diff --git a/src/claude_agent_sdk/_internal/message_parser.py b/src/claude_agent_sdk/_internal/message_parser.py
index 2363bb31..9f99b99f 100644
--- a/src/claude_agent_sdk/_internal/message_parser.py
+++ b/src/claude_agent_sdk/_internal/message_parser.py
@@ -107,7 +107,7 @@ def parse_message(data: dict[str, Any]) -> Message | None:
                             content_blocks.append(
                                 ThinkingBlock(
                                     thinking=block["thinking"],
-                                    signature=block["signature"],
+                                    signature=block.get("signature", ""),
                                 )
                             )
                         case "tool_use":
diff --git a/tests/test_message_parser.py b/tests/test_message_parser.py
index be5846bc..5540265b 100644
--- a/tests/test_message_parser.py
+++ b/tests/test_message_parser.py
@@ -274,6 +274,29 @@ def test_parse_assistant_message_with_thinking(self):
         assert isinstance(message.content[1], TextBlock)
         assert message.content[1].text == "Here's my response"
 
+    def test_parse_assistant_message_with_thinking_missing_signature(self):
+        """Test parsing an assistant message with thinking block missing signature."""
+        data = {
+            "type": "assistant",
+            "message": {
+                "content": [
+                    {
+                        "type": "thinking",
+                        "thinking": "Redacted thinking content",
+                    },
+                    {"type": "text", "text": "Here's my response"},
+                ],
+                "model": "claude-opus-4-1-20250805",
+            },
+        }
+        message = parse_message(data)
+        assert isinstance(message, AssistantMessage)
+        assert len(message.content) == 2
+        assert isinstance(message.content[0], ThinkingBlock)
+        assert message.content[0].thinking == "Redacted thinking content"
+        assert message.content[0].signature == ""
+        assert isinstance(message.content[1], TextBlock)
+
     def test_parse_assistant_message_with_usage(self):
         """Per-turn usage is preserved on AssistantMessage.