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Installation

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Overview

The @ai-billing/google-vertex package provides middleware for tracking token usage and calculating costs when using Google Vertex AI’s Gemini models with the Vercel AI SDK, via @ai-sdk/google-vertex’s createVertex. Vertex reports the same Gemini-native usage shape as @ai-sdk/google, but under providerMetadata.vertex.usageMetadata instead of providerMetadata.google.usageMetadata. Reasoning (“thoughts”) tokens are reported separately from the visible completion tokens (thoughtsTokenCount), and this package adds them back into the billed completion count so reasoning usage is never under-billed.
Vertex also reports trafficType ('ON_DEMAND' for pay-as-you-go calls vs 'PROVISIONED' for committed-throughput calls). ModelPricing has no field for a provisioned-throughput rate, so this middleware bills every call at the same resolved rate regardless of trafficType — this is a known, intentional limitation, not a bug. The raw trafficType value is still surfaced on every emitted billing event’s usage.subProvider field, so you can filter or re-rate provisioned traffic downstream if you need to.

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Usage

To use the middleware, wrap your Vertex model using wrapLanguageModel from the ai package and pass createGoogleVertexMiddleware.
1

Initialize the Google Vertex AI provider

First, set up the provider using createVertex from @ai-sdk/google-vertex. Vertex authenticates via Application Default Credentials, so make sure GOOGLE_VERTEX_PROJECT, GOOGLE_VERTEX_LOCATION, and GOOGLE_APPLICATION_CREDENTIALS are set in your environment.
2

Define model pricing

Set up a price resolver to define the costs for the models you’ll be using. For Vertex’s Gemini models, you can specify costs for standard prompt/completion tokens, cache-read tokens, and reasoning tokens (internalReasoningTokens).
3

Create the billing middleware

Initialize the Google Vertex billing middleware. You need to provide a destination (such as consoleDestination) where billing events will be sent, along with your priceResolver.
4

Wrap the model

Use wrapLanguageModel from the ai package to apply the billing middleware to your Vertex model.
5

Use the wrapped model

Finally, use the wrapped model with AI SDK functions like generateText or streamText. The billing middleware will automatically track tokens, handle reasoning metrics, and calculate costs.