If Notion returns invalid_grant on the OAuth code exchange, compare the redirect_uri sent in the authorize call with the one in the token call. Make them identical, or omit the parameter from both calls entirely, and the exchange will go through.

Context: Stack Overflow #67782515 (accepted answer, 13 votes): the asker completed the Notion OAuth authorize step fine but got invalid_grant when exchanging the code at /v1/oauth/token. The accepted answer found the mismatch: the authorize URL and the token request disagreed on redirect_uri. Removing the redirect parameter from both calls, so neither call sent it, made the exchange succeed. Notion validates that the two calls agree on the value.