Error when trying to add a Custom Connector with MCP SSE as Actions.

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Posted on by CU23031722-0

When adding a custom connector operation which produces: text/event-stream for MCP SSE communication, clicking the operation in Copilot Studio causes a TypeError: Cannot read properties of undefined (reading 'find'). The connector matches the MCP SSE transport specification and the Copilot Studio sample, works in Power Apps, and the server responds correctly.

The Custom Connector Test functionality works fine with 200 status and schema validation successful. But when adding an Action with the custom connector is showing this error on Browser Console.

Uncaught (in promise)

	message: "Cannot read properties of undefined (reading 'find')"
name: "TypeError"
stack: "TypeError: Cannot read properties of undefined (reading 'find')\n at https://content.powerapps.com/resource/makerx/static/js/module/ActionCreationWizard~~697903b6.3ca264bd.chunk.js:2:55495\n at Generator.next (<anonymous>)\n at m (https://content.powerapps.com/resource/makerx/static/js/module/ActionCreationWizard~~697903b6.3ca264bd.chunk.js:2:54268)\n at a (https://content.powerapps.com/resource/makerx/static/js/module/ActionCreationWizard~~697903b6.3ca264bd.chunk.js:2:54471)"
[[Prototype]]: Object

	

onenotemcpserver....

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Calling actions from Copilot Studio

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sandeepstw 312 Moderator on at

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You are getting an error TypeError: Cannot read properties of undefined (reading 'find') when adding an action using a custom connector that uses Server-Sent Events (SSE) with text/event-stream in Copilot Studio. The connector works in Power Apps and passes all tests, but fails in Copilot Studio with a browser console error.

1. Copilot Studio might not fully support text/event-stream responses in custom connectors, especially when used directly as an action. This response type is streaming and may not be parsed properly.
2. Check if your operation in the connector returns a well-defined schema. Even though it's streaming, define a response like:

json >
{
"type": "object",
"properties": {
"message": {
"type": "string"
}
}
}
3. Go to the custom connector JSON file or definition, check responses section for the operation and ensure there's a 200 response with correct schema.
4. Avoid using text/event-stream directly. Try modifying the backend to respond with application/json temporarily to test. If it works, issue is with SSE not being handled by Copilot Studio UI.
5. If possible, switch the SSE call to a polling-based API (e.g., fetch latest message via GET every few seconds). This avoids streaming and works with Copilot.
6. Check if the connector exceeds response size limit (500 KB), Copilot Studio has strict limits.
7. If you need to keep using SSE, do not define it as an action. Handle it outside Copilot Studio or via Power Automate, then pass results to Copilot Studio using environment variables or a data source.
8. Use browser dev tools > Network tab to confirm Copilot Studio is sending request correctly and backend is returning proper headers (Content-Type: text/event-stream).
9. As a workaround, build a proxy API that converts the SSE stream to regular JSON responses.
10. If nothing works, open Microsoft support ticket with full connector definition, console error, and steps to reproduce.

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CU23031722-0 2 on at

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I was following this Post
Extend your agent with Model Context Protocol (preview) - Microsoft Copilot Studio | Microsoft Learn

It says Currently, Copilot Studio only supports the Server-Sent Events (SSE) transport, may be the changes have not been rolled out to all regions ?

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