When Copilot fails to authenticate, you may see an error referencing a PKCE code verifier mismatch. This occurs when the cryptographic challenge sent by your client application does not match the verifier expected by the Microsoft identity platform. The mismatch blocks the token exchange and prevents Copilot from accessing Microsoft 365 data. This article explains the root cause of the PKCE code verifier mismatch and provides diagnostic steps to identify and resolve the issue.
Key Takeaways: Diagnosing PKCE Code Verifier Mismatch in Copilot
- PKCE code verifier and challenge mismatch: The SHA-256 hash of the code verifier must match the code challenge sent in the authorization request.
- Azure AD app registration > Authentication > Advanced settings: Ensure PKCE is enabled and the redirect URI matches your client configuration exactly.
- Browser developer tools > Network tab: Inspect the authorization and token endpoints to verify the code_verifier and code_challenge parameters.
Why the PKCE Code Verifier Mismatch Occurs
Proof Key for Code Exchange (PKCE) is an OAuth 2.0 extension that prevents authorization code interception attacks. During the authorization flow, your client application generates a random code verifier string. It then sends a SHA-256 hash of that verifier, called the code challenge, to the authorization endpoint. When the client exchanges the authorization code for tokens, it must send the original code verifier. The Microsoft identity platform hashes the verifier and compares it to the stored code challenge. If the two do not match, the token exchange fails with a PKCE code verifier mismatch error.
Common Causes of the Mismatch
The mismatch usually stems from one of three issues:
- Incorrect code verifier generation: The client generates a verifier that does not match the challenge sent earlier. This can happen if the verifier is regenerated between the authorization request and the token exchange.
- URL encoding differences: The code verifier must be sent URL-encoded in the token request. If the encoding method differs between the authorization and token endpoints, the hash will not match.
- Proxy or middleware interference: An intermediate service modifies the code verifier or challenge parameters, breaking the cryptographic link.
Diagnostic Steps to Identify the Mismatch
Follow these steps to locate the source of the PKCE code verifier mismatch. Perform each step in order.
- Capture the authorization request and token exchange with browser developer tools
Open your browser’s developer tools (F12) and go to the Network tab. Reproduce the Copilot authentication failure. Filter for requests tologin.microsoftonline.com. Locate the authorization request (HTTP 302 or 200) and the token exchange request (POST to/oauth2/v2.0/token). - Extract the code_challenge from the authorization request
In the authorization request URL, find thecode_challengeparameter. Copy the full value. It should be a base64url-encoded SHA-256 hash without padding characters. - Extract the code_verifier from the token request
In the POST body of the token request, find thecode_verifierparameter. Copy the full value. This is the plaintext string your client sent. - Recompute the SHA-256 hash of the code_verifier
Use a command-line tool or online SHA-256 generator to hash the code verifier. Then base64url-encode the result without padding. Compare this value to the code_challenge you captured. If they differ, the issue is in the client’s verifier generation or encoding. - Verify URL encoding consistency
Check that the code_verifier in the token request is URL-encoded exactly as the client generated it. Common encoding mismatches include treating+as a space or failing to encode=characters. - Inspect the redirect URI for exact match
In your Azure AD app registration, go to Authentication > Redirect URIs. Confirm the URI matches the one in the authorization request exactly, including trailing slashes and protocol.
If Copilot Still Has Authentication Issues After the Diagnostic
Token request returns 400 Bad Request with error invalid_grant
If your computed hash matches but the token exchange still fails, the issue may be a stale authorization code. Authorization codes expire after 10 minutes. If your client delays the token exchange beyond that window, the server rejects the request. Regenerate the authorization code and retry the exchange within the time limit.
Client logs show code_verifier parameter missing
Some client libraries omit the code_verifier in the token request if PKCE is not explicitly enabled. Verify that your OAuth library supports PKCE and that you have set the usePKCE option to true. For Microsoft Authentication Library (MSAL), this is enabled by default for single-page applications.
Error appears only in specific network environments
Corporate proxies or VPNs may strip or alter the code_verifier parameter. Work with your network team to allowlist the Microsoft identity platform endpoints and disable SSL inspection for login.microsoftonline.com and all subdomains.
PKCE Code Verifier Mismatch vs Other OAuth Errors: Key Differences
| Item | PKCE Code Verifier Mismatch | Other OAuth Errors |
|---|---|---|
| Error message | invalid_grant with description referencing code verifier | invalid_request, unauthorized_client, or access_denied |
| Root cause | Cryptographic hash mismatch between challenge and verifier | Expired authorization code, wrong redirect URI, or invalid client ID |
| Diagnostic focus | Inspect code_challenge and code_verifier values | Check token expiration, redirect URI, and app permissions |
| Fix approach | Correct verifier generation or encoding | Update app registration settings or regenerate authorization code |
By following the diagnostic steps in this article, you can identify the exact cause of a PKCE code verifier mismatch in Copilot authentication. Start by capturing the network traffic and comparing the cryptographic values. If the hash matches, investigate authorization code expiration or network interference. For persistent issues, enable detailed logging in MSAL to capture the raw parameters sent to the token endpoint.