[BUG] Claude cannot process images sent via MCP or get images from URL

Environment

• Platform (select one):
• [ ] Anthropic API
• [ ] AWS Bedrock
• [ ] Google Vertex AI
• [*] Other: Claude Desktop
• Claude CLI version: Claude 0.12.55 (d55c63) 2025-07-25T17:43:32.000Z
• Operating System: macOS 15.6
• Terminal:

Bug Description

Building remote MCP server that needs to send image in a response to tool call. Image is scaled down and sent (as base64 encoded, verified the format is ok). The response with the image is 400kB. I'm getting toast ephemeral (non copyable) error about "connection problems or conversation length". Im on paid plan and conversation is fresh.

To do a workaround, I uploaded image to web address (on the same domain as MCP server) and sent the URL. I'm getting error about "url cannot be accessed if the user has not directly submitted the URL to chat". Well, that's bummer, but ok, I copied and pasted the URL of the image to the chat and - I got another error, that Claude cannot read and analyze image from URL.

So my MCP server needs at some point the ability to sent to an assistant the image to identify something on the image. There is virtually NO WAY how to do it (except asking the user to download image and paste it manually to the chat)?

I triple checked with MCP Inspector all the responses and I do not see any problems or warnings.

THe response (is sent to Claude Desktop MCP client in less than 10 sec (which was mentioned in another research by Claude that it should be, but nowhere I can find as human the documentation).

I understand that MCP needs quardrails and security, but, I'd expect that:

• there should be any information about the maximum size of the image data sent in the MCP response to MCP Client (claude desktop). There is nowhere any info and Claude doing research tells me "less than 1MB". But even 400kB is too much.
• the MCP server domain should be whitelisted for accessing files or MCP should present (in a way like Content Security Policy) what should be safe to read.

Steps to Reproduce

Scenario A

1. sending image in response. The whole response (with text, stuctured content and image) is not more than 400kB

Scenario B

1. upload image to the same domain as MCP and send url in MCP response. Claude will NOT be able to use this url (it was not inserted by the user in the chat restriction)

Scenario C

1. Ask claude explicitly to analyze the image and paste URL to the chat. Claude is not able to analyze image.

Expected Behavior

Claude Desktop should support images in MCP responses (at reasonable resolution) or document how big they can be at max.