Vercel AI Gateway `only` provider filter is ignored

Description

Vercel AI Gateway provider filtering still appears to be ignored in OpenCode CLI 1.18.30 + Desktop 1.18.31.

This is the same behavior previously reported in #21130, which was automatically closed after 60 days of inactivity. The original feature request was #2153.

With this config:

{
  "$schema": "https://opencode.ai/config.json",
  "provider": {
    "vercel": {
      "models": {
        "zai/glm-5.3-flash": {
          "options": {
            "only": ["particle"]
          }
        }
      }
    }
  }
}

However, Vercel AI Gateway shows that requests are actually served by other providers. I observed Baseten and Fireworks instead of Particle.

OpenCode logs confirm both requests use the expected Vercel model:

2026-09-17T09:02:43
providerID=vercel modelID=zai/glm-5.3-flash small=true agent=title

2026-09-17T09:02:44
providerID=vercel modelID=zai/glm-5.3-flash small=false agent=build

The Vercel AI Gateway dashboard nevertheless shows these requests
being served by Baseten and Fireworks despite only: ["particle"].

Expected behavior: the request should either use Particle or fail if Particle is unavailable. It should not fall back to another provider.

The current OpenCode Vercel documentation lists only as a supported routing option.

Plugins

No response

OpenCode version

1.18.30

Steps to reproduce

Add the following to the OpenCode config:

{
  "$schema": "https://opencode.ai/config.json",
  "provider": {
    "vercel": {
      "models": {
        "zai/glm-5.3-flash": {
          "options": {
            "only": ["particle"]
          }
        }
      }
    }
  }
}

Verify the effective config:

opencode debug config

It shows:

"only": ["particle"]

Restart OpenCode.

Run:

opencode run -m vercel/zai/glm-5.3-flash "Reply only OK"

Inspect the resulting request in Vercel AI Gateway.

The serving provider is not Particle. In my tests the requests were routed to Baseten and Fireworks.

I reproduced the same behavior with OpenCode Desktop and the CLI.

Screenshot and/or share link

Operating System

macOS 15.7.9

Terminal

Terminal.app