diff --git a/tests/app/services/auth/test_token_service.py b/tests/app/services/auth/test_token_service.py
index f5e47af5b41..55e7cdbe3bd 100644
--- a/tests/app/services/auth/test_token_service.py
+++ b/tests/app/services/auth/test_token_service.py
@@ -5,6 +5,14 @@
 
 from invokeai.app.services.auth.token_service import TokenData, create_access_token, verify_token
 
+# Minimum token length to safely modify middle characters for testing
+# JWT tokens have format header.payload.signature and are typically >180 characters
+MIN_TOKEN_LENGTH_FOR_MODIFICATION = 50
+
+# Minimum signature length to safely modify middle characters for testing
+# JWT signatures are typically 43 characters (base64-encoded HMAC-SHA256)
+MIN_SIGNATURE_LENGTH_FOR_MODIFICATION = 10
+
 
 class TestTokenCreation:
     """Tests for JWT token creation."""
@@ -151,10 +159,13 @@ def test_verify_token_with_modified_payload(self):
 
         token = create_access_token(token_data)
 
-        # Try to modify the token by changing a character
+        # Try to modify the token by changing a character in the middle
         # JWT tokens are base64 encoded, so changing any character should invalidate the signature
-        if len(token) > 10:
-            modified_token = token[:-1] + ("X" if token[-1] != "X" else "Y")
+        # Note: We change a character in the middle to avoid Base64 padding issues where
+        # the last character might not affect the decoded value
+        if len(token) > MIN_TOKEN_LENGTH_FOR_MODIFICATION:
+            mid = len(token) // 2
+            modified_token = token[:mid] + ("X" if token[mid] != "X" else "Y") + token[mid + 1 :]
             verified_data = verify_token(modified_token)
             assert verified_data is None
 
@@ -288,11 +299,14 @@ def test_token_signature_verification(self):
         assert verify_token(token) is not None
 
         # Modified token should fail verification
-        if len(token) > 50:
+        if len(token) > MIN_TOKEN_LENGTH_FOR_MODIFICATION:
             # Change a character in the signature part (last part of JWT)
             parts = token.split(".")
-            if len(parts) == 3:
-                modified_signature = parts[2][:-1] + ("X" if parts[2][-1] != "X" else "Y")
+            if len(parts) == 3 and len(parts[2]) > MIN_SIGNATURE_LENGTH_FOR_MODIFICATION:
+                # Modify a character in the middle of the signature to avoid Base64 padding issues
+                # where the last few characters might not affect the decoded value
+                mid = len(parts[2]) // 2
+                modified_signature = parts[2][:mid] + ("X" if parts[2][mid] != "X" else "Y") + parts[2][mid + 1 :]
                 modified_token = f"{parts[0]}.{parts[1]}.{modified_signature}"
                 assert verify_token(modified_token) is None