[Limitation]PowerAutomate: Unable to use Custom Connector with actions having parameterized GET requests. Error: “Fix Invalid expression(s) for the input parameter(s) of operation ‘ActionName'”.

Recently, I was working on creating a custom connector that has got one action which is a GET request and accepts parameters. The purpose was to use this custom connector in Power Automate which could then have been executed on demand or create/update of record.

Below is the definition of custom connector:

And here’s the request details(Used Import from sample to construct this):
Verb: GET
URL: CalculateRollupField(Target=@target,FieldName=@fieldName)?@target={‘@odata.id’:’accounts(96ca73be-14e5-eb11-bacb-0022481016fb)’}&@fieldName=’new_noofcontacts’

I have passed 2 parameters in the URL @target and @fieldName. So, finally the request looks like as shown below:

Then I tested the custom connector and it was working fine. It refreshed the roll up field as expected.

So, I thought of using it in Power Automate so that I can refresh the roll up field on create/update of a child record as shown below:

When I started putting any value for any of the parameter the Power Automate started complaining with error: “Fix Invalid expression(s) for the input parameter(s) of operation <Action Name>”. It didn’t allow me to save. The reason of the issue is the parameter name starts with “@” which is a special character in Power Automate.

I tried to pass value to those parameters in different ways but none of them worked. Finally, I raised a support request with Microsoft to find out if there is any workaround. However, there is none at the moment and Microsoft acknowledged that this is a limitation.

As per Microsoft, “This is a design behavior from flow, where flow designer does not allow parameters containing special character @ in their name.”

As per Microsoft, The product group has been actively working on the code fix, and planning to complete the process at the 1^st week of September.

I’ll keep this post updated as soon as I hear back from Microsoft on this. Hope it helps !!