google-vertex-anthropic provider can't use the 1M context window for Opus 4. #14003

Description

The google-vertex-anthropic provider can't use the 1M context window for Opus 4.6. I had OpenCode explore a bit why, as I have Opus 4.6 [1M] working correctly with Claude Code on the same provider.
It seems to be because Vertex AI requires the beta flag as a body parameter ("anthropic_beta": ["context-1m-2025-08-07"]), not an HTTP header. Setting headers in provider config sends it as an HTTP header, which Vertex rejects with "Unexpected value(s) for the anthropic-beta header". There's no config option to inject body parameters. Confirmed via curl that the body param form works on the same endpoint.

# Note: I am on a regional endpoint
curl -X POST \
  -H "Authorization: Bearer $(gcloud auth application-default print-access-token)" \
  -H "Content-Type: application/json" \
  "https://$GCLOUD_REGION-aiplatform.googleapis.com/v1/projects/$GCLOUD_PROJECT/locations/$GCLOUD_REGION/publishers/anthropic/models/claude-opus-4-6:rawPredict" \
  -d '{"anthropic_version":"vertex-2023-10-16","anthropic_beta":["context-1m-2025-08-07"],"messages":[...],"max_tokens":20}'

Claude Code seems to handle this by auto-converting the header to a body param in its Vertex SDK adapter — the @ai-sdk/google-vertex/anthropic package used by OpenCode doesn't do this.
All these issues seem to be related, just not for Vertex but for Zen/Bedrock: #12452, #12338, #13199

Plugins

No response

OpenCode version

1.1.60

Steps to reproduce

1. Configure google-vertex-anthropic with claude-opus-4-6 and set limit.context: 1000000
2. Add "headers": {"anthropic-beta": "context-1m-2025-08-07"} to provider options
3. API returns 400: "Unexpected value(s) context-1m-2025-08-07 for the anthropic-beta header"
4. Without the header, it works but caps at 200K

Screenshot and/or share link

No response

Operating System

macOS Sequoia 15.7.2

Terminal

kitty