Custom MCP Federated Connector returns expired above certain response size

Question

Custom MCP Federated Connector returns expired above certain response size

Thomas Broatch 0

I'm building on a custom federated connector. I setup the connector successfully, everything works from an authentication / tool discovery standpoint on CoPilot chat.

Doing a small tool call works fine, but at a certain size, CoPilot says that Expired is returned. I can't get the exact size of failure but I estimate it to be around 50 KB.

I could consistently reproduce the bug by getting timeseries data from one of my tools, and progressively increasing the date range until it failed (see attached screenshots), but any tool where you can vary the response size based on inputs should be able to reproduce the problem.

There's a few bugs here

The error message is wrong since the tool call isn't actually expired, the problem stems from response size. This also has the side effect of confusing copilot since it believes there's an expired connection.
The response size which causes a failure is quite small which significantly limits the usefulness of our custom federated connector. ChatGPT and Claude can handle response sizes much larger than this without problem.

Interestingly, researcher mode in copilot chat can also handle response sizes that caused this error regular copilot chat mode, but because of how long it takes researcher to respond, it isn't really practical for our use case.

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Prasad-MSFT 10,546 Reputation points Microsoft External Staff Moderator

2026-09-03T06:18:04.08+00:00
This sounds like a platform limitation rather than an actual authentication/expiration issue.

If small payloads work consistently and failures occur only as the response size grows, the reported "Expired" error is likely misleading and masking an underlying response size/token limit.

The fact that Researcher mode can handle the same payloads is another indicator that the connector itself is healthy and the limitation is specific to the standard Copilot Chat execution path.

Unfortunately, there isn't much publicly documented around maximum response sizes for federated connector tool calls, so this may require confirmation from Microsoft.

As a workaround, consider:

Returning summarized/aggregated results instead of raw timeseries data.

Paginating large datasets.

Supporting date-range chunking so Copilot can request smaller windows of data.

Returning a link/reference to the full dataset rather than the entire payload inline.

Your answer

Prasad-MSFT 10,546 Reputation points Microsoft External Staff Moderator

2026-09-03T06:18:04.08+00:00

This sounds like a platform limitation rather than an actual authentication/expiration issue.

If small payloads work consistently and failures occur only as the response size grows, the reported "Expired" error is likely misleading and masking an underlying response size/token limit.

The fact that Researcher mode can handle the same payloads is another indicator that the connector itself is healthy and the limitation is specific to the standard Copilot Chat execution path.

Unfortunately, there isn't much publicly documented around maximum response sizes for federated connector tool calls, so this may require confirmation from Microsoft.

As a workaround, consider:

Returning summarized/aggregated results instead of raw timeseries data.

Paginating large datasets.

Supporting date-range chunking so Copilot can request smaller windows of data.

Returning a link/reference to the full dataset rather than the entire payload inline.