Vision: local image reads via Read tool fail with omniroute/cc provider, but same image works fine when sent directly as base64 to the proxy

Bug description

When using the Read tool to load a local image file, the model reports it cannot process the image ("this model does not support image input"), even though the active model (omniroute/cc/claude-sonnet-5) is registered as vision-capable.

Diagnosis performed

1. Confirmed the model is registered with "vision": true in the OmniRoute /v1/models endpoint.
2. Sent the exact same local image directly to the OmniRoute proxy as a base64-encoded image_url data URI via curl, bypassing OpenCode entirely:

curl -s http://localhost:20128/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "cc/claude-sonnet-5",
    "messages": [{
      "role": "user",
      "content": [
        {"type": "text", "text": "What do you see in this image?"},
        {"type": "image_url", "image_url": {"url": "data:image/png;base64,<...>"}}
      ]
    }]
  }'

3. Result: the proxy call succeeded and the model correctly described the image contents. Latency ~1.4s, cost ~$0.005, no errors.

Conclusion

The OmniRoute proxy and the underlying vision-capable model both work correctly. The bug is isolated to how OpenCode packages/sends the image content block when reading a local file via the Read tool with an openai-compatible custom provider (OmniRoute in this case) — the image content is either not being converted to the expected image_url/base64 format, or is being dropped before the request reaches the provider.

Environment

• OpenCode via custom openai-compatible provider (OmniRoute, local proxy at localhost:20128)
• Model: omniroute/cc/claude-sonnet-5 (registered as vision-capable by the provider)
• OS: macOS (darwin)

Expected behavior

Reading a local image via the Read tool should work identically to sending the same image as a base64 image_url directly to the same provider/model, since the underlying model and proxy both support and correctly process vision input.