fix(anthropic): dedup trailing /v1 in api_base before appending /v1/messages

AnthropicMessagesConfig.get_complete_url appended /v1/messages to any
api_base that didn't already end in it, so a base ending in /v1 (e.g. an
Anthropic-compatible gateway root) produced /v1/v1/messages and a 404.
The openai_like and deepseek messages configs already normalize this;
mirror their rstrip/removesuffix handling and add the matching
parametrized tests.

Fixes #36956

litellm/llms/anthropic/experimental_pass_through/messages/transformation.py

@@ -225,9 +225,13 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
         stream: bool | None = None,
     ) -> str:
         api_base = AnthropicModelInfo.get_api_base(api_base) or "https://api.anthropic.com"
-        if not api_base.endswith("/v1/messages"):
-            api_base = f"{api_base}/v1/messages"
-        return api_base
+        # Normalize like OpenAILikeAnthropicMessagesConfig: an api_base that
+        # already ends in /v1 must not become /v1/v1/messages.
+        base = api_base.rstrip("/")
+        if base.endswith("/v1/messages"):
+            return base
+        base = base.removesuffix("/v1")
+        return f"{base}/v1/messages"
 
     def validate_anthropic_messages_environment(
         self,

tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_messages_get_complete_url.py

@@ -0,0 +1,41 @@
+import pytest
+
+from litellm.llms.anthropic.experimental_pass_through.messages.transformation import (
+    AnthropicMessagesConfig,
+)
+
+
+@pytest.fixture
+def config() -> AnthropicMessagesConfig:
+    return AnthropicMessagesConfig()
+
+
+@pytest.mark.parametrize(
+    "api_base, expected",
+    [
+        # default when no api_base is configured
+        (None, "https://api.anthropic.com/v1/messages"),
+        # bases without a version suffix
+        ("https://host", "https://host/v1/messages"),
+        ("https://host/", "https://host/v1/messages"),
+        # bases that already carry /v1 must not become /v1/v1/messages
+        ("https://host/v1", "https://host/v1/messages"),
+        ("https://host/v1/", "https://host/v1/messages"),
+        ("https://api.groq.com/openai/v1", "https://api.groq.com/openai/v1/messages"),
+        # full endpoint passes through untouched
+        ("https://host/v1/messages", "https://host/v1/messages"),
+        # provider-prefixed path segments are preserved
+        ("https://gateway.example.com/anthropic", "https://gateway.example.com/anthropic/v1/messages"),
+        ("https://gateway.example.com/anthropic/v1", "https://gateway.example.com/anthropic/v1/messages"),
+    ],
+)
+def test_get_complete_url_deduplicates_trailing_v1(config, api_base, expected, monkeypatch):
+    monkeypatch.delenv("ANTHROPIC_API_BASE", raising=False)
+    url = config.get_complete_url(
+        api_base=api_base,
+        api_key="sk-test",
+        model="claude-sonnet-4-5",
+        optional_params={},
+        litellm_params={},
+    )
+    assert url == expected