Slack OAuth refresh token invalid_grant — What it means & how to fix it

When integrating with Slack via OAuth 2.0, the "invalid_grant" error typically arises during token rotation, leading to failed syncs and user actions until re-authorization occurs. This error, indicated by an HTTP 400 response, suggests the refresh token is expired or revoked, often due to reasons like disabled token rotation, using stale tokens, expired access tokens, app uninstallation, or mismatched client credentials. To resolve this, ensure token rotation is enabled, store the latest refresh token, refresh tokens before expiration, and re-authorize when necessary. To prevent future issues, it's crucial to adopt practices such as enabling token rotation, persistently storing new tokens, refreshing ahead of expiration, and maintaining a re-authentication process. Tools like Nango can simplify managing OAuth token lifecycles by automating refreshes and providing clear re-authentication signals.