Fix AADB2C90018: 3 Fast Steps for Client ID in 2025

Step 2: Audit Your Application's Configuration File

Now that you have the correct Client ID on your clipboard, it's time to check where your application is actually getting it from. This is, by far, the most common point of failure. A tiny discrepancy between the portal and your code's configuration is all it takes to trigger AADB2C90018.

The location of this setting depends on your application's framework.

For ASP.NET Core (appsettings.json)

In modern .NET applications, you'll find this in your appsettings.json file, or an environment-specific version like appsettings.Development.json.

{
  "AzureAdB2C": {
    "Instance": "https://yourtenant.b2clogin.com",
    "Domain": "yourtenant.onmicrosoft.com",
    "ClientId": "12345678-abcd-1234-abcd-1234567890ab", // <-- PASTE THE CORRECT ID HERE
    "SignUpSignInPolicyId": "B2C_1_signup_signin",
    // ... other settings
  }
}

Action: Paste the Client ID you copied from the Azure portal directly into the ClientId field. Check for any hidden characters or spaces that your editor might have added.

For Legacy ASP.NET (Web.config)

For older .NET Framework applications, your settings are likely in the Web.config file under the <appSettings> section.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <appSettings>
    <add key="ida:Tenant" value="yourtenant.onmicrosoft.com" />
    <add key="ida:ClientId" value="12345678-abcd-1234-abcd-1234567890ab" /> <!-- PASTE HERE -->
    <add key="ida:SignUpSignInPolicyId" value="B2C_1_signup_signin" />
    <!-- ... other settings -->
  </appSettings>
  <!-- ... -->
</configuration>

Action: Replace the value for the ida:ClientId key with the one from the portal. Be extra careful with XML syntax.

Pro Tip: Are you working with multiple environments (Dev, Staging, Prod)? Double-check that you are editing the correct configuration file or using the correct environment variable. It's incredibly common to have a valid Production Client ID accidentally configured in your Development environment, which points to a different B2C tenant.

Step 3: Validate the B2C Tenant and Policy

If you've verified the Client ID is a perfect match and you're still seeing the error, the issue might be one level higher: you could be talking to the wrong B2C tenant entirely.

Remember the error message: "...not registered in tenant '[your_tenant_name]'." The Client ID is unique per tenant. A Client ID from your `mycompany-dev` tenant will not be recognized by your `mycompany-prod` tenant.

In your configuration file, look for the setting that defines the B2C instance or authority URL. It usually looks like this:

• Instance: https://yourtenant.b2clogin.com
• Authority: https://yourtenant.b2clogin.com/yourtenant.onmicrosoft.com/B2C_1_your_policy

Action: Compare the yourtenant part of this URL with the tenant name you see in the Azure portal where you copied the Client ID from. If you copied the ID from `mycompany-prod.onmicrosoft.com` but your config file points to `https://mycompany-dev.b2clogin.com`, you've found the problem. Your app is using a valid key on the wrong lock.

Ensure that the tenant name in your config URL matches the tenant where the App Registration exists. This simple cross-reference solves a surprising number of AADB2C90018 errors that persist after checking the ID itself.

Quick Fix Table: Common Mistakes & Solutions

Here’s a quick reference table to help you spot the issue faster.

Conclusion: Banish AADB2C90018 for Good

The AADB2C90018 error feels daunting, but it almost always boils down to a simple configuration mismatch. By systematically checking these three areas, you can quickly resolve the issue and move on to more important work.

To recap our three fast steps for 2025 and beyond:

1. Source of Truth: Get the exact Application (client) ID from the correct App Registration in the Azure portal.
2. Application Config: Paste that ID into your application's configuration file (appsettings.json or Web.config), ensuring you're editing the right file for your environment.
3. Tenant Context: Verify that your application's configuration points to the correct B2C tenant URL—the same tenant where the app is registered.

Next time you see this error, don't panic. Take a deep breath, follow these steps, and you'll have it fixed in minutes. Happy coding!