[BUG] Vertex/Bedrock subagents silently downgraded to older models (Sonnet 4.5, Opus 4.1)

Bug Description

When using Claude Code with Vertex AI (CLAUDE_CODE_USE_VERTEX=1), subagent model resolution silently downgrades abstract model aliases to significantly older versions compared to first-party API users. This is undocumented and misleading.

Root Cause

The B8() function resolves abstract aliases (sonnet, opus, haiku) by calling provider-specific functions that hardcode older models for non-firstParty providers:

// CN() - "Current Sonnet"
function CN() {
  if (process.env.ANTHROPIC_DEFAULT_SONNET_MODEL) return process.env.ANTHROPIC_DEFAULT_SONNET_MODEL;
  if (E_() !== "firstParty") return b8().sonnet45;  // Vertex/Bedrock → Sonnet 4.5
  return b8().sonnet46;                              // Anthropic API → Sonnet 4.6
}

// tI() - "Current Opus"
function tI() {
  if (process.env.ANTHROPIC_DEFAULT_OPUS_MODEL) return process.env.ANTHROPIC_DEFAULT_OPUS_MODEL;
  if (E_() === "firstParty") return b8().opus46;     // Anthropic API → Opus 4.6
  return b8().opus41;                                 // Vertex/Bedrock → Opus 4.1 (!!)
}

Effective model mapping

| Alias | firstParty (Anthropic API) | Vertex/Bedrock |
|-------|---------------------------|----------------|
| sonnet | Sonnet 4.6 | Sonnet 4.5 (claude-sonnet-4-5@20250929) |
| opus | Opus 4.6 | Opus 4.1 (claude-opus-4-1@20250805) |
| haiku | Haiku 4.5 | Haiku 4.5 (same) |

Why This Is a Problem

1. Opus downgrade skips two major versions — Vertex users get Opus 4.1 instead of 4.6, despite 4.6 being GA on Vertex.
2. Undocumented — The Vertex AI docs don't mention that abstract aliases resolve to older models.
3. Misleading UI — The /model picker shows "Sonnet 4.6" and the system prompt references "Sonnet 4.6" for Vertex users, but subagents spawned with model: "sonnet" silently get 4.5.
4. Affects all plugins — Any plugin agent definition using model: sonnet or model: opus (e.g., feature-dev, pr-review-toolkit, code-review) is affected.
5. Models are available — Google's own docs confirm Sonnet 4.6 and Opus 4.6 are GA in us-east5, europe-west1, asia-southeast1, and global endpoints.

Environment

• Claude Code version: 2.1.68
• CLAUDE_CODE_USE_VERTEX=1
• CLOUD_ML_REGION=us-east5
• Platform: macOS (darwin arm64)

Workaround

Setting explicit env vars bypasses the downgrade:

export ANTHROPIC_DEFAULT_SONNET_MODEL='claude-sonnet-4-6'
export ANTHROPIC_DEFAULT_OPUS_MODEL='claude-opus-4-6'

Expected Behavior

Abstract aliases should resolve to the same model generation across all providers, or at minimum this behavior should be documented and visible in the UI when a downgrade occurs.

Related

• #18674 (model version override env vars)