Unable to send message to MCP server

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Posted on by KR-10021350-0

I have created a very basic MCP server that provides one tool - a calculator that performs addition/subtraction operations on two numerical values.

I have created a custom connector for that MCP server based on the documentation (https://learn.microsoft.com/en-us/microsoft-copilot-studio/agent-extend-action-mcp)

As a result my custom connector has two actions

- acquire a connection to MCP server (via /sse path)

- handling of messages to MCP server (via /messages path)

However, I am facing some issues when trying to use the custom connector based actions in Copilot Studio agent:

- when creating an action in Copilot Studio, neither of the actions have any input parameters (when I use the action from the custom connector in Power Automate, input parameters are showing up correctly)

- using an action to acquire a connection to MCP server ends up in agent becoming unresponsive (this is probably due to the fact that /sse endpoint is jsonrpc and does not work in immediate request/response fashion)

- when using an action to send a message to MCP server, I get no response from the agent

Am I missing something? I believe the problem is most likely with my custom connector definition, but I've been trying many things and nothing worked. I can provide the connector definition if needed.

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Romain The Low-Code... 391 on at

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Unable to send message to MCP server

Hello,

An easy way is to include your custom connector inside a flow. And make the flow the action. Work for me everyday when i work with no code user. Power automate as action work very well.

The most elegant way is to use the correction from https://rajeevpentyala.com/2024/10/11/step-by-step-using-custom-connectors-in-copilot-studio-2/ like some already said
zankard Moderator on at

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Unable to send message to MCP server

@KR-10021350-0 - Please add the MCP as an Action, not in a Topic and ensure Generative orchestration is enabled.

Let us know if this resolves your issue!
KR-10021350-0 on at

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Unable to send message to MCP server

Thank you for the response. It really helps.

We have just now create a custom connector again based on a YAML definition (we changed the hostname).

As a result, connector has only one action, related to sse endpoint.

Still, when adding the action to the agent, it does not have any inputs (even though SessionId input is visible in custom connector wizard and in Power Automate)

Moreover, after adding an action to the topic, agent becomes unresponsive to any kind of prompt (not even Greeting, nor Conversation Boosting seem to work). Agent does not reply at all, there is no movement on the activity map

Are we missing something here?
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zankard Moderator on at

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Unable to send message to MCP server

Hi @KR-10021350-0 - thanks reaching out! Please make sure you are using the YAML schema provided here. If you use that YAML, your connector will result in 1 action (not 2).

Please let me know if this resolves your issue!
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Michael E. Gernaey 44,766 Super User 2025 Season 1 on at

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Unable to send message to MCP server

Hi @KR-10021350-0

Can you see if this helps as it worked for others.

[Step by Step] Using Custom Connectors in Copilot Studio | Rajeev Pentyala - Microsoft Power Platform

If this helps resolve your issue, please Mark as such and maybe a like :-)