fix(anthropic): detect invalid thinking signatures from Vertex AI error format

Remove the `"block"` keyword requirement from is_anthropic_invalid_thinking_signature_error.
The Vertex AI error format uses "messages.N.content.M.thinking.signature.str: Input should be a valid string"
which contains "invalid", "signature", and "thinking" but not "block". The triple combination
is already specific enough to avoid false positives.

Fixes #26005

litellm/llms/anthropic/common_utils.py

@@ -763,8 +763,9 @@ def is_anthropic_invalid_thinking_signature_error(error_text: str) -> bool:
     Detect Anthropic 400 when encrypted thinking signatures in history do not match
     the current deployment (e.g. user rotated API key or switched model endpoint).
 
-    Example API message:
-    messages.N.content.M: Invalid `signature` in `thinking` block
+    Example API messages:
+    - messages.N.content.M: Invalid `signature` in `thinking` block
+    - messages.N.content.M.thinking.signature.str: Input should be a valid string
     """
     if not error_text:
         return False
@@ -773,7 +774,6 @@ def is_anthropic_invalid_thinking_signature_error(error_text: str) -> bool:
         "invalid" in lower
         and "signature" in lower
         and "thinking" in lower
-        and "block" in lower
     )
 
 

tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py

@@ -1153,6 +1153,20 @@ class TestAnthropicThinkingSignatureSelfHeal:
         )
         assert is_anthropic_invalid_thinking_signature_error(raw) is True
 
+    def test_is_anthropic_invalid_thinking_signature_error_vertex_ai(self):
+        """Vertex AI returns a different error format for invalid thinking signatures."""
+        from litellm.llms.anthropic.common_utils import (
+            is_anthropic_invalid_thinking_signature_error,
+        )
+
+        raw = (
+            '{"type":"error","error":{"type":"invalid_request_error",'
+            '"message":"messages.1.content.0.thinking.signature.str: '
+            'Input should be a valid string"},'
+            '"request_id":"req_vrtx_011CaB4qJPWdYMoM8yyQUQ5C"}'
+        )
+        assert is_anthropic_invalid_thinking_signature_error(raw) is True
+
     def test_is_anthropic_invalid_thinking_signature_error_negative(self):
         from litellm.llms.anthropic.common_utils import (
             is_anthropic_invalid_thinking_signature_error,