fix(anthropic): propagate api_base to count_tokens endpoint

When a deployment configures a custom api_base, /v1/messages/count_tokens
was silently routing to api.anthropic.com instead. The token counter read
api_base from litellm_params but never passed it to the handler; the handler
had the parameter but used it as a raw URL without appending the path.

Fixes: propagate api_base (and ANTHROPIC_API_BASE env fallback) from
AnthropicTokenCounter.count_tokens into the handler, and add
_build_count_tokens_url to AnthropicCountTokensConfig to correctly append
/v1/messages/count_tokens to any base variant.

litellm/llms/anthropic/count_tokens/handler.py

@@ -68,8 +68,8 @@ class AnthropicCountTokensHandler(AnthropicCountTokensConfig):
 
             verbose_logger.debug(f"Transformed request: {request_body}")
 
-            # Get endpoint URL
-            endpoint_url = api_base or self.get_anthropic_count_tokens_endpoint()
+            # Get endpoint URL; append the count_tokens path when a custom base is given
+            endpoint_url = self._build_count_tokens_url(api_base)
 
             verbose_logger.debug(f"Making request to: {endpoint_url}")
 

litellm/llms/anthropic/count_tokens/token_counter.py

@@ -63,11 +63,16 @@ class AnthropicTokenCounter(BaseTokenCounter):
             verbose_logger.warning("No Anthropic API key found for token counting")
             return None
 
+        api_base: Optional[str] = litellm_params.get("api_base") or os.getenv(
+            "ANTHROPIC_API_BASE"
+        )
+
         try:
             result = await anthropic_count_tokens_handler.handle_count_tokens_request(
                 model=model_to_use,
                 messages=messages,
                 api_key=api_key,
+                api_base=api_base,
                 tools=tools,
                 system=system,
             )

litellm/llms/anthropic/count_tokens/transformation.py

@@ -19,14 +19,22 @@ class AnthropicCountTokensConfig:
     - Response: {"input_tokens": <number>}
     """
 
-    def get_anthropic_count_tokens_endpoint(self) -> str:
-        """
-        Get the Anthropic CountTokens API endpoint.
+    _COUNT_TOKENS_PATH: str = "/v1/messages/count_tokens"
+    _DEFAULT_ENDPOINT: str = f"https://api.anthropic.com{_COUNT_TOKENS_PATH}"
 
-        Returns:
-            The endpoint URL for the CountTokens API
-        """
-        return "https://api.anthropic.com/v1/messages/count_tokens"
+    def get_anthropic_count_tokens_endpoint(self) -> str:
+        return self._DEFAULT_ENDPOINT
+
+    def _build_count_tokens_url(self, api_base: Optional[str]) -> str:
+        """Return the full count_tokens URL, appending the path when a custom base is provided."""
+        if not api_base:
+            return self._DEFAULT_ENDPOINT
+        base = api_base.rstrip("/")
+        if base.endswith(self._COUNT_TOKENS_PATH):
+            return base
+        if base.endswith("/v1/messages"):
+            return f"{base}/count_tokens"
+        return f"{base}{self._COUNT_TOKENS_PATH}"
 
     def transform_request_to_count_tokens(
         self,

tests/test_litellm/llms/anthropic/test_anthropic_count_tokens_transformation.py

@@ -93,3 +93,50 @@ def test_transform_no_system_no_tools():
 
     assert "system" not in result
     assert "tools" not in result
+
+
+def test_build_count_tokens_url_no_base():
+    """No custom base -> default Anthropic endpoint."""
+    config = AnthropicCountTokensConfig()
+    assert (
+        config._build_count_tokens_url(None)
+        == "https://api.anthropic.com/v1/messages/count_tokens"
+    )
+
+
+def test_build_count_tokens_url_bare_base():
+    """Plain domain base gets full path appended."""
+    config = AnthropicCountTokensConfig()
+    assert (
+        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_messages():
+    """Base already ending in /v1/messages gets only /count_tokens."""
+    config = AnthropicCountTokensConfig()
+    assert (
+        config._build_count_tokens_url("https://my-proxy.example.com/v1/messages")
+        == "https://my-proxy.example.com/v1/messages/count_tokens"
+    )
+
+
+def test_build_count_tokens_url_already_full():
+    """Base already containing the full path is returned as-is."""
+    config = AnthropicCountTokensConfig()
+    assert (
+        config._build_count_tokens_url(
+            "https://my-proxy.example.com/v1/messages/count_tokens"
+        )
+        == "https://my-proxy.example.com/v1/messages/count_tokens"
+    )
+
+
+def test_build_count_tokens_url_trailing_slash_stripped():
+    """Trailing slash on the base is ignored."""
+    config = AnthropicCountTokensConfig()
+    assert (
+        config._build_count_tokens_url("https://my-proxy.example.com/")
+        == "https://my-proxy.example.com/v1/messages/count_tokens"
+    )