@
+---
+"@ai-sdk/google": patch
+---
+
+feat(google-vertex): add support for streaming tool arguments input

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

@@ -345,6 +345,15 @@ The following optional provider options are available for Google Vertex models:
 
   Consult [Google's Documentation](https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/add-labels-to-api-calls) for usage details.
 
+- **streamFunctionCallArguments** _boolean_
+
+  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).
+
+  Consult [Google's Documentation](https://docs.cloud.google.com/vertex-ai/generative-ai/docs/multimodal/function-calling#streaming-fc) for details.
+
 You can use Google Vertex language models to generate text with the `generateText` function:
 
 ```ts highlight="1,4"
@@ -454,6 +463,60 @@ const result = await generateText({
 
 The optional `retrievalConfig.latLng` provider option provides location context for queries about nearby places. This configuration applies to any grounding tools that support location context.
 
+#### Streaming Function Call Arguments
+
+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.
+
+```ts
+import { vertex } from '@ai-sdk/google-vertex';
+import { type GoogleLanguageModelOptions } from '@ai-sdk/google';
+import { streamText } from 'ai';
+import { z } from 'zod';
+
+const result = streamText({
+  model: vertex('gemini-3.1-pro-preview'),
+  prompt: 'What is the weather in Boston and San Francisco?',
+  tools: {
+    getWeather: {
+      description: 'Get the current weather in a given location',
+      inputSchema: z.object({
+        location: z.string().describe('City name'),
+      }),
+    },
+  },
+  providerOptions: {
+    vertex: {
+      streamFunctionCallArguments: false,
+    } satisfies GoogleLanguageModelOptions,
+  },
+});
+
+for await (const part of result.fullStream) {
+  switch (part.type) {
+    case 'tool-input-start':
+      console.log(`Tool call started: ${part.toolName}`);
+      break;
+    case 'tool-input-delta':
+      process.stdout.write(part.delta);
+      break;
+    case 'tool-call':
+      console.log(`Tool call complete: ${part.toolName}`, part.input);
+      break;
+  }
+}
+```
+
+<Note>
+  This feature is only available on the Vertex AI API. It is not supported on
+  the Gemini API. When used with the Google Generative AI provider, a warning
+  will be emitted and the option will be ignored.
+</Note>
+
 #### Reasoning (Thinking Tokens)
 
 Google Vertex AI, through its support for Gemini models, can also emit "thinking" tokens, representing the model's reasoning process. The AI SDK exposes these as reasoning information.

examples/ai-e2e-next/app/api/chat/vertex-streaming-tool-calls/route.ts

@@ -0,0 +1,63 @@
+import { vertex } from '@ai-sdk/google-vertex';
+import { type GoogleLanguageModelOptions } from '@ai-sdk/google';
+import { convertToModelMessages, streamText, UIDataTypes, UIMessage } from 'ai';
+import { z } from 'zod';
+
+export const maxDuration = 60;
+
+export type VertexStreamingToolCallsMessage = UIMessage<
+  never,
+  UIDataTypes,
+  {
+    showWeatherInformation: {
+      input: {
+        city: string;
+        weather: string;
+        temperature: number;
+        description: string;
+      };
+      output: string;
+    };
+  }
+>;
+
+export async function POST(req: Request) {
+  const { messages } = await req.json();
+
+  const result = streamText({
+    model: vertex('gemini-3.1-pro-preview'),
+    messages: await convertToModelMessages(messages),
+    system:
+      'You are a helpful weather assistant. ' +
+      'Use getWeatherInformation to fetch weather data, then use showWeatherInformation to display it to the user. ' +
+      'Always show the weather using the showWeatherInformation tool.',
+    tools: {
+      getWeatherInformation: {
+        description: 'Get the current weather for a city',
+        inputSchema: z.object({ city: z.string() }),
+        execute: async ({ city }: { city: string }) => {
+          const conditions = ['sunny', 'cloudy', 'rainy', 'snowy', 'windy'];
