I already had Google Cloud billing.

I did not want another API key, another quota surface, and another place to explain to my team why the coding tool bill was showing up somewhere else.

The annoying part was that Claude Code speaks Anthropic's Messages API, while the budget I wanted to use was sitting in Vertex AI .

So I wired the two together through a local gateway.

The mismatch

If you try to connect Claude Code to Vertex AI directly, the shapes do not line up cleanly.

Claude Code expects this kind of world:

POST /v1/messages

Anthropic headers

Anthropic content blocks

Anthropic streaming semantics

Vertex AI is a different world.

For Claude models on Vertex, the request goes to a publisher endpoint like:

.../publishers/anthropic/models/claude-sonnet-4-6:rawPredict

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For Gemini models on Vertex, it goes to:

.../publishers/google/models/gemini-2.5-pro:generateContent

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That meant I needed something in the…