OpenCode Go: union-alpha fails with 503 'Upstream request failed: Endpoint is unavailable' on tool calls

Description

When using opencode-go/union-alpha, plain prompt requests (without tools) work as expected, but any turn that involves tool calling (e.g. read, write, bash) consistently triggers a 503 Service Unavailable from the provider gateway:

{"type":"error","error":{"type":"api_error","message":"Error from provider (Console Go): Upstream request failed: Endpoint is unavailable."}}

Steps to Reproduce

Tested on OpenCode 1.18.31 (and third-party agents like Pi):

1. Run plain prompt (succeeds):

curl -s -H "x-api-key: $OPENCODE_GO_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "x-opencode-session: session-test" \
  -H "content-type: application/json" \
  https://opencode.ai/zen/go/v1/messages \
  -d '{"model": "union-alpha", "max_tokens": 50, "messages": [{"role": "user", "content": "Hi"}]}'
# Returns 200 OK

2. Run request with tools attached or via CLI:

opencode run --model opencode-go/union-alpha "Create a test file test.txt and read it back"

Actual output:

← Write test.txt
Wrote file successfully.
Error: Error from provider (Console Go): Upstream request failed: Endpoint is unavailable.

Environment

• OpenCode version: 1.18.31
• Model: opencode-go/union-alpha
• Route: https://opencode.ai/zen/go/v1/messages (Anthropic-compatible endpoint)
• Related historical issues: #44300, #42757