+          return {
+            city,
+            weather: conditions[Math.floor(Math.random() * conditions.length)],
+            temperature: Math.floor(Math.random() * 50 - 10),
+          };
+        },
+      },
+      showWeatherInformation: {
+        description:
+          'Show weather information to the user. Always use this tool to present weather data.',
+        inputSchema: z.object({
+          city: z.string(),
+          weather: z.string(),
+          temperature: z.number(),
+          description: z
+            .string()
+            .describe('A brief description of the weather conditions.'),
+        }),
+      },
+    },
+  });
+
+  return result.toUIMessageStreamResponse();
+}

examples/ai-e2e-next/app/chat/vertex-streaming-tool-calls/page.tsx

@@ -0,0 +1,116 @@
+'use client';
+
+import { useChat } from '@ai-sdk/react';
+import ChatInput from '@/components/chat-input';
+import {
+  DefaultChatTransport,
+  lastAssistantMessageIsCompleteWithToolCalls,
+} from 'ai';
+import { VertexStreamingToolCallsMessage } from '@/app/api/chat/vertex-streaming-tool-calls/route';
+
+export default function Chat() {
+  const { messages, status, sendMessage, addToolOutput } =
+    useChat<VertexStreamingToolCallsMessage>({
+      transport: new DefaultChatTransport({
+        api: '/api/chat/vertex-streaming-tool-calls',
+      }),
+
+      sendAutomaticallyWhen: lastAssistantMessageIsCompleteWithToolCalls,
+
+      async onToolCall({ toolCall }) {
+        if (toolCall.toolName === 'showWeatherInformation') {
+          addToolOutput({
+            tool: 'showWeatherInformation',
+            toolCallId: toolCall.toolCallId,
+            output: 'Weather information displayed to user.',
+          });
+        }
+      },
+    });
+
+  let lastRole: string | undefined = undefined;
+
+  return (
+    <div className="flex flex-col py-24 mx-auto w-full max-w-md stretch">
+      <h1 className="text-lg font-bold mb-4">
+        Vertex AI — Streaming Tool Call Arguments
+      </h1>
+
+      {messages?.map(m => {
+        const isNewRole = m.role !== lastRole;
+        lastRole = m.role;
+
+        return (
+          <div key={m.id} className="whitespace-pre-wrap mb-2">
+            {isNewRole && (
+              <strong className="block mb-1">{`${m.role}: `}</strong>
+            )}
+            {m.parts.map((part, i) => {
+              if (part.type === 'text') {
+                return <span key={i}>{part.text}</span>;
+              }
+
+              if (part.type === 'tool-showWeatherInformation') {
+                if (part.state === 'input-streaming') {
+                  return (
+                    <div
+                      key={i}
+                      className="p-3 my-2 rounded border border-blue-300 bg-blue-50"
+                    >
+                      <div className="text-xs font-mono text-blue-600 mb-1">
+                        streaming tool args…
+                      </div>
+                      <pre className="text-sm">
+                        {JSON.stringify(part.input, null, 2)}
+                      </pre>
+                    </div>
+                  );
+                }
+
+                if (part.state === 'input-available') {
+                  return (
+                    <div
+                      key={i}
+                      className="p-3 my-2 rounded border border-yellow-300 bg-yellow-50"
+                    >
+                      <div className="text-xs text-yellow-700 mb-1">
+                        tool call complete — awaiting result
+                      </div>
+                      <pre className="text-sm">
+                        {JSON.stringify(part.input, null, 2)}
+                      </pre>
+                    </div>
+                  );
+                }
+
+                if (part.state === 'output-available') {
+                  return (
+                    <div
+                      key={i}
+                      className="p-4 my-2 rounded border border-green-300 bg-green-50"
