[BUG]: interactive image file part is persisted but not always forwarded to the model

Description

An image attached to an interactive session can be stored as a valid image/png file part, while the next assistant response still says it cannot inspect the image and asks for a local path.

I confirmed this from a sanitized session record: the user message contains a type: "file" part with mime: "image/png", a data: URL, and source metadata. The immediately following response used a vision-capable OpenAI model but behaved as if no image content reached it.

This appears intermittent or specific to an interactive input path. As a control, both of these succeed on the same installation and accurately read text, colors, and shapes from a synthetic PNG:

opencode run --pure --model "openai/gpt-5.6-luna" "Describe the attached image" --file test.png
opencode run --model "openai/gpt-5.6-sol" "Describe the attached image" --file test.png

Expected: if the session has a valid image/* file part and the selected model supports image input, the image is forwarded as multimodal input. The model should not ask for a filesystem path.

Actual: the attachment is visible in persisted session parts, but the model sometimes responds as though it was not provided.

Related issues checked, but they cover different boundaries: #29216 (non-vision model/MCP delegation), #20802 and #33542 (custom OpenAI-compatible capability metadata), #37581 (application/octet-stream in Desktop), and #34318 (run -f MIME detection).

Plugins

oc-codex-multi-auth (the successful non---pure control also used the configured plugins)

OpenCode version

1.18.5

Steps to reproduce

1. Start an interactive OpenCode session with a model whose capabilities.input.image is enabled.
2. Attach or reference a PNG so that the resulting user message contains an image/png file part.
3. Ask the model to describe an unambiguous detail visible only in the image.
4. Inspect a sanitized export with opencode export --sanitize <session-id>.
5. Observe that the file part may be present as an image/png data URL even though the response says the image is unavailable and asks for its path.
6. Run the same model and image through opencode run --file; in my control this works correctly.

I cannot determine from the persisted session metadata whether the failing interactive message originated from TUI, Desktop/Web, or another client because no client/frontend field is stored. Adding that field to sanitized diagnostics would also make this class of issue easier to isolate.

Screenshot and/or share link

Not included because the original image/session may contain private data. The finding above comes from --sanitize output; no data URL or local path is included.

Operating System

macOS 26.5.2 (arm64)

Terminal

iTerm2