diff --git a/EnvelopeGenerator.API/Extensions/ReceiverClaimExtensions.cs b/EnvelopeGenerator.API/Extensions/ReceiverClaimExtensions.cs
index 205be48d..05f3c878 100644
--- a/EnvelopeGenerator.API/Extensions/ReceiverClaimExtensions.cs
+++ b/EnvelopeGenerator.API/Extensions/ReceiverClaimExtensions.cs
@@ -11,6 +11,10 @@ namespace EnvelopeGenerator.API.Extensions;
 /// </summary>
 public static class ReceiverClaimExtensions
 {
+    private static readonly string[] EnvelopeIdClaimTypes = [EnvelopeClaimTypes.Id, "envelope_id", "EnvelopeId"];
+    private static readonly string[] EnvelopeUuidClaimTypes = [ClaimTypes.NameIdentifier, "envelope_uuid", "EnvelopeUuid"];
+    private static readonly string[] ReceiverSignatureClaimTypes = [ClaimTypes.Hash, "receiver_sig", "ReceiverSignature"];
+
     private static string GetRequiredClaimOfReceiver(this ClaimsPrincipal user, string claimType)
     {
         var value = user.FindFirstValue(claimType);
@@ -27,15 +31,32 @@ public static class ReceiverClaimExtensions
         throw new InvalidOperationException(message);
     }
 
+    private static string GetRequiredClaimOfReceiver(this ClaimsPrincipal user, params string[] claimTypes)
+    {
+        foreach (var claimType in claimTypes.Where(t => !string.IsNullOrWhiteSpace(t)).Distinct())
+        {
+            var value = user.FindFirstValue(claimType);
+            if (!string.IsNullOrWhiteSpace(value))
+                return value;
+        }
+
+        var identity = user.Identity;
+        var principalName = identity?.Name ?? "(anonymous)";
+        var authType = identity?.AuthenticationType ?? "(none)";
+        var availableClaims = string.Join(", ", user.Claims.Select(c => $"{c.Type}={c.Value}"));
+        var message = $"Required claim(s) '{string.Join("', '", claimTypes)}' are missing for user '{principalName}' (auth: {authType}). Available claims: [{availableClaims}].";
+        throw new InvalidOperationException(message);
+    }
+
     /// <summary>
     /// Gets the authenticated envelope UUID from the claims.
     /// </summary>
-    public static string GetEnvelopeUuidOfReceiver(this ClaimsPrincipal user) => user.GetRequiredClaimOfReceiver(ClaimTypes.NameIdentifier);
+    public static string GetEnvelopeUuidOfReceiver(this ClaimsPrincipal user) => user.GetRequiredClaimOfReceiver(EnvelopeUuidClaimTypes);
 
     /// <summary>
     /// Gets the authenticated receiver signature from the claims.
     /// </summary>
-    public static string GetReceiverSignatureOfReceiver(this ClaimsPrincipal user) => user.GetRequiredClaimOfReceiver(ClaimTypes.Hash);
+    public static string GetReceiverSignatureOfReceiver(this ClaimsPrincipal user) => user.GetRequiredClaimOfReceiver(ReceiverSignatureClaimTypes);
 
     /// <summary>
     /// Gets the authenticated receiver display name from the claims.
@@ -57,7 +78,7 @@ public static class ReceiverClaimExtensions
     /// </summary>
     public static int GetEnvelopeIdOfReceiver(this ClaimsPrincipal user)
     {
-        var envIdStr = user.GetRequiredClaimOfReceiver(EnvelopeClaimTypes.Id);
+        var envIdStr = user.GetRequiredClaimOfReceiver(EnvelopeIdClaimTypes);
         if (!int.TryParse(envIdStr, out var envId))
         {
             throw new InvalidOperationException($"Claim '{EnvelopeClaimTypes.Id}' is not a valid integer.");