Vertex Anthropic routing: google-vertex sends claude-* to publishers/google/ (404), and built-in subagent model bindings ignore user config

Summary

Two related bugs in google-vertex provider handling of Claude (Anthropic) models on opencode 1.18.5:

1. Wrong Vertex publisher namespace: google-vertex provider routes claude-* model requests to publishers/google/models/<claude-model> on Vertex, but Anthropic models on Vertex live at publishers/anthropic/models/<claude-model>. All Claude calls under google-vertex return Publisher model ... was not found.
2. Built-in subagent model bindings are hardcoded and ignore user config: The built-in subagents explore, general, junior-software-engineer, ops-debugger, qa-engineer are pinned to google-vertex/claude-haiku-4-5 (a non-existent model id — the current catalog only has claude-haiku-4-5@20251001). No user-config path can override this binding:
   • agent.<name>.model in opencode.jsonc — silently ignored
   • model: frontmatter in ~/.config/opencode/agents/<name>.md — silently ignored (verified by setting it to FAKE/xyz with no effect on the resolved config)
   • Adding mode: subagent to the same block — mode field is accepted but model is still ignored
   • Removing google-vertex from enabled_providers — still stuck

The same config shape works perfectly for any custom agent name (verified by adding an agent mycustom with the same config — resolved correctly).

Environment

• opencode 1.18.5 (brew, /opt/homebrew/bin/opencode)
• macOS 14.5 (Darwin 25.5.0)
• Vertex project: uses ADC auth via gcloud auth application-default login
• providers enabled: google-vertex-anthropic, google-vertex, github-copilot

Reproduction

~/.config/opencode/opencode.jsonc:

{
  "$schema": "https://opencode.ai/config.json",
  "model": "google-vertex-anthropic/claude-sonnet-4-6",
  "enabled_providers": ["google-vertex-anthropic", "google-vertex"],
  "provider": {
    "google-vertex-anthropic": {
      "options": { "project": "<PROJECT>", "location": "global" }
    }
  },
  "agent": {
    "explore":   { "model": "google-vertex-anthropic/claude-haiku-4-5@20251001" },
    "mycustom":  { "model": "google-vertex-anthropic/claude-haiku-4-5@20251001" }
  }
}

Run:

GOOGLE_VERTEX_PROJECT=<PROJECT> GOOGLE_VERTEX_LOCATION=global opencode debug config

Actual

agent.explore.model  = google-vertex/claude-haiku-4-5     ← ignored user config
agent.mycustom.model = google-vertex-anthropic/claude-haiku-4-5@20251001   ← honored

Invoking explore as a subagent then fails at runtime:

AI_APICallError: Publisher model
`projects/<PROJECT>/locations/us-central1/publishers/google/models/claude-sonnet-4-6`
was not found or your project does not have access to it.

and separately:

ProviderModelNotFoundError: Model not found: google-vertex/claude-haiku-4-5.
Did you mean: claude-haiku-4-5@20251001?

Note the location us-central1 in the URL even though region: "global" is set in the provider options — the region field also seems to be ignored for google-vertex.

Expected

• google-vertex provider should either route claude-* models to publishers/anthropic/ OR reject claude-* model ids with a clear "use google-vertex-anthropic instead" error.
• agent.<name>.model config for built-in subagent names should override the built-in defaults.
• Built-in subagent default model ids should reference model versions that actually exist in the current catalog (e.g., claude-haiku-4-5@20251001 rather than the versionless claude-haiku-4-5).
• provider.<name>.options.region should be respected.

Impact

Any user with a Vertex-anchored Claude setup can't use the built-in Haiku subagents (explore, general, junior-software-engineer, ops-debugger, qa-engineer) at all. Delegated work through these agent names fails immediately. Custom-named "shadow" agents work as a workaround.

Workaround

Duplicate each built-in .md under a new name (e.g., explore-vx.md with name: explore-vx) with model: google-vertex-anthropic/claude-haiku-4-5@20251001, then always call them via subagent_type=<name>-vx from the task tool. Full config for the custom name is honored. Users must remember to prefer the shadow, and skills/scripts that hardcode built-in names still break.

Related

• Bundled @ai-sdk/google-vertex/anthropic provider (google-vertex-anthropic) works correctly out of the box for Claude on Vertex — this is the code path the built-in agents should be using.
• strings /opt/homebrew/bin/opencode | grep -c "publishers/anthropic/models" returns matches, so the correct code path is compiled in.