+                    >
+                      <h4 className="font-semibold mb-1">{part.input.city}</h4>
+                      <div className="flex gap-3 text-sm">
+                        <span>🌡 {part.input.temperature}°C</span>
+                        <span>☁ {part.input.weather}</span>
+                      </div>
+                      {part.input.description && (
+                        <p className="mt-1 text-sm text-gray-600">
+                          {part.input.description}
+                        </p>
+                      )}
+                    </div>
+                  );
+                }
+              }
+
+              return null;
+            })}
+          </div>
+        );
+      })}
+
+      <ChatInput status={status} onSubmit={text => sendMessage({ text })} />
+    </div>
+  );
+}

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

@@ -0,0 +1,53 @@
+import { vertex } from '@ai-sdk/google-vertex';
+import { streamText } from 'ai';
+import { z } from 'zod';
+import { run } from '../../lib/run';
+import { saveRawChunks } from '../../lib/save-raw-chunks';
+
+run(async () => {
+  const result = streamText({
+    model: vertex('gemini-3.1-pro-preview'),
+    prompt: 'What is the weather in Boston and San Francisco?',
+    tools: {
+      getWeather: {
+        description: 'Get the current weather in a given location',
+        inputSchema: z.object({
+          location: z.string().describe('City name'),
+        }),
+      },
+    },
+    includeRawChunks: true,
+  });
+
+  for await (const part of result.fullStream) {
+    switch (part.type) {
+      case 'tool-input-start':
+        console.log(`\n[tool-input-start] ${part.toolName} (${part.id})`);
+        break;
+      case 'tool-input-delta':
+        process.stdout.write(part.delta);
+        break;
+      case 'tool-input-end':
+        console.log(`\n[tool-input-end] (${part.id})`);
+        break;
+      case 'tool-call':
+        console.log(`\n[tool-call] ${part.toolName}:`, part.input);
+        break;
+      case 'text-delta':
+        process.stdout.write(part.text);
+        break;
+      case 'finish':
+        console.log('\nFinish reason:', part.finishReason);
+        console.log('Usage:', part.totalUsage);
+        break;
+      case 'error':
+        console.error('Error:', part.error);
+        break;
+    }
+  }
+
+  await saveRawChunks({
+    result,
+    filename: 'google-vertex-stream-function-call-args-default.1',
+  });
+});

examples/ai-functions/src/stream-text/google/vertex-stream-tool-call-object.ts

@@ -0,0 +1,62 @@
+import { vertex } from '@ai-sdk/google-vertex';
+import { streamText } from 'ai';
+import { z } from 'zod';
+import { run } from '../../lib/run';
+import { saveRawChunks } from '../../lib/save-raw-chunks';
+
+run(async () => {
+  const result = streamText({
+    model: vertex('gemini-3.1-pro-preview'),
+    prompt: 'Cook me a lasagna.',
+    tools: {
+      cookRecipe: {
+        description: 'Cook a recipe',
+        inputSchema: z.object({
+          recipe: z.object({
+            name: z.string(),
+            ingredients: z.array(
+              z.object({
+                name: z.string(),
+                amount: z.string(),
+              }),
+            ),
+            steps: z.array(z.string()),
+          }),
+        }),
+      },
+    },
+    includeRawChunks: true,
+  });
+
+  for await (const part of result.fullStream) {
+    switch (part.type) {
+      case 'tool-input-start':
+        console.log(`\n[tool-input-start] ${part.toolName} (${part.id})`);
+        break;
+      case 'tool-input-delta':
+        process.stdout.write(part.delta);
+        break;
+      case 'tool-input-end':
+        console.log(`\n[tool-input-end] (${part.id})`);
+        break;
+      case 'tool-call':
+        console.log(`\n[tool-call] ${part.toolName}:`, part.input);
+        break;
+      case 'text-delta':
+        process.stdout.write(part.text);
+        break;
+      case 'finish':
+        console.log('\nFinish reason:', part.finishReason);
