“A required Metadata item with key \””ClientId\”” was not found in the TechnicalProfile with id …” error in Azure AD B2C

I was creating a custom policy when I got this error:

A quick look at AppInsights and I see:

""Exception"": {
     ""Kind"": ""Handled"",
     ""HResult"": ""80131500"",
     ""Message"": ""A required Metadata item with key \""ClientId\"" was not
       found in the TechnicalProfile with id \""AAD-UserReadUsingEmailAddress\""
       in policy \""..."" of tenant \""tenant.onmicrosoft.com\""."",
     ""Data"": {
     ""IsPolicySpecificError"": true,
     ...

WTF! There is no clientID in UserRead? And Mr. Google had nothing to offer 😢

So I did a bit of poking around and realised that one of the attributes that I was reading was an extension attribute. Which rang a bell …

As per the docs, you need to configure this:

<TechnicalProfile Id="AAD-Common">
    <Metadata>
        <!--Insert b2c-extensions-app application ID here, for example: 11111111-1111-1111-1111-111111111111-->
        <Item Key="ClientId"></Item>
        <!--Insert b2c-extensions-app application ObjectId here, for example: 22222222-2222-2222-2222-222222222222-->
        <Item Key="ApplicationObjectId"></Item>
    </Metadata>
</TechnicalProfile>

So when I tried to read the extension attribute, the clientID it referred to was the one in the AAD-Common technical profile. A somewhat misleading error message!

All good!