Power Platform Community Forum Thread Details

Hi all,

I'm working on integrating a .NET-based MCP server with Copilot Studio using a custom connector and am running into two possibly related issues. Hoping someone here has insights.

Setup Summary:

MCP server is implemented in .NET, with POST /mcp endpoint (synchronous, non-streamable HTTP, not SSE).
Fully compliant with JSON-RPC 2.0 and MCP spec.
Auth uses API key passed in the header.
2 tools are exposed by the server (confirmed with Postman).
Created a custom connector with security set to API key, imported via GitHub (recommended workaround).
Custom connector is added to a Copilot Studio agent and configured successfully.

Issue 1: Tools Not Displayed in Agent UI

When configuring tools in the Copilot Studio UI, no tools appear even though the MCP server is functional.

Observed Behavior:

The UI calls "initialize" and "notifications/initialized" methods on POST /mcp, but never calls "tools/list".
When I manually call tools/list from Postman, it returns the expected 2 tools.

Issue 2: First Interaction Always Fails

When testing in the Copilot Studio editor:

The first user message results in an error: "Sorry, something went wrong. Error code: SystemError. Conversation ID: xxx-xxx-xxx. Time (UTC): 7/30/2025 1:03:39 PM."
Typing the same message again succeeds - "tools/list" and "tools/call" are invoked as expected.
If I clear the chat, the error returns on the next first message, and the cycle repeats.

Observed Behavior:

On first message: only "initialize" and "notifications/initialized" are called.
On second message: "tools/list" and "tools/call" are triggered, and everything works normally.

Questions:

Why doesn’t Copilot Studio call tools/list during the tool setup UI?
What might cause the first message to fail, and subsequent ones to work? Is there a timing/initialization bug related to non-streamable MCP servers?
Is this a known issue when using a custom connector as the MCP client?

Any help is appreciated - especially from anyone who has successfully connected a non-streamable MCP server via a custom connector.

Categories:

Model context protocol

Do you mean Streamable HTTP? That's what this spec is (and SSE is deprecated anyway)

In my experience if you can connect to the MCP server with MCP Inspector, it will work with a custom connector

https://modelcontextprotocol.io/legacy/tools/inspector

It does appear to initialize the MCP tools as soon as the user has started a session (or at least when it is trying to orchestrate the first reply)

Perhaps that initialize is failing for some reason (you would see this in the console of a web channel or in the Save Snapshot if you're using Copilot Studio to test it)

I've gotten to the point where I test my MCP server in the MCP Inspector, and then just refresh the tools and publish. I'm not even sure if publishing is required since it seems to initialize each time. This works quite well - usually updating the instructions or the description of each tool will help the orchestrator figure out how to use the MCP Server