Streaming support for partial images (similar to incomplete link handling)

[pokroman]

Problem Statement

When streaming markdown, ‎streamdown currently removes incomplete images (e.g. ‎![alt](https://exampl) instead of rendering a placeholder. This works fine for final output, but in streaming UIs it introduces a noticeable delay: nothing is shown until the full image URL is emitted and the markdown becomes valid. In contrast, links are handled more gracefully: incomplete links are replaced with a placeholder URL (‎streamdown:incomplete-link), so the user can already see the link text while the model is still streaming.

For image-heavy content, this behavior makes the streaming experience feel less responsive compared to how links are handled.

Proposed Solution

Introduce a streaming-friendly image handling mode, similar to incomplete link handling. For example:

• When an image is incomplete (unterminated URL or closing ‎) missing), render a placeholder image or a “loading” image state instead of dropping the node.

• Optionally provide a configurable placeholder strategy (e.g. built-in default, custom React component, or callback).

• Once the full image URL has been streamed, update the rendered node in place to the real image.

The goal is that the user can see that “an image is coming” immediately, even before the markdown for the image is fully complete.

Alternatives Considered

• Keeping the current behavior and waiting until the full image URL is streamed. This keeps the implementation simple but gives a worse UX for streaming, especially when models emit long or slow URLs.

• Post-processing the model output on the host side to inject a custom placeholder when ‎![ is detected without a closing ‎). This is possible but reimplements logic that ‎streamdown (and ‎remend) already understand, and is easy to get wrong for edge cases.

• Using text-only placeholders in the original markdown (e.g. “Loading image…”), then replacing them later. This requires changing prompts / model behavior and doesn’t leverage the existing markdown/image pipeline.

Use Case

In a React app that streams AI responses using ‎`streamdown`, we often generate content with multiple images (e.g. visual step‑by‑step guides, design reviews, or diagrams). As the model streams:

- Links are visible right away thanks to incomplete link handling.

- Images, however, simply don’t appear until the entire image markdown is complete, which can be several seconds later.

Priority

Nice to have

Contribution

• I am willing to help implement this feature

Additional Context

No response