Opencode V2. Provider request failed with HTTP 400 when using vertex AI Anthropic models

Description

With V2, I cannot use Vertex AI Anthropic models, the request fails with 400 http code. The same configuration and environment works fine with V1.

The following error trace is recorded at ~/.local/share/opencode/log/opencode.log:

timestamp=2026-09-20T00:52:24.520Z level=ERROR run=24cf9740 message="Failed to drain Session" cause="AI.Error: Provider request failed with HTTP 400\n    at zF (/$bunfs/root/chunk-e3c5fy3f.js:17:3539)\n    at <anonymous> (/$bunfs/root/chunk-e3c5fy3f.js:17:32
32)\n    at SessionStep.attempt (/$bunfs/root/chunk-w6191acq.js:366:3584)\n    at SessionStep.attempt (definition) (/$bunfs/root/chunk-w6191acq.js:351:10764)\n    at SessionRunner.runStep (/$bunfs/root/chunk-w6191acq.js:366:2624)\n    at SessionRunner.runSte
p (definition) (/$bunfs/root/chunk-w6191acq.js:366:2774)\n    at SessionRunner.drain (/$bunfs/root/chunk-71mat7h1.js:24:2279)\n    at SessionRunner.drain (definition) (/$bunfs/root/chunk-w6191acq.js:366:352)\n    at ServerProcess.start (/$bunfs/root/chunk-fn
k4cbc1.js:2:6383)\n    at ServerProcess.start (definition) (/$bunfs/root/chunk-w6191acq.js:437:22432) {\n  [cause]: AI.Error.InvalidRequest: Provider request failed with HTTP 400\n      at ba (/$bunfs/root/chunk-e3c5fy3f.js:17:1117)\n      at zF (/$bunfs/roo
t/chunk-e3c5fy3f.js:17:3550)\n      at <anonymous> (/$bunfs/root/chunk-e3c5fy3f.js:17:3232)\n}" role=server sessionID=ses_f43b336fdffeavWqXLMZZfhGBR

Plugins

No response

OpenCode version

v2.0.10

Steps to reproduce

Use an Vertex AI Anthropic model and ask something

Screenshot and/or share link

No response

Operating System

Linux Fedora 44

Terminal

Wezterm

Start by reproducing the failure with a Vertex AI Anthropic model on OpenCode v2.0.10 using the configuration that works with V1, then inspect the provider request path and compare the resulting requests. Done means the same Vertex AI Anthropic request succeeds under V2 without the HTTP 400 error.