[Bug] Chat Completions Vision Format Crash

Describe the issue

Foundry Local's /v1/chat/completions endpoint severely mishandles standard OpenAI-style image_url inputs. Sending an image in the standard Chat Completions shape triggers a massive request-format bug. The engine attempts to allocate tens of gigabytes of memory, immediately crashing the entire engine.

-Impact: The standard /v1/chat/completions endpoint cannot be used for vision tasks.

(Working alternative for this: route the same image through the proprietary /v1/responses endpoint instead, using a distinct input_image field with raw base64 (no data: URI prefix). This works reliably but is undocumented as the required path for vision and isn't OpenAI-compatible, breaking drop-in compatibility with existing OpenAI client code.)

To reproduce

1. Construct a standard OpenAI Chat Completions request with an image supplied via the image_url content-part shape (as used by the official OpenAI SDK and every OpenAI-compatible client).

2. POST it to Foundry Local's /v1/chat/completions endpoint with the vision model loaded.

3. Observe the engine attempting a massive (tens-of-GB) memory allocation and crashing outright this takes down the whole service, not just the request.

4. Working alternative: route the same image through the proprietary /v1/responses endpoint instead, using a distinct input_image field with raw base64 (no data: URI prefix). This works reliably but is undocumented as the required path for vision and isn't OpenAI-compatible, breaking drop-in compatibility with existing OpenAI client code.

Urgency

Medium. Makes the standard/documented endpoint unusable for vision and forces consumers to use an undocumented, proprietary endpoint. But there is also a working alternative.

Platform and architecture

Windows X64

OS Version

Windows 11

Installation type

Released package/binary

Foundry Local version

0.10.2

API or surface area

REST API

Hardware acceleration/backend

CPU

Backend/runtime version

No response

Start by reproducing the standard OpenAI-style image_url request against the /v1/chat/completions endpoint with a vision model on Foundry Local 0.10.2. Compare this with the working /v1/responses input_image path, then trace the REST API request handling. Done means a standard image_url request no longer causes a massive memory allocation or engine crash and remains compatible with OpenAI client formats.