+        console.log('Usage:', part.totalUsage);
+        break;
+      case 'error':
+        console.error('Error:', part.error);
+        break;
+    }
+  }
+
+  await saveRawChunks({
+    result,
+    filename: 'google-vertex-stream-function-call-args-default.1',
+  });
+});

packages/google/src/__fixtures__/google-stream-tool-call-arguments.chunks.txt

@@ -0,0 +1,8 @@
+{"candidates":[{"content":{"role":"model","parts":[{"functionCall":{"name":"getWeather","willContinue":true},"thoughtSignature":"CiMBjz1rX25KieIB4d4AwFn8/WbsHTRNHBXso88PtemwfsSfRAp6AY89a1/e2FrmrE2HtlOxvV7vy8Kn5Og9n6CepcBKYW9aBT5QdXtTD4PGv9pDDWLEDAfsd86Et6k7HDEV976M3kC0ahLS8FCrwRntUKr+GF5uiUfPvgpzqNl5m8d8EwYykfwo1VwuTStR7q3mr/TzDcNzepafpVLdZloKYQGPPWtfkohmdau0GqGi3DuPbFTUBe5Ac/UbCvUk+P7KFbCGb93vFmnjmmTV7228DcAddqtWfe4iVePtSumw3JptYE6PIeVao2r8q7FXZKazzEiA9c04nmq6Vv9lOtt5NsgKrwEBjz1rX1ajIx3yeG/QI78uO+MsKoqgwrCI7PAEaDRqbyDCW/NX/4Vfuz1wp96oT3h/ttJ5ejzQyGamOrYreUUFvJBR/0TvFBdWOXwwCOWtvJ0Jh7SinV9HNqBDM1WWH8e+tGWY0r7xU/o/M1Fkwd73bvyp3BmSm5orrQL4jUZ1TN6ECqw7T7wHEkIfuBXGMZhQwAPzaapGVCdn0LjFJwd1aVReWmt6pE4ut993/K+0CqsBAY89a1+ts0Nggt7GZxVw/dQhH2Gc6KxRbAZKFMTuYRcNufJ5CyTvq85rMWEhTPWQxlQe1V7NIOlsQzyDhlejiCVbq0chDVrwRY23BmLrhSLi+SfKukEc0M+V9LdFiqxxLgUBrVdCvxA17g11j6HGdmXiNze2WDKm7ZXgMYvEiVmglpRZOb4JeJzxNPFsxhv2k6VkKB+GRtO48XwNz/i2taQ8Nq+LO5tED6HhCp4BAY89a18wBKoSkBT2IRo530FeNq/lpVozY0+k+p+H//TAtKoWZBamKCwra81idaMm1TTaWZGlX85Pq0kBvx1sQLFEhILeQrbH0O3Iuk9JlKLKctAwnBj8BZUGJEGtbigTKaSgWcMBhW+jfKWi1xwSxyJLRkIcXmE8U3FE/XHCLeiBy0NRa0yKn3xh62JU9087Q2nOMClxy5Zy5VsZ0qQ="}]}}],"usageMetadata":{"trafficType":"ON_DEMAND"},"modelVersion":"gemini-3.1-pro-preview","createTime":"2026-04-02T17:03:50.399550Z","responseId":"dqHOab6xGLzWodAPkPuViA4"}
+{"candidates":[{"content":{"role":"model","parts":[{"functionCall":{"partialArgs":[{"jsonPath":"$.location","stringValue":"Boston","willContinue":true}],"willContinue":true}}]}}],"usageMetadata":{"trafficType":"ON_DEMAND"},"modelVersion":"gemini-3.1-pro-preview","createTime":"2026-04-02T17:03:50.399550Z","responseId":"dqHOab6xGLzWodAPkPuViA4"}
+{"candidates":[{"content":{"role":"model","parts":[{"functionCall":{"partialArgs":[{"jsonPath":"$.location","stringValue":""}],"willContinue":true}}]}}],"usageMetadata":{"trafficType":"ON_DEMAND"},"modelVersion":"gemini-3.1-pro-preview","createTime":"2026-04-02T17:03:50.399550Z","responseId":"dqHOab6xGLzWodAPkPuViA4"}
+{"candidates":[{"content":{"role":"model","parts":[{"functionCall":{}}]}}],"usageMetadata":{"trafficType":"ON_DEMAND"},"modelVersion":"gemini-3.1-pro-preview","createTime":"2026-04-02T17:03:50.399550Z","responseId":"dqHOab6xGLzWodAPkPuViA4"}
+{"candidates":[{"content":{"role":"model","parts":[{"functionCall":{"name":"getWeather","willContinue":true}}]}}],"usageMetadata":{"trafficType":"ON_DEMAND"},"modelVersion":"gemini-3.1-pro-preview","createTime":"2026-04-02T17:03:50.399550Z","responseId":"dqHOab6xGLzWodAPkPuViA4"}
+{"candidates":[{"content":{"role":"model","parts":[{"functionCall":{"partialArgs":[{"jsonPath":"$.location","stringValue":"San Francisco","willContinue":true}],"willContinue":true}}]}}],"usageMetadata":{"trafficType":"ON_DEMAND"},"modelVersion":"gemini-3.1-pro-preview","createTime":"2026-04-02T17:03:50.399550Z","responseId":"dqHOab6xGLzWodAPkPuViA4"}
+{"candidates":[{"content":{"role":"model","parts":[{"functionCall":{"partialArgs":[{"jsonPath":"$.location","stringValue":""}],"willContinue":true}}]}}],"usageMetadata":{"trafficType":"ON_DEMAND"},"modelVersion":"gemini-3.1-pro-preview","createTime":"2026-04-02T17:03:50.399550Z","responseId":"dqHOab6xGLzWodAPkPuViA4"}
+{"candidates":[{"content":{"role":"model","parts":[{"functionCall":{}}]},"finishReason":"STOP"}],"usageMetadata":{"promptTokenCount":26,"candidatesTokenCount":23,"totalTokenCount":181,"trafficType":"ON_DEMAND","promptTokensDetails":[{"modality":"TEXT","tokenCount":26}],"candidatesTokensDetails":[{"modality":"TEXT","tokenCount":23}],"thoughtsTokenCount":132},"modelVersion":"gemini-3.1-pro-preview","createTime":"2026-04-02T17:03:50.399550Z","responseId":"dqHOab6xGLzWodAPkPuViA4"}