diff --git a/tests/PLATFORM-QUIRKS.md b/tests/PLATFORM-QUIRKS.md
index d849007..3f1fd6c 100644
--- a/tests/PLATFORM-QUIRKS.md
+++ b/tests/PLATFORM-QUIRKS.md
@@ -19,6 +19,18 @@ editor after upload.
 
 ## Adobe Sign API Quirks
 
+### Token refresh uses a separate endpoint `/oauth/v2/refresh` (2026-04-16)
+**Symptom:** Calling the standard OAuth2 token endpoint (`/oauth/v2/token`) with
+`grant_type=refresh_token` returns `{"error":"invalid_request","error_description":
+"Invalid grant_type refresh_token"}` — even with a valid, freshly-issued refresh token.  
+**Root cause:** Adobe Sign uses a non-standard separate endpoint for token refresh:
+`/oauth/v2/refresh` (not `/oauth/v2/token`). This differs from the OAuth2 spec and
+virtually all other OAuth2 providers.  
+**Fix applied:** `adobe_api.py` now uses `REFRESH_URL = .../oauth/v2/refresh` for
+refresh requests and `TOKEN_URL = .../oauth/v2/token` only for the initial auth code
+exchange.  
+**Note:** `redirect_uri` is not required in the refresh request and should be omitted.
+
 ### Company/Title contentType returned with `SIGNER_` prefix (2026-04-15)
 **Symptom:** When Company and Title fields are set via the Adobe Sign UI (the API
 rejects `COMPANY`/`TITLE` as contentType values), the API returns them as