fix(anthropic): self-heal on missing thinking-signature errors from Bedrock/Vertex (#33719)

* fix(anthropic): self-heal on missing thinking-signature errors from Bedrock/Vertex

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* fix(anthropic): narrow thinking signature error marker

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* test(router): stabilize prompt caching fixture size

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* chore: re-trigger CI

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Krrish Dholakia <krrishdholakia@berri.ai>
Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/llms/anthropic/common_utils.py

@@ -906,16 +906,17 @@ def strip_advisor_blocks_from_messages(messages: List[Any], replace_with_text: b
 
 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).
+    Detect Anthropic 400 errors caused by missing or invalid thinking signatures.
 
-    Example API message:
+    Known error formats:
+    {"message":"messages.2.content.0.thinking.signature.str: Input should be a valid string"}
+    messages.N.content.M.thinking.signature.str: Input should be a valid string
     messages.N.content.M: Invalid `signature` in `thinking` block
     """
     if not error_text:
         return False
     lower = error_text.lower()
-    return "invalid" in lower and "signature" in lower and "thinking" in lower and "block" in lower
+    return "thinking" in lower and "signature" in lower and ("invalid" in lower or "valid string" in lower)
 
 
 def strip_thinking_blocks_from_anthropic_messages(messages: List[Any]) -> List[Any]:

tests/local_testing/test_anthropic_prompt_caching.py

@@ -172,7 +172,7 @@ def anthropic_messages():
             "content": [
                 {
                     "type": "text",
-                    "text": "Here is the full text of a complex legal agreement" * 400,
+                    "text": "Here is the full text of a complex legal agreement" * 500,
                     "cache_control": {"type": "ephemeral"},
                 }
             ],

tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py

@@ -1261,6 +1261,23 @@ class TestAnthropicThinkingSignatureSelfHeal:
         )
         assert is_anthropic_invalid_thinking_signature_error(raw) is True
 
+    def test_is_anthropic_invalid_thinking_signature_error_positive_bedrock(self):
+        from litellm.llms.anthropic.common_utils import (
+            is_anthropic_invalid_thinking_signature_error,
+        )
+
+        # Real user-reported Bedrock scenario
+        raw = '{"message":"messages.2.content.0.thinking.signature.str: Input should be a valid string"}'
+        assert is_anthropic_invalid_thinking_signature_error(raw) is True
+
+    def test_is_anthropic_invalid_thinking_signature_error_positive_vertex(self):
+        from litellm.llms.anthropic.common_utils import (
+            is_anthropic_invalid_thinking_signature_error,
+        )
+
+        raw = "messages.4.content.1.thinking.signature.str: Input should be a valid string"
+        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,
@@ -1271,6 +1288,11 @@ class TestAnthropicThinkingSignatureSelfHeal:
             is_anthropic_invalid_thinking_signature_error("rate limit exceeded")
             is False
         )
+        assert (
+            is_anthropic_invalid_thinking_signature_error("invalid_request_error: model not found")
+            is False
+        )
+        assert is_anthropic_invalid_thinking_signature_error("thinking signature is malformed") is False
 
     def test_strip_thinking_blocks_from_anthropic_messages(self):
         from litellm.llms.anthropic.common_utils import (