Anthropic models on Vertex AI fail with FAILED_PRECONDITION (ChatCompletions not supported)

[kitadesign]

Summary

The Vertex AI Model Garden adapter added in #2287 routes all requests through the
OpenAI-compatible /chat/completions endpoint, but Anthropic Claude models on
Vertex AI do not support that endpoint — only the Anthropic-native
:rawPredict / :streamRawPredict endpoints work.

As a result, publisher: anthropic on Vertex is effectively unusable today.

Reproduction

agent.yaml:

models:
  claude:
    provider: google
    model: claude-sonnet-4-6
    provider_opts:
      project: <YOUR_GCP_PROJECT>
      location: us-east5
      publisher: anthropic

agents:
  root:
    model: claude
    instruction: say hi

Prerequisites:

• Anthropic Claude enabled on Vertex Model Garden for the project
• roles/aiplatform.user granted to the caller
• gcloud auth application-default login done

Run:

docker agent run agent.yaml --exec --yolo "hi"

Expected

Model responds.

Actual

❌ model failed: error receiving from stream:
HTTP 400: POST ".../endpoints/openapi/chat/completions": 400 Bad Request

With debug logging / direct curl the underlying error is:

{
  "error": {
    "code": 400,
    "message": "The deployed model does not support ChatCompletions. endpoint_id: ..., deployed_model_id: ...",
    "status": "FAILED_PRECONDITION"
  }
}

All Claude variants tested fail the same way:
claude-sonnet-4-6, claude-sonnet-4-5, claude-sonnet-4-5@20250929,
claude-opus-4-6, claude-haiku-4-5.

A direct :rawPredict call with the Anthropic body format returns 200 from the
same project/location, confirming access is configured correctly — only the
OpenAI-compat endpoint refuses Anthropic models.

Suggested fix

When provider_opts.publisher equals anthropic, route via the Anthropic-native
endpoint (:streamRawPredict + vertex-2023-10-16 body schema) instead of the
OpenAI-compat path in pkg/model/provider/vertexai/modelgarden.go. This mirrors
how the Bedrock adapter already talks to Anthropic models natively.

Environment

• docker-agent: v1.46.0
• OS: Linux (Cloud Workstation)
• Region tested: us-east5 (+ us-east4, us-central1, us-west1, us-south1, europe-west1, europe-west4, asia-northeast1, asia-southeast1 — all same error)

[dgageot]

@kitadesign Could you try #2469?

[elgreco247]

@dgageot I think you meant #2476 .