MCP Image Return

[eerobert]

Which version of LM Studio?
LM Studio 0.4.2

Which operating system?
Ubuntu 24.02

What is the bug?
MCP server reply with JSON + 3 images. But the client wrongly name the 1st 2 jpg using the same filename.

Screenshots

[eerobert]

I checked with MCP Inspector, confirm that the MCP server do reply 3 different images.
I believe the bug is the filename assignment by LM-Studio

[mikhel01]

LMStudio 0.4.11 (Build 1)

I found this bug when i was developing an image generation plug-in to LMStudio, it return 4 images, but i sometimes got the same image in duplicate, after hours of struggling i realized that the filenames is milliseconds and started suspect that LMStudio is at fault here,

--The text below '-- --' is a bug report template i asked Claude to write and have added and changed myself.--
--The sample code is also written with help of LLM to have a minimum code to reproduce the problem--

When an MCP tool returns multiple image content items in a single response, LMStudio saves them to disk using
millisecond timestamps as filenames (e.g. image-1775997858666.jpg). If two or more images are processed within the
same millisecond, they receive identical filenames, causing later files to overwrite earlier ones. The result shown to
the user contains duplicate references to the surviving file instead of the distinct images.

Steps to Reproduce

1. Configure an MCP tool that returns 4 image content items in a single response (e.g. an image generation tool).
2. Call the tool from an LMStudio chat session.
3. Observe the saved filenames in the LMStudio project folder.

Expected Behavior

4 distinct files are created, one per image content item.

Actual Behavior

Only 3 distinct files are created. Two items receive the same filename (e.g. image-1775997858666.jpg), the second
write overwrites the first, and the chat result references the same file twice:

image-1775997858663.jpg image 1
image-1775997858665.jpg image 2
image-1775997858666.jpg image 3
image-1775997858666.jpg image 4 (collision — overwrites image 3)

---

Root Cause

Filenames are generated from Date.now() (or equivalent millisecond timestamp). When the response array is iterated
synchronously, consecutive items can land within the same millisecond.

---

Environment

• Interface: LMStudio 0.4.11 (Build 1) (desktop) Windows
• Transport: MCP stdio
• Response type: image content items returned as base64 in a single tool result array

I Include a sample in imgbug.zip that contain

server.mjs
package.json
image0.jpg
image1.jpg
image2.jpg
image3.jpg

That should reproduce the bug

imgbug.zip

[Legend-etf]

Just adding some additional information that may be relevant.

I have ran into this myself on LM Studio 0.4.16 (Build 2) and while investigating, I found that images uploaded via the UI appear to have a filename generation method that avoids collisions like this due to the filenames for images uploaded via the UI (located in the "user-files" directory) appearing to generate with 3 additional, (random?) numbers at the end. For example:

(".lmstudio\user-files\1782233451888 - 242.jpg") - Image uploaded via the UI (3 extra numbers)
vs
(".lmstudio\working-directories\1782138137455\image-1782143661202.jpg") - Image from MCP server (no extra numbers)

Changing the MCP server response filename generation to also have the random suffix could work as a solution?