ChatGPT App added via “+” is recognized by Instant/Thinking, but GPT-5.5 Pro often cannot 'see' the app or its tools

Hey everyone!!

Wondering if anyone’s come across this weird model-specific MCP app/tool discovery issue I’m seeing in ChatGPT.

The Problem

With the same published MCP app, added to the same ChatGPT conversation using the “+” button in the composer:

• Works consistently in ChatGPT Instant & Thinking : the app is recognized and the model can list/use the app’s tools.

• Fails intermittently in GPT-5.5 Pro : around 8 out of 10 times, the model behaves as if the MCP app/tool manifest is not available, even though the app was added to the conversation context explicitly with the little “+” icon.

This happens whether Developer Mode is enabled or disabled (I believe I tried them both quite extensively). The MCP app is already published to the app directory, though it’s on Enterprise.

Example failed response from GPT-5.5 Pro

After adding the app via the “+” button and asking it to list the app tools, GPT-5.5 Pro responded:

I’m looking into whether I can list the tools that work with the app, but I don’t see the specific “api_tool” mentioned by the developer. It seems this tool might be hidden or not available in my current setup. I’ll be honest here and let the user know that I can’t access the tool list for this app.

This is notable because the same app/context works reliably in Instant and Thinking.

It looks like GPT-5.5 Pro may be receiving some app/developer context that references the app/tool, but intermittently not receiving the corresponding MCP tool manifest or runtime tool access.

What I’m trying to understand

1. Is GPT-5.5 Pro expected to handle MCP app context/tool manifests differently from Instant or Thinking?

2. Could there be a model-specific issue where GPT-5.5 Pro receives app-related context but does not reliably receive the app’s available tools after the app is added via the composer “+” button?

3. Are there known cases where published MCP apps are visible to some ChatGPT models but not others?

Reproduction

1. Open a new ChatGPT conversation.

2. Select GPT-5.5 Pro.

3. Use the “+” button in the chat composer to add your custom published MCP app to the conversation.

4. Ask the model to list the tools available for that app.

5. Observe that GPT-5.5 Pro often says it cannot see or access the app/tools.

6. Repeat the same flow with ChatGPT Instant or ChatGPT Thinking.

7. Observe that Instant and Thinking consistently recognize the app and can list/use the tools.