fix(anthropic): handle /v1-suffixed api_base in _build_count_tokens_url

api_base values ending in /v1 fell through to the catch-all branch and
produced a doubled path (/v1/v1/messages/count_tokens). Add an explicit
branch to append /messages/count_tokens in that case.

litellm/llms/anthropic/count_tokens/transformation.py

@@ -34,6 +34,8 @@ class AnthropicCountTokensConfig:
             return base
         if base.endswith("/v1/messages"):
             return f"{base}/count_tokens"
+        if base.endswith("/v1"):
+            return f"{base}/messages/count_tokens"
         return f"{base}{self._COUNT_TOKENS_PATH}"
 
     def transform_request_to_count_tokens(

tests/test_litellm/llms/anthropic/test_anthropic_count_tokens_transformation.py

@@ -140,3 +140,12 @@ def test_build_count_tokens_url_trailing_slash_stripped():
         config._build_count_tokens_url("https://my-proxy.example.com/")
         == "https://my-proxy.example.com/v1/messages/count_tokens"
     )
+
+
+def test_build_count_tokens_url_base_with_v1_only():
+    """/v1-suffixed base gets /messages/count_tokens appended (not doubled /v1)."""
+    config = AnthropicCountTokensConfig()
+    assert (
+        config._build_count_tokens_url("https://my-proxy.example.com/v1")
+        == "https://my-proxy.example.com/v1/messages/count_tokens"
+    )