read tool returns 2-year-expired DashScope OSS signed URL to vision model → model hallucinates image content

Description

When a vision-capable custom provider model (DashScope / Alibaba qwen-vl-max via openai-compatible) reads a local image through the read tool (in a subagent), the tool result contains a pre-signed DashScope OSS URL that expired ~2 years ago instead of base64 image data. The model cannot fetch the dead URL, and instead of erroring it hallucinates a plausible but completely fabricated description of the image.

Evidence

read tool result for D:\730_864.jpg (a real local file, 2560x1440, verified with Get-FileHash):

{
  "type": "image",
  "content": "https://qwen-vl-max-imgs.s3-accelerate.amazonaws.com/730_864.jpg?Expires=1723425439&OSSAccessKeyId=LTAI5tGhLdJbYkxTfHrUyvXK&Signature=PZ%2FmN0uWpBQvRnC8MfVq3ZoKg4c%3D"
}

• Expires=1723425439 = 2024-08-12 (today is 2026-08-11) — the signature is stale by ~2 years.
• Fetching that URL returns 403 Forbidden.
• The vision model then "describes" the image with fabricated content (twice, two different hallucinations: once a webpage UI, once an indoor room — neither matches the actual image).

Diagnosis performed

1. Confirmed the image file itself is fine: System.Drawing loads it (2560x1440 JPEG), Get-FileHash works.
2. Sent the exact same image directly to DashScope's OpenAI-compatible endpoint (https://dashscope.aliyuncs.com/compatible-mode/v1/chat/completions, model qwen-vl-max) as a base64 image_url data URI via Invoke-RestMethod — works perfectly, returns an accurate description (a CS2 IEM Cologne 2026 Major collectible coin UI popup).
3. So the API/model are fine; the bug is opencode's image pipeline: it hands the model an expired signed URL instead of base64 bytes.

Steps to reproduce

1. Configure DashScope as a custom openai-compatible provider with a vision model (e.g. qwen-vl-max, attachment: true, modalities.input: ["text","image"]).
2. Create a subagent with that model (or read an image in the main session on that model).
3. read a local image file (ideally one whose filename matches a previously-uploaded DashScope object, e.g. 730_864.jpg — see note below).
4. Observe the tool result URL is expired; the model either errors or hallucinates.

Note on suspected root cause

The returned URL object name equals the local filename (730_864.jpg), and the expiry is from 2024 — strongly suggesting opencode (or its DashScope upload path) caches the signed OSS URL keyed by filename (or served a stale cached URL) rather than uploading fresh bytes per read. Renaming the file to a new unique name may reproduce differently — worth checking whether the URL is regenerated per request.

Expected behavior

Reading a local image via read should deliver the actual image bytes (base64 data URI) to the vision model — same as sending base64 directly to the same provider, which works.

Environment

• OS: Windows 11 (native, no WSL)
• opencode: latest as of 2026-08-11
• Provider: custom openai-compatible → DashScope qwen-vl-max